4 Rules of Fired Heater Operation

Paper from the AFRC 2015 conference titled 4 Rules of Fired Heater Operation

Four commonsense "rules" have been suggested for operating fired heaters that are easy to remember and can help focus attention on principles that are critical for safe, efficient, and environmentally-friendly operation while minimizing unplanned shutdowns. Rule one is to keep the flames in the firebox. While this may seem obvious, examples exist where that rule is violated. A common violation is for flames that periodically come outside the heater, for example, due to furnace pulsations usually referred to as huffing. Rule two is to keep the flames off the tubes. This refers to flame impingement which can cause serious operational problems if not corrected. Initially, flame impingement causes the hydrocarbon process fluids to coke on the inside of the tubes. This insulates the tube metal from the cooling of the process flow causing the metal to overheat and eventually can lead to a tube rupture which is both dangerous and costly. Rule three is to keep the process fluid in the tubes. This is related to rule two where flame impingement can cause a tube rupture. This can lead to large quantities of hydrocarbon fluid pouring into the heater, generating fires and thick black smoke. The lack of sufficient oxygen to burn all of the hydrocarbons leaking into the heater makes this very dangerous as those hydrocarbons may burner uncontrollably outside the heater. Rule four is to keep flammables out during light-off. This refers to purging the heater prior to light-off to avoid igniting a flammable mixture in the heater that could be present without proper purging. Since most heater incidents occur at light-off, this is an important rule for safe operations. The paper describes these rules in more detail and includes examples of what happens when the rules are not followed.

4 Rules of Fired Heater Operation Chuck Baukal, Ph.D., P.E. and Bill Johnson John Zink Hamworthy Combustion (Tulsa, OK) Roger Newnham Onquest (Alberta, Canada) Abstract Four commonsense "rules" have been suggested for operating fired heaters that are easy to remember and can help focus attention on principles that are critical for safe, efficient, and environmentally-friendly operation while minimizing unplanned shutdowns. Rule one is to keep the flames in the firebox. While this may seem obvious, examples exist where that rule is violated. A common violation is for flames that periodically come outside the heater, for example, due to furnace pulsations usually referred to as huffing. Rule two is to keep the flames off the tubes. This refers to flame impingement which can cause serious operational problems if not corrected. Initially, flame impingement causes the hydrocarbon process fluids to coke on the inside of the tubes. This insulates the tube metal from the cooling of the process flow causing the metal to overheat and eventually can lead to a tube rupture which is both dangerous and costly. Rule three is to keep the process fluid in the tubes. This is related to rule two where flame impingement can cause a tube rupture. This can lead to large quantities of hydrocarbon fluid pouring into the heater, generating fires and thick black smoke. The lack of sufficient oxygen to burn all of the hydrocarbons leaking into the heater makes this very dangerous as those hydrocarbons may burner uncontrollably outside the heater. Rule four is to keep flammables out during light-off. This refers to purging the heater prior to light-off to avoid igniting a flammable mixture in the heater that could be present without proper purging. Since most heater incidents occur at light-off, this is an important rule for safe operations. The paper describes these rules in more detail and includes examples of what happens when the rules are not followed. Introduction There are many potential "rules" or guidelines for safely operating process heaters. API and API 5352 provide many useful recommendations for operating heaters and burners, respectively. Companies normally develop their own detailed procedures based on API recommendations and on their own experiences and best practices. The intent of this paper is to present four simple and easy-to-remember "rules" for operating process heaters. These are intentionally broad to encompass many of the more detailed procedures developed for a particular heater. They are especially useful for new operators when they are learning all the many facets involved with safely running process heaters. These commonsense rules are not intended to replace a company's procedures, but to provide a framework to make it easier to remember some of the more important factors in the safe operation of heaters. The focus here is 5601 1 on safety,3 and not specifically, for example, on minimizing pollution 4,5 or maximizing efficiency6,7 and uptime, although these often result from following the principles presented here. Examples are provided for the