Design Tips 7

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 low motor line current. Standby is 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 > ΔPoil. 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

• Avoid, a common spare pump serving 2 different services. May save a little money. But inadvertent wrong line up + inability to trip the pump on LALL/ PAHH have caused accidents and losses

• Seals: Check proper seals are specified. (1) using a redundant pump with seal not suitable for the pumped fluid (2) Pumping light ends with a pump specified for heavies as in a Flare KOD have caused accidents. Light HC may leak in seal system designed for heavies