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ExxonMobil ProprietaryCONFIDENTIAL

FIRED HEATERS Section Page

DECOKING OF FIRED HEATERS TUBES VIII-I 1 of 21

DESIGN PRACTICES January, 2005

This information is considered CONFIDENTIAL and shall not be released to or discussed with any persons except (a) employees of ExxonMobilAffiliates who have an appropriate research agreement with ExxonMobil Research and Engineering Company (EMRE), and (b) consultants,contractors, or employees of third parties with whom proper secrecy agreements have been executed with EMRE or such ExxonMobil Affiliates.ExxonMobil Research and Engineering Company

CONTENTSSection Page

1 SCOPE ....................................................................................................................................................... 3

2 REFERENCES............................................................................................................................................ 3

3 BACKGROUND.......................................................................................................................................... 33.1 METHODS OF DECOKING ........................................................................................................... 33.2 WHEN TO SPECIFY DECOKING FACILITIES.............................................................................. 4

4 APPLICATIONS ......................................................................................................................................... 44.1 PIG DECOKING PROCEDURE ..................................................................................................... 4

4.1.1 Equipment Description ............................................................................................................. 44.1.2 Heater Preparation ................................................................................................................... 54.1.3 Cleaning Procedure.................................................................................................................. 54.1.4 Checking for Completeness ..................................................................................................... 6

4.2 NORMAL STEAM-AIR DECOKING PROCEDURE........................................................................ 64.2.1 Pre-decoking Boilout ................................................................................................................ 64.2.2 Heater Preparation for Spalling ................................................................................................ 74.2.3 Spalling..................................................................................................................................... 74.2.4 Burning ..................................................................................................................................... 84.2.5 Reverse Flow Decoking............................................................................................................ 8

4.3 ONSTREAM DECOKING PROCEDURE ....................................................................................... 84.3.1 Heater Preparation ................................................................................................................... 94.3.2 Reduce Firing Rate................................................................................................................... 94.3.3 Replace Hydrocarbon Feed With Steam .................................................................................. 94.3.4 Monitor Progress ...................................................................................................................... 94.3.5 Reduce Firing Rate Prior to Introducing Feed .......................................................................... 94.3.6 Return Heater to Normal Operation.......................................................................................... 9

4.4 LOW VELOCITY DECOKING PROCEDURE .............................................................................. 104.4.1 Equipment Preparation........................................................................................................... 104.4.2 Soaker Spall ........................................................................................................................... 104.4.3 Soaker Steam Shocking ......................................................................................................... 114.4.4 Heater Spalling....................................................................................................................... 114.4.5 Heater Steam Shocking.......................................................................................................... 114.4.6 Air Shocking/Burning .............................................................................................................. 114.4.7 Post-burn Shocking ................................................................................................................ 12

5 DESIGN CONSIDERATIONS................................................................................................................... 125.1 PIG DECOKING........................................................................................................................... 125.2 NORMAL STEAM-AIR DECOKING ............................................................................................. 12

5.2.1 Fired Heater ........................................................................................................................... 125.2.2 Decoking Facilities.................................................................................................................. 13

Changes shown by �






6 DESIGN PROCEDURE FOR NORMAL STEAM-AIR DECOKING .......................................................... 14

7 SAMPLE PROBLEM ................................................................................................................................ 16

FIGURES

Figure 1 Typical Piping Arrangement for Steam-Air Decoking ...................................................................... 18

Figure 2 Typical Piping Arrangement for Pig Decoking ................................................................................. 19

Figure 3 Pig Decoking Piping Detail .............................................................................................................. 20

Figure 4 Details of Steam-Air Decoking Facilities.......................................................................................... 21

Revision Memo

12/04 Highlights of this revision are:

1. Additional details on pig decoking

2. Modified decoking facility requirements.

3. Minor revision of Figure 3. Pig Decoking Piping Detail.






1 SCOPE

Coke deposits inside heater tubes can lead to higher tube metal temperatures, shorter run lengths, and can increase the potentialfor tube failure. Methods for removing coke include pig decoking, steam-air decoking, onstream decoking, and "Sand jetting". Thissection of the Design Practice describes some of the mechanisms leading to coking, and provides design considerations andprocedures for applying these methods of coke removal. A definitive guide to major aspects of decoking is contained in ReportEE.79.E87, Guide for Fired Heater Decoking. Decoking procedures for Steam Cracking furnaces are maintained by ExxonChemicals and are outside the scope of this document.

2 REFERENCES

�

� Matta, G.M., Guide for Fired Heater Decoking, ER&E Report No. EE.79E.87.� Juedes, D.L., Monitoring Fired Heater TMTs with Infra-Red Imaging Systems, ER&E Report No. EE.48E.88.� Juedes, D.L., Decoking Guidelines for Visbreaker Fired Heaters, ER&E Report No. EE.44E.82.� Quilan, T.E., Delayed Coker Heater Coking - Commercial Correlations Study, Report No. 2004RESID12

3 BACKGROUND

Coke can be defined as a carbonaceous deposit on the inside tube walls of fired heaters. Coke is composed primarily of carbon,with smaller amounts of hydrocarbon, sulfur and inorganic compounds. Its composition and mechanical properties will vary widelydepending on heater feed properties, process conditions, and the length of time it has been in place. Coke can either be graduallydeposited over a number of years, or may be formed suddenly following a process upset. A change in feed composition can alterthe rate of coke formation. Nearly any hydrocarbon feed can form coke under the right combination of conditions; however,permanent decoking facilities are normally specified only if the service is such that coking is likely to occur during normal operation.

Coking may be caused by many factors, including tube-side flow maldistribution, excessive heat flux (general or localized),contaminants in the feed (asphaltenes, salts, inorganic compounds, particulates, etc.), incompatible feed blends, excessively highcoil outlet temperatures, or a poor job of coke removal during the previous decoking.

3.1 METHODS OF DECOKING

� Pig Decoking, also referred as "mechanical decoking", employs polyurethane projectiles, or pigs, and water to mechanicallyremove coke from the tubes. The pigs have metal appendages, or pins, attached radially along its exterior for cutting the coke. Thepigs are propelled through the tubes with water. Compared to the other off-line decoking methods, pig decoking is generallypreferred because of its shorter downtime requirements, the avoidance of severe operating temperatures, the lower probability oftube damage, and for the reduction of capital investment cost. A specialty contractor conducts pig decoking, so it may not beavailable at some locations. This method of decoking can remove very hard inorganic deposits left in the coil from years ofdecoking by other methods, and can also remove deposits located in areas (i.e. return bends) where removal by steam-airdecoking is not possible. A clear benefit of pig decoking is that the furnace does not have to be fired, allowing other work to becarried out within and around the heater during decoking. Prior to entering the heater during pig decoking, the heater's conditionshould be evaluated to assess risks associated with entry (i.e. potential for injury from falling refractory, etc.).

