Section 3 — Process Safety & Loss Prevention

Relief Disposal Systems

IPE Engineering Practice IPE-EP-3-7-2

Document number: IPE-EP-3-7-2 · Section: 3 — Process Safety & Loss Prevention

SCOPE

• This Practice covers the sizing, design, installation and maintenance of closed relief disposal systems and provides guidelines for the use of atmospheric relief disposal systems. This Practice does not apply to marine recovery and vapor processing systems that comply with the

U.S. Department of Transportation regulations administered by the Coast Guard.

• The requirements of this Practice shall be met when any addition or modification is made to an existing relief disposal system.
• Any deviations to this Practice shall be in accordance with the procedures given in EP 1–1–3.
• An asterisk (*) indicates that a decision by the Owner or Owner’s Engineer is required or that additional information is provided by the Purchaser.

2.0 REFERENCES

The latest edition of the following Standards and Publications are referred to herein.

STANDARDS AND PUBLICATIONS

STANDARDS AND PUBLICATIONS (CONTINUED)

DEFINITIONS

• Atmospheric Relief Vent – A vertical pipe on a pressure relief valve outlet for atmospheric discharge of vapor or gas, or both, to a safe location.
• Blowdown System – A system that disposes of discharges of liquids or condensable vapors from units, such as fluid catalytic crackers (FCCs) and cokers, as well as liquid discharges from pressure relief valves.
• Blowdown Valves – Manually, electrically, or pneumatically operated valves used to remove liquids and heavy condensable vapors from units such as cokers and FCCs.
• Built–up Back Pressure – The pressure in the discharge header that develops as a result of flow after the pressure relief valve opens. Built–up back pressure is caused by flow from the particular device and others, if any, which simultaneously discharge into the disposal system. This type of back pressure is variable.
• Closed Pressure Relief System – A system for the collection of vapor and gas discharges from pressure relief valves, vapor depressuring valves, unit pressure control vents, and onsite liquid knockout drums. The system consists of leads, laterals, unit headers, main headers, on–site knockout drums, flare header, flare knockout drum, flare stack, and flare tip. These systems may also utilize scrubbers, absorbers, quench towers, dump tanks, etc in lieu of or in conjunction with flares.
• Coanda – An aerodynamic skin adhesion effect in which gas follows the profile of a curved surface, entraining air up to twenty times its own volume.
• Combustion Support – The addition of fuel gas to an effluent stream to assist in flaring. This may be done for any of the following reasons:
• To increase fuel concentration in order to make the effluent flammable.
• To increase the volume of the effluent or the flare tip velocity, in order to avoid burn–back in the flare tip, flame pulldown outside or a lazy flame situation that could damage an adjacent flare tip.

• To maintain an adequate slot velocity in a flare tip that uses the Coanda effect.
• Contractor – Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Fire Zone – The zone within a process unit in which a surface fire can be isolated by curbing, roads, drainage, or fixed fire protection (active or passive). The defined zones should be consistent with the capabilities of the surface draining systems, so that it is not possible for fire to spread beyond the boundaries of the fire zone through burning oil spreading on top of draining firewater.
• Flame Pulldown – Describes the condition when the flame from the flare is pulled down along the downwind side of the tip and stack during periods of low flare gas flow. This condition is caused by the formation of a low pressure zone on the downwind side of the stack.
• Flare – A means of safe disposal of waste gases by combustion.
• Flare Header – The line from the flare knockout drum to the flare stack.
• Flare Knockout (KO) Drum – A horizontal or vertical offsite knockout drum that receives the discharge from the main flare headers in order to remove liquids that condense in the closed pressure relief system.
• Flare Stack – A vertical pipe that transfers discharged vapor, gas, or both from the flare header to a safe, elevated location for burning at a flare tip.
• Flare Tip – The section of pipe, on top of the flare stack, fitted with pilots and igniter to ensure burning of the discharged vapor, gas, or both. Generally, the flare tip is fitted with a steam injection device for smokeless burning of 10 to 30 percent of the maximum emergency relief load, depending on the type of gas or vapor discharged.
• Ground Flare – A flare system mounted on the ground or in a pit. Such flares are typically enclosed by a refractory wall or a dike.
• Highly Viscous Materials – Any materials which will congeal at flare line operating temperatures.
• Individual Pressure Relief Load – The discharge rate from a particular pressure relief or depressuring device as a result of on overpressure emergency. Individual pressure relief loads for various failure cases are listed in the Relief Load Summary.
• Internal Burning – The burning of gas within the flare tip caused by the combination of wind action and very low gas flow rates.
• Involuntary Relief – Vapor or liquid discharges from pressure relief valves and rupture disks.
• Laterals – The piping that collects the discharge of two or more pressure relief valve leads and directs it into a unit header.
• Leads – The piping between the pressure relief valve discharge nozzle and the tie–in to either a lateral or header.
• Main Header – The main offsite line which receives vapor from onsite knockout drums or unit headers. The main header terminates at the flare knockout drum.

• Manufacturer – The recipient of a direct or indirect purchase order for materials and/or equipment. In this context, a direct order is one issued to a Manufacturer by a Contractor or the Owner. An indirect order is one issued to a Manufacturer by a vendor (recipient of a direct order) for materials, fabricated components, or subassemblies.
• Mast – A guy–supported structure.
• Maximum Smokeless Rate – The maximum flare flow rate which is required to be burned smokelessly.
• Onsite Liquid Knockout (KO) Drum – A horizontal or vertical knockout drum that retains the unflashed liquid discharge from a unit liquid header and discharges vapor to the closed pressure relief vapor system.
• Owner – Inflection Point Engineering, LLC.
• Owner’s Engineer – A Inflection Point Engineering, LLC appointed engineer.
• Plant Pressure Relief Load – The maximum pressure relief load into the relief disposal system during a plant wide emergency, such as an electrical power failure, or the single largest unit relief load, if greater.
• Pressure Relief Valves – A generic term applied to relief valves, safety valves and safety relief valves.
• Pressure Vessels – Includes drums, towers, spheres, bullets, heat exchangers, and all other items required to conform to the ASME Boiler and Pressure Vessel Code.
• Protective Instrumentation – Instrumentation provided to prevent losses of all kinds, particularly in process upsets or emergencies, as distinct from instrumentation provided for normal control.
• Purge Rate – The minimum rate of inert or combustible gas injection required to prevent the oxygen concentration within the flare stack from exceeding a specified level.
• Reliability Analysis – A mathematical technique for assessing in probability terms the performance of a component or system.
• Restricted Access Zone – A volumetric zone around a flare flame that is designated by placards as a restricted area to minimize the risk of injury to personnel due to thermal radiation or heat related exhaustion.
• Self–Erecting Flare – A flare which can be erected without the use of cranes.
• Smokeless Burning – Burning without emitting visible smoke.
• Superimposed Back Pressure – The static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate.
• Tower – A self–supporting structure.
• Unit Header – The main vapor pressure relief line within the battery limit of a unit. Unit headers collect relieved vapors and/or gases from leads, laterals, or both, for discharge to the main header.
• Unit Liquid Header – The main liquid pressure relief line within the battery limit of a unit. Unit liquid headers collect relieved liquids for discharge to an onsite liquid knockout drum, liquid blowdown system, or lower pressure vessel capable of handling the liquid discharge.

• Unit Pressure Relief Load – The maximum pressure relief load in the unit header as a result of a unit wide emergency, such as a fire or an instrument air failure, or the individual pressure relief load, if it is larger.
• Vapor Depressuring – A method of reducing gas or vapor pressure to protect vessels and piping should fire occur.
• Vapor Depressuring Load – The volume of vapor release during depressuring required to protect vessels and piping from failure in the event of fire.
• Vapor Depressuring Valve – A manually, electrically, or pneumatically operated valve that is controlled from a remote location, should a fire occur, to reduce pressure in a vessel or in a process train.
• Voluntary Reliefs – Vapor or liquid discharges from vapor depressuring valves, vents, and liquid blowdown valves.

SELECTION OF RELIEF DISPOSAL SYSTEMS

• General

The choices for relief disposal systems are a closed or an open system. Preference shall be given to a closed relief disposal system for all flammable and toxic reliefs in accordance with paragraph 4.2. Atmospheric discharge may be permitted provided that none of the requirements of paragraphs 4.2 and 4.3.4 are satisfied.

• Use of Closed Relief Disposal Systems
• Any involuntary or voluntary emergency relief meeting any of the following criteria shall be taken to a closed relief disposal systems:
• The relief is a vapor that may result in the formation of a flammable atmosphere at grade or on elevated structures. This is a function of the discharge velocity and can be evaluated in accordance with API RP521 and may require a vapor dispersion study in some instances.
• The relief may pose an unacceptable safety and/or environmental hazard due to the presence of toxic or corrosive materials. This shall be evaluated per API RP521 and may require a vapor dispersion study in some instances.
• The relief will expel a liquid or result in the significant condensation of toxic, corrosive or flammable material in the atmosphere.
• Ignition of the relief stream will result in direct flame impingement upon equipment or unacceptable levels of radiant heat at operating positions, normal paths of travel, emergency exit routes, off plot areas or off site areas. Ignition sources such as lightning, static electricity and autoignition should be reviewed in accordance with API RP521.
• The relief will consist of unusually valuable materials that should be recovered for reprocessing.
• The relief would be illegal with respect to local, state or federal regulations.
• Voluntary reliefs such as normal venting of flammable and toxic materials arising from controlled process variations and sustained discharges for plant operability shall be taken to a closed relief disposal system.
• Selection of Closed Relief Disposal Systems

