Design Tips 12

Relief

• Fire RV: For vessels with large liquid, traditionally assumed that all heat goes to boil liquid. i.e., Release = Qfire/λ, ignoring ≈ 80% of heat is to liquid. Hysys blowdown model can provide likely RV area that keeps vessel pressure more or less constant at Prelief. In BDV calcs, selected size brings pressure down within 15 minutes. Select a size that keeps pressure at relief pressure. Trial-and-error. 2 trials. Gives relief temperature and rate. Usually D orifice. This method is not usual. May need owner approval. Take traditional route if third party certifier wants it

• Fire RV: In vessels with low liquid inventory and high relieving pressure, say compressor KODs, liquid will boil-off in minutes. Use Gas Relief calculation

• Fire RV - Flange Rating: Repeated Query. Select flange on design temperature, as for base vessel. Use relief temperature calculated for RV sizing; add a note in RV data sheet. RV will see relief temperature whereas base vessel metal will be hotter due to ∆Tgas + ∆Tmetal, about 300-400°C more than relief temperature. If you specify relief temperature for RV flange rating, such a RV may not be available! Fire incident pictures show vessels twisted out of shape, deformed, split or even fallen flat with RVs still sitting pretty. In most cases, RVs won't pop

• Pool or Jet Fire: Size for pool fires. Jet fire results in localized heating and get extinguished by blowdown that takes away the jet’s fuel. Jet fire impact area, as in a blow torch are small. Not right to use jet fire rate and pool fire area

• Thermal Release: Estimate based on trapped volume between 2 blocked valves*(Liquid Density @ initial conditions - Liquid Density @ RV release pressure and Blackbody Temperature). RV is likely to pop only once, as after the relief, the mass in the pipe is less even when it is heated again the next day

• Sparing: Some operators prefer single RV without inlet valve + a common LO outlet valve for all RVs/ BDVs for spared equipment and trains. If it takes time or difficult to replace a RV go for installed or warehouse spare. A single RD or RV can be isolated, removed and replaced under Admin Control. See ASME Sec VIII Div.1 Appendix MM-5.6

• Sparing: Fire or thermal RVs are single. Low probability events. A few operators still demand 1 + 1 RVs as they don’t want Admin control for testing and maintaining RVs. Admin control requires an operator near the RV for manual intervention during the RV is servicing/ testing. It has inherent risks as discussed in “Stationing an operator at a pressure safety valve bypass”, Jonathan R Webber, Digital Refining, PTQ Q3, 2022. See also TreviTesting or PreVenTest (www.ventil.nl) - online testing/ recalibration for fire and clean service RV

• Sparing: For dirty fluid, provide inlet isolation for visual inspection of fouling, cracks, corrosion, gumming etc. There is no annual shutdown as in old days. All plants are not equal in maintenance, as you observe when you walk in a plant. Production pressures delay maintenance. An Oil Major’s plant in Country A is not run the same way in Country C. These factors should decide - to spare or not!

• Check valve: May get blocked with debris or can get stuck shut and not permit flow. It should not be in the relief path, especially when an upstream equipment is protected by a downstream RV

• Viscous / Congealing Oil: Heat trace RV inlet and outlet to unit blowdown/CDD provided with a heating coil

• 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’