Steam-Air Decoking is the removal of tube-side coke by the combined action of steam and air. The complete process isconducted while the heater is out of service and consists of two parts, "spalling" and "burning." During spalling, steam orcondensate is admitted to the heater coil at high rates while the heater is fired. The coke is removed by: (1) the cracking andbreaking of the coke due to the expansion of the tubes caused by higher than normal tube metal temperatures, and (2) the scouringaction of the high-velocity coke particles. With proper operation, nearly all the coke can be removed in this manner. During theburning phase, air removes the remainder of the coke by supplying oxygen to support combustion of the coke. Steam is injectedalong with the air to control the combustion and tube metal temperatures. In some cases, it is necessary to reverse the direction offlow (i.e., coil outlet to coil inlet) during spalling and burning in order to increase the decoking effectiveness. This is known as"reverse-flow" decoking. This requires additional facilities that cannot generally be justified unless the service is unusually severe.






Onstream Decoking applies the spalling phase of steam-air decoking on a pass by pass basis while the heater is in operation.While it is not normally as effective as conventional steam-air decoking, it allows run length of some types of heaters to be extendedwithout requiring a unit shutdown. It has been applied with considerable success in delayed coking service.

Low Velocity Decoking utilizes lower steam spalling velocities and a slower heat up rate to produce an effluent plume which is lessheavily loaded with coke. It has been applied in heaters that were very heavily loaded with coke, such as Visbreakers, as well aslocations in which stack pluming is regulated by local authorities.

The Sand-Jet method of decoking requires an abrasive agent such as flint or steel shot that is propelled through the heater coil by anitrogen carrier. It was developed by the Union Carbide Company and has been used intermittently in the past with limited successthroughout ExxonMobil. It is a highly costly process and has not proven to be as effective as other decoking methods. However, itis can be applied to heater coil configurations that cannot be pig decoked and have no permanently installed steam-air decokingfacilities. Due to its limited application, no additional information is provided in this Design Practice section.

3.2 WHEN TO SPECIFY DECOKING FACILITIES

� Decoking facilities shall be specified for all fired heaters in normal coking services, such as atmospheric and vacuum pipestills, anddelayed cokers. When available locally, pig decoking facilities should be specified for atmospheric and vacuum pipestills. Acombination of pig decoking and single-direction steam-air decoking facilities are recommended for delayed cokers. Site preferenceor past experience may dictate otherwise. For heaters in heavy coking service, such as thermal cracking and Visbreaker heaters,reverse-flow decoking facilities may be required. This need should be established based on experience with the feed and expectedoperating conditions. If there is no previous experience available, the Heat Transfer Equipment Section of EMRE's PlantEngineering Division should be consulted.

Decoking facilities should not be specified for services known to be non-coking under normal or common upset conditions. Thisusually includes heaters processing all vapor, such as POWERFORMER or Treat Gas heaters, and most liquid hydrocarbonservices with coil outlet temperatures below 600°F (315°C). Liquid streams containing asphaltenes, salts, or solids may be prone tocoking at lower temperatures than other services. For coker heaters, see report 2004RESID12 for coking correlations. If in doubt,feed samples can be tested in the laboratory under simulated field conditions to determine the stream's susceptibility to coking.Connections for future pig or steam-air can be specified during the design stage for a relatively small incremental investment.

4 APPLICATIONS

4.1 PIG DECOKING PROCEDURE

4.1.1 Equipment Description

� A contractor specializing in pig decoking carries out the operation. Pigging contractors generally are equipped with all necessaryspecialty equipment to complete the pigging process. This includes the pig propulsion pumps, hoses, valves, data recorders,reservoir and strainer tanks, pig launchers and pigs. The refineries are generally required to supply electrical connection, water(typically from nearby fire hydrant), crane, compressed air or nitrogen, diesel fuel, miscellaneous hoses, spool pieces and otherfittings for water supply and drainage. Additionally the refinery must supply a vacuum truck to remove coke and other depositsfrom the contractor's equipment after decoking is complete.

Pig Launchers are used to launch and receive the pigs. They are installed at the inlet and outlet of the pass being decoked.Locating launchers at grade reduces set-up and dismantling time, and eliminates repetitive climbs to the launcher during the piggingprocess. Full-port ball valves allow the pig to pass to and from the launcher. Separate connections on the launcher allow thepressurized water from the propulsion pump to push the pig through the coil. The same connections on the pig "catcher" serve asdrain lines to carry waste water and deposits to a strainer tank. A quick-opening cover is provided to enable pigs to be changedrapidly. A decoking skid, also provided by the contractor, contains the propulsion pump, clean water suction tank, a strainer tank,and instrumentation, which includes a pressure trend recorder. This equipment is illustrated in Figures 2 and 3.






Several different pigs will be used during the decoking operation. A light foam pig is used first for sizing of the initial decoking pigand establishing that the flow path is clear. It is followed by decoking pigs, which have a number of metal appendages or pinsprotruding radially from the base of the pig. The size of the decoking pigs will increase as coke is removed from the inner tube wall.Soft pig technology is preferred over the older hard pigs. Like hard pigs, soft pigs have a relatively hard polyurethane shell but alsohave inner core filled with foam, gel or air. The soft pigs with their inner core navigate plug headers and return bends better thanhard pigs, are not likely to damage the tubes and are more likely to rupture from the water pressure if the pig is temporarily lodgedin the coil. Upon rupturing, the broken pig can be washed away, which avoids a potential downtime extension to remove a solidlylodged pig. Also the older hard pigs generally use large appendages, commonly with a hard tungsten-carbide coating, and arerigidly attached to the body of the pig. These pigs had been a subject of past controversy over damaging the internal side of piping.Soft pigs tend to use smaller appendages that are inserted shallowly into the molded polyurethane shell. These appendages mayhave the tungsten-carbide coating, but rather than gouging the tube metal, these appendages will either compress towards the softinner core or tear away from the pig when the pig is pressed forcefully against the metal.

4.1.2 Heater Preparation

� Depending upon the coil length and pig decoking equipment available, tube passes can be decoked either in series or individually.Many of the pigging contractors have two pig propulsion pumps on one truck, which is called a "double-pumper" arrangement. Thispermits two independent passes to be decoked simultaneously.