• Vapor reliefs should be directed to a system or vessel of lower operating pressure. If this is not practical it shall be directed to a flare, scrubber, absorber or other collection system as required by local, state and federal regulations. The system shall be protected by onsite knockout drums, if there is a possibility of liquid carryover into the vapor relief disposal system.
• Liquid reliefs shall be directed to one of the following:
• Process vessels of lower operating pressure.
• The upstream liquid reservoir of pump suctions.
• A vapor relief disposal system equipped with an onsite liquid knockout drum sized to retain the required liquid relief discharge rate for a minimum of 20 minutes (blowdown system), or to a closed liquid relief disposal system with.
• A liquid burner equipped flare.
• To an oily–water sewer if the liquid discharges are non–flammable, non–toxic, and are at a temperature below 140ºF.
• Condensable reliefs should be directed to vessels of lower operating pressure. If this is not practical they should be directed to a vapor relief disposal system equipped with an onsite quench drum.
• Since hydrogen sulfide (H2S) is a highly toxic material that is frequently encountered, general practice rules for selection of disposal means have been established and should be applied as shown in Table 1.
• (*) Specific arrangements shall be made for corrosive reliefs, and these shall be subject to approval by the Owner’s Engineer. Discharge of corrosive substances will normally involve special materials of construction, and the economics of segregating such discharges shall be evaluated. These releases should generally be directed to vessels of lower operating pressure.
• Low temperature reliefs of fluid shall be segregated from wet streams to avoid freezing of lines. For other than atmospheric discharge, a separate knockout drum and closed vent system shall be provided using materials of construction specifically selected for low–temperature service and/or in accordance with a materials analysis. Independent lines to the knockout drum should be provided for liquids of distinctly differing boiling point, i.e., for propane and butane. Where appropriate, consideration shall be given to the provision of methanol injection facilities to prevent hydrate formation and to prevent freezing of condensate that can be trapped in low points.
• Releases of highly viscous materials should be discharged to lower pressure vessels through heat traced lines. If this is not possible the reliefs shall be to an onsite knockout drum through heat traced and insulated lines.
• Exceptions to the selection of relief disposal systems are as follows:
• All vapor depressuring valves shall discharge to a closed pressure relief system.
• Liquid blowdown facilities should only be installed to handle the relatively few involuntary liquid discharges from pressure relief valves (see items 3 through 5).
• Liquids from desalter relief valves should preferably discharge to the crude tower flash zone. Otherwise, they should discharge to a liquid blowdown system.
• The relief valves on liquid filled vessels in gas plants and product treating plants should discharge to an onsite liquid knockout drum that vents vapor to the closed vapor relief disposal system.

• Coke drum pressure relief valves should discharge to a flare via a wax scrubber tower, fin fan condenser, and knockout drum. Handling of normal coker blowdown is a process design consideration and is not covered in this Practice.
• Non–volatile liquid discharge from thermal relief valves may be piped to sewer drains provided that the sewer system has adequate capacity, is properly sealed and vented, and complies with National Pollutant Discharge Elimination System (NPDES) requirements. Caution should be exercised to avoid discharging volatile, toxic, or hot fluids into a sewer.

DESIGN AND INSTALLATION OF ATMOSPHERIC RELIEF DISPOSAL SYSTEMS

• General
• The design of piping associated with atmospheric discharge of relief devices shall be in accordance with EP 5–6–4 and any supplemental requirements in this Practice.
• All flammable and toxic discharges shall comply with the requirements of paragraph 5.3 of this Practice.
• Noise limits shall be maintained in normally manned areas to meet the special limits given in the Owner’s Recommended Environmental Guidelines.
• The distance from a pressure relief valve atmospheric discharge to adjacent equipment may need to be increased dependent on design considerations, such as the following:
• Volume and nature of discharge
• Prevailing wind direction
• Height, type, and frequency of occupancy of adjacent structures
• Noise
• Heat density at grade and on adjacent equipment and structures should ignition of the pressure relief valve discharge occur
• Non–Hazardous Atmospheric Discharge

(*) Safety relief devices discharging air, steam or other non–flammable and non–toxic gases shall discharge to atmosphere at a safe location, as approved by the Owner’s Engineer.

• Flammable and Toxic Atmospheric Discharge
• (*) The duration of atmospheric relief discharge should be limited by the use of protective instrumentation per EP 3–7–4, as specified by the Owner’s Engineer.
• (*) The following criteria shall be met for flammable reliefs, and toxic reliefs as defined by the Owner’s Engineer.
• The discharge velocity should be sufficient to reduce the concentration of flammable material to below the lower flammable limit in the momentum jet, but not so high that a buildup of static electricity might arise. The additional effect of wind assisted dispersion between the jet and any source of ignition may be taken into consideration subject to the approval of the Owner’s Engineer. The use of pilot assisted relief valves may be necessary to achieve adequate jet velocities.
• To maximize dilution in atmospheric discharge, every pressure relief device relieving to atmosphere should have its own discharge line. The discharge line shall be adequately supported, and should be sized to give an exit velocity of a minimum of 500 ft/s up to a maximum of 0.9 Mach No. at the minimum discharge capacity of the device.

• There shall be no unacceptable safety and environmental hazards from the dispersion of toxic material.
• There shall be no significant condensation of flammable or toxic material as determined by calculation in each case.
• There shall be no direct flame impingement or unacceptable radiation levels at operating positions if ignition of the relief stream is required to be taken as a design condition.
• The calculation methods used to justify these general criteria shall be subject to approval by the Owner’s Engineer.
• Multiple pressure relief devices should be considered for atmospheric relief installations. The set pressures of multiple relief valves can be staggered to assist in maintaining a high discharge velocity and in minimizing chatter of pressure relief valves.
• An alternative means of obtaining high discharge velocities and minimum blowdown is to use pilot assisted pressure relief valves. The design shall be such that, in the event of failure of the pilot device, the main unloading valve will open automatically at the system design pressure and will discharge its full capacity. To enable the valve to act in this way, the pilot set pressure must not be less than 5% below the system design pressure.
• (*) The possibility of ignition of a relief discharge, coincident with the presence of an operator in the vicinity shall be considered, and specific means for operator protection or escape provided where necessary. These shall be subject to approval by the Owner’s Engineer. Suitable methods for estimating thermal radiation intensities will be found in API RP521.
• (*) When specified by the Owner’s Engineer, steam or inert gas connections shall be provided for atmospheric reliefs at ambient temperatures or above. These connections are only for extinguishing any ignited release. This shall be by hand control from grade level using double block and bleed valves, connected to the vent after the pressure relief device. A drain hole, left permanently open, shall be provided in the vent line. The vent line drain hole shall be fitted with a short line to a safe location, or be located to discharge away from any operating platform. Such locations shall be subject to approval by the Owner’s Engineer.
• At discharge temperatures below 32ºF, extinguishing connections shall be inert gas.

DESIGN AND INSTALLATION OF CLOSED RELIEF DISPOSAL SYSTEMS

• General
• (*) Where specified by the Purchaser, the feasibility of installing a flare gas recovery system shall be investigated for economic or environmental reasons. Where a flare gas recovery system is installed, a free path through a liquid seal to a flare shall be provided to allow for failure of the recovery system and emergency relief.
• On–site knockout drums, designed and installed in accordance with Section 8.0, shall be provided where it is necessary to prevent liquid being carried over to the closed relief disposal system.
• Quench drums, designed and installed in accordance with Section 7.0, shall be provided where it is necessary to prevent condensables from being carried over to the closed relief disposal system.
• The state (vapor, liquid or condensable vapor) and nature of the relief material shall determine the type of closed relief disposal system to be used.

• Heat tracing or other positive precautions shall be taken where the danger of line, drum or valve freezing exists (see EP 5–6–8, EP 13–12–1 and EP 14–1–1).
• All piping shall be pitched a minimum of 1/4 per 10 ft. and shall drain as follows:
• Leads – Towards the onsite or flare knockout drum.
• Laterals – Toward the onsite or flare knockout drum.
• Unit Headers – Toward the onsite or flare knockout drum.
• Main Headers – Towards the flare knockout drum.
• Flare Header – Towards the flare knockout drum.
• Pressure Relief and depressuring valve inlet piping – Towards the protected piece of equipment.
• Where a constant downward slope is not possible (i.e., line must go over a roadway) an offsite knockout drum shall be provided at the lowest point. This drum shall be designed and sized in accordance with Section 8.0.
• Special Consideration For Disposal to Lower Pressure Systems
• Care shall be taken to ensure that the material being discharged is chemically compatible with the material within the receiving vessel and with the materials of construction of the receiving vessel.
• When relieving cold materials (liquid or vapor) into a hot vessel and vice versa the effect of rapid vaporization shall be considered in the relieving capacity of the system and the design of the receiving vessel. The effects of the liquid flashing as it enters the receiving vessel shall also be taken into consideration.
• When a relief system is designed to prevent pump overpressure, the relief should preferably be directed to the vessel from which the pump takes suction. The relief may discharge directly to the pump suction if adequate cooling is provided to prevent the recycled liquid from overheating the pump or if a constant displacement pump is used.
• When selecting the receiving vessel it shall be assured that the receiving vessel will not be affected by the same non–fire event which may cause the protected vessel or equipment to become overpressured. The receiving vessel may be in the same fire area as the discharging vessel. However, the pressure relief of the receiving vessel must be adequately sized to protect both vessels simultaneously in this case.
• The maximum allowable working pressure (MAWP) of the receiving vessel shall be adequate to contain the expected relief and shall not be lower than that of the protected vessel or equipment, unless it can be shown that the MAWP of the receiving vessel will not be exceeded for the duration of the relief under all operating, emergency, startup and shutdown conditions. Selection of the receiving vessel shall consider the possibility of internal damage due to the impingement of the relief stream on the internal parts of the receiving vessel.
• The pressure relief of the receiving vessel shall be sized to discharge the combined flow rate of all process inflows and the inflow from the relieving vessel or equipment, unless it can be shown that the discharge outlet of the receiving vessel cannot be blocked in while the relieving equipment vessel is in operation.
• Pressure Relief Device Inlet Piping

• The inlet piping to pressure relief devices shall be designed and installed in accordance with EP 5–6–4.
• Inlet piping to pressure relief devices shall be drained in accordance with EP 5–6–4 and paragraph 6.1 of this Practice.
• The installation of isolation valves in the inlet piping to the pressure relief device shall be in accordance with EP 5–6–4.
• Sizing of Closed Relief Disposal System Piping
• The design of the pressure relief device discharge piping shall start with the determination of the maximum relief load that each section of piping may experience in accordance with

paragraph 6.5 of this Practice. The individual components of the relief disposal system shall be sized as follows:

• Leads – The maximum individual pressure load from the pressure relief device served by the lead. This is the maximum load listed in summary of the Relief Loads per EP 3–7–3.
• Laterals – The largest individual pressure relief load of the pressure relief devices served by the lateral or the largest relief load which could be imposed on the lateral by the simultaneous operation of multiple pressure relief devices, whichever is greater.
• Unit Header – Unit pressure relief load per paragraph 6.5.3 of this Practice.
• Main Header – Plant pressure relief load per paragraph 6.5.4 of this Practice.
• Flare Header – Plant pressure relief load per paragraph 6.5.4 of this Practice.
• It should be noted that the maximum relief/disposal load is the load that will impose the largest pressure drop within the system and not necessarily the load with the largest mass/volume flow.
• Determination of Relief Loads
• Individual Pressure Relief Load – The individual pressure relief load shall be determined in accordance with EP 3–7–1.
• Vapor Depressuring System Load – The depressuring load of individual vapor depressuring systems shall be determined in accordance with EP 3–7–1 and API RP 521, Paragraph 3.19.
• The unit pressure relief load shall be the sum of the unit’s normal venting load and the greatest emergency pressure relief load arising from a single unit wide event. The following events should be considered when calculating the emergency pressure relief load:
• The maximum pressure relief load of any single vessel or piece of equipment within the unit. Note that the pressure relief load of a vessel or piece of equipment with multiple pressure relief devices would be the summation of the individual pressure relief loads of all its pressure relief devices.
• Unit Cooling Water Supply Loss – Cooling water loss shall be considered as the following:
• (*) The mechanical failure of one operating cooling tower pump or electrical power failure to all motor driven cooling tower pumps. All spare, steam turbine, driven cooling tower pumps should be considered to automatically start if so equipped. The cooling tower pump drivers shall be specified by the Owner’s Engineer.
• In a partial failure it will be assumed that each service requiring cooling water shall lose the same percentage of cooling water circulation.

• The complete loss of cooling water due to a break in the main cooling water header or due to a closed block valve if the cooling water supply is considered unreliable or if the header is vulnerable to physical damage.
• Loss of level in cooling tower bay will cause all pumps to lose suction and shall be considered.
• Unit Power (Electrical) Failure – Loss of electrical power to the unit shall be considered as follows:
• Determine the effect of a unit wide loss of electric power on all individual equipment relief systems. Partial power loss shall also be considered.
• Sole source continuously operated steam and gas drivers shall be considered to remain in operation.
• Consideration shall be given to the auto–start of steam drivers and instrumentation action as follows:
• Normal operation of all single loop controllers, plus
• Non–operation of either one master controller, or one turbine drive auto–start device, whichever gives the highest relieving rate, plus
• Non–operation of one auto–lockout device on the unit that gives the highest relieving rate.
• Unit Fire – The effect of a unit fire shall be considered as follows:
• Fire zones established in each unit shall be calculated based on an examination of the drainage, curbing, and other methods of segregation in the unit. Particular attention should be given to those pieces of equipment that could release hydrocarbons into two drainage areas simultaneously.
• The maximum relief load for any fire zone shall be the total of the simultaneous discharges from all pressure relief devices in that fire zone.
• The unit fire load shall be the largest of the relief loads calculated for each of the zones.
• No credit shall be taken for air–cooled or water cooled exchangers within the fire zone.
• Credit can be taken for air–cooled or water cooled exchangers outside the fire zone provided that they cannot be blocked off.
• Unit Steam Supply Loss – The complete loss of steam to the unit shall be considered as follows:
• (*) Loss of steam to the unit steam header shall be considered if the source is considered unreliable or the header is vulnerable to physical damage. Operator action shall not be considered as a mitigating factor but the steam decay rate may, if substantiated by calculations approved by the Owner’s Engineer.
• Loss of steam supply to the individual equipment shall be considered assuming that the block and/or control valve in the branch line from the main unit header is closed.
• Unit Instrument Air Supply – The effect on the unit pressure relief loads shall be calculated for loss of instrument air assuming simultaneous movement of all control valves to the failure position.
• Failure of the distributed control systems based on a review of:
• Power supply reliability and vulnerability.
• Data/control wiring reliability and vulnerability.
• Vulnerability of control processing unit with respect to process equipment.

• Others – All major situations applicable to the unit should be considered.
• The plant pressure relief load shall be the sum of the plant’s normal venting load and the greatest emergency pressure relief load arising from a single plant wide event. The following events should be considered when calculating the plant pressure relief load:
• Plant Maximum Relief Rate from a Single Source – Obtain the largest relief load calculated for an individual pressure relief device among all the units connected to a common plant relief disposal system.
• Plant Cooling Water Supply Loss – Obtain the largest relief load calculated for any single unit among all the units connected to a common relief disposal system. When two or more units are connected to a single cooling tower, the sum of the unit relief loads shall be used if it is the largest relief load.
• Plant Power (Electrical) Failure – Obtain the sum of the relief loads calculated for each unit connected to a common relief disposal system.
• Plant Fire – Obtain the plant fire relief load based on the largest fire load or vapor depressuring load calculated for any one individual unit connected to a common flare (Only a single plant fire shall be considered to occur at any time).
• Plant Steam Supply Loss – Obtain the sum of the relief loads calculated for each unit connected to a common relief disposal system.
• Plant Instrument Air Loss – Complete loss of plant instrument air shall be considered. The relief load to any flare shall be the sum of the relief loads calculated for each unit connected to a common relief disposal system.
• When multiple reliefs are related, the resulting condition shall be considered as occurring simultaneously for calculating relief loads.
• Pipe Size Selection and Design for Single Valve Disposal
• If the outlet is connected to a lower–pressure system, the allowable pressure drop in the disposal system shall be hydraulically calculated to ensure that the MAWP of the receiving vessel or equipment is not exceeded. These calculations shall account for the process in flow and out flow of the receiving equipment or vessel.
• The built–up back pressure shall be limited:
• To 10% of the set pressure for conventional pressure relief valves.
• By the recommendations of API RP520, Part I and the Manufacturer’s curves for the effects of back pressure. Additionally, the back pressure should not exceed the rating tabulated in API Std 526, which may be lower than the outlet flange rating.
• The maximum allowable working pressure of all relief disposal system components.
• (*) In calculating the built–up back pressure and determining the proper piping size, the designer may use the Fanning fluid flow equations and kinetic energy correction data from isothermal compressible gas flow (see API RP 520, Part I); the Darcy equation; or a suitable computer program approved by the Owner’s Engineer.
• The designer shall not place any nominal size restrictions on any piping except that the piping shall have a diameter equal to or greater than the outlet of the pressure relief device.

• The line shall be sized for the loads described in paragraph 6.4 using a pipe roughness of 0.018 inch instead of the normally adopted value for clean steel pipe of 0.0018 inch. This reflects the Owner’s experience of increased roughness in relief headers. For existing headers, the excessive calculated back pressures due to using a pipe roughness of 0.018 inch may be re– evaluated on a case by case basis, taking into consideration the service and condition of the header.
• Where back pressure is not significant the line shall be sized to limit the maximum velocity to

0.75 Mach.

• The design temperature under fire conditions shall not be used to set the unit header metallurgy. However, the design of the unit header shall allow for expansion under fire conditions.
• The requirements for layout, mechanical design and fabrication, and pressure relief piping isolation shall be in accordance with EP 5–1–2 and EP 5–6–4.
• It is generally economical to combine the discharges from several pressure relief devices into a common relief disposal system. When considering a common relief disposal system, special consideration should be given to isolating certain relief streams. This involves the following situations:
• Relief streams containing corrosive materials.
• Significant differences in the operating and rated pressures of the equipment connected to the system.
• Pressure relief or depressuring streams that may subject piping to abnormally high or low temperatures.
• Multiple valve disposal systems that handle combustible vapors should not be used for venting air or steam. This is necessary to avoid the formation of flammable mixtures within the relief disposal system.
• In terms of paragraph 6.6.9.2, it may be more economical to up rate the low pressure vessels rather than to size the disposal system for the lowest back pressure or to use two separate relief disposal systems.
• Steam or Electric tracing per EP 5–6–8 or EP 13–12–1 shall be provided on all lines and valves which are expected to handle:
• Streams which may contain large quantities of water in areas where the ambient conditions may cause freezing.
• Streams which may contain highly viscous materials or materials with relatively high pour points.
• Streams which may be subject to hydrate formation.
• Small bore piping connected to disposal systems that is susceptible to blockage by hydration and/or solidification, should be protected by placement, heating or frequent cleaning and replacement.
• Flare headers intended to receive cold relief (i.e. liquid propane) shall be constructed of materials that are not prone to brittle fracturing.
• Pipe Routing
• (*) Flare lines shall be routed to avoid areas of high fire risk. If this is not practical, the lines and their supports shall be protected in a manner acceptable to the Owner.

• See EP 5–1–2 and EP 5–6–4 for further guidance in piping layout and routing.
• Construction
• (*) The thermal expansion and contraction of relief disposal lines shall be accommodated for by providing flexibility in the piping layout or by expansion loops. The use of sliding expansion joints shall not be permitted and the use of piping bellows shall be subject to the Owner’s approval.
• Laterals and leads should enter the unit header from above and at an angle of 30–45 degrees horizontally from the header centerline to discharge in the downstream direction. The same applies for the connection of the unit header to the main header.
• Individual pressure relieving devices within closed systems shall be located above the unit header.
• Relief disposal systems serving multiple units shall be provided with isolating block valves with flushing connections at the unit battery limits, in accordance with EP 5–6–4.
• Where headers with different materials of construction are connected together the higher quality material shall be used for at least 35 feet upstream of the change in the process conditions. This is necessary to prevent damage in the event of a possible backflow.
• Pipe supports and anchors shall meet the requirements of EP 5–5–3.

DESIGN AND INSTALLATION OF QUENCH DRUMS

• General
• Quench Drums shall be in accordance with EP 7–1–1 and shall have a minimum design pressure of 50 psig.
• Quench Drums shall be of the direct quench design as described in Paragraph 4.6.1.3.1 of API RP521. They shall be sized to reduce the temperature of the exit liquid and vapor to 150–200ºF and to vaporize no more than 40–50% of the quenching liquid per API RP521.

