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Relief
Existing RVs with more than 3% inlet ΔP - to replace or not. See API 520 Ed Force Balance methodology for acceptability/ stability
RV outlet ρV²: Support tail pipes properly. High ρV² can auto-close outlet LO valve and tail pipe vibrations can break LO valve padlock and self-close. See Safety Alerts. High velocity in tail pipe occurs, not during group loads, but when RV opens alone with low backpressure (1) Remove LO valve handle to minimize torque (2) Keep ρV² at 100,000 kg/m/s² (3) Keep valve spindle vertical (4) Locate valve after outlet reducer - larger valve (5) Go for extra strong locking plates - supplier standard ones are not adequate and (6) Go for high torque ball or consider gate valve
RV chattering: Can damage bellows leading to line rupture, local release and explosion. See Safety Alerts. Avoid large sluggish PCV that responds slowly. Avoid larger RVs; high inlet pipe ΔP; high backpressure and multiple RVs with the same set pressure (1) Good gap between PCV/PAH, PAHH and RV set points. Remember - RV reseats below its set point due to blowdown (2) Level settings are based on time gaps. Similarly, check time available between PIC/ PAHH to detect and respond, instead of deciding by % margins. Use Hysys Dynamics (3) Don’t size PCV for full flow. Dynamic simulation can help select right size (40-60%). Smaller PCVs provide better control and response. Ramp it fast in DCS to 60% or use a quick-opening characteristic valve (4) Multiple RVs: Stagger set points, to avoid chattering. Set the smaller one, usually for fire/ thermal case, at 100% and rest at 105% to prevent all opening at the same time. (5) Route bonnet vents to a common header to “safe area”. (6) If service conditions allow, no rust or hydrate, go for pilot type RV
Rupture disk upstream of RV: Common in fouling, slurry, polymerizing and coking services. Solids accumulation under the RD and blocking its inlet is common, even in services with liquid or N2 purge below the RD to inhibit solids build up. Not a safe design, as it depends on continuous supply of flush fluid and its effectiveness. Relocate RD close to source or better eliminate inlet pipe by protruding RD into source vessel. Better than a flush-mounted RD outside the vessel. Bonus: This can eliminate flush fluid flow control instrumentation and reduces operator attention to monitor flush
RD + RV: Cavity in between should be atmospheric pressure. Add a PAH/ PG to alert of pin hole leak that can equalize pressure across RD affecting its burst. Pin hole leaks or ruptured RD will allow dirty/ gooey/ polymerizing fluid to choke RV inlet. See Safety Alerts - popcorn polymer blocking RV inlet
RD: Add a P&ID Note: “Fatigue failure or aging. Replace RDs as recommended by manufacturer”.
RV outlet: In atmospheric discharge outlets, provide drain or weep hole directed away from the vessel. Where the weep hole gets plugged / choked with rust, water can freeze and block RV outlet or water column can add to backpressure
BDV isolation: LO/ LC valves as in RV. Outlet LO valve for partial stroke testing. Inlet LO valve to isolate and replace a passing or stuck BDV
Blowdown: No liquid relief along with blowdown load for sizing headers. Liquid relief for RV blocked outlet cases only. 1 train at a time. Don’t add loads that don’t occur at the same time
Reactor Relief Sizing Mistakes: Runaway reaction cases ignored (Use Adiabatic calorimetry data for runaway reaction). Only fire cases considered. Vents and catch tanks made of unsuitable materials; not rated for relief pressure and temperature. Vent sizing based on isothermal and not adiabatic temperature. Using water properties than that of reactants. Ignored two phase relief. Ignored solidification and blockage in vents. Ignored pressure drop and choke points in vents. Ignored simultaneous power or cooling water failure. IChemE, ‘Pressure relief venting systems - examples of good and bad practice’
Flare