In preparation for pigging, each pass should be freed of hydrocarbon and isolated from the remainder of the process piping. Allorifices or protruding instruments (i.e. thermowells) that may impede passage of the pig must be removed. Pig launchers areinstalled at the inlet and outlet of the pass being decoked. This may require piping modifications, although in some cases, it mayrequire only the removal of a spool piece. Note that pig decoking requires a continuous path throughout the coil. If radiant andconvection sections have a different number of passes, or intervening manifolds, launchers must be installed to avoid thesediscontinuities; however, non-branching coils that increase in tube size along its length as often is the case on vacuum pipestillheaters can be successfully pig decoked. The pig launcher is then hooked up to the propulsion pump and drain systems.

4.1.3 Cleaning Procedure

� Basic procedures for pig decoking are outlined below. However, since each fired heater is different, they should be modifiedaccordingly.� Line up valves at launchers to provide water flow to one launcher and return from the other.� After filling clean water (suction) tank, start pump, open appropriate valves and fill heater coil with water. For stainless steel

tubes, adding soda ash or other neutralizer to the water is required to prevent polythionic acid stress corrosion cracking. Makeup water should be continuously fed into the clean water tank during the initial filling of the heater coil to prevent pumpcavitation.

� Flush heater coil with water until clean of oil and coke debris.� Conduct an initial water flow test on the coil. This flow test generally consists of three water flow rate measurements. The

resulting pig propulsion pump speed and water pressure are recorded for each flow rate. The test results will be compared to afinal flow test, which will be conducted after pigging is complete.

� After coil is filled, flushed and flow tested, shut down the pump, close valves, and install full sized foam pig.� Realign valves as above, and start pump. The pig's location in the coil can be determined by observing the trend on the

pressure recorder (a "spike" will occur when the pig reaches each U-bend).� Maintaining a pig speed of approximately 8-12 ft/s (2.4-3.7 m/s) push the pig through the coil until it reaches the other launcher.

Shut down pump, close valves, and drain pig receiver.� Remove the foam pig and inspect to size the initial decoking pig and to determine if thermowells or other obstructions were

accidentally left in the coil.� If no problems occurred with the foam pig, install a decoking pig and make one pass. Remove pig from receiver and inspect for

damage and confirm that the pig traveled the entire coil. Reinstall and continue shuttling the pig through the coil, reversing flowdirection as necessary by manipulating the water valves. After several passes back and forth, replacing the pig will benecessary because of physical damage to the pig or a larger pig is required. This process will continue until an oversized pig(generally ¼" (6 mm) larger than tube inner diameter) can smoothly travel the entire coil without indications of remaining coke.






� Afterwards, several of the pigging contractors will follow up with either a "bolted" (bare soft pig that has short aluminum screwsrandomly screwed into the polyurethane body) or silica-coated polishing pig to complete the decoking process.

� After decoking is determined to be complete (see below), conduct final water flow test, drain water from heater and install afoam dewatering pig in the launcher. After ensuring that all personnel are out of heater, use nitrogen or air to pressure pigthrough coil to other launcher, which will remove residual water. Shut off nitrogen or air supply and disconnect launchers.

4.1.4 Checking for Completeness

� The progress of decoking can be checked by monitoring the pressure profile and the condition of the effluent water. When anoversized pig travels the entire coil length producing only a small amount of dirty effluent and when a relatively constant pressureprofile can be observed across all tubes then the coil can be considered clean. For multiple pass heaters, the final water flow testshould be approximately equal for all passes. Other methods for checking tube cleanliness are radiography of suspected cokedareas and boroscope of the tubes, which is currently limited to the inlets and outlet tubes. Measuring tube temperature rise withinfrared thermography after introducing warm water has been used successfully to identify areas of remaining coke when tubes arefree of external scale. Acoustic monitors have been employed as well.

4.2 NORMAL STEAM-AIR DECOKING PROCEDURE

The general steam-air decoking procedure given below applies to both normal and severe coking services. It should be used as aguide only with the understanding that modifications for individual heaters may be necessary. In addition, the procedure for a givenfired heater may require modifications depending on the properties of the coke deposits. In some cases, a combination of normalsteam-air decoking, followed by pig decoking, may be required. To be successful, steam-air decoking must be closely observedand controlled by personnel who are familiar with all aspects of the operation. A schematic of the typical piping arrangement isgiven in Figure 1.

4.2.1 Pre-decoking Boilout

When the coke deposit contains a large amount of water soluble salts, a pre-decoking boilout is performed to ensure that alldeposits be removed. After the heater coil has been drained and purged of hydrocarbons, it should be blinded off from theupstream and downstream equipment and filled with treated, fully softened, water. The coils must be vented to the atmosphere toprevent pressure buildup during the boilout process.

The heater should then be fired to maintain a bridgewall temperature of 250°F (120°C) for about 12 hours, using as many burnersas possible to ensure even heat distribution. Each coil should be refilled with water as necessary to maintain level. The effluentfrom the boilout should be periodically tested for sodium and chloride content. When the salt concentration in the retained effluenthas reached a maximum, the boilout can be considered to be complete. The coils should then be flushed with fresh water anddrained. Any remaining water (in vertical coils) can be removed by increasing bridgewall temperature to about 400°F (200°C) untilsteaming subsides. The bridgewall temperature should then be increased to 650°F (340°C) for final dryout. The heater may thenbe prepared for conventional steam-air decoking.

Note that the procedure above applies to a static boilout only, and may not be fully effective in removing deposits from convectionsection tubes. If significant convection section deposits are suspected, or if there are significant salt deposits in the preheatexchanger train, a hot water rinse using boiler feedwater or a recirculating boilout can be performed. However, this procedure ismore complex and is not addressed here. See Report EE.79E.87 or consult the Heat Transfer Equipment Section for moreinformation.






4.2.2 Heater Preparation for Spalling

The key steps in preparing the heater for decoking are isolating the coil from the remainder of the process, and heating the fireboxsuch that uniform temperature distribution is obtained. The following steps are required:� Isolate coil from upstream and downstream process piping and connect to decoking system.� After purging the firebox, light off burners following normal procedures using gas fuel (if available). To obtain uniform heat

distribution in the box, high excess air and as many burners as possible should be used.� When the bridgewall temperature reaches 400°F (200°C), decoking steam may be introduced. Adding steam to the coil when

the box is cold can cause some or all of the steam to condense.� Begin increasing steam rate until a mass flow rate of 18 lb/sec-ft2 (90 kg/s-m2) is achieved in all passes, while increasing firing

until a coil outlet temperature of 500°F (260°C) is reached.