DESIGN AND INSTALLATION OF KNOCKOUT DRUMS

• General
• Knockout drum design shall be in accordance with EP 7–1–1 and sizing calculations shall be in accordance with Paragraph 5.4.2 of API RP521.
• Drums shall be designed for full vacuum and shall have a design pressure of at least 50 psig.
• Vaporization Facilities shall be provided on knockout drums when it is necessary to:
• Vaporize light hydrocarbons for liquid disposal.
• Vaporize and superheat cold vapor before it enters the main flare header to prevent congealing or condensation.
• Steam tubes used in vaporization facilities in freezing climates shall be:
• External to the drum, or
• Constructed with a considerable corrosion factor.

• (*) Demister pads should not be used to reduce drums sizes due to the potential for blockage from scale or wax deposits. Their use shall be restricted to clean systems where there is no practical alternative and must be approved by the Owner’s Engineer.
• (*) Knockout drums shall be provided with automatic hydrocarbon liquid removal, unless otherwise approved by the Owner’s Engineer.
• (*) Facilities for water or heavy hydrocarbon removal shall also be provided by automatic or manual means as appropriate. The disposal route and facilities for these liquids shall be approved by the Owner’s Engineer and particular attention should be made to insure that the design does not create a flammable or toxic hazard. Where this is to be done by pumping a two–pump system, different types of drivers (i.e., one steam driven and the other electric) for each pump, shall be provided. Each pump should be sized to empty a half–full knockout drum within two hours and should have a minimum capacity of 25 US gpm. A separate water drain with a separation pot may be provided beneath the knockout pot, if required.
• Instrumentation and control systems for the knockout drum shall be in accordance with Section 12.0 of this Practice and EP 12–1–1.
• Piping systems entering and leaving the knockout drum shall be in accordance with EP 5–1–1.
• Winterization shall be provided for the knockout drum in accordance with EP 5–6–8, EP 13–12– 1 and EP 14–1–1.
• Facilities shall be provided for isolation, venting and purging, inspection, maintenance and cleaning of the knockout drum.
• (*) Specific attention shall be given to the requirements of inspection, maintenance and cleaning where the associated plants cannot be shut down. Proposals shall be submitted for Owner’s Engineer’s approval.
• Onsite Knockout Drums
• An onsite knockout drum shall be provided in all cases where significant quantities of liquid can be relieved from within the battery limit.
• The liquid storage capacity of the drum shall allow for a minimum of 20 minutes hold–up at maximum liquid in–flow to the knockout drum. This capacity shall be provided between the maximum normal liquid level (i.e., the pump trip–in level) and the maximum level allowable in the drum, taking into account:
• Any simultaneous requirements for vapor/liquid separation.
• Any flashing of the relieved liquid at the knockout drum pressure.
• Offsite Knockout Drum
• An offsite knockout drum shall be provided for each flare. The drum shall be located as close as practical to the flare, but no closer than 50 feet, taking into consideration any access requirements and the possible use of a liquid seal downstream of the drum.
• The offsite knockout drum shall be sized to remove liquid droplets above 600 microns at the maximum emergency gas flow rate and to remove droplets above 150 microns at the maximum smokeless flaring rate. This requirement may be waived if the flare has been equipped to burn larger size liquid droplets.
• Knockout Drums In Cold Service

• In this context, a “cold” stream is defined as a stream at a temperature which could cause freezing in a knockout drum, or on mixing with a stream containing free or dissolved water. The situation is most likely to occur in plants handling liquefied gases or gas streams at high pressure.
• (*) Wherever practical, separate systems shall be provided for cold streams with segregation maintained until the streams become compatible. The flare Manufacturer’s proposals shall be submitted for the Owner’s approval.
• Knockout drums intended to receive cold reliefs (i.e. liquid propane) shall be constructed of materials that are not prone to brittle fracturing.
• The liquid hold–up area, all instrument bridles and pumpout connections, shall be heat traced to prevent freezing per EP 5–6–8, EP 13–12–1 or EP 14–1–1.
• Knockout Drums for Highly Viscous Materials
• Knockout drums intended to handle highly viscous materials shall be provided with heating facilities to maintain the material in the liquid state until it is pumped off. In addition, all pump suction lines and instrument bridles shall be heat traced per EP 5–6–8 or EP 13–12–1.
• Demister pads are prohibited in knockout drums intended for highly viscous materials.
• Precautions shall be taken to ensure that highly viscous materials are not heated to the point of vaporization to prevent congealing and blockages downstream in the vapor relief disposal systems.

FLARES

• Flares and Associated Components
• The design of a flare on a closed relief disposal system requires consideration of the following components to be added to the system:
• Flare, consisting of:
• Flare tip or flare burners
• Flare stack (if elevated) or enclosure (if ground flare)
• Stack support system
• Continuous pilots
• Pilot igniters
• Piping
• Ignition system
• Flame supervision
• Flashback prevention
• Purge system
• Isolation system
• Smoke suppression control system
• Gas sampling system
• Oxygen sampling system
• Flow, temperature and level measurements and alarms
• Pump out facilities for drums
• Fire protection
• Insulation

• Heating and heat tracing
• Cold liquid/vapor vaporization and heating system
• Flare gas recovery system
• General Design Considerations

The following points shall be given specific attention in the overall design of the flare system.

• The safety and well being of all personnel in the vicinity (both on–site and off–site) under all conditions of flare operation. This shall include start–up, purging, operational and emergency flaring, shutdown, inspection and maintenance of all or parts of the flare system.
• The protection of plant and equipment in the vicinity of the flare system under all conditions.
• The protection of the flare from damage by external events (i.e., fires).
• The inherent safety of the flare system itself, especially in respect to the following:
• Flammable or explosive mixtures
• Blockages or flow restrictions
• Toxic components
• Chemical reactions
• Mechanical damage
• Corrosion, erosion and hydrogen embrittlement
• Flare flame stability
• Security of ignition
• Security of pilots
• (*) The flowrate, composition, molecular weight, temperature, frequency and duration of process streams discharging to the flare system at any one time and any inherent restrictions imposed (i.e., allowable back pressure). Particular attention should be paid to depressurizing flowrates, especially if depressurizing is activated because of a fire. The Owner will approve or provide design data such as flare gas composition, molecular weight, flow rates and services available.
• Materials of construction for the flare should be selected to be suitable for operation at the minimum temperature of the system, allowing for any auto–refrigeration from depressurizing.
• The required life of the flare and its components.
• Meteorological and any other relevant environmental conditions pertaining to the site.
• Any local, state and federal regulations, particularly concerning smokeless flaring, flare visibility, pollution, noise restrictions and aircraft lighting.
• The requirement for a cold liquid/vapor vaporization and heating system in situations, where a cold flare cannot be justified.
• Multiple elevated flares may be provided to maintain capacity during partial shutdown for inspection or maintenance. Where this is the case, the flares should be sited sufficiently far apart to reduce the possibility of thermal damage, and so that these activities can be safely carried out on one flare while the others remain in operation.
• (*) If this arrangement is not feasible (e.g., due to space limitations) a tower supported multiple flare system may be proposed for the Owner’s Engineer’s approval. In such an arrangement, each individual flare shall be designed to be dismantled and lowered while the adjacent flares remain in operation. The time required and cost of such dismantling should be evaluated at an early stage in the design. Where necessary, protection against thermal radiation shall be provided for personnel involved in dismantling, inspection, and maintenance.

• Types of Combustion Devices
• (*) The type of combustion device or flare to be used will be specified by the Purchaser or shall be proposed by the flare Manufacturer for the Owner’s Engineer’s approval.
• The selection shall be based on the following:
• Nature, frequency and quantity of relief
• Space available
• Effect on surrounding plants and neighborhood
• Environmental requirements regarding smoke, pollution, noise, radiation and emissions.
• The following combustion devices may be considered for use:
• Steam–Assisted Gas Flares

Steam–assisted flares may be of the external, internal, combined steam jet or Coanda types. Only the external type is permitted in freezing climates.

• Air–Assisted Gas Flares

(*) When smokeless combustion of a gas stream is required and where this requirement can only be satisfied by the use of an assisting fluid, air may be used if it is more convenient or more economical than using steam, high–pressure gas or water. The means by which the air is provided will be specified by the Purchaser or shall be proposed by the flare Manufacturer for approval by the Owner’s Engineer.

• High Pressure Gas–Assisted Flares
• High pressure flares should be considered when it is advantageous to minimize the heat radiated from the flare, or when there is a possibility of liquids passing through the flare.
• When high pressure Coanda type flares are used, the flare radiation calculations shall take into account the lower heat radiation factor and the shorter, stiffer flame produced by these flares, especially internal Coanda type flares.
• Water–Assisted Gas Flares
• (*) Water may be used for inspiration of air, particularly for installations at ground level if the relatively large volumes of water required are available. Proposals to use water shall be submitted for approval by the Owner’s Engineer before proceeding with design.
• The use of water–assisted gas flares shall not be permitted in freezing climates.
• Unassisted Gas Flares (Pipe Flares)

Pipe flares shall be used only for duties where there are no restrictions on radiation or smoke production. Pipe flares generally consists of pilots, igniters, wind deflectors and a flame stabilizer.

• Multipoint (multiple burner) flares

Multipoint (multiple burner) flares may be utilized to provide smokeless burning where it is not economical to provide utilities such as steam, air or water. These flares may also be utilized where flare radiation and noise is a concern.

