Process Heaters Operation and Control

Engineering Encyclopedia Saudi Aramco DeskTop Standards

PROCESS HEATERS OPERATION and CONTROL

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain or may not be copied, disclosed to third parties, otherwise used inreproduced, whole, or insold, part, given, withoutorthe written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Mechanical File Reference: MEX-105.02

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Inroduction to Process Heaters Process Heaters Operation and Control

Section

Page

INFORMATION ............................................................................................................... 4 PROCESS HEATER OPERATING VARIABLES, MONITORING AND CONTROL ................................................................................................ 4 Typical PFD.......................................................................................................... 4 Heater Charge Flow ............................................................................................. 7 Pass-Flow Balancing ............................................................................................ 7 Tube-Skin Temperature - Tube Metal Temperatures ........................................... 8 Variables Affecting Tube-Metal Temperature ....................................................... 8 Variables Affecting Coke Deposits ..................................................................... 10 Heater Outlet Temperature................................................................................. 11 Heater Charge Temperature............................................................................... 11 Stack Temperature ............................................................................................. 11 Fuel Variables..................................................................................................... 12 Firebox Draft....................................................................................................... 13 Flue Gas Oxygen................................................................................................ 14 Excess Air/Oxygen ............................................................................................. 14 Air Leaks.................................................................................................. 16 Operating Guidelines ............................................................................... 18 Consequences of Inadequate Control ................................................................ 19 Tube Failure - (Overheating).................................................................... 19 Fuel Accumulation and Ignition................................................................ 21 PROCESS HEATER SAFETY SYSTEMS .................................................................... 22 Alarm Systems ................................................................................................... 22 Emergency Shutdown (ESD) Systems ............................................................... 24 Startup Interlocks................................................................................................ 24 Over-Pressure Control........................................................................................ 26 Typical Fuel Gas System.................................................................................... 26 MAJOR STEPS FOR SAFE PROCESS HEATER STARTUP....................................... 28 Fuel Shutoff ........................................................................................................ 28 Establish Tubeside Process Flow....................................................................... 28 Purging ............................................................................................................... 29 Lightoff Pilots And Burner Ignition ...................................................................... 29 Process Heatup Rate ......................................................................................... 30

Saudi Aramco DeskTop Standards

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MAJOR STEPS FOR SAFE PROCESS HEATER SHUTDOWNS................................ 32 Emergency Shutdown ........................................................................................ 32 Normal Shutdown ............................................................................................... 32 PROCESS HEATER STEAM-AIR DECOKING............................................................. 33 Steps and Control............................................................................................... 33 Sample Decoking Procedure .............................................................................. 33 GLOSSARY .................................................................................................................. 34 ADDENDUM ................................................................................................................. 35 ADDENDUM A: RT CRUDE UNIT................................................................................ 36 Furnace Operations ............................................................................................ 36 Furnace Firing Check List................................................................................... 45 Furnace Fireman Certificate ............................................................................... 47 Crude Unit Furnace Burner Design .................................................................... 48 Location of Furnace Thermocouples .................................................................. 49 ADDENDUM B: RT CRUDE UNIT – DECOKING ATMOSPHERIC FURNACES........................................................................................ 51 ADDENDUM C: RT CRUDE UNIT – DECOKING VACUUM FURNACE...................... 56 ADDENDUM D: RT CRUDE UNIT – FURNACE OPERATING LIMITS........................ 65 REFERENCES.............................................................................................................. 67


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List of Figures Figure 1. Example of Direct Fired Reboiler Controls/Safety Devices ............................. 4 Figure 2. Naphtha Reforming Unit, Ras Tanura ............................................................. 6 Figure 3. Heat Transfer Resistance................................................................................ 9 Figure 4. Furnace Natural Draft Profile......................................................................... 13 Figure 5. Optimum Excess Air for a Fired Heater......................................................... 15 Figure 6. Typical Combustibles Emission from Fired Heaters ...................................... 15 Figure 7. Furnace Air Leaks ......................................................................................... 16 Figure 8. Cost of Furnace Air Leaks............................................................................. 17 Figure 9. Short Term Overheating................................................................................ 20 Figure 10. Extreme Short Term Overheating ............................................................... 20 Figure 11. Burner Safety System - Natural Draft Furnace............................................ 25 Figure 12. Naphtha Reforming Unit, Ras Tanura ......................................................... 27

List of Tables Table 1. Operating Guidelines...................................................................................... 18


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INFORMATION PROCESS HEATER OPERATING VARIABLES, MONITORING AND CONTROL Typical PFD Figure 1 is a typical PFD for a process heater. Furnace controls and safety devices vary considerably depending on the furnace service and the refinery a plant location. Figure 1 shows an example control system for a directfired heater. It should not be considered complete, but only representative of the type of instrumentation that should be carefully considered in designing a control system for a furnace service. For example Figure 1 shows no tube-skin temperatures or flue gas oxygen measurement.

