Design Tips 11

Piping

• FI: Show upstream and downstream straight piping requirement. With hire-and-fire project staffing, the assigned piper may not know such requirements. Rework has ripple effect. Do it right the first time

• FI - TI: Locate TTs 10D downstream of FI. ΔP thru flow orifice gives good mixing and prevents velocity profile distortions to get average temperature

• Vacuum Breaker: Add a goose neck with vent on downcomer gravity flow lines to avoid vacuum and vibration

• Piping model review: Sit with piper. Avoid long PSV inlet and pump suction runs. Avoid dead legs associated with vessel liquid outlets - LCV bypass, drain lines and low points. Avoid piping blocking manholes or access. PGs/TGs protruding into ladder access

• Slope: Closed Drain 1:100 to 1:250. Depends on flow rate to meet friction head. Lower slopes reduce piping cost; OK for clean liquid. Higher slope for Oil & Gas plant drains with sand, dirt and muck accumulation. Flare - 1:250 to 1:500. A consultant has suggested reverse slope towards unit KOD to reduce flare header elevation and costs

• LO/LC Valves: PSV/ RD/ BDV valves are LO/LC. When a PSV on one vessel protects another, add a "LO" valve in between. Sometimes a start-up HP line, say fuel gas sourced from a pipeline, can cause overpressure during normal operation if kept "LC", due to leaks. Preferred option is drop-out spool piece or swing elbow for positive isolation. Some facilities use excessive "LC" and "LO" valves that are not safety-critical. Could be NC or NO - normally open or closed or CSO or CSC - Car Seal Open/ Closed are OK. Have an internal valve by valve review and decide

• Liquid Spray: Use nozzles to mix inhibitor etc with gas or liquid. ΔP = 7 bar is a magic number for nozzles - service water hose or oil burners or Chemical Injection Quills. See www.spray.com

• Piping Vs Instrumentation Valves: Piping engineers specify valves MOC, heat treatment, ASTM spec etc to suit service. In a few companies, Control + SDV valves are specified and procured by Instrumentation engineers, with less emphasis on valve body construction and more emphasis on actuator etc. This may lead to poor performance of Control and SDV. Instrumentation Engineers at times end up procuring wrong type of valves - example, RF (Raised Face) while Piping Spec is for FF (Flat Face) that requires rework. Instrumentation Engineers should refer to piping specs for the valve body and specify only actuators, PID logic, etc. Refer to piping specs for instrumentation valves

• 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

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