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Separators/ Vessels
Gas Blowby: To downstream LP Vessel. Design Basis: Agree early with Owner. [A] Flow area: Either LCV or its bypass or both. Both may be open when bypass is opened when LCV malfunctions - undersized LCV/ flow more than design or debris/ mill scale reducing LCV capacity. [B] Flow rate (1) Gas alone or Gas + Liquids in proportion to inflow (2) LP Vessel PSV sizing: Gas only or Gas and liquid or Gas volume equivalent liquid [C] Source Pressure - NOP, PAH or PAHH or PSV. [D]: PSV Relieving Pressure at 110% of DP or Hydrotest Pressure. Each client has his own say
Gas Blowby: Relief flow may reduce if you consider ΔP in LCV outlet + LP Sep at PSV Relieving Pressure. At times gas rate can be high - more than normal upstream gas inflow. If gas outflow > inflow, upstream Separator pressure will fall. May not be able to sustain gas blowby rate. Wrong to assume liquid will keep accumulating and send gas only. Accumulating liquids will build level and stop gas blowby. Cv available for gas = Cv full minus Cv normal taken by liquid. Plant on turndown can increase Cv available for gas. Dynamic simulation helps to get a good picture
Gas Blowby: If LCV + bypass impacts LP flare size (1) RO in bypass to limit Cv to that of main LCV (2) 2x50% LCVs with independent LICs. No bypass. With varying water cut over years, may help level control too (3) Blowby PSV load to HP Flare (4) Increase LP vessel DP to HP Sep PAHH/1.3 or same DP. No credit for mechanical stop/limit on LCV to limit flow. A clamp-on stop may work loose; a welded one may break loose - after chattering-hits. A new or clueless operator may remove it a few years down the road. Organizations have no memory
Gas Blowby: Occurs when operators are fatigued - start-up/ transient conditions when some control valves are on manual, LALL bypassed to establish level etc. As they have 100 things to do with so many alarms and lights flashing, they may forget to switch to auto in one particular vessel. Ensure time available between sand pad and LALL is good to close outlet SDV. Many sizing programs ignore it
Gas Blowby to CDD: HP vessel say, at 100 bar may route condensate to CDD under on/off control. Fully rate outlet piping and run it direct to CDD. JT effect - ice/hydrate formation and blockage. Pipe MOC to suit MDMT. Heat Tracing maintains pipe wall temperature but does not heat bulk fluids inside pipe. Chilling inside the pipe after a blowby is an instantaneous with which any heating arrangement can NOT cope. Add inhibitor injection point downstream
Sand: Based on sand load, decide sandjet nozzles/ sand disposal options
Blocked Outlet PSV: Earlier for gas only assuming oil, gas and water outlets will not close at the same time. Then for 2 streams, Gas + Oil or Gas + Water. While oil can go with gas, downstream Gas KOD cannot handle oil flow! Water can go with oil, but API says no credit for favourable response of a control valve to take additional fluid. Present practice: Size for all 3 streams based on Safety input that PAHH or LAHH can close all 3 outlet SDVs while inlet SDV may fail to close
Blocked Outlet PSV: On high backpressure, inflow may reduce. [In HP gas wells downstream choke pressure may not have any impact on flow]. Model from reservoir to PSV to find flow. Individual flowline PAHH will shut wells before Separator PSV pops but SIL studies may show a few wells may fail to close. Some consider 2 out of 5 wells failing to close, an old rule of thumb. As non-closing wells may be high flow ones, inflow may not be 40% but 60-70% design. High backpressure may reduce to say 50% design. Varies. Some employ a PC in Production Manifold to manipulate chokes while a few go for individual flowline PCs. Some close liquid outlet SDVs only on LALL. Not on PAHH/ LAHH. Situation specific. If PSV size/ flare loads are high, opt for HIPPS
Heavy Viscous Oil Relief: Released oil may solidify downstream of PSV, losing heat to header pipework or due to heater failure or heater inadvertently switched off. Best to design LP Sep/Coalescer to upstream HP Sep DP to avoid relief here. Impact on plate thickness and cost is marginal. Liquid relief from upstream HP Sep may have the same issues. If so, HIPPS to close HP Sep inlet SDV can eliminate blocked outlet release
No Credit for Trip Action: In PSV sizing, no credit for trip. Trips could have latently failed. Trip failure is known ONLY when a demand is placed
PSV as Flare Dump PCV: A few use them with pilot PSVs. Wrong. You don't use a bodyguard for errands
PFP: Can delay heating and high vessel wall temperature. Thickness based on likely unattended fire exposure time and allowable metal temperature/rupture stress. Should withstand fire water hose impact without dislodging. Good for large all liquid filled vessels e.g., C3 accumulator
Storage Tanks
See Safety Alerts: Hydrotest on tanks on soft soil, inlet valve leak, rapid in/out flow. Roll Over
Inlet velocity: Limit for non-conducting fluids like Naphtha to avoid static electricity spark
Static Mixer: If products develop flammable vapours, go for floating roof to avoid vapor space or inert gas padding
Level Trip Failure: P&ID 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 LTs on trips is difficult to spot
Blanket Gas: Inlet PCV: Product outflow + Thermal. Outlet PCV: Flash gas + product inflow + Thermal. PVV: PCV Failure. Emergency Gauge Hatch/Manhole: Gas blowby/ control valve/ heater failure. Frangible roof-to-shell joint: Fire. Set inlet PCV low and exit PCV high to minimize blanket gas demand. Manifold blanket gas inlets/ outlets of similar tanks - service and DP, to conserve blanket gas
PVRV: Introduces air. To avoid, go for blanket gas. No flame propagation in “closed” systems under blanket gas pressure
Venting: (1) PVV with or without Flame Arrestor (FA) (2) Vent with FA for flammable vapor. Avoid FA with viscous or cutback or asphalts that plug. Avoid FA with PVRV, an emergency path to keep it free of fouled and plugged FA. PVRV, a secondary path and unlikely to be on demand at all times. PCV or vent in use all the time and may require a FA, depending on fluid. If its FA is fouled, PVRV protects. Check regularly FA for plugging. Provide access. Alternate, glycol or water seal; check regularly seal presence
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