With permission from the Gas Processors Suppliers Association. Source: Engineering Data Book.

Figure 1. Example of Direct Fired Reboiler Controls/Safety Devices


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Figure 2 shows a simplified PFD for a Reforming Unit at Ras Tanura. The knock out drums on the fuel streams remove liquid and solid entrainment that could interfere with proper burner operation. The emergency isolation valve (ZV) is controlled by the emergency shutdown device (ESD). There is a separate pilot gas system for the pilots in each burner. The firing control valve is controlled by a temperature controller on the combined process fluid outlet. The heater F - 101 has four coils and six burners. The flow through each coil is manually controlled using a flow indicator nearby.


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Figure 2. Naphtha Reforming Unit, Ras Tanura


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Heater Charge Flow The heater charge flow is the flow rate of the process fluid. This flow rate is very important since it cools the metal of the heater tubes. The heater charge rate is alarmed when it is below a minimum for that heater. A flow rate below the minimum rate will result in tube overheating which could result in coke formation, deformation of tubes, and possibly, rupture of tubes.

Pass-Flow Balancing The flow through each parallel tube pass should be maintained at the same rate. This is referred to as pass-flow balancing. Each pass of a heater has a flow indicator and at least a manual control valve. The flow through each pass must be equal for proper control of tube metal temperatures. An imbalance in flow can result in coking and tube overheating. A reduced flow will result in less heat being removed from the tubes in that pass. Since the heat flux on all tube passes is similar, the tube pass with the reduced flow will have a higher fluid temperature because there is less fluid to absorb the same amount of heat. A higher fluid temperature will result in a higher metal temperature. A higher fluid temperature can also cause coking that will further increase the metal temperature. The outlet of each coil or pass has a thermocouple to measure the temperature. The coil outlet temperatures must be within at most 10ºF for pass-flow balancing. If the flows indicate passflows are balanced but the temperatures do not, then the flow meters and thermocouples should be checked to resolve this discrepancy.


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Tube-Skin Temperature - Tube Metal Temperatures The ability of the tubes to contain an internal pressure is dependent on the basic allowable stress of the tube metalurgy. As the temperature of the tube metal increases the basic allowable stress decreases, which in turn reduces the internal pressure handling capability of the tube. Metal temperatures can also affect the thermal stability of the material in the tube resulting in hydrocarbon coking at high temperatures. For these reasons the tube metal temperature is often monitored using tube-skin thermocouples. Additionally, high tube temperatures can also provide an additional indication of low flow or ineffective pass-flow balancing. Naphtha Reforming Unit charge heater F - 201 has a tube-skin thermocouple near the inlet in the radiant section and three other tube - skin thermocouples in the radiant section on each of four coils. The Ras Tanura Crude Unit 15 furnaces each have four tube-skin thermocouples on each of the radiant coils. Each atmospheric furnace has eight coils and each vacuum furnace has four coils.

Variables Affecting Tube-Metal Temperature The tube-metal temperature is a function of the heat transfer resistances as previously discussed. Figure 3 summarizes the resistances to heat transfer. Anything that affects these resistances will affect the metal temperature. In the radiant section the resistance on the outside of the tubes is low since almost all the heat is transferred by radiation.


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Figure 3. Heat Transfer Resistance

The following changes in operation are the usual causes of tube - metal temperature changes: •

•

•

•

•

A change in firing rate: the change in the firing rate will change the radiant section heat flux (Btu/hr ft 2), which will change the metal temperature. A change in flame pattern: the flame pattern can result in local overheating. A change in heater outlet temperature (change in firing rate (heat flux) and temperature of fluid in tubes). A change in process fluid rate which will change both the heat flux and the inside tube heat transfer resistance. A change in fouling resistance inside tube: the change in fouling resistance inside the tube is usually due to deposits such as coke.