4.2.3 Spalling

The majority of the coke should be removed during the spalling process. One key factor in determining the operation's success ismaintaining a sufficiently high tube metal temperature on all tubes to ensure that they expand sufficiently to mechanically crack thecoke. Key elements of the spalling procedure are listed below.� Begin water quench of effluent to maintain a temperature of approximately 500°F (260°C). This temperature should be held

during the entire decoking process.� � Increase coil outlet temperature (COT) in increments of approximately 200°F (110°C) until spalling begins, as indicated by a

slightly black stack plume from the decoking pot and/or a darkening of a condensed sample of steam effluent. Usually, thisbegins around 1000 to 1100°F (540�C to 590�C).

� Hold COT constant until spalling subsides. At this time, the COT should be slowly increased until spalling resumes, or themaximum allowable Tube Metal Temperature (TMT) is reached. Note that this limit is typically set by metallurgicalconsiderations rather than by the tubes' mechanical strength. Typical TMT limits are as follows:

� Carbon steel 1275°F 690°C Dark Cherry Red

2-1/4% chrome 1325°F 720°C Bright Cherry Red

5% chrome 1350°F 730°C Bright Cherry Red

9% chrome 1375°F 745°C Bright Cherry Red

18 Cr - 8 Ni 1700°F 925°C Light Orange

HK-40 (25 Cr - 20 Ni) 2050°F 1120°C Light Yellow

Note that it is not essential that the actual TMT be continuously measured at this time, as radiant TMTs will typically be 35-50°F(20-30°C) higher than the Coil Outlet Temperature. However, TMTs do provide a reference point to ensure good heatdistribution in the heater. In some cases, temporary thermocouples, such as those employed by heat treating contractors, canbe used to monitor a larger portion of the firebox. In critical applications, instrumentation can be supplemented with portableinfrared imaging equipment. Tube color can also be monitored to estimate the magnitude of the temperature.

� If spalling subsides, it can be re-initiated by using short bursts of air or by temporarily reducing the steam rate. Air bursts ofapproximately 1.2 lb/sec-ft2 (6 kg/s-m2) of increasing duration (from 3-20 seconds) should be used. If this fails to initiatespalling, steam flow to the troublesome coil can be reduced to about 1/2 the normal rate for a short period of time (about 30seconds).

� Spalling is considered to be complete when it cannot be re-initiated by further air bursts or steam flow changes, while atmaximum TMT. This should have removed the majority of the coke from the tubes. The remaining coke will be removedduring the burn phase of the process.






4.2.4 Burning

In this phase of the process, air is introduced into the coil to burn out the coke not removed during the spall. Steps in the burnprocedure are:� Reduce steam mass flow rate to 12 lb/sec-ft2 (60 kg/s-m2) with COT at the maximum spalling temperature.� Admit air to all passes at the rate of 0.3 lb/sec-ft2 (1.5 kg/s-m2), while monitoring the tubes for hot spots and measuring the CO2

content of a dry effluent sample.� When the CO2 level falls below 1-2% (% by volume), the air rate is increased in increments of 0.3 lb/sec-ft2 (1.5 kg/s-m2) until

the maximum level of 1.2 lb/sec-ft2 (6 kg/s-m2) is reached.� When the CO2 level drops below 0.5% the burn is considered to be complete.

� Verify the completeness of coke removal through radiography of selected tubes in the heater. This is an optional step that willhelp determine if a change in decoking procedure is needed.

4.2.5 Reverse Flow Decoking

Reverse flow decoking is not necessary for most fired heaters. Some locations may use reverse spalling (without the burn) toremove stubborn coke deposits. The process may begin after lining up the piping and removing any orifices or instruments in thecoil inlet piping which might be damaged by high-velocity coke particles. The same procedure is used for spalling and burning,except that the limiting temperature is now the bridgewall temperature. This should be limited to 1275°F, which will ensure thatradiant and convection tube metal temperatures will not be excessive for the lowest grade carbon steel.

To adequately monitor reverse flow decoking, it is desirable to have an indication of both the steam and flue gas temperature atboth the inlet and outlet of the convection section. If the steam temperatures indicate that effective spalling is not occurring, caremust be exercised during burning to ensure that the tubes and convection inlet piping are not overheated. Reverse flow decoking isa case where additional temporary thermocouples might be justified.

4.3 ONSTREAM DECOKING PROCEDURE

� In the onstream decoking method, the heater is decoked one pass at a time while the remaining passes are kept in service. Thereis no burn phase and thus the coke removal is not as complete as with a normal steam-air decoke. However, it does enable theinterval between steam-air decokes, and downtimes, to be extended. In general, this method is typically applied to heaters in heavycoking service, such as delayed cokers and vacuum pipestills, and it has been applied to other services following process upsets.However, not all heaters are suitable for this procedure; it works best if passes are arranged in discrete zones, or cells, for whichfiring rates can be separately controlled.

Compared to conventional steam-air decoking, onstream decoking carries greater risk. The hydrocarbon feed must be removedfrom the operating pass, quickly replaced with steam, then introduced back into the coil after the process is complete. A very smallamount of hydrocarbon leaking through inlet block valves during decoking can cause more severe coking or even blockage of thatpass. In addition, re-introducing feed into a pass that has been at decoking temperatures must be done quickly, or a new layer ofcoke will be formed. To prevent these types of problems from occurring, a detailed procedure must be written specifically for eachheater that is to be onstream decoked. The following paragraphs provide guidance in developing these detailed procedures.






4.3.1 Heater Preparation

The heater's inlet and outlet block valves must be able to ensure complete shutoff of flow. These should preferably be of the doubleblock variety, with a bleeder downstream to check for leakage. While this arrangement was normally provided as part of the flowcontrol hardware, the bypass around the control valve in past designs often had only a single block valve. Experience has shownthat a double block valve on the bypass is also required to avoid inadvertent leakage of feed, which can cause tube pluggage andheater shutdown. The heater's tube metal thermocouples should also be in working order; if this is not the case, another means ofmonitoring TMT should be made available. A means of monitoring spalling progress, such as a sample tap on the outlet of the passbeing decoked, should also be provided.

4.3.2 Reduce Firing Rate

Most fired heaters will require the firing rate (and throughput) to be reduced to prevent excessive TMTs from occurring when thefeed is replaced with steam. Some heaters, such as delayed cokers, have individual firing control valves for each pass; in thesecases, only the firing rate on that pass need be reduced to permit onstream decoking.

