James Hay explains why rupture discs are your most important safety devices
A rupture or bursting disc is a non-reclosing, fail-safe device. It is among the most misunderstood and least appreciated safety device used in systems where pressure or vacuum are present and where excess pressures or vacuum may need to be controlled and relieved when conditions exceed the safe design parameters.
The first rupture discs were very simple flat pieces of metal, weakened to open, sometimes with very little accuracy or predictability, at a supposedly fixed pressure. Over the years the technology employed in controlling pressures in the various types of oil, gas, petrochemical and other downstream industries has improved and developed to high levels of safety, mostly through the use of electronics and pneumatics to maintain a safe working pressure regime in the various plants.
In addition, the development of safety valves has also moved forward, again raising the safety level to one where many design and plant personnel lost sight of the most important safety device in the plant, the rupture disc.
Today operators and end users have been lulled into a false sense of security that the design engineers and system process/instrumentation designers have covered all the possibilities with sophisticated control and monitoring systems - nothing can go wrong so why do we need a rupture disc? In fact there are many reasons why we need rupture discs. These include:
* Fast opening times, typically <2ms, reacts to sudden pressure rises far faster than any valve
* Full-bore opening, unlike safety valves where the orifice restricts the flow
* 100% isolation of process fluids; no leaks until the disc opens
* Isolate safety valves from corrosive process media when installed upstream of the safety valve, can also be used downstream to protect the outlet
* Prevents rogue emissions, blockage of the safety valve and can be used to pop test the safety valve in situ.
Unfortunately some designs of rupture discs have not changed much over the past 80 years and we still find the old and unreliable scored or indented discs being specified today. The lack of understanding in the advancement of laser technology in the manufacture of rupture discs leads to the mistaken belief that the older and less reliable designs are the only ones available.
Rembe is setting out lead the way in advanced laser technology. It produces rupture discs with advanced design features that increase their reliability and operational life far beyond those of scored discs.
Busting the rupture disc myths
So why do so many design engineers get it wrong when they set out to do the FEED work? It is mainly because they were never taught (or had an explanation of) what a rupture disc does and does not do.
Rupture disc selection is based on the information given in the datasheet of the RFQ or tender. Sometimes this is minimal, whereas in reality the volume of information needed is much higher to ensure that the rupture disc is engineered and manufactured to meet the needs for that application. As many as 53 different points are needed to ensure that the disc is engineered to the full specification so it will operate correctly; on average only eight points are given to make the design. There is more emphasis given to painting specs, packaging, how many copies of paperwork, the inspection, etc. than to the specification of the disc itself.
With regard to 'disc failure', the term normally applied in the industry to the disc opening, operators try to avoid them, when in fact the disc opening means it is doing its job, protecting the plant. If you run the plant above its rated capacity, something that is done regularly, but you still use the same disc design characteristics for normal operations, why blame the disc for opening? If you have new pressures and temperatures then the disc needs to be designed for those parameters, not the old ones.
To recap, a rupture disc (and its holder) is your most important safety device, as long as it has been specified correctly in the design stage. It is fail-safe: whatever else goes wrong a properly designed and engineered disc opens at its set pressure. Electronics, pneumatic and mechanical devices can fail, human error can cause operational errors in the pressure system, yet the rupture disc still opens, making it your best device to protect your plant.
James Hay is with Rembe.
For more information, visit www.engineerlive.com/process