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Variables Affecting Coke Deposits Coking is a polymerization reaction and is controlled by time at a temperature. An equal amount of coking can occur with a relatively low temperature (750 - 800ºF) if given long residence time and with a relatively high temperature (900ºF) and a short residence time. A coker operates at temperatures in the 750 800ºF range and the coke drum provides a long residence time. Certain chemical compounds have a greater tendency to coke than others. Materials with many olefins and heavy hydrocarbons (resids) are more likely to coke. The heavy hydrocarbons crack easily at 750ºF forming olefins that readily polymerize to form coke. High tube-metal temperatures will cause coking even at the relatively low residence time in a heater tube. A heater is always more susceptible to coking when operating at low velocities because the residence time is longer. Steam is sometimes added to the heater coil to maintain high velocity and a low residence time. Once coking has started it increases the tube-metal temperature because the cooling process fluid is insulated from the tube by the coke. Increased tube - metal temperatures due to coking can result in tube failure. Coke deposits can be observed by the change in tube color due to the higher metal temperature.


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Heater Outlet Temperature The heater outlet of each coil is monitored to verify that the flow through the coils is balanced or evenly divided. The heater outlet temperature of the combined coil outlets is controlled at a desired value for the process by controlling the firing rate. The control of this variable needs to be tuned to avoid cycling the firing rate which can result in tube overheating. The firing rate controlled controlling fuel gas or fuel oil flow, butcan notbe both. Usuallybythe gas flow either is controlled and oil is fired under manual flow control.

Heater Charge Temperature The heater charge temperature is monitored because it can have a major effect on the heat duty required to raise the process fluid to the heater outlet temperature. Changes in heat duty affect firing and heat flux, which then affect tube-metal temperature. Changes in the heater charge temperature also will change the temperature of the fluid in the tubes. The metal temperature will be affected by both the firing rate and the fluid temperature in the tubes.

Stack Temperature Stack temperature is an indication of the amount of waste energy is going up the stack. The primary control of this variable is proper design of heat exchange in the heater. A leaking or ruptured tube can result in a sudden increase in this temperature because the process fluid is burning and the flow is uncontrolled.


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Fuel Variables The fuel variables for process heaters include fuel gas pressure, fuel oil pressure, fuel oil temperature, fuel flow and the steam/fuel oil differential pressure. Fuel flow is controlled to meet heater outlet temperature requirements. Fuel flow can change due to changes in the heater outlet temperature and changes in the heating value of the fuel. Fuel flowpressure should not function of fuel supply pressure. Supply to be theacontrol valve should have an independent control. Fuel flow will be shut off by the emergency isolation block valve. Shutdown events will close this valve. One such event is a low low heater charge flow which if continued would result in tube failure because of tub overheating. On heaters with the ability to burn both gas and oil fuels, the temperature control of the heater outlet will control one rate but not both. When both fuels are fired the oil rate is usually manually controlled by the number of oil burners in service and the gas rate is controlled by the heater outlet temperature. Changes in the heater charge temperature also will change the temperature of the fluid in the tubes. The metal temperature will be affected by both the firing rate and the fluid temperature in the tubes. For example, too low fuel gas pressure can result in an unstable flame and possible flame failure. Too high gas pressure can result in flame lift-off and possible flame failure. Flame failure can result in fuel gas accumulating in the firebox and being ignited by hot refractory with explosive force.


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Firebox Draft The firebox draft is a critical variable in a natural draft furnace because it controls the available pressure drop of air across the burner. A minimum pressure drop across the burner is required for proper mixing of air and fuel. Changes in the draft can also change the amount of combustion air to the burner. Low draft can result in smoking and long flames. High draft can result in a low thermal efficiency because too much air is flowing through the furnace. Process heaters are designed to operate with a draft (negative pressure) in the firebox. The view ports are not covered with fire resistant glass for pressurized operation. Figure 4 shows a typical draft profile for a natural draft process heater.

With permission from the Gas Processors Suppliers Association. Source: Engineering Data Book.

Figure 4. Furnace Natural Draft Profile


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Flue Gas Oxygen Flue gas oxygen is the amount of oxygen in the flue gas. It is not excess oxygen although it is frequently erroneously referred to as such by many. Flue gas oxygen can be measured by an analyzer in the stack, an analyzer on the ground, or by taking a sample to the laboratory.