potential negative consequences of not following each rule. 1. Keep the Flames in the Box Keeping the flames inside the firebox seems to be an obvious rule since the goal is to heat something inside the heater, not outside. This rule is important for both worker safety and equipment integrity. This includes keeping not only flames in the heater but also the hot gases generated in the combustion process. Flames and hot gases could exit any heater openings including sight ports, burner air inlets, and cracks in the heater shell. Figure 1 shows an example of a flame outside a heater. In that example, the flame periodically exited the heater when the pressure inside went positive which was on a fairly regular basis. Figure 1. Flames outside a heater. Hot gases exiting a heater may or may not be visible depending on the conditions. For example, on a bright sunny day it may be difficult to see hot gases exiting heater openings. This means it is critical to ensure no conditions exist that could force flames and hot gases outside a heater since they may not always be seen. The high temperature gases can injure personnel even wearing flame-retardant clothing. Prolonged emission of hot gases can also damage equipment. Two common causes that can force flames and hot gases out of a heater are positive pressure and pulsations which are each discussed next. Heater Draft Process heaters are designed to have a slightly negative pressure at the top of the radiant section,8 also referred to as the arch or bridgewall. Figure 2 shows a typical process heater draft profile. The least negative (lowest draft) location inside the heater is at the arch. However, it is possible for this pressure to go positive. The primary device used to control heater draft is the 2 stack damper. If that damper is not sufficiently open, that would cause the pressure to go positive. A plugged convection section could also cause the same effect. Note that it's possible the heater pressure could be negative near the bottom of the heater, but positive near the arch. This is why heater pressure should be checked before attempting to look through sight ports, especially those located where the pressure is most likely to be positive if the heater is not operating as designed. Draft measurement at the arch is essential and automatic draft control is recommended to ensure the heater pressure does not go positive. Figure 2. Heater draft profiles. Flame Stability While there are many types of unstable flames, 9 those that are pulsing (often referred to as huffing or woofing) can force flames and hot gases to go outside of a heater. The transient nature of this condition can cause the pressure in a heater to fluctuate between positive and negative. Severe pulsing has been known to cause sight port covers and explosion doors to lift or flap. Pulsating flames can not only cause flames and hot gases to come out of a heater, but could possibly cause flames to temporarily go out by lifting them off completely. If there is an ignition source inside the heater, such as any part of the heater being at a temperature above autoignition,10 the flames could reignite. This is potentially very dangerous as the heater would likely be full of flammable gases that could ignite explosively causing severe overpressurization. One of the possible causes of pulsing flames is the fuel pressure to the burners exceeding the maximum design pressure.11 High fuel pressure increases the fuel/air mixture velocity exiting 3 the burner. If that velocity significantly exceeds the burning velocity, the flames can lift and even go out completely. The flames become over-strained because the gases can't react fast enough to maintain stable flames. In diffusion burners (also known as raw gas or nozzle-mixed), the mixing of the fuel and air begins at the exit of the burner. Higher than designed fuel exit velocities can also delay the mixing process which can also increase the likelihood the flame will become unstable. Before the flame blows off completely, it often begins to pulse. This is because the high velocity gases slow down as they expand after exiting the burner, which can allow them to burn back toward the burner. Assuming the gas velocities are not so high the flame blows off completely, this can set up a pulsation where the flame burns back toward the burner but then the higher-than-designed gas velocities push the flame away from the burner. This cycle continuously repeats until something changes. The pulsations cause the heater pressure to pulsate which can cause flames and hot gases to exit the heater. There are other possible causes of flame pulsations such as damaged flame stabilizers (e.g., metal cone flameholders or ceramic tile ledges) or something wrong with the fuel injection system.9 Flame stabilizers are designed to anchor the flames close to the burner outlet. Damaged stabilizers (see Figure 3) can lead to unstable flames. There are several possible fuel injection system problems that could produce unstable flames such as plugged or damaged tips (see Figure 4), improperly aligned tips (see Figure 5), or the wrong tips installed (see Figure 6). Properly installing and maintaining the correct burner tips is essential for proper burner operation to prevent pulsations that could cause flames and hot gases to exit a heater. Figure 3. Damage burner cone (flame stabilizer). 