Process Unit Relief Headers

Table of Contents

1. Purpose

This procedure describes the Inflection Point Engineering practice for specifying process unit relief headers.

The process unit relief header detail is shown on the Piping and Instrument Diagram legend. The detail provides general information concerning header purging, header isolation, heat tracing (where applicable), materials of construction (when other than killed carbon steel is required), and other construction details. It does not apply to the main relief header, which has additional design requirements and is generally the responsibility of the contractor. If header sizing is included within our scope, a Pressure Relief Valve (PRV) or Offsite Specialist must be assigned to do the extensive and specialized calculations.

All deviations from the process unit relief header details included herein must be reviewed by a PRV or Offsites Specialist before being implemented. This ensures that proposed deviations do not compromise process safety and that changes comply with API recommended practice.

2. Types of Process Unit Relief Headers

Four types of process unit relief headers may be described in our P&IDs depending on service requirements. These include:

• Standard relief header
• Acid gas relief header
• Acid relief header
• Cryogenic relief header

When multiple relief header services are required for an individual process unit, include separate header details in the P&ID legend for each service. Indicate on the P&ID the relief header to which each PRV discharges and use a consistent header designation in the 807 project specification for each valve.

Occasionally, a customer requests specific designations for the process unit relief header. For example, a customer may want separate high and low pressure relief headers. For those cases, the standard relief header detail can be used and re-labeled to be consistent with the customer’s description.

Consult the PRV, Offsites, or Technology Specialist if you are unsure which relief header detail to use.

It is typical for Oleflex units to use the term “wet relief header” instead of the term “standard relief header” and to use the term “dry relief header” instead of the term “cryogenic relief header.” These terms are acceptable provided the entire Schedule A package uses the same nomenclature.

2.1 Standard Relief Header

Relief valves that discharge flammable or toxic fluids are typically routed to the standard relief header for safe disposal. Both liquid and vapor can be discharged to the standard relief header, but liquid knockout facilities may be required within the process unit to prevent liquid from entering the main relief header. If a liquid knockout drum is required, its design is generally the contractor’s responsibility. Consult the Offsite Specialist if the liquid knockout drum is included in Inflection Point Engineering’s design scope.

All process unit relief headers are provided with a continuous fuel gas purge to exclude air from the header piping and to sweep toxic or corrosive material from the header during normal operation. In addition, purge connections are required to “flush” hydrocarbon vapor from the header prior to process unit isolation during a shutdown. (These same purge connections are used to “flush” air from the header prior to re-establishing the connection between the process unit relief header and the main relief header during start-up.) The P&ID detail for the standard relief header is given in . This detail is shown in module F050005. Inflection Point Engineering allows the contractor to select the metallurgy and corrosion allowance for the standard relief header. In addition, the contractor decides if heat tracing is required to prevent freezing or embrittlement given the lowest ambient air temperature listed in the BEDQ.

If the relieving fluid autochills to a temperature between a -20ºF (-29ºC) and -50ºF (-45ºC), a note should be added to the header detail requiring impact-tested killed carbon steel.

2.2 Acid Gas Relief Header

An acid gas relief header is typically specified for Sour Water Stripping, Amine Treating, Amine Guard FS, and the Selexol process units. When the Sulfur Recovery Unit (SRU) is taken off-line, gas from the Sour Water Stripper and Amine Stripper is automatically diverted to the acid gas relief header for safe disposal until such time that the SRU is returned to service or the process units get shut down.

The PRV’s in these units (particularly those protecting the Amine Stripper and Sour Water Degassing Drum) generally should be routed to the standard relief header. If the relieved vapor has a concentration of hydrogen sulfide too high for a standard relief header (generally over 50 wt%), then the PRV can be routed to the acid gas relief header, unless the liquid knockout drum at the base of the acid gas flare cannot handle the liquid loads obtained during a blocked outlet case. Do not route a PRV discharge to the acid gas relief header without first consulting the PRV or Offsites Specialist.

Because a number of process units utilize an acid gas flare, the P&ID detail for the acid gas header may have to be modified for specific applications. Design concerns associated with the acid gas relief header are: corrosion, freeze protection, plugging with ammonium salts, and liquid knockout. A generalized P&ID detail for the acid gas header is given in . The preferred metallurgy for this header is 316L SS which provides corrosion protection. (Upon customer request, steam-jacketed killed carbon steel can be used instead of 316L, but this design is inherently less safe. This design utilizes a steam-jacket to maintain the killed carbon steel piping above the water dew point of the relief stream. Thus, corrosion protection will be compromised if steam is lost or if the steam trap fails closed.)

