Design Tips 3

Separators/ Vessels

• 2 Phase Sep: If required, check for future 3-phase separation to take bulk water out as water cut increases. Provide support channels for future internal weir

• Sizing: Separator sized for 100,000 BLPD with 10 to 90% water cut ends up sized on 90,000 BWPD + 90,000 BOPD. Second or oil draw off compartment is usually 1m long. A variable height weir can reduce overall size. Initially low weir with a thicker oil layer. Later increase weir height as water cut increase for thinner oil layer. Provide support channels for weir height increase. Or size on 90% water cut and vary LAL/NLL/LAH in initial years 

• Level Settings - FPSOs: FPSOs are subject to wave action. 150 mm gap between alarm and trip is low. Internal waves may cause premature trip. Calming baffles minimize wave action. Add time delays on trips. Sense level in vessel centre

• Level Trip Settings: LT nozzles to have a minimum gap from vessel bottom/ top for a minimum angle to shell. LALL: Allow for mud/ sand layer. Liquid volume above sand to allow liquid outlet SDV time to close to avoid gas blowby. LALL to avoid vortex suck-in at outlet nozzle

• Liquid Outlet Nozzles: Project 6” above vessel bottom to stay clear of sand or dirt accumulation

• Level Trip Failure: In all vessels, add a note “LTs of LC to track LTs of trips and provide a discrepancy alarm, to detect latent failure of trips”. LIC-LTs are always on service and their failures noted immediately. Latent failure of trip LTs is difficult to spot. See Safety Alert on Naphtha Tank Overflow. Better option is for all the 3 LTs - LIC/LAHH/LALL) to track each other; use average for LIC and 2 oo 3 voting for trip. Note: PT failures can be spotted by another PT upstream or downstream

• Separate Transmitters for LL and HH Trips: A few ask to allow maintenance. Not warranted. Smart self-checking transmitters. HH and LL demands are mutually exclusive scenarios. You can track and compare outputs of trips (SIS) and control (DCS) transmitters to get a discrepancy alarm

• Coalescer internals: Added to enhance oil-water separation. Foul and get blocked by sand and well mud. Reduces flow area. Just because Hysys output does not show them, you can’t ignore sand and muck

• Viscous Oil: If pour point is above minimum ambient temperature, heat trace LG/LT. Go for radar type LT. Add spray nozzle above vane pack / mist pad for online cleaning/ de-waxing with hot water or diesel/ kerosene. Heat trace and insulate drain lines and PSV inlets to last valve. Heat trace bypass lines

• Foaming Service: Have a Foam Detector with alarm

• Liquid Outlet SDV: Earlier SDV was at LCV outlet as a battery limit valve. Now, at vessel nozzle to minimize inventory loss should a rupture occur downstream of LCV. Safety requirement. LCV outlet is flashing 2 phase flow + mill scale + sand, high erosion zone. SDV downstream of LCV may have its seat seals damaged impacting it TSO. Suggested SDV+Check+LCV and not LCV+Check+SDV or LCV+SDV+Check. When downstream vessel is elevated to provide pump NPSH, then add a check valve at the inlet of downstream vessel, to protect its emptying  if LCV outlet ruptures - usually at a bend due to erosion. Case by case analysis

• Avoid Quick Opening Doors. Have resulted in accidents

• Condensate Filter: In wellfluid service with muck and mud, it is likely to be plugged in no time. Check with client their operating experience

• H2S Service: P&ID Caution Box: "H2S forms pyrophoric compounds on metal parts, mill scales and sludge that are prone to spontaneous fire on exposure to air. Keep metallic parts wet before, during and after opening vessel, piping, and filter etc. Follow company procedures to avoid pyrophoric fire"

• Utility Connection: Blind flange + check valve + ball valve at nozzle to avoid hose rupture and release. Expecting operators to attach a check valve to the hose, may not work. See Safety Alert

• Vent/drain spec break: If source > 35 bar (600# piping) (1) Fully rate tail pipe to header. Spec break at header. Check tail pipe pressure in Flarenet. Will exceed 150# rating when a 2" globe valve is fully open. Do not use 1" or ½" vent lines in HP service to reduce flow and tail pipe pressure. Small bore pipes prone to rupture or break on dropped loads. Minimum 2" pipe

• Vent: Not a PSV bypass. To depressurize before draining or take out for maintenance. 2 valves in series - ball and globe. May have a bleed in between to remove residual pressure after venting to LP flare backpressure. Bad practice to install a closed spec blind downstream of vent valve to stop fugitive emission thru a passing valve. Unsafe to expect an operator to swing a spec blind against a single passing valve on pressure (vessel) side and HC/H2S on flare side. Leaking process gas can jettison mill scale and injure operator. Spec blinds are for vessel isolation for manual entry and should be on vessel nozzle

• Drain: Bad practice to install a closed spec blind downstream of drain valve to stop fugitive emission thru a passing valve. See reasons above - vent line. Drain lines are in stagnant and corrosive service - handling muck and sand, prone to seat cutting, corrosion and leak. Spec blind in closed position, may get corroded and lose its integrity. See ‘Closed Drain Valving Philosophy’ in Training

• Drain: 2 ball valves, not a ball and a globe. Globe gives a wrong impression “throttling allowed”, viz draining under pressure that results in low temperature issues, embrittlement and over-pressurization of drain system and Closed Drain Drum. Resulted in gas blowby into drain header and lifting off CDD. P&ID Caution Box: “Vent first and then drain under gravity”

• Drain: Perform gas blowby calculations for all sources at 300# and above, one source at a time. Good to have CDD vapor outlet, sized to avoid CDD pressure exceeding its DP during a blowby