• Liquid Burners
• (*) Liquid burners may be steam, air or gas atomized, as approved by the Owner’s Engineer. They shall be easily removable for maintenance and have individual isolation valves for the liquid fuel and atomizing medium. Provision should be made for purging the liquid fuel side of the liquid burner before removal.
• The flare shall be sized to allow the maximum relief load to be handled with one liquid burner out of service.
• A permanent gas or light oil pilot with ignition system shall be located so that it will cross– light to all the liquid burners.
• Provision shall be made to allow for the draining or blow through of all liquid lines. The liquid and atomizing media shall be filtered to ensure long liquid burner life.
• Specific attention shall be given to the volatility of the liquids and to providing a means of preventing vapor lock if necessary.
• Low temperature liquids shall not be atomized with media that contain water, water vapor or any liquid likely to freeze at the lowest possible operating temperature.
• Low Pressure Gas Flares
• Low pressure gas flares may be either pipe or Coanda type.
• If the required purge rate cannot be adequately maintained, consideration may be given to the installation of a flame arrestor immediately upstream of the flare tip, subject to the restraints of paragraph 9.14 of this Practice.
• Where a flame arrestor is used, the type and location of the arrestor should be chosen taking into account:
• Ease of access for maintenance
• The need to minimize back pressure on upstream process equipment
• Where operational process vents are connected to a low pressure flare system, the materials of construction shall be suitable for the minimum temperature the system could reach during a possible cold relief.
• Mixed Pressure Flares
• Mixed pressure flares should be considered when space and weight are at a premium. These flares generally use a combination of pipe type and Coanda type flare tips.
• The combined sections of the tip and associated pipework should be designed so that the possibility of defects leading to leakage from the high pressure stream to the low pressure stream is avoided.
• The low and high pressure headers and their associated equipment upstream of the mixed pressure flare tip should be designed for independent operation.
• Smokeless Flaring
• Provision for smokeless flaring shall comply with any local, state or federal regulations applicable to the site.
• As a minimum, a smokeless flare shall burn smokelessly for:
• All cases of operational flaring, (i.e., a controlled release of fluid to the flare system for a continuous period exceeding 30 minutes).
• 10–15% of the maximum flaring capacity.

• (*) The requirements for smokeless flaring maybe relaxed by agreement with the Owner for periods of non–normal operation, (i.e., initial commissioning, start–up and shut–down), if permitted by local, state and federal regulations.
• (*) The flare Manufacturer shall propose and submit to the Owner’s Engineer for approval, the flow rates for both smokeless and non–smokeless flaring.
• Steam, high–pressure gas, air, or water may be used for smoke suppression in flaring.
• Smokeless flaring may be accomplished through the use of multipoint (multiple burner) flares. These flares do not draw on utilities (gas, air, or steam) to achieve smokeless burning.
• When using steam for smoke suppression, the following points shall be observed:
• The system shall be designed to provide dry steam at the flare tip with the steam pipework being suitably insulated.
• Drainage, with steam traps per EP 5–6–7, shall be provided at all the low points and the steam lines shall be frost protected in accordance with EP 5–6–8, EP 13–12–1 and EP 14– 1–1.
• The steam flow shall be automatically controlled either in relation to the gas flow or by the characteristics of the flame (see paragraph 12.3). The latter method is preferred.
• Steam lines should be suitably filtered as close to the flare base as practical, but upstream of the flow control valve.
• In order to cool the pipework at the tip, a minimum flow of steam shall be maintained through a bypass around the steam control valve.
• Sizing of Flares
• The flare capacity should be based on the plant pressure relief load as determined in paragraph 6.5.4 of this Practice, for emergency flaring conditions and on the normal flare load due to startups, shutdowns, equipment depressuring and equipment purging.
• Where the normal and emergency relief loads vary greatly, it may be advantageous to provide an operating flare and an emergency flare. In this arrangement the operating flare is designed for smokeless burning of the normal plant load. The emergency flare, which is connected to the flare header through a water diversion seal, will burn the excess relief load as the back pressure in the system exceeds normal operating conditions and forces the relief vapors through the water diversion seal.
• Multiple point flares with staged burners may be used where the normal and emergency relief loads very greatly. However, a diversion water seal shall be provided to ensure that full flaring capacity will be available in the event of a control failure.
• (*) The flare stack diameter shall be based on the maximum allowable pressure drop and the minimum velocity required to prevent the ingress of air at the tip. These calculations shall be approved by the Owner’s Engineer.
• (*) Flare tip velocity calculations shall be approved by the Owner’s Engineer. This velocity shall be chosen to satisfy requirements for flame stability, noise and dispersion.

• Siting of Flares
• Flares shall be sited in accordance with the following general principles:
• A flare should be as close as possible to the unit or units it serves. However, consideration should be given to possible future expansion requirements into what will become the Restricted Access Zone.
• The siting should consider the routing of headers and drum locations.
• The flares shall be located so that one flare can be maintained while the others remain in service, if practical, unless the simultaneous shutdown of all flares is an operational requirement.
• Prevailing wind direction should be taken into consideration in order to minimize environmental effects, whenever possible.
• The area required to contain excessive levels of thermal radiation should be considered in relation to the height and design rates of both operational and emergency flares (see paragraph 9.8 of this Practice).
• Flare siting shall also consider the additive effect of thermal radiation from other elevated flares that may be expected to operate simultaneously. The direction and magnitude of solar radiation should be included.
• Flare siting shall consider the possibility of burning droplets and hot coke particles being emitted from the flare tip.
• The calculations used to determine the height of elevated flares shall consider:
• The maximum allowable thermal radiation levels as specified in paragraph 9.7.1 of this Practice.
• (*) The adequate dispersion of toxic gases at ground level, even with the flare extinguished, such that their concentration shall be acceptable to any local, state or federal regulations. Calculations of ground level concentrations shall be submitted to the Owner’s Engineer for approval. The method of calculating the maximum concentration of polluting gases and their corresponding distance are given in the API Manual on Disposal of Plant Wastes, Volume II. The acceptable concentrations shall be based on the period over which the conditions leading to the release can be sustained and the health hazard which they represent.
• Any local, state, or federal height restrictions, (i.e., aircraft lighting).
• Permissible Flare Radiation Levels
• The height and siting of the flare shall be chosen to keep personnel exposures to thermal radiation below those listed in Table 2.
• The following notes apply to the use of Table 2:
• The figures given assume that all personnel, excluding those in administration areas, are wearing at least a single–layer of whole–body working clothing including a hard hat.
• The figures exclude solar radiation, which needs to be accounted for.
• Metal surfaces irradiated at any of the thermal radiation levels given may produce burns on contact with bare skin.
• Multipoint and internal Coanda type flares may be used to reduce thermal radiation.
• (*) If necessary, these design levels may be achieved by the use of displacement or shielding. The requirements for any shielding system and the type of system to be employed shall be approved by the Owner’s Engineer at the outset of the project.

• Ladders shall be provided on all towers or elevated structures where rapid escape is not possible. These ladders shall be mounted on the opposite side of the tower or structure from the flare to provide some protection against thermal radiation.
• (*) All access requirements shall be considered when using tower supported multiple flare systems. Shielding shall be provided when specified by the Owner’s Engineer.
• The effects of flaring on nearby equipment shall also be considered, using the same design levels as noted above, from the following aspects:
• High temperature from radiation,
• Large temperature gradients, between exposed and non–exposed surfaces,
• Corrosive action of pollutants,
• Possibility of burning droplets being emitted,
• Effect of hot gases.
• Calculation Methods for Flare Radiation
• (*) The flare Manufacturer or Contractor shall use a differential type of calculation method that will realistically predict the radiation levels. The flare Manufacturer or the Contractor shall indicate the basis for the calculations and supply calculated results for flame length, flame shape and emissivity. The Owner will specify the points where flare radiation calculations are required and the environmental and operating conditions.
• Where multiple flares are present (i.e., operating and emergency flares) the cumulative effect of thermal radiation shall be accounted for in the calculations.
• Restricted Access Zone
• To minimize the risk of injury to personnel due to thermal radiation or related heat exhaustion, a volumetric zone around the flare flame shall be designated a Restricted Access Zone. This area shall be designated by warning placards prominently placed where personnel may enter this zone at ground level and on elevated structures.
• Equipment may be located within a Restricted Access Zone provided that:
• It is designed such that it will not be damaged by the highest thermal radiation levels to that it could be exposed to during an emergency relief.
• The equipment requires no regular operator attention or maintenance while the flare is in operation.
• It is possible to carry out emergency maintenance without risk of injury from thermal radiation to personnel (wearing protective clothing or using radiation shields if necessary).
• The outer limits of this zone shall be defined by the larger of the distances calculated below:
• Operational (for periods of one shift or more): The radial distance from the flare tip beyond which the thermal radiation level does not exceed 500 Btu/(hr–ft2) based on the maximum operational rate and the wind speed determined by local environmental conditions.
• Emergency (for periods up to 60 seconds): The radial distance from the flare tip beyond which the thermal radiation level does not exceed 1500 Btu/(hr–ft2) based on the maximum emergency flaring rate (plant pressure relief load) and the wind speed determined by local environmental conditions.
• Blowdown (for periods up to 30 minutes): The radial distance from the flare tip beyond which the thermal radiation level will not exceed 1000 Btu/(hr–ft2) based on the maximum blowdown flowing rate and the predominant wind speed determined by local environmental conditions.

• Solar radiation needs to be accounted for.
• Elevated Flare Construction and Installation
• The following general requirements apply to the construction of elevated flares.
• The structural design of elevated flares, whether they are guyed, mast or tower supported shall be carried out by specialists in this field with a proven record of experience. Structural steel shall meet the requirements of EP 4–5–1.
• (*) The design, detailing, supply and erection of the flare should preferably be the responsibility of a single Contractor, but in any case the materials of construction, standards for fabrication, inspection and the nominated Manufacturer shall be subject to Owner’s Engineer’s approval.
• The flare Manufacturer shall verify the meteorological conditions and all other relevant conditions likely to be encountered.
• (*) The following construction requirements shall be satisfied, unless otherwise approved by the Owner’s Engineer.
• All load carrying connections shall be bolted,
• All steelwork and bolting shall be galvanized or aluminum sprayed after fabrication.
• The design of the flare stack shall take into account the proposed method of transportation and erection.
• (*) The structural calculations shall be submitted to the Owner’s Engineer for review and shall demonstrate that the design loadings are in accordance with EP 4–1–1, including:
• Static wind loading
• Dynamic wind effects including the dynamic response of the structure, the vortex shedding phenomenon, and the dynamic/galloping response of guys.
• Ice loading on the structure and its effect on the static and dynamic response.
• Local stress at guy attachment points and local stress due to the choice of structural element, i.e., in the case of tubular joints, punching shear stress.
• Radiant heat and its effect on the riser, guys, upper guy fixings and upper members in the case of a structure.
• The effect of fatigue.
• Shell and strut buckling for the static and dynamic load.
• In addition to the loadings given in paragraph 9.10.1.6, the structure shall be designed to withstand the thrusts from liquid slugs (where these can occur), the thrust of the gas discharging from flare tips, and the imposed structural and equipment loads. The structure shall also be designed to withstand maintenance loads such as a spare flare tip, additional scaffolding, lifting beams, tools personnel, etc.
• The specification for the bolting of flanges shall take fatigue into consideration.
• The following requirements apply to the construction of elevated flare foundations:
• The foundation design should be completed by the main civil engineering Contractor, to the loads and moments specified by the designer of the supporting structure and in accordance with EP 4–2–3.
• In cases where guys are used, specific attention shall be paid to the possibility of differential settling of the main foundation and those of the deadmen. Grounding of the flare structure and riser shall be independent of the foundation reinforcement and piling.
• Templates for anchor bolts shall be supplied and delivered to site in time for associated civil works to commence.
• The following requirements apply to the construction of the Flare Supporting System:

• Guylines and terminations shall comply with EP 4–1–1, EP 4–5–1 and the following:
• A radiation shield in Type 321 stainless steel shall be provided where the effect of the incident heat flux will reduce a termination efficiency by more than 5%.
• Sufficient articulation shall be provided in the connections between guy rope terminations at one end and rigging screws at the other end, to ensure that no bending moment is transmitted to their respective attachment points due to the effect of wind blowing the guylines sideways as well as other changes in the catenary form.
• When the diameter of the riser is small, necessitating a large number of guys, or where multiple risers are required, a guyed supporting mast that encompasses the risers may be used.
• Structural components shall be designed to ensure that allowable stresses will not be exceeded at the temperatures which may be reached due to thermal radiation, hot gas flow and, if applicable, flame impingement. In carrying out this analysis, specific attention should be given to wind effects.
• (*) To reduce erection costs, self–erecting flares may be used for high elevated flares, unless specified otherwise by the Purchaser.
• The following requirements apply to the construction of flanges for guyed flares:
• Flanges for the risers of guyed flares shall be of forged weld neck type with flat faces. The jointing faces of the flanges should be machined after welding the flanges to the pipe. The accuracy shall be such that after assembly the deviation of the center line from vertical shall not be greater than 3 inches in 10 inches.
• The gas inlet to guyed flares should be of the same size as the riser and may be in the form of either a “tee” branch or a bend.
• In both cases, sufficient reinforcement shall be provided to transmit the vertical loads in the riser from above the inlet to below the inlet without exceeding allowable stress levels.
• Guyed flare risers shall utilize full face gaskets with supporting inner and outer rings.
• The selection and construction of flare tips shall comply with paragraph 9.11 of this Practice.
• The selection and construction of platforms and ladders shall comply with the following:
• Ladders and platforms shall be located to provide access for maintenance and inspection of the tip and on guyed flares for the inspection of guy attachment points. These ladders and platforms shall be in accordance with EP 4–5–3.
• Where two or more stacks are within 500 feet of each other, the access ladder shall be placed so that the stack shields the ladder from the radiant heat of the other stacks.
• The preference for tip removal shall be the use of onsite mobile lifting equipment. However, where the use of such mobile equipment is not possible or practical, a derrick shall be used. The derrick shall be:
• Lowered below the level of the top platform or the gas seal, if fitted during normal flaring operations. This storage method shall protect the derrick from thermal radiation and not require full load testing of the derrick before subsequent re–use.
• Provided with a suitable lifting tackle to raise the derrick to its lifting position. The wire ropes used for lifting the derrick, the tip, and the seal, if fitted, shall be replaced by receiving lines and removed to storage during normal flare operation.
• Designed for the temperatures to which it will be subjected to during flare operation without significant deterioration and remain operational after exposure.

• provided with a moveable winch or winches as needed.
• Flare Tips
• (*) The smokeless capacity and total capacity of the flare tip(s) will be specified or approved by the Owner’s Engineer. The flare tips shall be designed to burn with a stable flame for the full operating range at all anticipated wind speeds.
• Flare tips shall be designed for minimum maintenance with the objective being that of no maintenance between unit overhauls.
• Flare tip design and materials selection shall be made to minimize the potential damage to the tip and ancillaries due to high temperatures and corrosion. The design shall be suitable to survive flame lick or pulldown.
• (*) The materials of construction shall be suitable for the full range of metal temperatures to be encountered. These are dependent on the design of the tip, gases burned, flow rate, cooling effect of the smoke–suppressing steam or air, burn–back at low flows or purge flow, etc. In all cases the materials shall be subject to approval by the Owner’s Engineer.
• (*) Where internal burning cannot be prevented or is anticipated, internal refractory lining of the tip should be considered. The type of refractory and method of application shall be subject to approval by the Owner’s Engineer.
• Where flame pulldown along the outside of the flare tip cannot be prevented under all weather and flaring conditions the refractory lining of the tips exterior surface should be considered. Pilot and other lines should be protected by this refractory lining.
• Ground Flare Construction and Installation
• A ground flare generally consists of a refractory lined box of a cylindrical or rectangular shape enclosing multiple burners. The following general requirements apply to the construction of ground flares:
• (*) For conditions where the emission of light and noise to the surroundings has to be reduced, box type multi–burner ground flares may be used subject to the approval of the Owner’s Engineer. The air inlet at the bottom of the box shall be screened by a wall or a fence to cut off the direct route for light and noise to the surroundings.
• To allow for greater turndown, burners may be divided into staged zones with some of the zone being smokeless, and the others non–smokeless. The staging shall be done automatically and may incorporate valves. Liquid seals shall be installed parallel to the staging controls in order to retain full flaring capacity in the event of primary control system failure.
• On sites where space for the Restricted Access Zone and environmental regulations are not limiting, low level, single burner ground flares may be used (these may be conventional pipe flares). These may be non–smokeless, totally smokeless, or partially smokeless up to a specified burning rate.
• The flare line leading to low level single burner flares shall not be buried.
• (*) The ground flare shall be capable of providing stable combustion performance for the gas flow and stream composition rates specified, and shall satisfy the maximum allowable emission requirements. These will be provided by the Purchaser.
• (*) The designed operating temperature and residence time within the ground chamber shall ensure complete combustion of all incoming gases and hydrocarbon fuels. These parameters shall be approved by the Owner’s Engineer.

• (*) The proposed height for the ground flare shall be submitted for the Owner’s Engineer’s approval.
• The exit area of the ground flare shall provide for adequate dispersion of all combustion products.
• The construction of the chamber shall meet the following requirements:
• The structural design and construction shall meet the requirements of EP 4–5–1 as a minimum.
• The casing plate shall be seal welded to prevent the infiltration of air and water.
• The design of the ground flare, particularly the design of the structural steel, shall allow for thermal expansion and contraction.
• All ladders, platforms, and their associated parts, shall be hot–dip galvanized in accordance with EP 10–3–1.
• Consideration shall be given to the possibility of condensation forming within the ground flare and its affect upon the structure. This is particularly important where low temperatures can drop below the acid dew–point.
• The lining shall be constructed in accordance with the following requirements:
• The ground flare shall be lined with an acid–resistant material that is capable of withstanding a temperature excursion of 300ºF above the normal maximum flue gas operating temperature. The lining material shall also accommodate rapid temperature changes.
• Walls, arches and floors shall be designed to allow for the thermal expansion of all parts under design conditions. The expansion joints within multi–layer linings shall be staggered.
• Ceramic fiber casings shall have an internal protective coating to prevent corrosion and to provide a vapor barrier. All joints shall be staggered between adjacent layers.
• Access doors shall be protected from direct radiation by a material equivalent to the adjacent liner.
• Ground flares shall be provided with wind fences to prevent flame extinguishment and the formation of vortices within the ground flare. These wind fences shall:
• not hinder the dispersion of combustion products.
• have a minimum of two openings if a forced air purge is not to be used. These openings, with closeable gates, shall be located opposite to each other and parallel to the predominant wind direction. This is necessary to allow for the purging of high molecular weight gases prior to start–up.
• be provided with access ways through the wind fence and into the ground flare for maintenance and inspection purposes.
• The design and construction of the burners shall meet the following requirements:
• The gas burner(s) shall be capable of stable firing within the specified range of gas flow and composition.
• Combustion of the waste gases shall take place within the combustion chamber with no visible flames outside of the flare stack during normal operation.
• The gas jets of the main burners shall be sized sufficiently to remain free from blockage during all operating conditions. The sizing of the gas jets shall also allow for the burning of all expected qualities of gas without filtering.

• The burners should be capable of turndown from the maximum flare gas flow to the minimum purge gas requirements or they shall be staged by automatic controls to provide for efficient burning at all relief disposal rates. However, rupture disk bypasses shall be provided to ensure full flare capacity in the event of a control failure.
• The materials of construction shall meet the following requirements:
• The materials of the structures and accessories shall be adequate for all load conditions at the lowest specified ambient temperature.
• Furnace refractories shall conform to the relevant grade in ASTM C 401 or C 155.
• (*) All construction materials shall be inherently protected against atmospheric corrosion or properly coated to prevent atmospheric corrosion. These materials and/or coatings shall be selected by the Manufacturer and approved by the Owner’s Engineer.
• (*) All components that may come into contact with corrosive gases, including those which may come into contact as a result of downwash, shall be inherently protected against acid attack or coated to prevent acid attack. All ladders and platforms shall be coated in addition to being galvanized. All materials and coatings shall be selected by the Manufacturer and approved by the Owner’s Engineer.
• The use of brittle materials (i.e., cast iron, spheroidal graphite cast iron, malleable iron) and low melting point materials (i.e., copper or aluminum and their alloys, plastic) is not acceptable for use in any burner parts which are under pressure or their associated supports, bolts, nuts, springs, etc.
• (*) Any proposal to pre–purge the ground flare with air shall be submitted to the Owner’s Engineer for approval. Fans provided for this purpose shall have anti–vibration mounts and must be approved by the Owner’s Engineer.
• Noise levels shall be in accordance with paragraph 9.15 of this Practice.
• In addition to the general instrumentation requirements in Section 12.0, each ground flare shall be provided with:
• Flame detection for each burner zone – the flame detectors shall be selected and positioned so that they can discriminate between the zone being monitored and adjacent zones, and between the main burner flames and pilot burner flames. The loss of the main flame shall be annunciated in the control house responsible for the flare.
• Chamber temperature and draft detection.
• Ignition Systems
• All ignition systems shall utilize:
• Pilot burners at the flare tip or burners capable of igniting the flare gas under all relevant flow and ambient conditions.
• A reliable system for igniting the pilot burners under all relevant ambient conditions.
• Pilot burners shall be provided as follows:
• Pilot burners shall be of an energy efficient design and proven by at least two years of operation in similar service. They shall be capable of remaining lit in winds of up to 80 mph or higher if appropriate for the site.
• Elevated flares shall be provided with a minimum of 3 evenly spaced pilot burners for each flare tip. Ground flares shall be provided with a minimum of 2 pilot burners per main burner zone.