Excess Air/Oxygen Excess air is defined as the amount of air in the stack divided by the amount of air theoretically required for complete combustion. Excess air is normally expressed as a percentage. Excess air and excess oxygen are numerically equivalent because both numerator and denominator are multiplied by the same conversion constant to convert from one to the other. The percent oxygen in the stack is not the percent excess oxygen. Burners are not 100% efficient and need some excess air to operate properly. Figure 5 and Figure 6 show that a burner has an optimum range for excess air. The optimum will be different for different burners. Forced draft burners usually have lower optimum range than natural draft burners. Oil burners normally have higher optimum range than gas burners.


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Figure 5. Optimum Excess Air for a Fired Heater

Figure 6. Typical Combustibles Emission from Fired Heaters


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Air Leaks

High excess air can be caused by air leaks into the furnace. Figure 7 shows likely sources of furnace air leaks.

Figure 7. Furnace Air Leaks


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The cost of air leaks is demonstrated in Figure 8 for an open 5 x 9 inches observation door with fuel costing $5.00/million Btu.

Figure 8. Cost of Furnace Air Leaks


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Operating Guidelines

Both the stack damper and the burner air registers are used to control both the excess air and the furnace draft. The operating guidelines in Table 1 below can be used to judge which control should be used.

Low Draft

High Draft

Low Excess Air (O2)

Open Damper

Open Burner Air

High Excess Air (O2)

Close Burner Air

Close Damper

Table 1. Operating Guidelines


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Consequences of Inadequate Control Inadequate control will result in damage to the heater. The damage can be serious to catastrophic. The most common results are listed below. Tube Failure (Overheating)

Tube overheating is the most likely result of inadequate control. If the metal temperature is too high, the elastic stress limit will be exceeded, the tube will bulge and may burst. Tube overheating may result from coking, operating upsets, low draft, low excess air, poor flame patterns and poor burner load distribution. The consequences of tube overheating are shown in Figure 9 and Figure 10.


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Figure 9. Short Term Overheating

Figure 10. Extreme Short Term Overheating


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Fuel Accumulation and Ignition

Improper control of burner operation, especially during startup, can result in a furnace explosion that can result in significant damage to the furnace. The most likely cause of a furnace explosion is from fuel that was admitted into the furnace without ignition at the burner and ignited later by flame from another source or by hot tubes and refractory. Furnace startup procedures are carefully written and interlocks used to avoid combustibles (explosive mixture) in the furnace when flame is introduced into the furnace. A flame failure can also result in a furnace explosion, given the right circumstances, because uncombusted fuel is released into the furnace firebox. Furnace burners are monitored by flame detectors and the fuel is shut off if no flame is detected to avoid furnace explosions.


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PROCESS HEATER SAFETY SYSTEMS The most common process heater safety systems include alarm systems, emergency shutdown systems, startup interlocks and a fuel control system to make sure fuel is not introduced into the heater until ignition can be controlled.

Alarm Systems The process heater burner safety system has the following alarms (pre-alarm before emergency shutdown) as specified by SAES-J-603: •

•

•

•

Fuel gas K.O. (Knock Out) drum high level. Indicates that there is a danger of carryover of liquid into the gas fuel system. Liquid carryover could cause a flame failure of the gas burners or result in liquid burning across the heater floor. Pilot gas low or high pressure. Indicates that the pilot gas pressure is low or high enough that stable operation of the pilot flame cannot be assured. Main fuel gas header pressure low or high. Indicates that the main fuel gas header pressure is such that stable operation of the gas burners cannot be assured. Instrument air pressure low. Indicates that air pressure is low enough that reliable operation of control valves may not be assured.

•

•

Process fluid high outlet temperature. The process temperature is high enough that high tube-metal temperatures may result in stress limits being exceeded. The temperature controller is unable to control the outlet temperature by reduced firing. This may also indicate a fire due to a leaking tube. Process fluid low flow. The low flow will result in high tubemetal temperatures.


