Rupture discs and buckling pin relief valves (BPRVs) are the two major primary relief devices used to protect a wide range of applications from potentially serious over-pressurisation.
According to ASME VIII Division I, either may be used as a primary pressure relief device, so dependent on the application, process conditions or customer preference, one solution may be selected over the other.
With increased customer expectations on the performance of a pressure relief solution in relation to quality, efficiency, maintenance and the environment, it is not surprising that manufacturers have made considerable technological advancements to these devices.
Worryingly, many customers do not fully understand the advancements in these technologies and selection may be based on out-of-date information. Here, we challenge the common misconceptions of pressure relief technology, discussing the pros and cons of both solutions to give further insight into which pressure relief solution is right for a particular application.
Common misconceptions
With both technologies benefiting from instantaneous, full-bore opening in the event of an over-pressure situation, each offers considerable advantages over relief valves.
However, there is still a concern in the marketplace over certain performance characteristics, including fatigue.
Customers worry that both solutions may fatigue over time, increasing replacement costs and maintenance time.
Firstly, buckling pins cannot fatigue as they are not subject to process conditions and only buckle at the set point.
Traditional, forward-acting rupture discs were more susceptible to this problem, however, the latest reverse-acting solutions outperform the original designs.
Many reverse acting discs have been tested to over 1 million life cycles as well as benefiting from 3% tolerances, meaning 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 three-year warranty compared to the traditional 12 months.
Concerns on tolerances such as manufacturing/operating, temperature and set point have been driving customers to opt for buckling pins in instances where operating as close to the set point as possible is critical.
Modern buckling pins typically offer maximum operating pressures of 95% to 98% of the set point.
Where original rupture discs didn't offer such characteristics, leading manufacturers’ reverse-acting discs are available with 3% tolerances and 97% operating ratios; the benefit of these tighter tolerances are that there is a less dramatic effect on the operation of the rupture disc due to temperature changes and in many instances stock can be rationalised, dramatically reducing purchasing costs.
Dependent on the application, process mediums and conditions can vary significantly. Corrosion is a real concern for many customers who want to avoid their investment failing early and increasing long-term costs.
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.
The buckling pin valve itself is typically sourced from materials compatible with the process medium, although this is typically limited to carbon or stainless steel. Although 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.
Although rupture disc devices are in contact with the process medium, they can be manufactured from a wide range of highly non-corrosive materials including; graphite, Hastelloy and tantalum.
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.
Using discs in combination with modern burst detection systems can ensure that if a disc fails safe due to corrosion an instantaneous alert can be made prompting for 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.
Installation concerns
Correct installation of pressure relief devices is a real concern for customers operating in safety-critical environments.
With many incidences reported by the HSE being related to ‘human factors' it is imperative manufacturers offer designs that remove this issue.
For buckling pins, it is almost impossible to install the pin improperly.
Commonly, customers 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 and RFID detection, such occurrences are almost eliminated.
Notification of function
Most manufacturers now encourage customers to take a burst detection system alongside the pressure relief device; this is the only safe and reliable way to get a notification that the product has functioned.
Although with the buckling pin it is visually apparent that the pin has activated, due to it having only two conditions – straight or buckled – it still relies on an employee noticing this state and doing something about it.
For rupture discs, companies cannot rely on the fact that they will hear the disc burst and 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 burst, and can even be wired to a control room to shut down a process as a further safety measure. This has several key advantages: firstly, it ensures a customer can get their process up and running a quickly as possible so they are not missing out on lost production; secondly, by stopping the process it ensures that no dangerous gasses are leaking into the environment; and thirdly, it enhances overall safety.
High maintenance costs
Most customers undertake regular and routine maintenance in line with their company's quality manual.
Reducing and improving maintenance schedules plays a critical part in improving site efficiencies. Buckling pins are relatively maintenance free and offer the major advantage that they can be inspected without breaking the line.
Replacement pins can also be installed in a matter of minutes without disrupting the line.
Although there is not a visible indication of rupture disc state without removing the disc from its holder, with the introduction of installation tools, 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 three-year warranty, increasing maintenance intervals accordingly.
Which solution?
As both rupture discs and BPRVs offer benefits that were not commonly understood, there is still the challenge of selecting the appropriate device for the application. Considerations of the size of the line and initial investment versus long-term investment can play a major role in this decision-making process. As modern day designs offer comparable performance characteristics, considerations based on an application would make for better decision making than worrying about these common misconceptions that on the whole have been eradicated.