• Flame detection, either a thermocouple or ionization detector, shall be provided on each pilot burner. This detector shall annunciate a local alarm upon pilot flame failure and be tied into the automatic ignition system (if provided) as described in paragraph 9.13.3 of this Practice.
• Ignitors shall meet the following requirements:
• The pilots burners on elevated flares shall be ignited through the use of a flame from a flame front generator. The flame front generator may utilize educted or compressed (plant) air. However, educted air type generators are recommended to reduce the system’s dependence upon utilities.
• Ignitors that place any equipment other than the burners (i.e., spark plugs, eductors) above grade level are prohibited. This is to ensure that the system can be maintained without taking the flare out of service.
• High energy igniters may be used in lieu of flame front generators on ground flares.
• The igniters shall be an integral part of each pilot head and shall be similarly proved (see paragraph 9.13.2.1 of this Practice).
• The ignitor controls shall be located at grade level in an area safe from high thermal radiation levels and liquid carryover. The controls shall be classified in accordance with the area electrical classification.
• (*) The gas–air mixture in the ignition chamber shall be ignited by a spark plug. This spark plug should be normally energized through a transformer/converter and provided with a back–up piezoelectric energy source. The ignition may be manual or fully automatic as specified by the Owner.
• The gas and air (if provided) lines in the ignition panel shall be equipped with stop valves, regulating needle valves, pressure gauges and check valves. A means of directing the flame front to each individual pilot in sequence shall be provided downstream of the ignition chamber.
• If fully automatic, the ignition system shall carry out the following:
• On press–button initiation – open the flow of the ignition gas and air (if provided), fire the spark plug, and ignite the first pilot.
• Determine if the pilot burner has ignited through the use of the pilot flame detector.
• If the pilot burner has failed to ignite automatically make three more attempts at ignition. If this fails, sound a local alarm and stop the ignition sequence.
• As each pilot is ignited, turn the distributing valve and repeat actions 1–3 to ignite each consecutive pilot, until all of the pilots are lit.
• In addition, the controller shall be equipped for full manual operation.
• The Pilot Gas Supply shall meet the following requirements:
• (*) The pilot gas supply shall be from a highly reliable source approved by the Owner. If a highly reliable source is not available a backup gas supply with automatic switchover should be provided.
• The Manufacturer shall notify the Owner’s Engineer of the molecular weights and colorific value ranges required for proper operation of the flame front generator. The air and gas fuel settings shall not require adjustment to cover these ranges.

• The pilot gas supply should be sweet and taken directly from the plant fuel gas main, a top mounted branch, with two filters, in parallel or a dual filter, should be used. A differential pressure gauge shall be provided to measure the pressure drop through the filter. The filter elements shall have a mesh size of approximately 0.020 inch. The filter, piping, and fittings downstream from the filters shall be of Type 321 or 347 stainless steel to prevent corrosion and jet blockages.
• The pressure reducing valve shall be placed downstream of the filters.
• Flashback Prevention
• A reliable method of flashback prevention shall be incorporated into the flare system design. The following methods may be used, either singularly or in combination:
• Gas Purge – Refer to paragraph 10.1 of this Practice.
• Liquid Seals – Refer to paragraph 10.2 of this Practice.
• Gas Seals – Refer to paragraph 10.3 of this Practice.
• Efflux Velocity Accelerators – Refer to paragraph 10.5 of this Practice.
• Where flare gas recovery is to be used both a gas purge and liquid seals shall be installed.
• Flame arresters may be used where none of the above methods will provide adequate flashback protection. However, their use must be in accordance with paragraph 10.4 of this Practice.
• (*) The method of flashback protection will be subject to approval by the Owner’s Engineer.
• Noise Levels
• When operating at maximum emergency flow, the noise level of the flare shall not exceed 115 dbA at positions normally accessible to personnel or along the plant fence line. When operating at maximum smokeless flaring rate the noise levels shall comply with the Owner’s Recommended Environmental Guidelines.
• The siting of the flare should be chosen to minimize noise through distance and by keeping areas requiring low noise levels out of the direct line of sight of the flare.
• (*) Flare Manufacturers, in their quotations, shall provide information on the noise emission from the flare at maximum emergency flow and at the maximum smokeless flaring rate. The noise emission data shall be submitted to the Owner’s Engineer for approval. The data shall include the sound power levels in octave bands from 31.5 Hz to 8 kHz. All measurements shall be taken in accordance with the Owner’s Recommended Environmental Guidelines.

• Auxiliary Flare Piping
• All the auxiliary piping required for flare operation shall be provided. This may include the following:
• Pilot gas line(s)
• Flame front igniter line to each pilot
• Steam line to the main smoke suppression system
• Steam line to the auxiliary system
• Oxygen sampling lines
• All piping shall be in accordance with EP 5–1–2, EP 5–6–4, and the following:
• The thermal expansion of the auxiliary flare piping shall be specifically accounted for in the design. The method preferred by the Owner for elevated flares is to have the piping anchored at the bottom of the flare tip and guided along the length of the stack. The expansion and contraction is then taken up by designing the horizontal piping at grade level to flex.
• Pilot gas piping and oxygen sampling lines shall be of type 321 or 347 stainless steel. The oxygen sampling lines shall have a minimum outside diameter of 1/2 inch.
• The piping on elevated flares shall be flanged at the base of the tip for ease of tip removal and provided with suitable flanges for grounding points at the base of the stack.
• (*) The type and location of the connecting joints at grade will be specified by the Owner.
• The auxiliary flare piping shall be protected by the refractory lining on the exterior of the tip, if provided.
• Heat Tracing

Heat tracing for winterization shall be in accordance with EP 5–6–8, EP 13–12–1 and EP 14– 1–1.

• Flare Sparing Philosophy
• (*) The time between overhauls will be specified by the Owner with the Manufacturer making adequate provision to enable the full specified range of continuous and intermittent flaring operations to be sustained during this period.
• (*) Sparing of flares shall be considered to allow for maintenance, inspection and breakdown of the flares. In addition, some form of sparing may be specified by the Purchaser when multiple, independent units are served by a flare system.
• Isolation of spare flares shall be in accordance with EP 5–1–2 and EP 5–6–4.
• Spare Equipment

(*) Spare equipment lists shall be compiled by the flare Manufacturer and submitted for the Owner’s Engineer’s approval. The list shall include the following as a minimum:

• Replacement for all the gaskets for all joints that have to be broken during construction or after testing.
• One set of spares to cover the first overhaul.
• One complete pilot burner.
• One complete set of spare thermocouples.
• A complete set of parts for the ignition panel.
• One set of spares for the smoke suppressant apportioning instrumentation.

FLARE PURGING AND SEALS

• Gas Purge
• (*) A continuous purge system shall be used, unless otherwise specified by the Owner’s Engineer.
• Flammable or inert gases may be used for purging with reliability and availability. The choice should be evaluated in terms of cost. When deciding to use an inert purge specific attention shall be given to the consequences of releasing unburned, toxic materials to the atmosphere.
• (*) The purge gas supply shall be from a highly reliable source, approved by the Owner’s Engineer. If a highly reliable source is not available an automatic backup supply should be provided.
• The minimum purge gas flow rate shall be such as to limit the oxygen content of the flare gases to less than 6% at a point 25 feet or 15 diameters, whichever is the less, down from the top of the flare tip. If the flare gases contain more than 85% hydrogen, the maximum oxygen content shall be reduced to 2%. This can be accomplished by the following:
• (*) The required flow rate of any lighter than air purge gas shall be calculated using

H. W. Husa’s formula, unless otherwise approved by the Owner’s Engineer.

• The minimum purge rate for heavier than air, flammable gases may be very low. This may cause burning inside the tip, which may lead to shorter tip life or flame extinguishment. In such cases, the cost of maintaining a minimum sufficient flow rate to prevent internal burning shall be evaluated against the following alternatives:
• Increasing the purge gas velocity at the tip to 0.5 – 1.0 ft./sec.
• Upgrading the material specification of the tip.
• Providing an alternative relief route to allow the tip to be replaced more often.
• Providing tip cooling.
• The oxygen content at the point specified in paragraph 10.1.4 may be continuously monitored as described in paragraph 12.6.
• During normal operations the purge gas shall be injected into the main header at the furthest point from the flare.
• To allow for purging of the relief disposal system prior to unit startups a purge gas supply shall be provided in each unit and plugged purge vents shall be provided as follows:
• On all leads immediately downstream of the pressure relief device.
• On the unit header, immediately upstream of the unit isolation valve and blind.
• Once the required purge rate is calculated a properly sized restricting orifice shall be provided to ensure that the supply remains constant and is not subject to maladjustment.
• Noncondensable emergency purge gas should be provided to the flare system, where appropriate, to prevent the formation of a vacuum and the ingress of air as the system cools following a release. This supply should be automatically initiated and controlled by pressure, temperature, or a combination of both.