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In addition the alarms on a heater may include the following: •

•

•

•

•

•

•

•

•

•

•

High stack temperature. This may indicate a leaking convection section tube with the leak burning. High tube-skin temperatures. This may indicate a coking deposit or that burners need adjustment. Low process fluid outlet pressure. This may indicate a burst tube. Low flow in convection section if process fluid is different than radiant section. Low or negative draft. Indicates an unsafe heater operation. High combustibles in the stack. This indicates smoking and may indicate a heater leak. Unbalanced flows through coils. This can be indicated by flow indicators and/or individual coil outlet temperatures being more than 10ºF different. Low flows can result in high metal temperatures. High pass inlet pressure. Indicates possible plugging of the coil. Low differential pressure between steam and oil. Indicates an unstable oil burner operation. Flame failure indicated by one of the flame detectors. Loss of electric power.

Operating limits for Ras Tanura Crude Unit furnaces are shown in Addendum D. These may be alarm or shutdown values.


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Emergency Shutdown (ESD) Systems An emergency shutdown will be initiated by the emergency shutdown system for the following conditions as per SAES-J-603: Low Low Heater Charge Flow. Low Low Pass Flow. High High Pass Outlet Temperature. •

• • •

• •

•

Low Low or High High Fuel Pressure. Low Low or High High Pilot Gas Pressure. Low Low Instrument Air Supply. Emergency shutdown button activated.

All shutdown signals will be initiated by a dedicated primary device such as a pressure switch as per SAES-J-601. The emergency shutdown system will shut the emergency isolation valves (tight shutoff) on fuel supplies. The emergency isolation valves will not reopen until the operator manually resets them. Emergency shutdown switches will not normally be bypassed. Bypass switches are provided in shutdown branch circuits for the following: •

•

•

Shutdowns that will shut the furnace down during startup if not manually bypassed. Shutdowns that will shut the furnace down at any time so that the instrumentation may be serviced and tested. Shutdowns that will shut major operating equipment down (charge pumps, lube oil pumps, etc.) so that the system can be serviced and tested.

Startup Interlocks SAES-J-603 specifies minimal interlocks for process heaters as noted in the major steps for process heater startup. These interlocks are intended to prevent heater explosions. The interlocks prevent further progress in the startup procedure until they are satisfied. Figure 11 shows the minimum interlocks for a natural draft furnace. For example, the emergency isolation valve (ZV) cannot be reset (opened) unless all supervisory cocks are closed.


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Figure 11. Burner Safety System - Natural Draft Furnace


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Over-Pressure Control Unlike boilers process heaters do not usually have safety valves. There are usually no valves between the furnace outlet and the receiving vessel so that the safety relief valve of the receiving vessel can be used to control the maximum pressure in the furnace. If there are valves between the heater outlet and the receiving vessel then these valves are chained open so that the receiving vessel safety relief valve can be utilized. If a control valve (manual or automatic) is between furnace outlet and receiving vessel then the furnace must have a safety valve.

Typical Fuel Gas System Figure 12 is a flow diagram of the fuel gas system for the Ras Tanura Naphtha Reforming Unit and is typical of fuel gas systems. It contains knock out drums to prevent liquids and solids from entering a gas burner, an emergency isolation valve (ZV) activated by the emergency shutdown system, a control valve to control firing based on the heater outlet temperature, and a manual control valve and supervisory cock for each burner. Hamer blinds are used here instead of double block and bleeds to make sure there is no flow of fuel until it is desired.


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Figure 12. Naphtha Reforming Unit, Ras Tanura


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MAJOR STEPS FOR SAFE PROCESS HEATER STARTUP Hydrostatic test water should be removed from the coils to the maximum extent before introducing the process fluid. All fuel lines should be steam blown to blowdown (not through burners) to air-free fuel lines and remove any liquids or solids that might plug burners. SAES-J-603 specifies the following minimum requirements for heater startup.

Fuel Shutoff A. The first sequence is to make sure that fuel is not leaking into the heater when you are trying to purge combustibles and that the airflow through the heater is not restricted. •

•

•

•

The main burner supervising cocks are checked closed. This check is an operator and logic (interlock) function. The pilot and main fuel gas emergency isolation valves are checked closed. This check is an operator and logic function. The stack damper is in proper open position. This is an operator function. The individual burner air registers are placed in their proper position. This action is an operator function.

Establish Tubeside Process Flow B. The process flow is established to cool the tubes when fire is introduced into the firebox. •

•

The process fluid flow is established and all coil passes are proven to be carrying the minimum flow. These conditions are a logic function. The individual pilot burner manual valves are checked closed. This check is an operator function.