4.3.3 Replace Hydrocarbon Feed With Steam

Steam is gradually introduced as the hydrocarbon feed is backed out of the pass to be decoked. Tube metal temperatures shouldbe monitored to ensure that the change is gradual, and that the temperatures remain in the normal spalling range as describedearlier. A further reduction in firing rate may be required to maintain temperatures in the desired range. The final steam rate isusually higher than that used during normal steam-air decoking; some locations have used mass flow rates as high as 40 lb/sec-ft2(200 kg/s-m2).

4.3.4 Monitor Progress

A monitoring procedure must be developed for each fired heater. If sample taps on each pass are available, monitoring methodswill be similar to those used for conventional steam-air decoking. In some cases, such as vacuum pipestills, the overhead gas fromthe vacuum tower is sampled for H2, CO, and CO2. The concentration of these gases will begin to rise as spalling begins, reachinga peak at some point in the procedure, and return to pre-spalling levels, indicating that the process is complete. Firing rate may beincreased and/or steam flow varied to raise the TMT to the material's metallurgical limit. Reforming times can range from 8 to 48hours, depending on the nature of the coke deposit.

4.3.5 Reduce Firing Rate Prior to Introducing Feed

When decoking is complete, the TMT of the pass being decoked must be reduced prior to introducing the feed. This isaccomplished by lowering the firing rate (temporarily reducing the feed rate or COT). In general, the TMT should not be muchhigher than that seen during normal operation. Feed must be re-introduced into the coil very quickly, while shutting off the decokingsteam.

4.3.6 Return Heater to Normal Operation

Once normal operation has been established, the TMT of the previously coked areas should be checked to confirm that theprocedure was successful.






4.4 LOW VELOCITY DECOKING PROCEDURE

Low-Velocity steam-air decoking was developed in the late 1970s to counter problems encountered when applying conventionalsteam-air decoking procedures to Visbreaker heaters. The very heavy coke deposits [up to 1 in. (25 mm) thick at end of run]resulted in erosion of tubes and a very heavy, black stack plume. It has recently been applied to other heaters in locations in whichthe opacity of the effluent stack plume was an issue.

Low-Velocity decoking uses a steam mass flow rate of about 25% of that for conventional steam-air decoking, and a heat-up ratethat is much slower. As a result, overall coke loading in the effluent spall steam is reduced by 50%. Additional "shocking" stepsafter the final burn are also used; these steps may also be applied after a conventional steam-air decoke. A brief description of theprocedures associated with this method follows. As with onstream decoking, a specific procedure must be prepared for each heater.For a more definitive treatment of Low Velocity Decoking, see Report EE.44E.82, Decoking Guidelines for Visbreaker FiredHeaters.

4.4.1 Equipment Preparation

Heater preparation is generally the same as for a conventional steam-air decoking procedure, except as follows:� Visbreaker heaters often have coils made of austenitic stainless steel, which requires protection from polythionic acid stress

corrosion cracking when exposed to air. A mixture of ammonia and nitrogen can be used for this purpose.� After lighting burners in soaker and heater cells (for Visbreakers), increase firing to raise bridgewall temperature to 500°F

(260°C), at a rate of 100°F (55°C) per hour.� Introduce steam to the process coil to provide a mass velocity of 10 lb/sec-ft2 (50 kg/s-m2), based on the inside diameter

resulting from the maximum expected thickness of coke. Nitrogen may be removed from the coil after steam flow isestablished.

� Increase firing in the soaker and heater cells (if applicable) to bring both Coil Outlet Temperatures (COTs) to 750°F (400°C), ata rate of 100°F (55°C) per hour. If ammonia is being injected, it may be removed when coil outlet temperatures reach 575°F(300°C).

4.4.2 Soaker Spall

Visbreakers differ from conventional fired heaters in that they contain a soaker section. This area of the heater often contains themost severe deposits, and requires modified spalling procedures, as described below.� Maintain heater COT at 750°F (400°C) during soaker spall.� Increase soaker COT to 930°F (500°C) at a rate of 50°F (30°C) per hour.� Reduce rate of temperature rise to about 30-35°F (15-20°C) until spalling begins. This usually occurs around 1020°F (550°C);

if spalling does not begin, hold this temperature for approximately 1 hour and decrease the steam rate slightly, by about 1lb/sec-ft2 (5 kg/s-m2).

� After spalling begins, maintain coil outlet temperature and steam rate. If spalling rate is too high, COT should be reduced asrequired to maintain an acceptable spalling rate. Do not decrease steam rate.

� When initial spall stops, try to re-initiate spalling by increasing COT by 20°F (10°C) per hour.� If spalling does not start after a 60°F (30°C) rise in COT, increase steam flow in 1 lb/s-ft2 (5 kg/s-m2) increments (based on

CLEAN tube area) to a maximum change of 3 lb/s-ft2 (15 kg/s-m2). Hold steam flow and temperature constant once spallingbegins.

� Continue re-initiating spalling as above (increasing steam rates only when a 60°F (30°C) rise in COT fails to produce a spall)until COTs reach 1330°F (720°C), or maximum coil TMT is reached. Steam rates should not have exceeded 12 lb/s-ft2(60 kg/s-m2) on a clean tube basis.

� Increase steam rate in 1 lb/s-ft2 (5 kg/s-m2) increments until a mass velocity of 12 lb/s-ft2 (60 kg/s-m2) is reached, holdingsteam rate constant during spalls.






4.4.3 Soaker Steam Shocking

After spalling is complete, the soaker coil is shocked with steam. Starting conditions should be as stated above. A summary ofprocedures is presented below.� Increase steam rate to 16 lb/s-ft2 (80 kg/s-m2) for a period of about two minutes, then reduce flow back to the initial rate until

spalling subsides. Repeat until three successive cycles fail to produce a spall.� Shock the coil by cycling as above, but to 20 lb/s-ft2 (100 kg/s-m2). Shocking is complete when three successive cycles fail to

induce a spall.

4.4.4 Heater Spalling

The heater section is spalled after the soaker steam shocking is complete. Steam flow rate should be 12 lb/s-ft2 (60 kg/s-m2), with aCOT of 750°F (400°C). Procedures are as defined below:� Reduce soaker firing as required to maintain a COT of about 1020°F (550°C).� Increase heater COT to 840°F (450°C) over a period of one hour. Continue heating up at a rate of 35°F (20°C) per hour until

spalling begins. Hold COT constant during spalls.� Increase COT at 20°F (10°C) per hour as required to re-initiate spalling. Heater spall is complete when COTs reach 1330°F

(720°C) or until coil is at maximum allowed TMT.

4.4.5 Heater Steam Shocking

Steam shock heater per the soaker shocking procedure.