The acid gas header is normally inactive unless the Sulfur Recovery Unit goes off-line. Although the control valve(s) that direct flow to the header are normally closed, they may leak, allowing water, ammonia, H2S and/or CO2 to enter the acid gas header and freeze (water) or solidify (ammonium bisulfide).

• If the lowest ambient temperature is below 32°F (0°C), heat tracing is required to prevent ice build-up in the header. When ice formation is an issue, set the objective temperature for the heat tracing at 40°F (5°C).
• If the acid gas contains both ammonia and hydrogen sulfide, ammonium bisulfide salt may build-up and restrict or block flow in the header. This hazard is addressed by tracing the acid gas header. Set the objective temperature for the heat tracing above the dissociation temperature of ammonium bisulfide, i.e. 185°F (85°C). Alternatively, the project engineer can perform calculations to establish that ammonium bisulfide will not solidify at the low ambient temperature. In this case, heat tracing is not required.

If you are unsure when heat tracing is required, consult the Technology Specialists for the various treating units to obtain the appropriate objective temperature for your conditions.

2.3 Acid Relief Header

An acid relief header is specified for the HF Alkylation Unit to safely dispose relief and vent streams containing significant amounts of hydrofluoric acid. The acid relief header connects to a Liquid Knockout Drum for coarse removal of liquids containing HF. The vapor stream exiting the knockout drum flows to a Relief Gas Scrubber where HF is neutralized using a potassium hydroxide (KOH) solution. The acid-free hydrocarbon vapor stream leaving the scrubber is then routed to the non-acid relief header (standard relief header) for safe disposal.

Both the acid relief header and the non-acid relief header are constructed of killed carbon steel, but the acid header requires a higher corrosion allowance than that provided for the non-acid header. Although the non-acid relief header is shown in the P&ID legend, a separate detail for the acid relief header is not necessary because this system is described in P&ID drawings for the HF Alkylation Unit. Depending on the HF acid concentration in the relief stream, individual relief valves will be routed to the acid relief header or the non-acid relief header. Consult the Technology Specialist for HF Alkylation if you are unsure where individual PRVs should discharge. Regardless of discharge location, the 807 project specification must be consistent with the P&ID designation.

2.4 Cryogenic Relief Header

Cryogenic relief headers are used when relief streams are sufficiently cold to cause embrittlement. If a process stream normally operates at low temperature or if it will auto-chill below -20ºF (-29º) when discharged to the relief header, the metallurgy must be upgraded. (Heat tracing may be provided to off-set a low ambient temperature, but heat tracing cannot supply enough heat to offset the cooling due to auto-chilling.)

If, in addition, the relief stream contains water with a water dew point above the auto-chill temperature, a separate water phase will form in the relief header. If the auto-chill temperature or lowest ambient temperature is below 32ºF (0ºC), this separate water phase will freeze. During normal operation, a leaking PRV may go undetected for a prolonged period, and ice may build-up in the relief header causing plugging. This hazard is managed by enlarging the PRV tailpipe to accommodate a small amount of ice build-up that occurs during a relief event, and heat-tracing the PRV and its tailpipe to prevent long term ice build-up due to leaking. In addition, a note should be added to the 807 project specification stating that ice build-up may occur, i.e. “Increase the PRV tailpipe one size larger than PRV outlet flange to accommodate ice build-up during relief event.”

The P&ID detail for the cryogenic relief header is given in . If the auto-chill temperature of the relieving stream is above -20ºF (-29ºC), route the PRV discharge to a standard relief header. If the flash temperature is below -50ºF (-45ºC), route the PRV discharge to a cryogenic relief header. But if the flash temperature is between -20ºF and -50ºF, preferentially route the PRV discharge to a cryogenic relief header, when available, or to an impact-tested killed carbon steel relief header (typically pipe class T1A1).

It is acceptable to add a sized or unsized valve (with blankoff) for methanol injection to allow methanol to be injected to deice a cryogenic relief header. Consult the Technology Specialist prior to adding the injection valve.

A cryogenic header is typically constructed of 304 SS to withstand low temperature embrittlement. To prevent ice plugging or hydrate formation, relief streams containing water are generally not discharged to the cryogenic header. When this design rule cannot be applied, the PRV, its tailpipe, and relief header must be heat traced. Refer to for guidance in selecting the appropriate relief header based on the adiabatic flash temperature and water dew point of the relieving stream.

Although this procedure primarily applies to PRV discharge, it can also be applied to depressuring operations and manual drains and vents routed to the relief header. If the venting/draining operation is expected to last for fifteen minutes or more, follow the logic given in . If manual venting is expected to take less time, it can be routed to the most convenient location, including a standard relief header. The thermal inertia of the piping should be sufficient to prevent embrittlement during short-duration operations.

