Design Tips 6

Fired Equipment

• Tubes: Above 450°C, tubes operate in creep or plastic deformation range. Designed for 100,000 hours of operation at DT and DP. As Operating P and T are less than design. , Pressure declines from inlet to outlet as the temperature increases. Due to lower P and T than design, tubes last longer than design life. See API 530 Remaining life calculations. For unlikely or infrequent pressure conditions such as pump shut-off that last for a short duration, go by elastic design for reduced design life in consultation with system designer/ Owner

• Bridge Wall Temperature: BWT, flue gas temperature at firebox exit is less than datasheet value due to < design load. Flux = F(T1^4-T2^4), where T1 = Av BWT; T2= Av TMT. Lower the heat density, lower is BWT. BWT is less due to heat given to shock tubes, part of convection section. BWT ≈ 50 - 90°C less than T1. BWT is the best indicator of firebox performance. BWT ≈ 600 - 850°C in normal heaters. ≈ 1,100°C in ethylene crackers or Hydrogen reformers, with high flux and temperatures. BWT indicates how much fired heat is absorbed in firebox. Along with stack temperature, indicates how much heat is picked in convection section. It lets you select radiant and convection tube supports material. BWT temperature changes are more obvious than stack temperature. Whenever heater load changes or Xs air increases, stack temperature may change by 10-15°C whereas BWT may change by 50°C, easy to spot. Like BP as to the state of your health, trending BWT over a period will tell a good story about heater’s health. [F is decided by the proportion of 2 radiating components CO2 and H2O in flue gas. Lower in oil firing c.f. gas. Lower with Xs air]

• BWT: May marginally go up due to high TMT - internal coking or external fouling. Sometimes with SS tubes; their higher unoxidized emissivity > assumed oxidised design emissivity. Will get OK once tube external surfaces get oxidized. SS pipes at lower operating temperature may stay bright (not oxidized) and have lower emissivity. Unlikely with CS tubes

• Flue Gas Temperature (FGT): FGT is convection section exit temperature. Ref: ‘Fired Heaters - Operations Cause & Effect’ in Training. FGT may change due to (1) Higher throughput = higher duty = higher TMT and hence high FGT (2) Higher Xs air as indicated by low CO2 or high O2. Tramp air, air that does not participate in combustion but get sucked in via peep doors and other openings can cause this (3) Internal coking or External fouling. (4) Tube leak and after burning. One can observe sooty stack gas or after burning at stack tip during night (5) If the heater is on 2 different services - HC in radiant and steam/ others convection section, firing is controlled by radiant service. Convection picks up available waste heat. Changes to FGT, will affect convection section heat pick-up

• Excess Air and FGT: Xs air reduces flame temperature. Flame temperature matters only in an adiabatic combustion viz. no heat transfer to surroundings. In fire box, BWT, temperature of gas cloud that surrounds the flame matters. BWT is decided by flux/ load/ firebox area and doesn’t change with flame temperature due air-preheat or low Xs air. Internal coking or external deposits increase TMT and marginally increase BWT. Marginal changes due to T^4 fourth power effect. As Xs air increases, H2O-CO2 concentration/ emissivity / F go down increasing BWT. This reduces heat given in firebox and increases convection section duty. As convection heat transfer area is fixed, FGT, stack gas temperature goes up to increase LMTD for higher convection duty. COT - TIC fires more to maintain heat duty

• FGT: Sulfur bearing fuels produce sulfurous acids in flue gas that on cooling can condense and corrode parts. See API 560 - acid dew point (ADP) based on fuel Sulphur level. Say ADP =140°C. API 560 suggests a margin of 8-14°C (para 6.2.2) and 25°C (para 13.2.8 d) giving a min FGT of 148-165°C. This will allow subsequent flue gas cooling due to stack heat loss and resultant FGT drop. Note: (1) FGT drop will be more at part load and in uninsulated stack. (2) A common mistake made is keeping FGT above dew point but introducing process fluid cooler below ADP. All metal parts in heater and air-preheater should be above ADP to avoid acid condensation on tubes + acid carry over/ acid rain via stack tip. Cases are known of acid condensation and corrosion in ID fan, flue gas duct and stack. An unlined boiler stack resulted in acid rain on neighbours. In concrete stacks, stack temperature drop specially on part load can be minimized by a stack in stack - externally lined steel stack inside a concrete stack. Preheating air to the air preheater with excess LP steam during part load can keep flue gas and Air Pre Heater metal hot. ID Fan is insulated to keep casing temperature above ADP + margin. Provide for off-line washing and neutralizing the acidic deposits

• Flue gas water dew point is about 60-70°C. Case Study: Process fluid below flue gas water dew point produced water in fire box

• Air preheater with glass or polymer tubes can cool flue gases below ADP, say 85°C (http://www.digitalrefining.com/article/1000962) allowing acid to condense to recover more heat. In condensing air preheaters, bulk of the acid is removed. Stripped flue gas has low or nil corrosion potential or condensation in downstream ID fan, ducting and stack

• Turn down: Heaters on start-up or SOR/EOR (Start/ End of Reaction) run on reduced duty. Turndown available with all burners on to maintain firing symmetry is decided by fuel pressure. Gas at 2 barg, 5:1 and oil at 5 barg, 3:1 based on minimum fuel pressure to sustain stable flames. i.e., 20-30% of design load. [Note: Lab Bunsen Burners operate at 0.02 barg (200 mm WC) and domestic LPG stoves at 150 mm WC]. Vaporizing service may have 50-60% turndown limit to get the right flow regime to ensure wetted wall. Unwetted wall results in hotter tubes/ fouling/ coking in sensitive stocks. Case Specific. During part load BWT will fall, increasing heat pick-up in radiant section. FGT will fall increasing thermal efficiency. Convection section will pick less with reduced LMTD and low flue gas velocity/ outside gas coefficient. Independent convection service, say steam may not get its full load. See API 560 G9

• Fuel PALL: Supply failure or control valve hunting action can result in low pressure at burners + flame out. API 556: Pre-mix burners flame out below 3 psig (20 kPag). Raw gas or low NOx burners, below 3.5 kPag. Moments later controller may send unburnt fuel gas that is likely to result in a firebox explosion. Constant burning pilot burner, flame scanner and PALL are layers of protection. A minimum stop on fuel gas PCV may ensure minimum firing. It is not reliable - may wear or drop or corrode out