Design Tips 6

Fired Equipment

• No of Burners on turndown: Below 20-30% load on start-up or during steam: air decoking or refractory drying switch off a few burners, still maintaining heat release symmetry. With N operating burners, theoretically you can go down to single burner, 5N:1 or 3N:1. Using bigger than standard pilot burners, one can switch to pilot burners only to provide more turndown. It doesn’t matter if BWT is below auto-ignition temperature of fuel. Your kitchen or room heating gas burner operates with room temperature. Heater refractory is dried at low firebox temperature

• No of Burners: Do rotate switched off burners. Remove or keep their guns cooled, as once fuel flow stops, the guns are not internally cooled. Keep their air registers totally blanked-off to avoid tramp air. Reduced flue gas flow during part load, reduces draft loss, increases available draft, leading to more Xs and tramp air. Modulate stack damper to keep arch draft low. Read about tramp air + unfired burners in ‘Fired Heaters - Operations’ in Training

• No of Burners: Even after you cut-off a few burners, the control valve hunting may impact fuel flow and flame out. Control valve may have minimum stop and charts may be developed for firing on minimum stop + a smaller number of burners. Don’t rely on min stops - clamp-on stop or welded one may break loose - after a few chattering hits. As fuel composition changes over a day in plants that make their own fuels, go for a small parallel FCV / PCV and bring it online at part load

• Heater treaters Vs Stabilizers: Treaters heat heavy/ high viscosity oil forming stable emulsion with water to break the stable emulsion. Stabilizers strip off of light ends from a condensate to maintain Reid Vapor Pressure (RVP)

• Emulsion Heaters: Go for a direct fired heater with emulsion in tubes. Avoid firetube bath heaters. Salt deposits on firetubes lead to overheating and failure. Case study

• Equal Flow Vs Equal COT: Go for equal flow in all passes. Usually, pass inlet FCV keeps equal flow. Minor variations in coil outlet temperatures (COT) are OK, fouling or coking in one or two tubes. If you reduce flow to a coked pass on manual control to increase its COT, it will coke more, a vicious cycle

• Draft Control: Read in ‘Fired Heaters - Operations’ in Training on adjusting both stack and burner/ Forced Draft (FD) Fan damper step by step. Duct/ stack are designed for 120% design load. At operating and part loads, draft losses are low. Partially close stack dampers to maintain arch at 1-2 mm WC. Instead of single blade, multiple blade stack dampers provide better control

• Draft Control: Some adjust stack damper alone, make arch pressure +ive to reduce burner draft and Xs air, as it is a pain to adjust individual burners. Some believe that by +ive draft, you can force heat into tubes. Firebox heat is by radiation. Sun is far way and its radiation reaches us across space without any huff and puff. +ive firebox pressure makes the flue gas flow thru arch refractory, heat its supports, warp roof casing and brings down the “roof”. Case study. Tale-tale brown spots on casing. No need for arch PAHH set at 0 mm WC. Alarm should do

• Draft: On part load operation, lower stack exit velocity may result in air incursion via stack tip. This recirculation may reduce stack gas temperature - reducing stack effect + acid gas condensation. Stack heat loss may also cool part load flue gases

• Draft: Prevailing wind Vs Draft Gauge orientation impacts draft reading by ±5mm (0.2” WC), especially in coastal areas

• Tramp Air: Tramp air gets in via unlit burners, open peep holes and tube sheet holes around tubes header boxes. Seal all openings to cut tramp air. Fire box has the highest draft or tramp air pulling capability. While O2 probe at arch is better than in stack, unless combustibles (CO/ HC) are measured, cutting Xs air on O2 alone will result in unburnt fuel and/or potential explosion due to tramp air

• Flame Impingement: Lower Xs air leads to longer flames. Longer flames or closely located burners lead to flame interaction causing flame lick on tubes. Most flame licks are due to bad burner maintenance. A coked oil tip not regularly cleaned can result in directed flames and hot gas. Gas guns also coke when there is liquid carry over. (Check regularly Fuel Gas KOD. Note: During start-up, cold piping can result in liquid in superheated gas too.) Missing or broken burner tile directs flame towards tubes. Mostly, it is hot gas rather than flame impingement. You can observe with saw or coal dust thrown in air. Dr RD Reed, "Furnace Operations". Use sodium bicarbonate, Daniel Gonnet in LinkedIn. Sparklers show flue gas path and impingement, if any