3. Design Details for the Relief Header

3.1 Free-Draining Requirement

The relief header and all piping associated with the PRV must be free-draining. If liquid is allowed to accumulate in the piping, severe damage to the relief header can occur when liquid slugs are accelerated downstream by vapor discharging into the header. In addition, free-draining helps minimize corrosion caused by standing pools of acidic water.

The PRV is generally elevated above the relief header to facilitate free-draining of the tailpipe to the header. (Figure 1).

If the PRV cannot be elevated above the relief header, its discharge piping may be routed to the process unit liquid KO drum, or to an intermediate liquid KO drum that is subsequently vented to the top of the relief header. However, the intermediate KO drum must be properly sized, its liquid level continuously monitored, and the collected liquid must be drained to a safe location. (Figure 2).

3.2 Liquid Knockout Facilities

Liquid may discharge to the relief header or it may form as condensable vapors contact the cold header piping. Regardless of the source of liquid, liquid knockout is required at the base of the flare stack, and may be required within the process unit. The contractor is responsible for the design of the liquid knockout facilities unless it is specifically included within our design scope. On those occasions, an Offsite Specialist should be assigned to the project to ensure a safe design of the liquid knockout facilities.

Relief events that require the discharge of high pour point liquids should be eliminated where possible because this liquid could solidify in the valve outlet, its tailpipe, or header piping. If the PRV leaks during normal operation, the viscous liquid will cool in the header, potentially causing a flow restriction or plugging. When possible, process equipment should be designed for pump shut-in pressure to eliminate the blocked outlet relief case. If liquid relief cannot be avoided, consider routing the high pour point material to another process location instead of the relief header. Regardless of discharge location, heat tracing is required for the relief valve, its inlet and outlet piping, and the relief header up to and including the process unit liquid knockout drum. The objective temperature for the heat tracing must be above the pour point temperature of the viscous fluid. In some cases, a liquid flush is provided to “sweep” the high viscosity material from the inlet and outlet piping for the PRV. Consult the Technology Specialist for specific guidance when handling high pour liquids. Similar design considerations apply to water when the lowest ambient temperature listed in the BEDQ is below 32 ºF (0 ºC).

3.3 Material Selection

For the Standard Relief Header, Inflection Point Engineering does not specify the metallurgy or the pipe class on our P&ID; the contractor has this responsibility. For an Acid Gas Header, the PRV tailpipe and the relief header should be specified as 316L SS on our P&ID because this design is inherently safer than a steam-jacketed killed carbon steel header. For HF service, the PRV tailpipe and relief header is specified as killed carbon steel with at least a 0.125 inch (3mm) corrosion allowance.

The metallurgy required for cryogenic service is based on the auto-chill temperature of the relief stream when flashed at the nominal header pressure, i.e. 5 psig (0.35 kg/cm2 g). If the temperature is above -20ºF (-29ºC), killed carbon steel is adequate. If the auto-chill temperature is between -20ºF and -50ºF (-45ºC), impact-tested killed carbon steel is used. But if the flash temperature is -50ºF or lower, 304 SS piping is required.

Note that the standard metallurgy for the PRV body is 316SS when the auto-chill temperature or the lowest ambient temperature is below -20ºF (-29ºC).

Material selection should be reviewed by the metallurgists in the event the fuel gas purge is deemed to be a wet H2S service.

3.4 Purging

A continuous fuel gas purge is introduced at the beginning of each process unit relief header while it is in service. The fuel gas purge maintains the header at slight positive pressure to keep air from entering the header and to sweep toxic or corrosive components out of the header.

Refinery fuel gas is typically dry and sweet. On this basis, the typical design for a process relief header is that isolating valves downstream of the relief valve do not require special trim. However, valve trim should be reviewed whenever the operating environment of the valves is severe. In addition, if the refinery fuel gas (or other purge gas) is found to be a wet H2S Service per NACE MR0103, then consult with the metallurgists or piping specialists for a review of the P&ID detail for the standard relief header.

In addition to the continuous fuel gas purge, an intermittent, high-volume fuel gas purge is provided on demand whenever hot or condensable vapor is discharged into the main or refinery relief header. This purge is activated upon high temperature in the header, or it may be initiated manually. If condensable vapor remains in the header after the relief event is over, it will eventually condense, creating a partial vacuum and potentially drawing air into the header via the flare stack. A similar argument applies to rapid cooling of hot vapor in the relief header. The high volume purge gas is intended to counteract the effects of partial vacuum caused by rapidly cooling material in the relief header. Design of the purge systems is the responsibility of the contractor unless the Client specifically requires a Inflection Point Engineering design. Contact a PRV or Offsites Specialist if required to design the purge system.