4 (a) (b) Figure 4. (a) Plugged tips, (b) Plugged burners. Figure 5. Improperly aligned tip (cone should be further left). Figure 6. Examples of tips that look similar but have different hole drilling patterns. 5 2. Keep the Flames Off the Tubes Fired heaters are used in the refining and petrochemical industries for the heating of various fluids. Figure 7 shows how heat is transferred to the outside of process tubes by both radiation and convection, through the tubes by conduction, and away from the inside of the tubes by convection. Flame impingement causes too much heat transfer to the outside of the tubes. Figure 7. Heat transfer to process tubes. Flame Impingement on Process Tubes Since the majority of process heaters have a feed that is a hydrocarbon, flame impingement (see Figure 8) can cause serious problems. Visual observation of the burners may show that the flames are contacting the tubes. In some cases a gradual increase in the tube metal temperature (TMT) could also indicate possible flame impingement. The operators should make it a point to look into each of their fired heaters at least once a shift to check for any problems with the flame patterns, particularly flame impingement (see Figure 9). Figure 8. Flame / hot gas impingement in a platformer furnace. 6 Figure 9. Mild flame impingement in a crude heater. Effect on Operations The reason that tubes do not overheat inside a furnace is because of the cooling effect of the fluid inside the tubes. This is why many heaters have carbon steel tubes. Once a tube starts to overheat there is a gradual build-up of carbon on the inside of the tube (see Figure 10). This layer of carbon acts to insulate the tube from the cooling effects of the process flow. This in turn makes the tube hotter. As the carbon continues to build, the flow area of the tube is reduced. If allowed to continue the carbon will choke the tube completely which can result in a tube rupture. F ahrenheit 2,372 1,309 1,300 1,280 1,260 C oking P attern 1,240 1,220 1,200 1,180 1,160 1,140 1,120 1,100 1,080 1,060 1,040 1,020 1,000 980 960 940 920 900 880 860 840 833 833 Th e m r o tekn ix Thermacam - 12 Bit 5:21:26 A M 8/17/2004 e : 0.75 Bg : 73.4°F Figure 10. Thermal image showing coke formation inside process tubes. 7 Hot spots will normally develop in progressive stages. When the flames contact the tube surface there is a cooling effect on the flame. This results in ash being laid down on the tube. This build up will lead to scale on the tubes (see Figure 11) as the outer layer of the tube starts to burn away. Figure 11. Severe scale on a process tube. Figure 12. Process tubes with various heat patterns. There are various stages in this process (see Figure 12): 1. Dark areas first start to appear from the carbon coating on the side of the tubes facing the burners. 8 2. Silver or light gray spots form within the dark areas. This is caused by the carbon being burned off. 3. These light gray spots will enlarge and cover more area. 4. As the coking continues red spots will begin to appear in the gray areas of the tubes. In some cases the tube will take on a "mirror" finish that looks almost like a chromed piece of pipe. 5. The tube will eventually start to bulge and then develop "pin hole" leaks. At this point the tube is ready to rupture and immediate action must be taken. Corrective / Preventative Actions The most important thing is to keep the flames off of the tubes! If flame impingement is noticed, the first step should be to adjust the burner causing the impingement to get the flame off the tube(s). • • Check the burner air register to confirm that it is open. Then look at the gas tips and determine if there is any plugging (see Figure 13) that would cause the flame to impinge on the tube. If there is plugging then remove the tips and clean them. Be sure the gas tips are properly orientated by looking at the burner drawing. Confirm that the excess oxygen and draft requirements are per the heater design specifications. Figure 13. Plugged gas tip due to coking. If the heater cannot be shut down, there are three options: 1. Take the burner out of service or reduce the firing rate by manually closing the block valve. 2. Increase the excess air to help cool the firebox. 9 3. Increase the process flow to the overheated pass. There are other options such as wrapping the tubes or clamping them while the heater is in service. These options would have to be considered extreme risk situations and approved by the safety department. 