Design Tips 12

Relief

• Fire PSV: Some set PSV low so that Gas relief temp = Prelief / Pop*Top is low. Wrong. Ignores metal temperature, hotter by 350°C, over gas. Fire flux = 20,000 Btu units. Gas coefficient 30 Btu units. ∆t gas = 350°C. For gas filled vessels real protection is by depressurisation, PFP and deluge systems

• Fire PSV: API - Wetted or unwetted area within 232 m² (2,500 ft²). Radius of 8.6m. Wetted Area - Horizontal Vessel: Liquid LAHH if within 7.6m or Centerline if above 7.6m high above pool fire. Vertical Vessel: Bottom head only if above 7.6m or to LAHH. Unwetted Case: Vessel height or 7.6m

• Fire PSV: 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 PSV 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 PSV: Vessels with low liquid inventory and high relieving pressure, say compressor KODs. Liquid will boil-off in minutes. Traditional method Gas Relief or Relief = Qfire/λ. λ = 50 Btu/lb

• 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

• Fire PSV - Flange Rating: Repeated Query. Select flange on design temperature, as for base vessel. Use relief temperature calculated for PSV sizing; add a note in PSV data sheet. PSV will see relief temperature whereas base vessel metal will be hotter due to ∆Tgas + ∆Tmetal, about 300-400°C more. Both see the same pressure. Which will fail first, RV or base vessel? If you specify relief temperature for PSV flange rating, such a PSV 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

• Thermal Release: Estimate based on trapped volume between 2 blocked valves*(Liquid Density @ initial conditions - Liquid Density @ PRV release pressure and Blackbody Temperature). PRV 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 PSV without inlet valve + a common LO outlet valve for all PSVs/BDVs for spared equipment and trains. If it takes time or difficult to replace a PSV go for installed or warehouse spare. A single RD or PSV can be isolated, removed and replaced under Admin Control. See ASME Sec VIII Div.1 Appendix MM-5.6

• Sparing: Fire or thermal PSVs are single. Low probability events. A few operators still demand 1 + 1 PSVs as they don’t want Admin control for testing and maintaining PSVs. Admin control involves stationing an operator near the PSV for manual intervention during the period the PSV is serviced or tested. It has its 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 PSV

• 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 PSV

• Heavy Viscous Oil Relief: Heat trace PSV inlet and outlet to unit blowdown/CDD provided with a heating coil

• Existing PSVs with more than 3% inlet ΔP - to replace or not. See API 520 Ed Force Balance methodology for acceptability/ stability

• PSV 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 PSV 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

• PSV chattering: Can damage bellows + local release + line rupture and explosion. See Safety Alerts. Avoid large sluggish PCV that responds slowly. Avoid larger PSVs; high inlet pipe ΔP; high backpressure and multiple PSVs with the same set pressure (1) Good gap between PCV/PAH, PAHH and PSV set points. Remember - PSV 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 PSVs: Stagger set points, to avoid chattering. Set the smaller one, usually 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/ hydrate, go for pilot type PSV

• Rupture disk upstream of PSV: 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 vessel. Bonus: This can eliminate flush fluid flow control instrumentation and reduces operator attention to monitor flush

• RD + PSV: 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 PSV inlet. See Safety Alerts - popcorn polymer blocking PSV inlet

• BDV isolation: LO/LC valves as in PSV. 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 PSV blocked outlet cases only. 1 train at a time. Don’t add loads that don’t occur at the same time