Safety Alerts 6

Storage Tanks

• Leak in pressurized C3/C4 storage area. Fire. 9 tanks damaged

• Natural Events: (1) Lightning: API discounts lightning as a low probability event. A few accidents in storage tanks, usually escalate to adjacent tanks. Lightning impact on power supply can cascade (2) Earthquake: Failed tanks and pipe breaks can result in dyke overflow. Cracked dykes cause subsoil/ aquifer contamination. Dykes may have holes (3) Tsunamis or hurricanes can wash away tank foundations, electrical system damage, break pipe connections and debris impact (4) Floods can spread oil in SWD around the plant; damage transformers located at low levels with resultant fires (5) Solar heat input caused PSVs on 1,000s of C3= cylinders pop that caught fire, launching the cylinders as missiles. Usually partially filled or ‘empty’ cylinders heat up fast (6) Low ambient temperature crystallized cyclohexane that ruptured a tank area pipe. Butadiene condensed, creating vacuum and collapsed the tank and (7) Overfilling, due to frozen water in a level detector has caused BLEVE in 3 storage columns in a liquid CO2 plant

• Oil & Gas Storage and Transportation Electrostatic Accidents https://iopscience.iop.org/article/10.1088/1742-6596/418/1/012037/pdf

• Bulk chemical shipment drums/ containers look alike but may carry a wrong chemical due to mix-up

• Tanker unloading: Tanker moving without first disconnecting the supply hose has led to many accidents. Add physical barriers to prevent the tanker moving without ensuring hose disconnection

• Railroad tanker had wrong type of lining for the monomer shipped. Reacted with iron, polymerized and exploded

• Railroad tanker gas liquified due to low ambient temperature. Vacuum. Tanker collapsed. N2 padding

• Tanker Truck: Regularly overfilled (Fill Ratio 99% Vs 53% max) Ammonia tanker truck exploded, releasing 22 tonnes of anhydrous ammonia - spreading a dense ammonia cloud. 129 died and 1,150 injured. In liquified gas service, over filling leads to overpressurization and stress

• Inspect storage dykes for spills and leaks. Pump out rainwater promptly from dykes. Dykes filled with rainwater can’t contain spills

• Excessive rain water accumulation on the roof of a double deck external floating roof tank caused the roof to sink, exposing a crude naphtha mixture. Flooded dykes limited access to roof drain. BP

• Carbon bed adsorbers for emission control. Heat generated by adsorption ignited flammable vapor in the adsorber that ignited the attached tank. Account for heat generation and add flame arrestor at adsorber inlet

Heat Exchangers

• Refrigerated LPG Absorption: Hot lean oil stopped to HX but cold rich oil continued to flow. Transient condition resulted in lower than MDMT. When hot lean oil started, thermal stress caused rupture and release. It was ignited by a heater 170 m away. Jet fire. Identify such issues during design

• Heat exchangers tube leak led to HC in Cooling Tower. Fire and explosion during welding in ‘water service’ Cooling Tower. Expect HC in Cooling Water network

• Usual to keep HP process fluid on tube and LP seawater on shell. May lead to corrosion of CS tube sheets behind titanium cladding on the shell side. HC release/ explosion. Go for seawater on tube side

• Leaking HX: To detect leak, after removing head, 8 bar gas was applied on shell side. Tube bundle moved out killing mechanics. Liquid instead of gas could have avoided fatalities

• HX tubes cleaning: With water at 1,000 bar. Foot operated Deadman switch was disabled to clean lower tubes. Cleaner tripped and the nozzle came out of the bundle and killed him. Use automated cleaning

• Similar, To push out a stuck tube bundle, steam was supplied to shell side and bundle blew out. WWW

• HX cleaning: Hot butene, a reactant in polymerisation reactor, was circulated. Catalyst traces on the walls caused a runaway reaction rupturing a plastic blowdown drum. Pure butane, instead of a mixture with other components, was very reactive

• Welding: Trapped fluids in plugged tubes may explode or emit obnoxious fumes when heated. Titanium tubes may melt; molten titanium readily burns in air. WWW

• C3= HX reached -40°C after depressurising. Was restarted (pressurized) and it ruptured. Brittle failure. Fire and explosion. Train operators on auto-refrigeration and MDMT issues

• Low Temperature Embrittlement: During a trip, C3= on the shell side of HX chilled down as its pressure fell. Water in tubes froze and tubes broke. C3= in the cooling water system blew out a section of 16” water line that was ignited by a furnace 40 m away WWW

• C3= fractionator reboiler isolated from PSV on column. On introduction of hot quench water to tube side, thermal expansion of trapped C3/C3= in shell. Release and fire

• Low Temperature Embrittlement: During a trip, C3= on the shell side of HX chilled down as its pressure fell. Water in tubes froze and tubes broke and blew out a section of 16” water line that was ignited by a furnace 40 m away. WWW

• BAHX (Brazed Aluminium Heat Exchangers). Service Life. Accumulated thermal fatigue caused rupture. Don’t go by ‘leak before failure’ but by service life to replace HX. In an Air Separation Unit, explosive burning of aluminium heat exchanger elements in the presence of liquid oxygen caused explosive rupture

• Mercury liquid metal embrittlement. Aluminium cold box nozzle failed leading to explosion

• Water Hammer: Condensate in steam supply line damaged the impingement plate in a reboiler and then broke HX tubes

• Condensate build-up in HX can reduce heat transfer, making the supply valve open fully to expel the condensate; then the supply valve may get throttled allowing condensate build up. Vicious cycle. Water hammer and corrosion. WWW

• Erosion: Thinning/hole observed in Cold Box piping bend, downstream of a liquid methane LV. Flashing and cavitation

• Closed blowdown valve resulted over-pressurization of an LNG plant HX that exploded

• HTHA: Severe failure of Naphtha HDT HX shell. 38-year-old unit designed when HTHA was not understood. HTHA can occur in the so-called safe region of Nelson curve in API 941. Operating conditions can get severe than design. Avoid CS in HTHA service and go for Cr-Mo steel 

• Graphite HX cracked, due to higher than allowed torque while hot-torquing bolts during a restart. Hydrogen Chloride gas was released. Torque for HX to piping was lower than adjacent piping to piping bolts