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Piping
Steam Traps: Bimetal traps sub-cool condensate. Difficult to maintain constant temperature. Accumulating condensate can corrode steam coils. On shutdown may freeze and split. Replace with float or thermostatic traps that avoid condensate accumulation (JR Risko, ChE, May 24)
An improperly specified "flex pipe" ruptured while pressure testing. Need to specify correctly and completely, rather than letting go off an incomplete specification hoping to catch up later
Gear operated valve. Gas leaked via stem into gear operator. Its vent was painted over. Top plate blew off
Natural gas was used to actuate its RV. Corroded/ over-pressurized and ruptured actuator blew off
Long bolt or sandwich, or wafer valves: On flame impingement longer bolts expand rapidly and lengthen, allowing both gaskets to leak, adding more fuel to fire than standard flanges. Avoid
Long bolts: Expanded on reaching operating temperature of 310°C. Flange opening. HC leak. Fire. BP
3 Part SS ball valve nuts cracked. Local release. Avoid lower grade SS 303/304. Use marine grade 316. Replace 3-part valves with solid block type
HC piping trunnion support: Subject to localized corrosion and release. Inspect regularly
Low Point: Peroxide/ alcohol mix trapped in a 100m elevated piping. N2 purging was ineffective to remove the reactive chemicals. Pipe was steamed. Chemicals decomposed and ruptured. Operators missed 2 low point drains ‘hidden’ in the pipe rack. Piping redone eliminating the pocket
Dead-legs: API 570 “Components of a piping system that normally have little or no significant flow. .. blanked (blinded) branches, lines with normally closed block valves, lines with one end blanked, pressurized dummy support legs, stagnant control valve bypass piping, spare pump piping, level bridles, pressure relieving device inlet and outlet header piping, pump trim bypass lines, high-point vents, sample points, drains, bleeders, and instrument connections. Dead-legs also include piping that is no longer in use but still connected to the process.” See Section 6.6.2 Dead-leg inspection
Dead-legs: Get corroded. During winter trapped water froze and cracked piping. HC release via a leaking valve. Conduct dead-leg reviews/ Winterization check/ Freeze protection
Dead-legs: Get corroded. 8” relief line failed and released 75 tonnes of flammable material. Avoid dead-legs associated with vessel liquid outlets - LCV bypass, drain lines and low points
Dead-legs: Isolated pump trapped oil and water for 190 days. During cold weather, water froze in piping dead-leg. Daytime temperature melted the ice. HC release via an ice-damaged valve flange. Drifted towards a fired heater. VCE. Identify and protect dead-legs. CSB
Dead-legs: HDS reactor outlet piping had a dead end that trapped injection water containing NH4OH and Chlorine. H2 leak. Explosion. Oil leak and pool fire. Inspect/ check wall thickness of dead end and low point piping
Dead-legs: Out of service, for 15 years, propane control station piping cracked. Fire. Refinery down. Isolate units/ equipment/ piping not in service, purge and blank off
Dead-legs: Out of service, acid pipeline. On opening a joint, acid under pressure sprayed out. Gas, a corrosion product had pressurized the acid. Trapped pressure. BP
Chloride stress corrosion cracking: During pressure equalization, HP Compressor suction 16” ball valve bottom trunnion SS304 screw failed and trunnion ejected. Gas release
Bypass line in RDS. NH4HS corrosion. Leak and fire
H2 Compressor Discharge Piping: Cut thru by a bolt on adjacent TG support. Watch during plant walks
Insulation: That is soaked in oil can reduce its auto-ignition temperature and cause fire. Replace such insulation ASAP. WWW
Insulation Flanges: Avoid inadvertent painting or removal. Keep a caution board. BP
Small Bore Pipe: Toppled LPG Cylinder ripped off LPG small bore filling line. Fire and explosion
Small Bore Pipe: Branches off large pipes have failed in a number of places. Poor support. Usually in compressor or pipe discharge lines. Vibrations. Avoid
Small Bore Pipe: Many small bore pipes fail by fatigue as they are free to vibrate. Small valves have been knocked open or have vibrated open. WWW
Small Bore Pipe: Heating oil piping insulation was being replaced. Guardrail of the scissor lift impacted a ½” threaded piping connected to 4" hot oil piping. Oil at 260°C leaked. Aerosol ignited by electrostatic. No provision for remote isolation. Plant closed. CSB
Small Bore Pipe: While installing a ceiling heater, a scissor lift impacted NH3 refrigeration piping and severed ½” threaded piping connection. NH3 release. CSB
Hose: No NRV at vessel nozzle. Attempt to purge vessel without depressurizing, ruptured N2 hose as the vessel was at 18 bar. Expecting operators to add a check valve to a hose may not work
Hose: N2 used to pressure test a compressor with a suction side hose. After the test, compressor outlet valve was opened to vent N2 to flare. Leak past a downstream check valve, pressurized compressor suction, bursting the N2 hose. BP
Hoses: Are frequently connected and disconnected. Not stored properly. Frequent flexing makes failure likely
Hoses: Mix-up. LPG hose used for NH3 failed. SS hose instead of Hastelloy C used in Chlorine service failed
Foam Debris: Dried foam choked a foam: water mix proportioning valve. Flush after every use
Gasket: For cathodic isolation of C2/C3. Glue between stud bolt washers and isolation sleeves extruded on sunlight, weakening the bolted joint. Failed and released gas. After cleaning off the glue, bolts maintained the original torque
Gasket: Mismatched/ locally procured gasket O ring failed. Gas plant fire. Remember space shuttle Challenger explosion due to O ring failure
Gasket: DGA column 36” line hydrotested at 135 bar. Poor quality gasket good for 82 bar broke at 106 bar. HP water jet with broken gasket hit a worker on his face
H2S: While modifying pipe known to contain H2S, a 2 m pit was dug. Workers who entered the pit were exposed to H2S. Fatality. Sumps, pits and trenches are confined spaces. PPE + Confined Space Entry Permit a must. BP
H2S: Plant operator was exposed to H2S, while removing a PG from a H2S piping. BP
Relief
Ensure atmospheric relief (open vent) discharges are to a safe location and are safe. Discharges can be toxic or may result in VCE (Vapor Cloud Explosion) (1) Toxic Methyl isocyanate discharge (2) Flammable chemicals VCE (3) Ethylene vapor (4) Methyl Mercaptan from a storage tank; poisonous liquid methyl mercaptan pump relief close to grade; Nitrogen discharge below pipe rack; Chlorine discharge below operating platform (5) Tetrahydrofuran vapors and (6) Isomerization unit vent stack
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’
RV Vent: Bonnet vents of conventional RVs are plugged in service. Bonnet vents of balanced bellows RV should not be plugged and should be left open to atmosphere. Take care. Difficult to know RV type by external appearance
RV: Inlet pipe in Debutanizer plugged with polymer. High pressure events deformed the inlet piping, as RV was unable to relieve. Butadiene is known to polymerize when concentrations build-up in dead-legs like RV inlet piping. Flush continuously or insert a RD in protected vessel to avoid build-up
RV: HCU RV malfunctioned and opened. To reseat it, operators tried to close its 6” inlet valve. Unable to reach, they closed its lower rated 20” outlet valve. Valve failed. H2 + HC release. Fire and explosion. CSB
RV: Inlet flange had minor leaks. When RV started simmering, the stud nuts fell off. Release and fire
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