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Purging C. The firebox is purged with steam and the absence of combustibles is verified before introducing a flame. •

•

Steam is introduced into the firebox to purge the heater until the heater is clear of combustibles. This is an operator function. Immediately after the purge is completed the firebox is checked manually for the presence of combustibles using a gas sniffer. This check is an operator function. If combustibles are detected another purge is required.

Lightoff Pilots And Burner Ignition D. Once all combustibles have been purged a flame can be introduced to light the first pilot and then light the associated first burner. •

•

The pilot emergency isolation valve (ZV) is opened (manually reset) pressurizing the pilot header up to the individual pilot burner valves. This action is an operator function. A hand torch is lit and placed near the pilot burner in the firebox. The pilot manual valve is opened, and the pilot flame is established and verified. These actions are operator functions.

•

•

When the pilot has been lit the main burner associated with that pilot may be fired. The main fuel gas emergency isolation valve is opened (manually reset). The supervisory cock is manually opened. The manual valve at the burner is opened. The main burner flame is established and visually verified. Manual draft adjustments are made as required. Steps 2 and 3 are followed to light the remaining burners. Main burners are not to be lit from other main burners.

Unlike boiler pilots, the pilots for a natural draft process heater burn continuously under all operating conditions.


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Addendum A is the Ras Tanura Refinery Furnace Operation Instruction Manual for Crude Unit furnaces 15F - 100A/B and 15F - 299A/B. This startup procedure shows a more conservative variation of the above procedure. The procedure calls for lighting off all pilots one at a time once the blind has been pulled on the pilot gas but the fuel gas header is still blinded. Then the blind is pulled on the fuel gas header and the main fuel gas burners are lit off one at a time from the pilots.

Process Heatup Rate Each startup may be a little different depending on the maintenance work that was done during the shutdown. For example if much refractory work was done, there maybe a restriction on the heatup rate that is more severe than normal. The heatup rate is restricted to allow the water that is within the refractory to migrate to the surface before it evaporates. More water must be removed from new refractory (castings, firebrick mortar) and the water is much deeper than old refractory that has been splashed with water during water washing of the tubes. If the heatup rate is too fast, the water will vaporize within the refractory and result in spalling (sloughing off layers) of the refractory. Addendum A shows that Ras Tanura Crude Unit heaters heatup rate is controlled so that the process fluid furnace outlet is increased by a maximum of 150ºF per hour. It takes about 5 hours to get to the normal heater outlet. The following activities are expected from the startup personnel during the refractory dryout and initial furnace operation. •

•

Check the performance of all the burners during refractory dryout. Monitor thermal movements of tubes, tube support systems, and refractory during dryout. Watch for debris on the heater floor. Debris may be from spalling of the refractory and indicate that the dryout rate is too fast.


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Inroduction to Process Heaters Process Heaters Operation and Control •

Investigate any performance data for the fired heater and attendant equipment that appears to differ from design specification values. Listed below are some of the more important general observations to be made and problems to look for during an initial startup. Coil and external piping movements. Lining condition as heater reaches operating temperature. − −

−

− −

− − −

− −


Pass flows, pass crossover, and outlet temperature. Tube hot spots and overheated passes. Burner and pilot combustion performance. Watch for problems such as fuel oil dripback, wet atomizing steam, burner orifice plugging, burner tip coking, uneven burner firing rates, leaning flames, flame impingement, burner noise, etc. Draft conditions, particularly at bridgewall (arch). Expansion joint movement. Damper positions. Stack vibration. Discuss special problems related to the specific fired heater in the operating manual.

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MAJOR STEPS FOR SAFE PROCESS HEATER SHUTDOWNS The major steps will depend upon whether the shutdown was an emergency or normal shutdown.

Emergency Shutdown During an emergency shutdown the fuel supply is shutoff (ESD System Function) by the emergency isolation valve. The process fluid heater charge may be shutdown (operator function) depending on the emergency (burst tube and/or fire within heater). Snuffing steam may be turned on. The heater is not cooled down before shutting off the fuel.