• Liquid Seals
• Liquid seals may be used:
• As flashback protection, if located between the base of the flare stack and the offsite knockout drum. These seals shall be as close to the base of the flare stack as possible.
• As a diversion seal to provide full relief capacity to a flare in the event of a flare gas recovery system failure.
• As a diversion seal as part of a flare staging system.
• The Freezing of Liquid Seals shall be prevented by:
• Using water seals only where the temperature of the incoming vapors cannot fall below 32ºF.
• Using automatic heating facilities in freezing climates or through use of antifreeze if make– up water requirements are not significant.
• Using glycol or another suitable material, either pure or with water, to protect against freeze up when in cold service (vapor temperatures of 32ºF or lower).
• The design of all liquid seals shall include:
• A vacuum leg of sufficient height, measured from the maximum liquid level in the drum, to handle the maximum expected vacuum in the header due to the cooling and/or condensation of hot vapors. However, it shall have a height of no less than 10 feet. In addition, the volume of liquid above the vacuum leg inlet shall be sufficient to fill the entire vacuum leg.
• The inlet pipe submergence depth based on the maximum exit back pressure allowable in the relief header.
• A free area for gas flow above the liquid that is at least 3 times that of the inlet pipe cross sectional area to prevent surging of the flare.
• (*) A design capable of flowing all quantities from maximum emergency flow down to 1/2000th of that flow without causing flow pulsations. Details of the dip leg design must be submitted to the Owner’s Engineer for approval.
• A minimum design pressure of 50 psig.
• All necessary equipment to maintain the design seal level and sized to replace the seal within 10 minutes.
• Provisions for:
• The prevention of hydrocarbon build up,
• The prevention of seal liquid displacement.
• The flare header that is sloped from the top of the vacuum leg back to the off–site knockout drum.
• Provisions for disposing excess seal liquids by:
• Draining it into a suitable sewer system.
• Drawing it off into a recirculation system taking into consideration the possibility of contamination by the relief stream (i.e., H2S). The latter system should be employed for static seals containing antifreeze.

• Gas Seals
• (*) Gas seals of the labyrinth type shall not be used. However, seals of the flow restriction type may be used with the Owner’s Engineer’s approval.
• A Buoyancy Seal (Mol Seal) uses the density difference between air and the relief gases to prevent air from flowing back into the flare header. These are permitted if the following installation requirements are met:
• The mol seal must be located on the flare stack just below the flare tip, not at grade.
• A drain to grade with a suitable seal for removal of trapped condensables must be installed and a handhole near the drain line must be permitted to allow cleaning of the drain line.
• The bottom of the mol seal and entire drain line shall be electrically traced or steam traced in areas where temperatures fall below 35ºF.
• All drain line valves should be self closing.
• Kinetic Seals are proprietary seals which are available from most major flare Manufacturers (John Zink’s Arrestor, NAO’s Fluidic Seal, Kaldair’s Diode Seal, McGill’s Venturi Seal, etc.). All of these seals use the upward velocity of the purge gases to reverse the flow of oxygen penetrating the tip. These seals are generally cheaper, lighter and have less pressure drop than other seals. They shall not be used, unless a highly reliable source of purge gas is available because they do not function if the gas flow stops.
• Flame Arresters

The following restrictions apply to the use of flame arresters:

• No other suitable method of flashback protection must be available.
• They may be used only on clean systems.
• A means of inspecting, maintaining and repairing them between unit and flare shutdowns, must be provided.
• (*) Their use must be approved by the Owner’s Engineer.
• Efflux VelocIty Accelerators

Flashback from the flare flame into a flare stack will not occur provided that the efflux velocity of the flare gases always exceeds the flashback velocity. The efflux velocity may be increased by the use of an orifice plate with single or multiple orifices.

11.0 SCRUBBERS, ABSORBERS AND OTHER EQUIPMENT

(*) This equipment shall be designed in accordance with Manufacturer’s recommendations and all applicable local, state and federal regulations. Their use shall be specified or approved by the Owner’s Engineer.

CONTROLS AND INSTRUMENTATION

• General

(*) The instrumentation in relief disposal systems shall be in accordance with EP 12–1–1, unless otherwise approved by the Owner’s Engineer. Attention shall be given to the effects of thermal radiation on the instrumentation.

• Pilot Flame Failure Detection

• Each pilot shall be fitted with flame failure detection, which is required to perform the following functions:
• Alarm to indicate a detector fault.
• Alarm on pilot flame failure.
• Indicate “pilot on.”
• (*) A common alarm shall be activated in a control room specified by the Purchaser for any of the above failures with type of failure being annunciated at the ignition panel.
• Automatic Smoke Control
• (*) When specified by the Purchaser, steam smoke suppression systems shall be equipped with automatic control systems to apportion the suppressant to the flare gas in order to produce clean burning without excess steam flow.
• The three types of control systems that may be used are listed below in order of preference:
• Flare tip mounted radiation sensors.
• Ground mounted, optical flare radiation sensor.
• Flare gas flow rate measuring systems which use flow sensing thermistors.
• Ground mounted, optical flare radiation sensors utilize optical monitors that are located at ground level at a moderate distance from the base of the flare stack. The optical monitors are trained on the base region of the flame to measure its intensity.
• The following guidelines apply to the use of optical monitors:
• The optical monitors shall be rugged, telescopic with a restricted field of view, and equipped with a photo–cell sensitive to near infra–red radiation. The telescope shall be waterproof and allow for regular cleaning of the lenses.
• These systems shall be avoided in areas prone to smog, fog or smoke.
• Flare tip mounted radiation sensors shall be designed to be maintenance free, non–optical and strongly constructed.
• Compensation for ambient variations (night/day, sun/cloud) may be required for both types of radiant heat measurement. Signals from the monitor shall operate the steam control valve via appropriate converters, adjustable for range and zero. Manual control shall also be provided.
• (*) Flare gas flow rate measuring systems shall contain facilities for on–stream inspection and maintenance of all the important parts of the system. A density measuring device shall be provided to compensate for density changes in the relief gas, if specified by the Owner’s Engineer.
• Burn–Back Detection

(*) Burn–back detection shall be provided by one or more thermocouples located in the thermowells of the flare tip, where burnback is possible. The thermocouples shall annunciate an alarm in a control room specified by the Owner. The locations of the thermocouples shall be approved by the Owner’s Engineer.

• Purge Control

The flow rate of the continuous purge system shall be based on the oxygen concentration in the stack as specified in paragraph 10.1.4 of this Practice.

• Oxygen Monitoring

Where oxygen monitoring is required on flares per paragraph 10.1 of this Practice, the following requirements shall be satisfied:

• The oxygen sampling probe shall be located 25 ft or 15 tip diameters, whichever is the less, below the tip exit. The probe piping shall be in accordance with paragraph 9.16 of this Practice.
• The oxygen analyzing installation should be located at the base of the stack or at the boundary of the Restricted Access Zone. If located in an area where radiation level may exceed 1500 Btu/(hr–ft2) it shall be provided with suitable shielding.
• The sample gas shall be withdrawn from the stack by a diaphragm type vacuum pump, fitted upstream with liquid knock–out pot, and returned to the stack above the sample point.
• (*) A portion of the sample gas shall be taken through a regulating needle valve to an oxygen analyzer of a type approved by the Owner’s Engineer, and exhausted to an environmentally safe location. Local and control room indications and alarms shall be provided as specified by the Owner’s Engineer.
• Flow Measurement
• Flow measurement of the flare gas may be required for two reasons:
• For control of the flow rate of the smoke suppressant.
• For general or operator information.
• (*) Flow measurement systems shall be of a type that uses flow sensing thermistor probes, unless an alternative is approved by the Owner’s Engineer.
• Flow sensing thermistor probes shall be mounted in the flare line downstream of the off–site knockout drum, inserted via a sealed housing and isolating valve, and capable of being withdrawn during flare operation.
• General Requirements for Instrumentation on Closed Relief Disposal Systems
• Steam smoke suppression systems, where installed, shall be provided with:
• Flow control: auto and manual
• Flow indication and recording
• When oxygen monitoring is required per paragraph 10.1 of this Practice, the flare stack internal atmosphere shall be monitored with:
• Oxygen sampling equipment and analyzer
• Oxygen content indication
• High oxygen content alarm
• Burn–back detection and alarm
• Each knockout drum shall be provided with:
• Level indication
• High level alarm
• Level switches for automatic operation of pumps
• Level gauge
• Upstream pressure gauge
• Temperature control, if required

• Each liquid seal shall be provided with:
• Level indication
• Low level alarm
• High level alarm
• Liquid temperature indication
• Temperature control
• Adequate instrumentation for liquid sump tank and liquid overhead drum, if fitted
• Flare pilot gas systems shall be provided with:
• Flow indication
• Pressure control valve
• Low flow alarm
• Indication of back–up supply in operation
• Pilot flame failure
• The purge gas system shall be provided with:
• Flow indication
• Flow control
• Low flow alarm
• The air supply to pilot ignitors, if provided, shall have:
• Flow indication
• Low flow alarm
• Pressure control valve
• Pressure indictor

13.0 SURFACE PROTECTION OF RELIEF DISPOSAL SYSTEMS

• (*) Unless otherwise approved by the Owner’s Engineer, the surface protection of the flare and the entire relief disposal system, (structural steel, flare, pipe, etc.) shall be in accordance with EP 10–3–1.
• Flare stacks of bolted galvanized construction should be overpainted except for those surfaces that will exceed 660ºF.
• Flares that serve multiple, independent units shall have surface protection applied after fabrication to provide 2–3 years of service between flare shutdowns.

14.0 TESTING

• The Manufacturer shall carry out the flushing, cold testing and static testing of the relief disposal system in accordance with EP 5–5–3 and provide any special equipment required for this testing.
• (*) The Manufacturer shall produce documentation for the Owner’s Engineer’s approval, listing the pre–commissioning and commissioning activities based on EP 5–5–3.
• (*) Any further requirements of the Manufacturer for the attendance of specialist, operators and service staff during the pre–commissioning, commissioning and performance testing of the flare system shall be specified by the Purchaser.

15.0 TABLES

TABLE 1 DISPOSAL OF H2S

NOTES:

• All releases shall comply with paragraph 9.1.
• (*) Disposal may be to the atmosphere, if calculations show acceptable ground level concentrations and when approved by the Owner’s Engineer.
• A continuous release, in the context, would be a release caused by the shutdown of a process unit for an extended period of time. An intermittent release would be the result of a pressure relief device operation.

TABLE 2

PERMISSIBLE EXPOSURE LIMITS FOR THERMAL RADIATION(1)

NOTE:

(1) Adopted from Table 3 of API RP521.

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