3. Keep the Process in the Tubes The general purpose of a process heater is normally to heat some type of fluid flowing through tubes inside the heater. In a refinery or chemical plant, the fluid is typically some type of hydrocarbon fluid such as crude oil. The purpose of the burners in the heater then is to heat the fluid. In reality, the burners heat the tubes which conduct the heat into the fluid flowing through them. The tubes are used to safely convey the fluid through the heaters. If those tubes are damaged, the fluid which is under pressure can leak out into the heater. This is particularly dangerous when the fluid is flammable because the heater is likely to be hot enough that the flammable fluid could ignite if there is sufficient oxygen in the heater. The most common cause for damaged process tubes is flame impingement (see Figure 14) which is related to rule #2. There are many possible reasons why flames may impinge on tubes. One or more burners could be misaligned and accidentally aimed at the tubes. The fuel tips in burners could be plugged (see Figure 4), misaligned (see Figure 5), or the wrong design (see Figure 6) that could cause flames to lean into the tubes. Burners could be located too close together causing the flames to coalesce and creating a much larger and longer flame that could impinge on tubes near or above the burners (see Figure 15). Figure 14. Flames impinging primarily on roof convection tubes. 10 Figure 15. Flames coalescing and causing impingement on convection tubes. Continuous flame impingement on a process tube causes the tube wall temperature to increase. If the temperature gets high enough, it can cause the hydrocarbon fluid flowing through the tube to break down and deposit coke (carbon) on the inside of the tubes. Tubes are designed to be cooled by the fluid flowing through them by transferring heat away from the metal by internal forced convection. However, if coke forms inside the tube, it acts like an insulator since its thermal conductivity is much lower than the metal's. This reduces the convective cooling of the metal which causes the metal temperature to rise. When internal coking is coupled with flame impingement, the tube temperature can continue to rise above its maximum design limit and eventually cause the tube to leak (see Figure 16) or even rupture (see Figure 17). This obviously needs to be avoided because the resulting fire from a significant tube leak can generate a large fire and huge quantities of thick black smoke because there will not likely be enough oxygen to fully combust all of the leaking flammable fluid. A further concern is the uncombusted leaking liquids and gases could burn outside the heater, increasing the risk of personnel injury and equipment damage. 11 Figure 16. Tube leak caused by flame impingement. Figure 17. Tube rupture caused by flame impingement 4. Keep Flammables Out During Lightoff Although this is listed as the 4th Rule, purging is the first action in the safe start-up of all direct-fired heaters.12 Therefore, purging is a mandatory requirement. This is covered in all the codes and standards around the world such as API 556, NFPA 85, 86 & 87, EN 298, CSA B149.3-10, and ANZ 3814. Depending on the jurisdiction / authority that the heaters are operating under, the purging function is not only mandatory, but also must be a "proven" function within the safety system that is operating the heater. Natural Purging In the past, and still in many installations today, the normal practice for purging a natural draft heater is to allow a period of time for ambient air to pass through the heater by natural draft, thus purging the heater of any fuel products (see Figure 18). Typically this takes about 20 minutes on a cold start and the time is at the discretion of the operator. The stack damper and burner air registers must be in the open position. 12 Figure 18. Natural purging. The major drawback with this method of purging is how to "prove" the purging function has taken place, and is complete. The ambient conditions have a big effect on the free-flow of air through the heater. In colder regions, the free-flow of air can be zero and hence no purging is actually taking place. For that reason, ambient purging based purely on time does not guarantee the heater has actually been purged and therefore should not be done. The choice of 20 minutes is not based on any empirical calculated method to ensure 4 or 5 volume changes is taking place, but is an industry pre-determined minimum time period. Most operating companies will supplement the 20 minute time period with an operator-actioned LEL check for any combustibles in the floor of the firebox. Steam Purging An alternative to using ambient air is to purge with steam (see Figure 19). However, caution must be taken to prevent steam condensation from affecting the operation of electronics in or near the heater such as the ignition and flame monitoring instrumentation. Steam purging involves injecting steam into the base of the radiant section. As this hot steam rises, it pushes any combustible gases up through the heater, while at the same time bringing in fresh air through the burner air registers or dampers. 