Finally, inert gas purge connections are provided to safely isolate the process unit relief header from the refinery relief header system during a shut-down operation. Inert gas is used to purge air from the process unit header prior to start-up, and it is used to purge fuel gas from the header during shutdown and isolation.

3.5 Isolation for Maintenance

Inflection Point Engineering recommendations for isolating individual process unit relief headers are based on codes and practices. All deviations from our recommended practice must be requested in writing by the customer and approved by an Offsites or PRV Specialist.

A locked-open gate valve and spectacle blind are provided to isolate the process unit relief header from the refinery flare system when the unit is shut down for maintenance. The valve stem of the isolating valve must be oriented horizontally to prevent a detached gate from blocking the header. Inflection Point Engineering does not permit check valves in the process unit relief header, consistent with API recommended practice. They are concerned that a check valve may remain stuck in the partially closed position and restrict flow in the header system.

Once the process unit is shutdown, the relief header can be isolated by following the sequence below:

• Depressure and de-inventory the process unit.
• Establish an inert gas purge upstream and downstream of the isolation valve.
• Stop and isolate the continuous fuel gas purge to the relief header.
• Unlock and close the isolation valve in the relief header.
• Verify that hydrocarbon is purged from the relief header using the test connection upstream of the spectacle blind.
• Swing the spectacle blind upstream of the isolation valve.
• Maintain the inert gas purge downstream of the isolation valve.

The relief header must be commissioned before the process unit is pressured up. The re-commissioning sequence is given below:

• Purge the header upstream of the spectacle blind with inert gas to remove air.
• Verify that air is purged from the system using the test connection upstream of the spectacle blind.
• Swing the spectacle blind to open position.
• Open the isolation block valve and lock it open.
• Reestablish the continuous fuel gas purge to the header.
• Close and isolate inert gas purge connections.

3.6 Hydraulic Design of the Relief Header

PRV tailpipes, process unit relief headers, and the refinery relief header are sized to limit the back pressure obtained at the outlet flange of each PRV during a relief event. The hydraulic analysis for the header system involves determining the maximum flaring rate for various relief contingencies, identifying back pressure constraints imposed by individual PRVs, and allocating the available pressure drop to the tailpipe, process unit sub-header, and the refinery relief header. Generally, the hydraulic and economic analysis required to size various sections of the relief header system is the responsibility of the contractor. If, however, the relief header system is included within our design scope, an Offsites or PRV Specialist must be assigned to do the complicated analysis.

3.7 Heat Tracing

Heat tracing and insulation are required when the low ambient temperature is below the freezing, pour point, or solidification temperature of fluids expected in the header, including hydrates. When freezing or plugging can occur, the PRV and its inlet and outlet piping should be heat-traced to the unit relief header as a minimum, and it may be required up to the process unit liquid knockout drum. The objective temperature for the PRV and its inlet and outlet should be the highest objective temperature specified for the fluid that is relieved through the valve. The objective temperature for the relief header should be the highest objective temperature specified for all non-flowing sections of the process unit. As mentioned in paragraph 2.4, heat tracing cannot used to prevent low-temperature embrittlement of the header. Rather, this hazard is addressed by the proper selection of metallurgy.

3.8 Cross-Contamination

Occasionally, a separate relief header is run to the knockout drum at the base of the flare stack to prevent contamination of high purity products. For example, the vents from the Raffinate and Extract columns in the Parex unit are typically routed to a vent tank with a separate vent line to the flare to minimize pressure surges that could back contaminants into the process.

Attachment 1: P&ID Detail for a Standard Relief Header

Attachment 2: P&ID Detail for an Acid Gas Relief Header

Attachment 3: P&ID Detail for a Cryogenic Relief Header

Attachment 4: Decision Tree for Selecting Header Metallurgy and Heat Tracing

Note 1. Relief and vent streams containing light liquids (C4 minus) will flash when entering the relatively low pressure relief header and auto-chill. If the auto-chill temperature is lower than -20ºF, carbon steel will be susceptible to brittle fracture. This hazard is addressed by upgrading the piping metallurgy.

Note 2. If the water dew point temperature is above the auto-chill temperature, a separate water phase will occur in the relief header. If the auto-chill temperature is lower than 32ºF, ice will form in the header. This hazard is addressed by enlarging the PRV tailpipe one pipe diameter to accommodate the ice build-up during a short-lived relief event.

Note 3. If the water dew point, lowest ambient, and the auto-chill temperature are such that ice will form when the PRV leaks, heat tracing is required to prevent an undetected ice build-up in the header piping. Since the PRV tailpipe size is not given on the P&ID, add a note to the 807 project specification instructing the contractor to enlarge the tailpipe at least one pipe size to accommodate ice build-up.