Buckling Pin Relief Valves (BPRFs) and rupture discs are widely used to protect against over-pressurisation in a range of applications. Information from ASME VIII Division I suggests the two are interchangeable — but which is best suited to your application? Here Elfab, a manufacturer of rupture discs, weighs up the pros and cons of both solutions:
Fatigue of device
Over time, devices can fatigue, leading to greater replacement and maintenance costs. But is this true of BPRFs and rupture discs?
Buckling pins are not subject to process conditions and buckle at the set point, so therefore cannot fatigue. Traditional, forward-acting rupture discs were more susceptible to this problem, however, the latest reverse-acting solutions outperform the original designs.
Reverse-acting discs benefit from 3% tolerances and have been tested to over one million life cycles. As such, this means there is little risk of fatigue in even the harshest operating conditions. Due to such advanced performance characteristics, some rupture discs are now offered with a 3-year warranty compared to the traditional 12 months.
Tolerances of devices
Buckling pins have become the obvious choice for those concerned about manufacturing, operating, temperature and set point tolerances, or those scenarios where operating as close to the set point as possible is critical.
Nowadays, buckling pins normally offer 95% to 98% maximum operating pressure, based on the set point. Where original rupture discs didn’t offer such characteristics, reverse-acting discs are available with 3% tolerances and 97% operating ratios. These tighter tolerances mean there is a less dramatic effect on the operation of the rupture disc due to temperature changes. Likewise, in many instances, stock can be rationalised dramatically, reducing purchasing costs.
Corrosion can be problematic, as it increases the likelihood of a device failing and costs rising. For buckling pin relief valves, the pressure relieving part – the pin – is external to the process conditions, meaning relatively cost-effective materials can be used for this component as it will never be in contact with the process media. This ensures low repurchase costs and confidence that the product should not be affected by this problem.
Usually, the buckling pin valve is constructed from compatible materials for the process medium, normally carbon or stainless steel. While sourcing the valve body in corrosive resistant materials may lead to a high initial investment cost, this is a long-term investment as the valve will typically last the length of the project.
Rupture discs can be manufactured from a range of non-corrosive material such as graphite, hastelloy and tantalum, despite not coming into contact with the process medium. For modern rupture disc designs, in the instance a rupture disc is affected by corrosion, the discs will fail-safe, meaning customers will be made aware there is a potential problem.
Rupture discs can be used in combination with modern burst detection systems to alert of disc failure as a result of corrosion. This allows for a quick review and change out. In addition, double disc assemblies can be used to allow manufacturing to continue until a suitable maintenance schedule is due to take place.
For optimum performance, it’s important that the devices are correctly installed. With many incidences reported by the HSE being related to human factors, it is imperative that manufacturers offer designs that irradiate this issue.
Given the design of buckling pins, it is very difficult to install them incorrectly. Commonly, customers have thought rupture disc designs could be installed incorrectly, however, modern solutions have eradicated this problem. With the introduction of a range of installation tools such as disc alignment tags, flange mounted installation locks (fig. 2) and RFID detection, such occurrences are almost eliminated.
It is widely encouraged to install a burst detection system to work alongside your pressure relief device. This is the only safe and reliable way to get a notification that the product has functioned.
You can clearly see when the buckling pin has been activated, as it has only two conditions: straight and buckled. However, it still relies on an employee noticing this state and doing something about it.
When it comes to rupture discs, listening for the disc burst is not a reliable method. You are therefore encouraged to install burst detection. Modern magnetic and reed switch burst detectors are non-invasive to the process and therefore not subject to spurious alarms, back pressures or damage during installation, unlike original designs. Such detectors give an instantaneous notification of a burst, and can even be wired to a control room to shut down a process as a further safety measure.
This brings numerous advantages, namely helping to get your process started quickly and minimising lost production. Secondly, by stopping the process it ensures that no dangerous gases are leaking into the environment. Thirdly, it enhances overall safety.
Cost of device maintenance
A company’s quality manual will outline the regular and routine maintenance a system requires. Reducing and improving maintenance schedules plays a critical part in improving site efficiencies.
Largely maintenance-free, buckling pins can be inspected without breaking the line. Replacement pins can also be installed in a matter of minutes without disrupting the line.
Inspection of rupture discs involves removing the disc from its holder. However, the introduction of installation tools means quick and simple checks of the rupture disc state can be done without stopping the line, making the maintenance of both solutions equally as rapid and as environmentally friendly. Some rupture discs now come with a standard 3-year warranty, increasing maintenance intervals accordingly.
Rupture discs vs BPRV: the verdict
While some are often overlooked, both rupture discs and buckling pin relief valves offer numerous benefits — but which should you choose?
There are a number of points for consideration, including line size, initial and long-term investment. As modern day designs offer comparable performance characteristics, considerations based on an application would make for a better decision-making process, rather than worrying about these common misconceptions that on the whole have been eradicated through continued advancements.