13 Figure 19. Steam purging using snuffing steam. Another way to purge using steam is with a steam educator (see Figure 20). With this type of purging, steam is injected into the stack, above the damper. As the steam rises, it draws air into the bottom of the heater, which flows up through the heater, expelling any combustible gases. Figure 20. Steam purging using a steam eductor. 14 Mandatory Purging with a Burner Management System The most important function with the use of a burner management system (BMS) is to prevent the possibility of an accumulation of combustible gas within the heater, prior to introducing a flame inside the heater.13 The issue becomes how to "prove" that a 4 to 5 volume change has taken place. With a steam purge, or steam to an eductor used for purging, this is possible. Since if it's proven that with stack damper open, and knowing the flow rate of purge steam, it can be determined how long it takes to evacuate the air and any combustibles from within the heater. With a natural purge this is not possible. Therefore, for facilities with no steam available, then the next resource is to install a purge fan to perform the purge function (see Figure 21). With this method and the known flow rate of air, the purge time period can be determined. Figure 21. Typical purge fan configuration. Figure 22 shows a typical flow diagram for the mandatory purge using an automated burner management system. 15 Figure 22. Purge cycle flow chart. Conclusions Four simple and commonsense rules have been presented for the safe operation of a process heater: (1) keep the flames in the box, (2) keep the flames off the tubes, (3) keep the process in the tubes, and (4) keep flammables out during lightoff. These are not intended to be comprehensive but are designed to be easy-to-remember, particularly for those with less experience operating process heaters. Companies will likely have their own detailed processes and procedures for operating specific heaters which should be followed. Failure to abide by these four rules can have very detrimental consequences including personnel injury and equipment damage. Since adverse events can happen in a hurry, heaters need to be constantly monitored, including regular visual inspections both inside and outside of the heater. This will help prevent 16 situations where flames are exiting the heater, flammable liquids are leaking out of the process tubes, or other detrimental incidents are occurring. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. ANSI/API Standard 560: Fired Heaters for General Refinery Service, Fourth Edition, August 2007, American Petroleum Institute, Washington, DC. API Recommended Practice 535: Burners for Fired Heaters in General Refinery Services, Third Edition, May 2014, American Petroleum Institute, Washington, DC. C.E. Baukal, "Safety," Chapter 1 in The John Zink Hamworthy Combustion Handbook, Volume 2: Design and Operations, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. C.E. Baukal, I-P Chung, S. Londerville, J.G. Seebold, and R.T. Waibel, "Pollutant Emissions," Chapter 14 in The John Zink Hamworthy Combustion Handbook, Volume 1: Fundamentals, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. C.E. Baukal and W. Bussman, "NOx Emissions," Chapter 15 in The John Zink Hamworthy Combustion Handbook, Volume 1: Fundamentals, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. C.E. Baukal and W. Bussman, "Thermal Efficiency," Chapter 12 in The John Zink Hamworthy Combustion Handbook, Volume 1: Fundamentals, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. R. Newnham, Direct-Fired Heaters: Improving Efficiency and Capacity While Reducing Emissions, Kingsley Knowledge Publishing, Alberta, Canada, 2013. W. Johnson, E. Platvoet, M. Pappe, M.G. Claxton, R.T. Waibel, and J.D. McAdams, "Burner/Heater Operations," Chapter 12 in The John Zink Hamworthy Combustion Handbook, Volume 2: Design and Operations, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. W. Johnson, E. Platvoet, M. Pappe, M.G. Claxton, and R.T. Waibel, "Burner Troubleshooting," Chapter 13 in The John Zink Hamworthy Combustion Handbook, Volume 2: Design and Operations, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. S. Londerville, J. Colannino, and C.E. Baukal, "Combustion Fundamentals," Chapter 4 in The John Zink Hamworthy Combustion Handbook, Volume 1: Fundamentals, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. R.T. Waibel, M.G. Claxton, and B. Reese, "Burner Design," Chapter 6 in The John Zink Hamworthy Combustion Handbook, Volume 2: Design and Operations, edited by C.E. Baukal, CRC Press, Boca Raton, FL, 2013. R. Newnham, Direct-Fired Heaters: Operator Training Manual, Kingsley Knowledge Publishing, Alberta, Canada, 2013. R. Newnham, Direct-Fired Heaters: A Practical Guide to their Design and Operation, Kingsley Knowledge Publishing, Alberta, Canada, 2012. 17