Tighter, quicker, better
A guide to bursting discs for the modern engineer

Bursting discs are changing with the times. Today’s bursting discs are available with a wide choice of rated burst pressures, tight burst pressure tolerances, zero fragmentation, and reliable burst detection.

In recent years, requirements for efficient, flexible process plant have forced plant engineers to look beyond the simple bursting disc and consider pressure-relief systems that are compatible with wider business issues. Traditionally, process plant was set up to run with a view to long-term stability. Today, the challenge is to increase flexibility. As a company’s business needs change, so do its plants and processes: plants switch to different products, processing conditions are altered, and different process operation steps are switched in and out of line. All of this takes place while new health, safety and environmental standards are enforced with increasing strictness. These factors have driven bursting disc manufacturers to improve their designs, and broadened the criteria for choosing bursting discs.

Bursting disc types
Bursting discs are categorized as forward- or reverse-acting, depending on whether the pressure acts on the concave (forward) or convex (reverse) face. Discs have conventionally been forward-acting, but as application demands have increased, manufacturers have started switching to reverse-acting designs. This is because, during normal operation, a reverse-acting design can support pressures much closer to its rated burst pressure than a forward-acting design. The ratio of a disc’s maximum continuous operating pressure to its rated burst pressure is called the operating ratio. Forward-acting discs typically have operating ratios of 80–85 percent, while reverse-acting discs can have operating ratios as high as 95 percent. The ability to operate at pressures closer to the vessel’s maximum can increase process yield without the need to buy a new reactor. Traditional disc designs are of the type known as fragmenting. When fragmenting discs burst, as the name suggests, pieces of the disc are released into the downstream pipework, along with the contents of the vessel. This can damage downstream equipment, including secondary pressure-relief valves, and complicate attempts to recover the vessel contents. An example of a fragmenting disc is the graphite bursting disc. Figure 2 shows the end-on view of a graphite disc as it bursts, photographed at 2000 frames per second. Frame 1 shows the intact disc. By frame 6, just 2.5 ms later, the pipe bore is fully open for venting.
Modern disc designs, both forward- and reverse-acting, are non-fragmenting. This is achieved by making the discs from metal or other non-brittle materials, and deliberately adding lines of weakness (“scoring”) so that the disc ruptures in a predictable manner without breaking into pieces.
Areas and discharge rates
In 2003, ISO 4126 started to replace the long-standing ISO 6718, and other standards such as BS 2915, as the European reference for pressure relief devices. ISO 4126 has seven parts, of which two (parts 2 and 6) apply directly to bursting discs. ISO 4126 can be used to demonstrate compliance with the essential safety requirements of the Pressure Equipment Directive (PED; 97/23/EC). Under the PED, bursting discs are classed as safety equipment and fall into Category IV, requiring the involvement of a government-notified body to review areas such as design and quality systems. Some of the most important information in ISO 4126 concerns the calculation of discharge flowrates. Knowing the discharge flowrate under given conditions, and comparing it with the required rate as calculated from a risk assessment, allows the disc to be sized so as to ensure a safe system. ISO 4126 classifies bursting disc systems as either “simple” or “complex”. In the former, the size of the bursting disc itself is the controlling factor that determines the discharge flowrate, and the calculations are relatively straightforward. A system is simple if it has:
-discharge directly to atmosphere;
-an upstream pipe length of less than eight pipe diameters from the vessel to the bursting disc, and a downstream pipe length of less than five pipe diameters;
b-bursting disc open area measuring at least 50 percent of the pipe cross-sectional area;
-nominal pipe diameters, both upstream and downstream of the bursting disc, equal to or greater than that of the bursting disc;
-single-phase flow; and
-a vessel nozzle.
Systems that do not meet all these requirements are classed as complex, and require iterative calculations to determine the discharge rate.
Burst pressure accuracy
The pressure at which a bursting disc ruptures depends on its thickness and the material of which it is made. Since it is not practical to manufacture disc membranes or “foils” in an infinite range of thicknesses, however, disc suppliers have developed ways to fine-tune the burst pressure to the exact value required.
In the case of reverse-acting discs, the burst pressure is generally set by the point at which the disc’s shape collapses, rather than by the limiting strength of the foil. Coupled with the introduction of computer-controlled production equipment, this has enabled bursting discs to be offered with a burst-pressure tolerance of 63 percent, a vast improvement on historical values of 610–15 percent. This increased accuracy can be used to deliver commercial benefits in addition to the obvious performance benefits. For example, it is not untypical for a plant to have two bursting discs in parallel, designed so that their burst pressure ranges overlap. It may be possible to replace these discs with a single disc of tighter burst pressure tolerance, with a saving in capital costs and inventory.
Burst detection
With today’s huge plants and increasing reliance on automation, it is becoming increasingly important to indicate disc bursts remotely. Disc burst indicators have been available for many years, but the original type based on the breaking of an electrical connection across the face of the disc tend to be unreliable. A better solution is the non-invasive burst detection system shown in Figure 3, which is based on magnetic field sensing. This system is mechanically robust, and is Atex-approved for use in Zone 0 areas. It also allows the operation of the detector to be checked while the process is running, simply by unscrewing the sensor from the disc holder so as to increase the distance between sensor and magnet.
Short lead times
When a bursting disc operates, the priority is usually to get the process running again as soon as possible once the system has returned to a safe condition. Most process operators therefore carry a stock of spare discs so that they do not have to wait for delivery from the manufacturer.
But with increasing emphasis on cutting the costs of inventory management, better approaches are starting to appear. The use of advanced manufacturing methods means that bursting disc manufacturers can now deliver discs in just two weeks, compared to historical lead times of 12–16 weeks. With these dramatically shorter delivery times, plus rationalization of bursting disc requirements made possible through tighter burst pressure tolerances, many process operators can now reduce their bursting disc stocks by two-thirds or even more. Modern bursting discs also offer: reliable protection against overpressure, with very fast response times; a very wide range of burst pressures, with tight tolerances; short delivery times; and built-in burst detection.