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Fired Equipment
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
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
GT Exhaust WHRU
Review/ specify WHRU low turbine exhaust temperature at part loads
Pumps
Auto-start: For critical service. If duty pump fails, standby pump switched on by discharge PAL/FAL or source LAH or motor line current. Standby not started by trips. Trips protect an equipment or plant. They get wired to SIS. Controls get wired to DCS
Blocked outlet/ Discharge Check Valve Stuck Closed: Credible Hazop scenario but a low probability event. Centrifugal pumps are designed for shut-off head - blocked outlet or check valve stuck closed. Overheating churning fluid in casing is an issue. Likely to get noticed by source vessel LAH/LAHH or downstream FAL/LAL
Shut-off head: Max suction + normal ΔH or Normal suction + max ΔH; highest density fluid that may flow (water?) and maximum impeller size that can fit in the casing. Add static head of users located in decks below pump, to find system design pressure
Discharge Check valve: May slam shut with a large shock wave while pumping up a hill when the pump is shut off. Install a hydraulic dampener
Alarms/ Trips: (1) Large MW pumps: Oil Export or Water Injection - suction PALL, discharge PAHH and recycle coolers. Suction PALL is for blocked inlet filter to avoid running dry and damage (2) Small and Medium pumps: If pumped fluid is clean dPG across inlet filter. For dirty fluid dPA. Discharge PAHH not required. PAHH may not work if pump curve is flat. Go for FALL
Min Flow: Go for RO for pumps < 35 kW. RO results in bigger pumps and wastes energy continuously. Discharge PCV for pumps with dropping curves. PCV costs less and responds faster than FCV. FCV for pumps with flat curves. Common FCV for medium pumps <200 kW. Large Pumps > 500 kW, individual FCVs and FALLs. Route minimum flow back to suction source and not to suction. See Hydraulic Institute recommendations
End of Curve Operations (EOC): Size motors for EOC operation to avoid motor overload. Motors come in standard sizes and cost differential is not much to oversize it. Piping and NPSHa for design load. Until vendor data is known - assume EOC as 120-130% design flow
Pump Suction Routing: Review. In every project, there is always a piper who routes it long!
Pump Suction: Rate suction valve and piping to discharge side rating
Pump Suction: Saturated or boiling liquids release vapour on pressure reduction. Flat top eccentric reducer avoids vapor accumulation in suction + avoid vapour lock. Min 5-10D straight length. Install suction gate valves horizontally to avoid vapor accumulation. Inform gate valve supplier, API-600 to machine to achieve good fit between guiding surfaces of disc and body. Otherwise galling occurs and valve requires repair/replacement
Emulsion Pumping: Oil: Water mix has a higher emulsion viscosity. Emulsion μ = Oil μ * CF, Correction Factor. Pipeline emulsion ΔP > ΔP-oil. Bigger pumps. Go by Hysys emulsion μ for preliminary sizing. Ask owner to give lab measured emulsion μ. High CF for low μ oils; low CF for high μ oils
Export Oil Pumping in Series: At low flow rates, switch off Export Pumps and carry on with upstream Transfer Pumps only. Add a check valve across Export Pumps
Flare KOD Pumps: At any given instant - flaring with high backpressure or no flaring and at atmospheric pressure, vapor pressure of fluid pumped = prevailing KOD pressure. For NPSHa calculations, liquid height between pump Centre Line and KOD LAL counts. Some base NPSH on water assuming light ends weather away or heated by heaters if provided as in viscous services. One can’t count on heating, as pumps will start at a pre-set level without waiting for heater to heat first. Light ends are high value condensate. No point in boiling them off. In offshore, the pump can be located in a lower deck level. In onshore + FPSO, it is expensive to elevate the KOD as it requires elevation of upstream flare header and pipe rack. Vertical Can Pump inside the KOD or in a boot or slow speed pumps with low NPSH help. Onshore pumps are in a pit. Onshore contractors may like to keep the cost low by running the large flare headers on ground sleepers and bury the KOD. Some may assume vapor pressure as 10 psia, as a compromise between water and hydrocarbon, but it has no basis. Sub-cooled liquid is not valid as the HC liquid in KOD is boiling in a given instant
Utility Connection at Pump Suction: UC to purge pump and piping after a shutdown before opening the pump casing - in Amine and Light Ends or hot and heavy viscous fluids to avoid congealing
Hot Fluids Pump: Add a RO + 2” ball valve across discharge check valve to keep standby pump warm and avoid its failure on thermal shock. Add a P&ID Note
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