4.4.6 Air Shocking/Burning

Both heater and soaker sections are air shocked as a unit prior to the burn phase of the process. Initial conditions should have thesoaker at a COT of 1330°F (720°C) or maximum TMT. It may be necessary to reduce heater firing to avoid excessive TMTs duringthis process. A steam rate of 12 lb/s-ft2 (60 kg/s-m2) based on clean tube area should be present in both sections. A description ofthe process follows:� Introduce short bursts of air into the coil at a rate of about 2 lb/s-ft2 (10 kg/s-m2). Initial bursts should last about three seconds,

and should be repeated when spalling subsides. When three successive 3-second bursts fail to re-initiate spalling, repeat with6, 12, and 30 second bursts, followed by a two-minute burst. Air shocking is considered to be complete when three 2-minutebursts do not re-initiate spalling.

� Air burning is then conducted per the procedures initially described for conventional steam-air decoking.






4.4.7 Post-burn Shocking

The final step is a post-burn shocking, to remove the last remaining coke. Procedures are described below:� Increase steam rates to 18 lb/s-ft2 (90 kg/s-m2). If spalling does not occur, or stops, cycle steam rate down to 12 lb/s-ft2

(60 kg/s-m2), then up to 25 lb/s-ft2 (120 kg/s-m2). Repeat until spalling begins, then resume initial steam rate as spallingproceeds.

� Repeat until five or six consecutive cycles fail to produce spalling.

5 DESIGN CONSIDERATIONS

5.1 PIG DECOKING

� Flow Arrangement The heater should have the same number of radiant and convection passes, with no interveningheaders. Other arrangements can be accommodated, but may require that radiant andconvection sections be decoked separately.

� Coil Construction Decoking will proceed more smoothly if return bend and intermediate tube welds do not intrudeinto the flow path. The design specification should state that over-penetration of welds shall notexceed 1/32 in. (0.75 mm).

� Spool Pieces Pig decoking apparatus must be attached directly to the inlet and outlet of each pass. Pipingshall be designed with removable spool pieces or swing elbows to facilitate attachment.Locating the launchers at grade reduces the time required for set up and dismantling, andeliminates repetitive climbs to the launcher during pigging The attachment flange to the piglaunchers needs to be a Class 300 raised face.

5.2 NORMAL STEAM-AIR DECOKING

5.2.1 Fired Heater

�

� Flow Arrangement Flow should be arranged such that radiant and convection sections have an equal number ofpasses.

� Return Bends Generally welded return bends of nominal thickness are specified in place of the moreexpensive, and leak-prone, plug headers. The exception would be for heaters that are routinelyspalled and steam-air decoked (Cokers). In these heaters plug headers could be specified onone end of the heater and hardfaced or thicker return bends on the other end. The plugheaders aids in the identification and removal of a plugged tube pass as a result of spalling.The hardfaced or thicker return bends protect against severe erosion caused by high velocityparticles during spalling.

� Vantage Points The burning period requires observation of tube color, to supplement thermocouple readings.Sufficient observation doors should be provided to ensure a complete view of all radiant andshield tubes (note that shield tubes may run hotter than radiant tubes).






� Heater Tubes Maximum allowable tube and effluent piping metal temperatures during the decoking for variousmaterials are as follows:

Carbon steel 1275°F 690°C Dark Cherry Red2-1/4% chrome 1325°F 720°C Bright Cherry Red5% chrome 1350°F 730°C Bright Cherry Red9% chrome 1375°F 745°C Bright Cherry Red18 Cr - 8 Ni 1700°F 925°C Light OrangeHK-40 (25 Cr - 20 Ni) 2050°F 1120°C Light Yellow

Note that for reverse-flow decoking, heater coil inlet piping must also be designed for thesetemperatures; this has a significant effect on piping layout, particularly for carbon steel, due to itsvery low strength at this temperature range.

5.2.2 Decoking Facilities

� Piping Arrangement Figure 1 shows a suggested steam-air decoking arrangement for a two-pass heater.� Steam Requirements

1. Source Steam is normally taken from the 110-125 psig (860 kPa) refinery supply.2. Flow rate The recommended steam mass velocity for spalling is 18 lb/sec-ft2 (90 kg/s-m2). Sufficient

steam should be available to supply all passes simultaneously at this rate. Mass velocity duringthe burning operation should be 12 lb/sec-ft2 (60 kg/s-m2).

� Air Requirements1. Source Burning air is normally supplied from the refinery's 50-100 psig (350-690 kPa) utility air supply. 2. Flow Rate A mass velocity of 1.2 lb/sec-ft2 (6.0 kg/s-m2) is recommended, with all passes being supplied

simultaneously. If this amount of air is not available, all passes may be burned at a lower airrate [but not less than 0.6 lb/sec-ft2 (3 kg/s-m2)]. If insufficient air is available to simultaneouslysupply all passes at this rate, passes may be burned sequentially.

� Instrumentation1. Steam and Air Flow indicators shall be provided in the steam and air headers.2. Temperature Provide temperature indicators in the following places:

� Coil outlet of each pass

� Bridgewall*� Stack*� Decoking effluent upstream and downstream of quench point� Tube metal temperatures (desired)*� Crossover piping (required for reverse-flow decoking only)

* Note: These instruments are usually provided for normal operation but are also neededfor monitoring the decoking process.

3. Pressure/Flow The valves, pressure, and flow indicators used to control process flow to individual passes maybe used during normal steam-air decoking. Inlet gate valves may also be used to balance flowbetween passes during reverse-flow decoking. However, these valves should remain fully openduring initial phases of reverse spalling to reduce erosion from coke particles.

� Isolating Process Lines Blinds, of either the "slip" or "spectacle" variety, are normally used to change from process todecoking conditions. Swing elbows, which allow the switch to be made more quickly, can bespecified at the owner's request. Decoking effluent lines should also be isolated from anyprocess flow meters or control valves to prevent them being eroded by coke-laden steam.






� Sampling It is necessary to repeatedly sample the effluent for coke during spalling and for CO2 contentduring the burning operation. Ideally, a sample tap is provided on each pass of the heater.During spalling, a small stream is typically quenched and allowed to run to a safe location,where it is periodically checked to determine the progress of the operation. During the burningprocess, a sample is collected from this same location, cooled to condense out water, andtested using a Draeger tube or other similar device. A detail of the sample tap is provided inFigure 4.

� Disposal of Effluent Four methods are available for disposal of decoking effluent. In each case, quench water willbe added to the effluent stream immediately after the sampling system to reduce downstreamvelocity, pressure drop, and temperature. Effluent piping shall be designed for a maximumvelocity of 1000 ft/s (300 m/s) or a maximum pressure drop of 5 psig (35 kPa), based ontemperatures of 1200°F (650°C) upstream of the quench site and 500°F (260°C) downstream.The quantity of quench water vaporized should be included in the downstream velocitycalculations.