Flares: Finalize first number and type of flares - HP, LP and AP. Acid Gas flared or vented. Spared or not. Demountable flare. Drone inspection to avoid stairs and ladders. Load reduction by HIPPS; staggered/ extended blowdown; steam drives & backup power. Dynamic Simulation helps reduce loads as all RVs don’t pop at the same time + flare header packing reduces load to flare tip
Adjacent Flares: (1) Will the existing flare operate when installing the second. Check to avoid flame lick from each other (2) Can a flare be taken for maintenance without radiation from the operating flare. Otherwise keep them apart. (3) Operating flare stack metal is kept cool by flared gas flowing inside. If only one operates, expansion issues for the other (4) If the 2 divide total flow, then Water Seal Drum (WSD) or PCV to avoid U tube phenomena air suck-in on low flows. Read API 521
Staged Flares: Water seal drums for LP and PCV/ SDV backed up by buckling pin for HP. Read API 521
Flare location: Based on plant layout. Some prefer upwind to avoid plant releases are not blown into open flame. API 14E downwind. API 51R downwind. Some prefer crosswind or in sides
Flare Gas Recovery Unit (FGRU): Standard packages available. Talk to suppliers. Read API 521. FGRU recovers continuous low pressure relief and RV/ PCV leaks. See typical leak rates discussed a few pages further down. FGRU is not designed for short duration, high flow and high backpressure releases. On high backpressure, FGRU is bypassed by breaking the seal in WSD or via an on/ off valve. Provide a RD or buckling pin parallel to the on/off valve if it fails to open. RD requires inlet/ outlet LO valves + flare system shutdown to replace RD. Buckling pin is easy to replace and low MTTR (mean time to repair)
Headers: Service conditions decide MOC. MOC decides number of headers. (1) HT/ LT - High or low temperature (2) Corrosive - Acid/ Sour gas, separate heat traced (3) CO2 Removal membrane rejects
Header: Avoid heating cold fluid to use CS header above its MDMT. Heater failures can result in brittle fracture of CS piping. Local release and fire. Use SS piping. See Safety Alerts
Header: Do not mix wet and cold streams in the same header. Can lead to hydrate blockage
LT/ HT Interface: LT minimum 10D or 3 m upstream of LT/HT joint. With LT sources, some specify minimum 50D or 10 m to cater to cold creep via metal conductivity. Cold upstream piping can cool inflow gas and reduce downstream temperature further, theoretically. Based on experience, each industry has its norm. For a definite answer, do transient analysis
KOD: Underground KOD can float up and damage attached piping after a violent storm fills the underground basin. See Safety Alerts. Avoid. Or add a caution note
KOD: PAHH against twisted or damaged flare tip to avoid ‘blocked outlet’. ESD-1. Causes: Flame lick from adjoining flare. Internal coking/ burning damaging tip. Google Images <Damaged Flare Tip>
KOD Sizing: Go by API 521 and client. Droplet size: 300-600μ. Higher in steam-assisted/ HP flares
KOD Liquids: Practices differ. Oil & Gas: Route to Inlet Sep. Valuable condensate that drops out on JT chilling or hot flared gas cooling due to heat loss to flare header metal. FPSO: Route to Slop Tanks. Refineries: Not to crude tanks, as light condensate flash adds to vapor load. Heavy liquids go to tanks. Ref: API 521 on hold-up time
Liquid Carry Over to Flare: API 521 5.7.9.4 - likely droplet sizes that cause burning rain. Estimate liquid fall-out distance to decide sterile area, apart from radiation. Some take it as 60m
DP: LP KOD/CDD operate at 0 to 34 kPa. DP min 345 kPa (50 psi), based on peak explosion pressure = 7-8 times operating pressure; legacy number from coal mines. Read API 521. Higher DP for HP Flare based on OP + margin
DT: Max relieving or black body temp in upstream Oil & Gas. Fire case temp ignored. Release temp is high in downstream Refinery or Petchem. DP based on DT. For 150# piping - at 40°C = 20 bar; at 150°C = 16 bar. As the change is not drastic, a few take DT as 150°C; good for system that get purged with steam for maintenance. DT impacts piping stress + expansion loops on large headers
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