Normal Shutdown A normal shutdown includes cooling off the process heater at a specified rate before shutting off the fuel to each burner and purging all hydrocarbon from the flue gas side of the heater, the fuel lines and the burners. The normal shutdown for Ras Tanura Crude Unit is shown starting on page A-8 of Addendum A. Blinds are set for each burner as well as for the fuel supply headers in this procedure. Preparation for maintenance would include draining and removing all hydrocarbons on the process side. Austenitic stainless steel tubes will have to be washed with a soda ash solution before opening to air to prevent stress cracking. All wash water should be drained.


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PROCESS HEATER STEAM-AIR DECOKING Steam-air decoking is a common method of removing coke deposits from process heaters. Heaters in units that have significant amounts of coke deposited such as crude units and coking units have short scheduled turnarounds to decoke the heaters via steam-air decoking.

Steps and Control Process heater decoking is accomplished by spalling and by air burning the remaining coke. Spalling is accomplished by passing steam through the furnace tubes while firing outlet temperatures above 800ºF with tubeskin temperatures of 1200ºF. The hot steam dries, shrinks and loosens coke deposits, which are swept out of the heater into the portable decoking vessel. The steam is water quenched between the outlet header and the decoking vessel. The coke and water are drained from the decoking vessel through a screen to the oily water sewer. When spalling begins, the water from the decoking drum turns from a milky color to gray and then black. Pressures are controlled to a low level (about 50 psig) because of the limited strength of the tubes at a metal temperature of 1200ºF. Air burning of remaining coke is accomplished by admitting air to the furnace tubes along with the steam. The tube walls are cooled to about 1000ºF before admitting air. The hot tube walls (1000 - 1200ºF) insure ignition of the residual coke. Combustion is controlled by varying the ratio of air to steam to stay within the tube-wall temperature limitations of about 1200ºF but no more than 1300ºF. Burning can be observed by changes in the tube color. The last page of Addendum B and the next to the last page of Addendum C give a means of estimating metal temperature based on color.

Sample Decoking Procedure Addendum B and C are decoking procedures for Ras Tanura Crude Unit furnaces. Addendum B is for the atmospheric furnace and addendum C is for the vacuum furnace.


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GLOSSARY decoking

Removal of coke deposited in furnace (heater) tubes

draft

Negative pressure in furnace (heater)

Emergency isolation valve

A positive shut off valve for fuel systems activated by ESD. Must be manually reset once closed.

ESD

Emergency shutdown system

furnace

Process heater

K.O.

Knock out or a K.O. drum to remove liquids and solids from a gas.

pass

A coil of a heater

pilot

Premix burner used to ignite main burners

process heater

Furnace

purge

Flow of non-condensible gas (steam) used to displace combustibles in a furnace (heater)

skin thermo-couple

Thermocouple used to measure tube-metal temperature

sniffer

A tester for combustibles such as a J-W gas tester.

ZV

Emergency isolation valve


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ADDENDUM ADDENDUM A: RT CRUDE UNIT .................................................................. 36 Furnace Operations ................................................................................. 36 Furnace Firing Check List........................................................................ 45 Furnace Fireman Certificate .................................................................... 47 Crude Unit Furnace Burner Design.......................................................... 48 Location of Furnace Thermocouples........................................................ 49 ADDENDUM B: RT CRUDE UNIT – DECOKING ATMOSPHERIC FURNACES .......................................................................... 51 ADDENDUM C: RT CRUDE UNIT – DECOKING VACUUM FURNACE ........ 56 ADDENDUM D: RT CRUDE UNIT – FURNACE OPERATING LIMITS .......... 65


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ADDENDUM A: RT CRUDE UNIT Furnace Operations Page 1 of 15


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Inroduction to Process Heaters Process Heaters Operation and Control Page 2 of 15


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Furnace Firing Check List Page 10 of 15


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Furnace Fireman Certificate Page 12 of 15


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Crude Unit Furnace Burner Design Page 13 of 15


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Location of Furnace Thermocouples Page 14 of 15


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ADDENDUM B: RT CRUDE UNIT – DECOKING ATMOSPHERIC FURNACES Page 1 of 5


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ADDENDUM C: RT CRUDE UNIT – DECOKING VACUUM FURNACE Page 1 of 9


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ADDENDUM D: RT CRUDE UNIT – FURNACE OPERATING LIMITS Page 1 of 2


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REFERENCES SAES-J-603

Process Heater Burner Safety System

NFPA 85C

Prevention of Furnace Explosions/Implosions

Ras Tanura Operating Procedures


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Process Heaters Operation and Control

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