• Tube Hot Spots: Can occur in any configuration, Vertical Cylindrical (VC)/ Box/ Cabin or vertical/ horizontal tubes, single/ double fired. Heater construction, coil layout, misdirected burner throw from dirty tips. Keeping burner tips clean is a dirty job but essential. Read Tony Barletta revamps.com/Revamps/documents/170.pdf. A few skin thermocouples may read higher, due to poor pad welding with an air gap between the tube and thermocouple. Hot gas impingement causes overheating/ coking. See ‘Fired Heaters - Operation’ in Training. In vertical heaters, tubes are equally exposed, even if you have 8 passes. In box heater with 4 passes, 2 on top and 2 at bottom of side walls. Bottom passes receive more heat + coking. Downward flue gases recirculate behind the tubes - between tubes and wall, promoting good mixing. Tall fireboxes promote higher flue gas recirculation

• FAL/ FALL: To avoid false reading due to tube rupture, locate FAL at the inlet and FALL at the outlet 

• Different Convection Service: On power failure, feed to convection section may stop while radiant section keeps running. Tube failure and fire. Provide FAL/ FALL/ TAH/ TAHH protection

• Finalize tube internal cleaning (decoking/ pigging/ turbining) and external cleaning options - robotic or water jetting of convection external surfaces. Ask for access provisions

• FD Fan Air Inlet: Specify minimum air intake height to suck air free of hydrocarbon from plant leaks. See LNG Plant blown off on Safety Alerts

• Flame Scanning: An air swept flame scanner will keep it cool and have its scanning face free of dust

• Coastal or plains location: Firebox puffing due to wind induced momentary air starvation, as wind ebbs and flows. Case study. Wind barriers or burners in a plenum or FD Fans help

• Fuel Supply SDVs: Adding a Solenoid Operated Valve (SOV) on FCV/ PCV to make it a SDV is no longer allowed. No protection against “control valve stuck open” scenario. A FC (Fail Closed) valve can still get stuck open. FCV/ PCV is not a TSO valve. Its control action results in seat wiredraw damage. Frequently operated valves such as control valves, wear easily and are a source of gas leak. Soft seated SDV offers better TSO. Quarter-turn ball and plug valves, used as block valves are better than gate and globe valves. Eventually all valves leak, including SDV. That’s why a DBB is provided, to vent any leak past the first valve. SDV is wired to SIS/ ESD system, a separate and independent layer of protection

• Firebox Purging: API. Before lighting burners, ensure air flow of 3 times firebox volume to purge. Usually, LP Steam is admitted @ 1 volume change in 5 minutes or running FD Fan. In warm countries, natural purging due to wind turbulence at stack tip is known to create a draft. You can feel air flow by opening a peep door and holding a handkerchief. Not reliable in cold countries. Ambient air might have warmed up but air in firebox may be cold and heavy. One can use steam or air educators too. See http://www.gasprocessingnews.com/features/201510/design-an-alternate-purge-system-for-an-lng-plant-fired-heater.aspx. HC detectors can ensure that there are no HC in the firebox

• Firebox Purging: See Safety Alerts on firebox explosion during light up. Add a Caution Board near heaters: "Purge the firebox, every time before a light up or after a failed ignition before attempting a reignition". Most of operators may not know about the explosion issue or fatigued to remember during start-up. Note: Critical if H2 is in the fuel gas. Low molecular weight H2 can easily leak into the firebox during shutdown. Wide flammability range

• Fuel change over: See common mistakes under Safety Alerts

• Firebox Purging: With N2 in Sour Gas Thermal Oxidizer/ Incinerators

• Off gas disposal in fire box: via burners or floor nozzles. Provide flashback protection in off gas line 

• Heater Feed Pumps: Spare pump with power from a separate system or turbine driven. Adequate hold-up in source vessel

• Pave the grade below fired heater sloped away with a toe-wall, to avoid leak accumulation below. See C2= Cracker incident in Safety Alerts