Overpressure risk follows hydrocarbons through every stage of an oil and gas operation. A separator, a gas compressor heat exchanger, a fixed roof storage tank and a downhole fracturing tool each carry a credible overpressure scenario, and each places different demands on the pressure relief devices that protect it. Specifying the wrong device family for the application is how relief systems fail. Rupture discs, relief valves, tank breather valves and flame arresters each belong to particular oil and gas applications, and the protection holds only when the match is right
Rupture discs: non-reclosing relief across the process
A rupture disc is a non-reclosing pressure relief device engineered to burst within a certified tolerance of its marked burst pressure, giving instantaneous, full-opening relief while holding a bubble-tight seal in normal operation. In oil and gas service, rupture discs protect tanks, transmission lines, compressors, evaporators, pipes and instrumentation in gas and liquid handling systems. They can serve as primary relief, as secondary relief, or in series with a pressure relief valve to isolate it from the process media.
Rupture discs work based on two different types of working principles, and each behave differently in service. Reverse-acting discs are oriented with the process media against the convex side, placing the disc in compression. They are commonly used in high-cycle, pulsing applications include separators, distillation columns, choke drums and isolation of a pressure relief valve. Forward-acting discs load the membrane discs in tension and suit primary and secondary relief on separators, storage tanks and wellhead protection.
There are two documented cases which show poor selection logic at work. On separator packages where poorly maintained relief valves were sticking, the OEM added a rupture disc as the secondary relief device, rated at 105% of the separator’s maximum allowable working pressure in accordance with the ASME Section VIII code. The disc has no moving parts and is replaced during routine maintenance of the separators, so a stuck valve no longer left the vessel without a working relief path. Another shows older-technology discs in offshore platform protecting gas compressor heat exchangers which fails from fatigue under pressure cycling. The solutions were to reengineer the installation with reverse acting rupture discs, double disc holders and a burst disc indicators which stopped the unplanned shutdowns they were causing.
Storage tanks: breathing, fire and emissions
A fixed roof atmospheric tank must breathe as it fills, empties and warms, or the shell fails by rupture or implosion. Pressure/vacuum relief valves are mounted on a roof nozzle to protect the tank in normal operation. In an application where the priorities are product conservation and prevent fugitive emissions, pilot operated relief valves offer lower leakage and better flow performance than weight or spring loaded designs, and they let the tank operate closer to its set pressure. Emergency relief valves cover the external fire case, with higher flow capacity than standard pressure/vacuum valves. Blanket gas regulators hold a constant positive pressure in the vapour space, keeping outside air and moisture out of the vessel and cutting evaporation losses from the stored product.
Flame arresters complete the tank protection layer. A deflagration flame arrester protects stored or process media where combustible gases move through low pressure lines, or sits at the top of a tank. A detonation flame arrester is specified where a pipeline restriction or turn can accelerate a flame front, and at compressor inlets and outlets to prevent flashback damage. In one tank farm where lightning strikes had caused tank fires at a neighbouring facility, Groth Corporation’s Model 8800A Combination Relief Valve and Flame Arrester (ATEX certified model 1200A-FA1) was retrofitted across the tanks. On a follow-up visit after severe thunderstorms, an arrester element had taken a lightning strike and needed replacement. The tank and its hydrocarbons content were protected.
Downhole: relief technology as an activation tool
Pressure activation is a common rupture disc application in downhole tools during drilling, completion and production. The Well Activation Safety Protection (WASP) rupture disc is manufactured with a ±2% burst tolerance and is used in downhole tools for cementing, completion and hydraulic fracturing. The WASP-Type A activates a downhole tool when pressure is applied to the annulus, while the WASP-Type I releases pressure from inside the drill string, casing or tool. For hydraulic fracturing pumps and high pressure, high temperature downhole applications, the MRA machined reverse acting disc combines the disc membrane and body in a single machined device that relieves pressure or activates tools at extremely high burst pressures.
Common Questions to ask when selecting relief devices in Oil & Gas
When should a rupture disc back up a relief valve? When the primary valve can stick, leak or corrode in the process media. A disc can be sized as the secondary relief device against the vessel code, as in the separator case above (105% of maximum allowable working pressure per ASME Section VIII), or installed in series to isolate the valve from the process media.
What is the difference between a deflagration and a detonation flame arrester? Application, mainly. A deflagration arrester suits tank tops and in-line service on low pressure lines carrying combustible gases. A detonation arrester is built for high speed flashback, where pipeline restrictions or turns are present and at compressor inlets and outlets.
Why specify a pilot operated relief valve for VOC storage? Conventional relief valves can vent before they are required to open, releasing volatile organic compounds. In a documented chemical plant case, Groth’s 1660A Pilot Operated Relief Valve with snap action met the seat tightness requirements of environmental regulations even with the operating pressure close to the set pressure of the valve, and the premature leakage stopped.
Specifying relief devices for Australian sites
Rupture discs are selected and installed in accordance with AS 1358. Engineers must go through rigorous checking on application data such as : set pressure, media, temperature, line size and the backpressure the device will see. Measurement Solutions Pty Ltd, A Bestech Company, is the authorised distributor for Continental Disc Corporation and Groth Corporation in Australia. With our recent integration into Bestech Australia, we now have the support of nation-wide sales and application engineers who support selection, sizing and commissioning locally. Send the set pressure, media, temperature and line size for a recommendation.
Send us an email or call +61 2 9894 4300 to speak with an application engineer.