Section 12 — Instruments and Controls

Control Systems for Fired Heaters

IPE Engineering Practice IPE-EP-12-4-2

Document number: IPE-EP-12-4-2 · Section: 12 — Instruments and Controls

SCOPE

• The purpose of this Practice is to present requirements for the design of a fired heating unit instrument and control system.
• Any deviation from this Practice must be approved by the procedure described in EP 1–1–3.
• A revision bar indicates all changes made to this Revision.

2.0 REFERENCES

The latest edition of the following standards and publications are referred to herein and shall be used with this Practice.

STANDARDS AND PUBLICATIONS

DEFINITIONS

• Contractor - Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Inspector - A Refining Company appointed engineer or inspector.
• 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.
• Owner - Refining Company.
• Owner’s Engineer - A Refining Company appointed engineer.
• Purchaser - The party placing a direct purchase order. The purchaser is the Owner’s designated representative.

QUALITY ASSURANCE

• The contractor shall operate a quality management system to ensure that the requirements of this specification are achieved. Each contractor shall state, as part of the quotation the quality assurance program and procedures used. may require demonstration of the quality system.
• The implementation of such a quality system shall be subject to monitoring by and in addition may be audited following an agreed period of notice. personnel shall have the right to inspect at all reasonable times and at all locations during the engineering, configuration, assembly, and testing of the fired heater controls, systems and cabinets.
• The system vendor and contractor shall ensure that technical and quality requirements specified in this specification are applied to all materials, equipment and services provided by sub– contractors and to free issue materials.

INSTRUMENTS AND CONTROLS

• General
• Detailed requirements for control equipment shall be as specified by in accordance with the applicable portions of EP 12–1–1 and EP 12–2–1. Electrical requirements shall conform to EP 13–1–1 and EP 13–2–1.
• The heater supplier shall be responsible for the satisfactory design and operating capability of the local instruments, controls and safety equipment associated with the furnace, and the supplier shall submit details to for approval before placing orders.
• The instrumentation specified in this section is the minimum required to enable the heater to be operated safely.
• This Practice does not contain any references to automatic combustion control. If a vendor supplied automatic control system is to be considered, check that they have similar systems operating under similar refinery conditions, and that these systems have been operating satisfactorily on several units for sufficiently long periods. Vendor supplied automatic control systems shall be approved by Owner.
• Furnace Conditions
• The following draft indicators shall be locally mounted at ground level, except where noted otherwise.
• In the plenum chamber(s), where fitted to natural draft burners. With forced draft burners, the common windbox pressure(s) shall be indicated locally and in the control room. In addition, individual burner air inlet pressures, either in the ducts downstream of individual burner dampers or the burner air casing shall be indicated locally.
• At burner level of the furnace.
• At the inlet to the convection section, with draft gauge located, or repeated, near the damper controls at grade.
• At the outlet from the convection section (if control point, control room indication also).

• Before the outlet damper or at the induced draft fan inlet, where installed, if the draft is significantly different from that at the convection section outlet (if control point, control room indication also).
• Downstream of the outlet damper.
• A plugged draft connection for test purposes shall be provided in the furnace arch at positions to be approved by .
• The following temperature indications shall be located in the control room, except where noted otherwise.
• Liquid fuel in the burner manifold as near to the burners as possible (local indication only).
• Heavy fuel oil in the individual heater return main downstream of the return flow indicator (local indication only).
• Flue gas at the inlet to the convection section.
• Flue gas at the outlet from the convection section.
• If a combustion–air preheater is utilized, the flue gas temperature at the exit from the preheater and the combustion air temperatures at the exit from the preheater and in the common burner windbox.
• Flue gas in the stack above the highest duct entry. The number and positioning of sensing points required for 5.2.2.c through 5.2.2.f of this Practice shall be such as to provide an adequate indication of the average flue gas temperature and shall be approved by
• Radiant coil tube skin metal. Thermocouples for this service shall be fabricated and attached according to EP 12–2–1. Fired reboilers and all heaters on services where coking may be expected shall have at least two skin thermocouples per pass. Other heaters shall have skin thermocouples on selected passes. The thermocouples shall be positioned where maximum metal temperatures are anticipated and shall be approved by the Owner.
• Where reverse steam–air decoking is intended, additional skin thermocouples shall be installed in each pass, on one of the shock tubes.
• Tube metal temperatures on cracking and reforming furnaces with cast tubes shall be measured by portable pyrometers; skin thermocouples shall not normally be fitted.
• The following pressure indications shall be located local to the heater, except where noted otherwise.
• Fuel gas and liquid fuel:
• Main supply line (control room indication also).
• Individual furnace supply upstream of the control valve.
• At each end of the burner manifold.
• Pressure difference across filters.
• Pilot gas:

• Main supply line, unless identical with 5.2.5.a.1 above (control room indication also).
• On the pilot burners manifold.
• Pressure difference across filters.
• Atomizing steam downstream of the pressure control valve.
• To monitor the leakage of the shut–off valves with a single main fuel, a pressure measuring connection shall be provided between the two safety shut–off valves. With multiple main fuels, a second measuring connection shall be provided downstream of the second safety shut–off valve. The pressure measuring connection, suitable for use with a sensitive test gauge to detect small leaks, shall be valved and blanked off accordingly.
• The following flow indications shall be located local to the heater, except where noted otherwise:
• Combustion air flow where forced draft burners are utilized.
• Liquid fuel to each heater supply, both supply and return, where applicable.
• Fuel gas to each heater (control room indication also).
• An additional flow element for pilot gas where it is piped independently of the main gas supply.
• Flue gas characteristics.
• A opacity measuring device, when required by, will be fitted in the flue at the outlet of each furnace, with the indication on the local firing panel and an alarm and trend history in the control room.
• A sample point, of a size to be specified by , shall be provided near the opacity measuring device for the insertion of solid and other emission sampling probes.
• An analyzer, or analyzers, of a type to be specified by in accordance with EP 12– 7–1, shall be provided for taking samples of the flue gas at points to be approved by . The measurements will, as far as is practicable, relate to flame conditions and, where a convection bank is installed, the probe/ sampler position shall be as near the fire box as is practical.
• Oxygen shall be measured and indicated on the local firing panel and in the control room.
• Sample points shall be provided for oxygen measurement level and at the inlet and outlet of the convection section in each cell. The size of the sample points shall be specified by and shall be provided with screwed closures.
• Where the furnace discharges into a common duct system, a provision similar to 5.2.7.b above shall be made in the combined flue gas duct or stack. Where statutory regulations require, an oxygen analyzer as in 5.2.7.c above must also be installed at the same position.
• Process Conditions
• The following temperature indications shall be located in the control room, except where noted otherwise.

• Process fluid in the common inlet manifold.
• Process fluid at the crossover between the radiant and convection sections with local indication only, and at the outlet of each pass. When steam–air decoking is specified, to avoid damage from spalling, the last named thermowells shall be located downstream of the swing bends. To measure the decoking effluent temperature, additional thermowells with installed thermocouples shall be locate in the decoking outlet lines as specified by .
• Process fluid in the common outlet manifold.
• Where provision is made for future reverse steam–air decoking, a thermowell with installed thermocouple shall be provided at the normal inlet end of each individual pass which will become an outlet during decoking. This thermowell shall be located in the decoking piping, not the process piping, on the side of the swing bend away from the heater. Compensating cable shall be run from these thermocouple heads and terminated at a locally mounted junction box to allow a temporarily– installed recorder to be connected.
• On multiple–pass furnaces, e.g. reforming furnaces, the measurement of individual tube process outlet temperatures shall be by the installation of in–tube thermocouples.
• The following pressure indications shall be located local to the heater, except where noted otherwise.
• Process fluid at the common inlet manifold.
• Process fluid at the inlet to each pass, if they are fitted with separate inlet control valves.
• Process fluid in the common outlet manifold.
• Flow indicators, controls.
• a indication with hand control valves, according to EP 5–3–17, on each pass, operable locally from grade. Flow indication shall be local at grade in sight of the control valves and in the control room.
• A low flow alarm per pass shall be provided in the control room.
• Heaters on liquid vaporizing service where coking is not anticipated, e.g. distillate reboilers, shall have pass flow indicators and valves as specified in 5.3.3 of this Practice, but the flows shall be indicated at the heater near to the valve operating location and in the control room with a low flow alarm per pass. The flow measuring elements shall be positioned where the flow through the element is wholly liquid.
• Heaters on liquid vaporizing service where coking is likely, e.g. crude, vacuum, shall have individual passes flow controlled. Preference should be given to a master total flow controller cascaded to individual pass flow ratio/bias controllers or pass balancing based on heater pass outlet temperatures.