1. Stack Decoking effluent can be discharged directly to the heater stack if it is located at grade (i.e., notmounted atop the convection section). The effluent line should enter the stack at a 45-degreeangle directed upward, impinging on a "target plate" which shall be provided to protect the stackfrom erosion. Coke falls down into the bottom of the stack and is periodically shoveled out fordisposal.Alternatively, a "deflector nozzle," which directs the effluent upward, can be used in place of thetarget plate.

2. KO Drum A coke knockout (KO) drum shall be provided if the main heater stack is mounted atop theconvection section. The purpose of the drum is to remove the larger particles of coke upstreamof the main heater stack. Designing the fluid conditions in the drum for high residence timesand low velocities minimizes particulate emission. The drum itself shall be sized according toDP V-A and VI-C. Inlet and outlet piping shall be sized to provide maximum velocities of about120 ft/s (35 m/s) for 20 pipe diameters upstream and 5 pipe diameters downstream of the drum.The effluent line shall enter the stack as described above. See Figure 4 for a detail of a typicalcoke knockout drum.

3. Total Quench In some unusual cases, it may be desirable to provide facilities to condense all of the effluentsteam from the decoking process. This prevents the smaller coke particles from going up thestack. Although quenching allows the coke to be sent directly to the sewer, it requires a largeamount of water and is not practical except for very small heaters. A secondary purpose forquenching is to provide emergency cooling water to maintain the drum at 500°F (260°C).

4. Offsite The rising cost of disposal of sludge from plant sewer systems has led some locations to use amobile disposal truck to capture the effluent. The quenched effluent is routed to a special,multi-compartment truck, where the solids are separated from the stream. In many cases, thismaterial can be disposed of more readily than material from the plant sewer system.

6 DESIGN PROCEDURE FOR NORMAL STEAM-AIR DECOKING

The design procedure for steam-air decoking facilities is relatively straightforward. It consists of determining the required flow ratesof steam and air, calculating the amount of quench water required, and determining the size of the decoking line and drum. Thefollowing paragraphs illustrate the steps to be followed.

Calculate Steam Rate - Sufficient steam is required to provide a mass flow rate in each pass of 18 lb/s-ft2 (90 kg/s-m2), unless theheater design is such that individual cells can be isolated from the rest of the box. Therefore:

Wsteam = 18 p Ax � 3600 or Metric: Wsteam = 90 p Ax Eq. (1)

where: Wsteam = Mass flow of steam required, lb/hr (kg/s)p = Number of parallel passesAx = Cross-sectional flow area through one tube, ft2 (m2)






Calculate Air Rate - Sufficient air should ideally be available to provide an air rate of 1.2 lb/sec-ft2 (6 kg/s-m2) in all passes,although passes may be burned sequentially, in which case sufficient air for only one pass need be provided.

Wair = 1.2 p Ax � 3600 or Metric: Wair = 6.0 p Ax Eq. (2)

where: Wair = Mass flow of air required, lb/hr (kg/s)p = Number of parallel passesAx = Cross-sectional flow area through one tube, ft2 (m2)

Determine Water Quench Rate - Sufficient water must be provided to quench the effluent stream from maximum decokingtemperature to 500°F (260°C). For the purpose of calculation, the effluent stream is assumed to be all superheated steam at 5.0psig (35 kPa), although the stream will contain coke particles, and during the burn phase, CO, CO2, etc. Some allowance may beadded to account for additional heat contained in the coke particles. Treating it as a simple mixing problem:

Wquench = (Wsteam + Wair) � (heff - h500) / (h500 - hquench) Eq. (3)

or Metric:

Wquench = (Wsteam + Wair) � (heff - h260) / (h260 - hquench) Eq. (3)M

where: Wquench = Mass flow of quench water required, lb/hr (kg/s)heff = Enthalpy of effluent stream at decoking temperature, Btu/lb (kJ/kg)hquench = Enthalpy of quench water, Btu/lb (kJ/kg)h500(h260) = Enthalpy of mixed stream at 500°F (260°C), Btu/lb (kJ/kg)

When quench water temperature is 60-100°F (15-40°C), and the effluent temperature is 1250°F (675°C),

Wquench = 0.3 (Wsteam + Wair) Eq. (4)

Determine Size of Decoking Line - The effluent piping should be designed for a maximum velocity of 1000 ft/s (300 m/s), andshould be short enough that the total pressure drop between the heater outlet and the knockout drum is less than 5.0 psi (35 kPa).For the purposes of velocity calculation, the temperatures upstream and downstream of the quench are assumed to be 1200 and500°F (650 and 260°C). First, the piping cross-sectional area which results in the proper velocity is calculated, then the next largerpiping size is selected. Remember that calculations downstream of the quench must include the quantity of water vaporized.

Ad = (W v) / 3.6 x 106 or Metric: Ad = (W v) / 300 Eq. (5)

where: Ad = Cross-sectional area of decoking line, ft2 (m2)W = Mass flow of effluent stream, including steam, air, and quench water as applicable,lb/hr (kg/s)v = Specific volume of effluent at conditions, ft3/lb (m3/kg)

After a preliminary line size has been determined, the pressure drop should be calculated, and the line size adjusted accordingly.

Coke Knockout Drum - The primary purpose of the knockout drum is to remove large particles of coke from the effluent upstreamof the heater stack. This is particularly important if the stack is mounted above the heater convection section. The proceduredescribed below should prove adequate for most applications.






Select Size of Inlet/Outlet Nozzles - Inlet and outlet piping should be sized to provide a velocity of 120 ft/s (35 m/s) for a distanceof 20 pipe diameters upstream and 5 pipe diameters downstream if the heater stack is mounted atop the convection section.Conditions at the knockout drum are assumed to be 500°F (260°C) and 5 psig (35 kPa). Assuming these conditions:

An = Wt / 15180 or Metric: An = Wt / 20.6 Eq. (6)

where: An = Cross-sectional area of knockout drum inlet/outlet piping, ft2 (m2)Wt = Mass flow of effluent stream, including steam, air, and quench water, lb/hr (kg/s)

Select Diameter of Coke Knockout Drum - In the past, this diameter has been set at 85% of the heater stack diameter, if thestack is mounted on the heater itself, or at least 3 times the diameter of the inlet and outlet nozzles, whichever is larger. DesignPractices DP V-A and VI-C offer guidance in the design of separation equipment.

