Design Tips 10

Piping

• Corrosion Under Insulation: Water and in association with Chlorides and Fluorides in insulation leads to CUI. May lead to External Stress Corrosion Cracking in SS piping operating at 50-175°C. Select suitable insulation material as per ASTM C795. Provide water barrier and inspection windows in the insulation

• DBB: Double Block and bleed arrangement. Shown for process piping may get missed out for instrumentation tappings. Include a separate DBB legend in P&ID Legend sheet. (R Sundar, ex-GASCO)

• Revamp projects hydraulic analysis: Take advantage of control valves partially open at full loads; motors/ turbines/ compressors/ pumps on partial loads; add a bigger impeller in pump casing and check if compressor suction pressure can be increased. Test runs can identify limiting elements that are easy to replace. Read ChE, Dec 21, p26-32

• ROV: Downstream units - refineries, petrochemical plants with large liquid inventories do not have as many Remotely Operated Valves as in upstream Oil & Gas plants. Review. See Safety Alerts

Relief

• RV Set and Relieving Pressure: You can set at or below MAWP. DP/MAWP = 100 units. 2 cases (1) SP = 100 and (2) SP = 90. Relieving Pressure, RP: 110 for non-fire; 116 for non-fire multiple RVs and 121 for fire cases. Overpressure = RP-SP gap. Case 1: 10/16/21 respectively Case 2: 20/26/31 (22/28.9/34%). Accumulation = Increase over MAWP. Same as OP, when SP = MAWP

• Why 21% for Fire?: Low probability event. Gives a smaller RV. No real difference as RV comes in discrete sizes. You select next standard size. Calculated, say = 1.5 sq.in with 21%; 1.65 with 10%. For both K 1.838 sq.in

• RV Back Pressure, BP: Adds to spring load. Increases SP. For constant BP, RV spring load is reduced correspondingly. SP = 100. Constant BP = 20. Spring selected = 80. Cold Differential Test Pressure (CDTP). If constant BP is 0, RV opens prematurely at 80. Specify constant BP only if it exists

• Built-up Backpressure (BBP): Pressure in RV outlet due to its flow. Acts as a temporary additional spring. Should be less than OP, so that RV upstream is maintained at SP

• BBP: As long as <10% for conventional and <40% for balanced RV, minor variations in BBP have no impact on size, as RV comes in discrete sizes. No need to calculate BBP to 3rd decimal accuracy

• Backpressure (BP): High BP reduces lift/ area/ capacity. Bellows RV help when BP is variable, by cancelling BP effect on valve seat. RV opens at set pressure

• BP: Process Engineers miss this. BP should be less than API 526 values. Allowable BP is lower for bigger RVs. Size D BP = 285 psig. Size R BP = 60 psig. Similarly, weak bellows limit BP. Say 150# 4M6. Max Inlet = 285 psig. Max BP = 80 psig and not 40% of 280 = 112. For 8T10 BP = 30 psig only

• BP: Take care of BP of non-relieving RVs too

• RV Flange Rating: Reduced set points for bigger RVs. For a few RVs less than flange ratings per ASME B16.34. See values in parenthesis API 526. Results in higher RV inlet flange rating than base vessel, usually for bigger sizes. Known issue

• HP Sources: Low ambient temperature may result in hydrate, ice + wax deposits and blockage in RV no-flow (dead leg) RV inlet pipes, specially pilot lines. Heat trace and insulate. Add a P&ID note to avoid insulation removal during maintenance to access valves

• Bellows RV: Bellows movement restricted in plugging and polymerizing service. Keep foreign matter out of bonnet in hydrate, solid, foaming and coking services. Bellows are fragile, prone to fatigue and pin-hole leaks. Leaks take away the ability to handle backpressure; hence bonnet is vented

• Fire RV: During a fire, vessel metal heats up. Ability to handle internal pressure falls. Yield stress at 200/400/600°C = 100/80/36 units. A vessel designed for 100 units will fail at 36 when wall reaches 600°C, before its RV pops at 100 units. BDV is the real protection as it can rapidly bring down internal pressure. Provide RV to meet code + Certification/ Insurance agencies. It is cheaper and schedule-effective to install a RV than go for protracted correspondence

• Fire RV: Some set RV 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 RV: 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 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