• The flow measuring elements shall be positioned where the flow through the element is wholly liquid and preferably downstream of any exchangers. In such cases, the system pressure shall be increased if necessary to suppress any vaporization at or through the flow element. Alternatively, and subject to the approval of , the measuring elements may be placed upstream of the exchanger bank in which vaporization starts, but this arrangement is not normally acceptable due to the need to split exchanger banks, loss of heat transfer through vapor separation, and possible damage to tubes from uneven heating. The flows shall be indicated both at the unit (at grade within sight of the control valves) and in the control room with a low flow alarm per pass. The flow controller shall also be in the control room.
• Heaters on non–coking service where two–phase flow at the heater inlet cannot be avoided,

e.g. hydrocracker heaters, shall be arranged as follows:

• Where a fully dispersed flow regime (as judged by the O. Baker parameters published in Oil and Gas Journal, July 26th, 1954) can be assured or arranged at the point of a split over the specified operating flow range, then uncontrolled symmetrical passes are acceptable.
• The splitting of passes within a vacuum heater shall also be governed by the above requirement. The injection arrangements of steam into the split headers of vacuum heaters shall also ensure even distribution between the passes.
• Where a fully dispersed flow regime cannot be achieved by any practical means, either a single pass shall be provided or the individual phases shall be flow controlled to each pass before mixing.
• Uncontrolled split flow is acceptable for gas heating service, subject to symmetry in design and to the pressure drop across each coil being sufficiently high relative to the variation in the static head along the manifold to maintain equal pass flows, particularly at turn–down.
• The ability to shut off total process flow to all heaters, regardless of service, shall be remote from the heater.
• Protective Instrumentation
• Alarms shall be provided in the control room, to be actuated by the following.
• The tube skin thermocouple sensing the highest temperature.
• Low and high pilot gas pressure at the pilot burners.
• Low pressure in the fuel mains, gas and liquid.
• Low and high pressure in the fuel lines downstream of control valves.
• Individual low flow alarms for passes of multiple pass furnaces.
• High outlet temperature on each pass.
• High opacity.
• Low atomizing steam pressure for fixed atomizing steam pressure burners and low differential pressure for burners using differential steam pressure.
• Low percentage oxygen in the flue gas.
• High percentage combustibles in the flue gas.
• Low combustion air flow on forced draft burners.
• High firebox pressure.
• High windbox pressure if forced draft burners are utilized.
• Low windbox pressure if forced draft burners are utilized.
• High stack temperature.

• Heaters with hydrogen recycle shall have an automatic shutdown which stops the main fuel flow upon loss of hydrogen recycle.
• All furnace burners shall be fitted with a “high stability” natural gas fired pilot.
• Minimum fuel gas flow bypasses shall not be used.
• A manually (operator initiated) activated shutdown system to isolate the main flow of fuel(s) to the firebox during a unit emergency shall be provided. The system shall be activated from a remote location. Unless another location is specified by the Owner’s Engineer, the system will be activated front he control house. The design of the system shall follow the requirements for a Category 2 Protective Instrumentation Systems, EP 12–10–1. The remote isolation of pilot gas is not recommended for a manually activated shutdown system.
• If the fired heater is included in an automated unit protective system (Category 2 as defined in EP 12–10–1 ), the automated system will provide for isolation of both the pilot gas and main fuel flows.

5.5 Flame Failure

If flame failure equipment is required, the type and manufacturer shall be approved by the Owner in accordance with API 550, Section 14.

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