7 SAMPLE PROBLEM

Given: Fired heater with 8 passes of 6 in. (150 mm) tubes (see Sample Problems 1 and 2, DP VIII-B)

Find: Steam and air mass flow requirements for normal steam-air decokingRequired quench water flowProper size of decoking line and drum

Assuming: All passes are decoked simultaneously

Solution:Calculate Steam Rate

Wsteam = 18 p Ax � 3600 or Metric: Wsteam = 90 p Ax [From Eq. (1)]= 18 x 8 x 0.1946 x 3600 = 90 x 8 x 0.01808= 100,900 lb/hr = 13.0 kg/s

Calculate Air RateWair = 1.2 p Ax � 3600 or Metric: Wair = 6.0 p Ax [From Eq. (2)]

= 1.2 x 8 x 0.1946 x 3600 = 6.0 x 8 x 0.01808= 6725 lb/hr = 0.87 kg/s

Wquench = (Wsteam + Wair) � (heff - h500) / (h500 - hquench) [From Eq. (3)]

Assuming superheated steam at 5 psig (35 kPa) and 1250°F (650°C):= (100,900 + 6725) � (1666 - 1287) / (1287 - 28)= 32,400 lb/hr

or Metric:

Wquench = (Wsteam + Wair) � (heff - h260) / (h260 - hquench) [From Eq. (3)M]= (13.0 + 0.87) � (3820 - 2993) / (2993 - 251)= 4.2 kg/s






Determine Size of Decoking LineAd = (W v) / 3.6 x 106 [From Eq. (4)]

Upstream of quench [at 5 psig and 1200°F (35 kPa and 650°C)]

Ad = 107,625 x 49.41 / 3.6 x 106

= 1.47 ft2 (choose 18 in. or 20 in. NPS line)

Downstream of quench [at 5 psig and 500°F (35 kPa and 260°C)]

= 140025 x 28.46 / 3.6 x 106

= 1.1 ft2 (choose 16 in. NPS line)

or Metric:

= (W v) / 300

Upstream of Quench

= 13.87 x 3.08 / 300

= 0.142 m2 (use 450 or 500 mm pipe)

Downstream of Quench

= 18.1 x 1.78 / 300

= 0.107 m2 (use 400 mm pipe)

Specify Size of Coke Knockout Drum & NozzlesInlet and Outlet nozzles (if heater mounted stack)

An = Wt / 15180 or Metric: An = Wt / 20.6 [From Eq. (5)]

= 140025 / 15180 = 18.1 / 20.6

= 9.2 ft2 = 0.88 m2

This would result in extremely large nozzles, greater than 40 in. (1 m) in diameter. Since this heater has a grademounted stack, inlet velocity will not be as critical, so the largest commercially available ANSI standard pipe size, 30in. or 36 in. (760 or 910 mm) will probably be sufficient.

Decoking Drum DiameterFrom Problem 6 in DP VIII-C, stack diameter = 8.94 ft (2.72 m).

Normally, drum diameter = 85% of stack diameter = 7.6 ft (2.3 m). However, to satisfy drum size criteria this wouldrequire 30 in. (750 mm) or smaller inlet nozzles.

A drum diameter of 9.0 ft (2.75 m) with 36 in. (900 mm) nozzles would satisfy the criteria.






Figure 1Typical Piping Arrangement for Steam-Air Decoking

�

TI

THA

PipingSpecificationBreak

Sample Tap(Note 2)

BackupWater Quench

"A"WaterQuench

PI

SeeNote 2

Coil OutletUsed to steamout prior to decoking

Min.

Coil Inlet

FI

Pipingspecificationbreak

TI

TI

"B"

TI(8)

(4)TI

(7)

FI

FI

PI

PI(6)

TI

Process CoilSteam (If req'd)

(5)

(2)(2)

FI

FI

100 Psig Air(690 kPa)(Note 5)

150 Psig Steam(890 kPa)(Note 5)(Note 8)

To Ground-Supportedstack or knockout drum(Note 3)

(1)

DP8If01

Notes:1. For flue gas temperature at bridgewall.2. These valves are installed only to save blind changes when flow is reversed.3. Locate as close as possible to heater header. Details are shown in Figure 4.4. Provide sufficient run of pipe for mixing of steam and air.5. Utility connections shall be arranged per IP 3-6-3.6. These valves shall be suitable for throttling/adjusting pass flows.7. (Temperature Indicators) TI shall be provided in each pass if reverse flow decoking is used.8. Steam and air can be individually piped to each pass to enable better monitoring of burn progress. If required, onstream

decoking capabilities can be provided.






Figure 2Typical Piping Arrangement for Pig Decoking

P

P

Pig Launcher

NC

Drain/VentLine

NC

Pig Receiver

HCV

Bypass Valve

Pig PropulsionClean Water Tank

H2OMake-up

N.C.

"Dirty" Water Tank

Strainer

To OilyWater Sewer

DP8If02






Figure 3Pig Decoking Piping Detail

N.C.

"Clean" Tank

Pig Propulsion PumpSteam(if needed)

Make-up H2O

Bypass Valve

N.C.

"Dirty" Tank

Drainto Sewer

Strainer

Position of These ValvesControls Direction of Pig

Pressure Trend Recorder

Effluent From Tubes Pig to Tubes

By Contractor

OwnerConnection

N.C.

DP8If03

Ø

Note: "Dirty" water tank is used to capture effluent which contains significant amounts of coke fines. "Clean" water tankcan be used when water is relatively free of coke.






Figure 4Details of Steam-Air Decoking Facilities

�

Inlet

To stack

H

To sewer(if used)

85% of stack diameter OR3 x H, whichever is larger

Coke Knock Out Drum

Line size:6" - 8" NPS

(150 mm - 200 mm)

Water(if used)

1.5 x H

3Dpipe

Water

Dpipe

Quench water nozzle

12 ft. (3.7 m) min. straightrun of piping req'd.downstream of nozzle

To sampling device(during burn phase)

Water

SampleCooler

Take sample here(during spall phase)

Sample tap detail

decoking line

one per pass (recommended)one per heater (min)

Access door

DP8If04

Notes:(1) Design Pressure = 1 psig (7 kPa) + Delta P between Drum and Stack; it should be less than 14.7 psig (100 kPa) if

possible. Design Temperature should be 900°F (480°C).(2) Material to be carbon steel with 1/8 in. (3 mm) corrosion allowance.(3) Locate at grade.(4) Drum height to be equal to drum diameter if drum is designed for continuous water washing of coke to sewer. Otherwise

provide additional height for hold-up of one decoking operation, removing 1/4 in. (6 mm) from all radiant tubes.

DP8If04

dp08i

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