Bursting discs for the modern engineer
July 1st 2007

In recent years, requirements for an 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. Ben Rutter, development engineer, Elfab looks at the issue

Traditionally, process plant was set up to run with a view to long-term stability. Today, the challenge is to increase flexibility, and as a company's business needs change, so do its plant 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 standards, both health and safety and environmental, are introduced and enforced with increasing strictness.

This has driven disc manufacturers to improve disc designs and increased focus on a broader range of selection criteria than may have been previously considered.

Fragmenting/non-fragmenting design Discs are available in fragmenting and non-fragmenting designs. The older, more traditional, disc designs have all been fragmenting. With these disc-types, when a burst event takes place, fragments of the disc are released into the downstream pipework, along with the contents of the vessel. This may cause fouling of downstream valves, such as secondary pressure-relief valves, and prevent them from properly re-seating, while also complicating attempts to recover or reprocess vessel contents.

An example of a fragmenting disc-type is the graphite bursting disc. The following photographs were taken by a high-speed video camera capturing images at 2000 frames per second. The view is 'end on', effectively looking down the bore of the pipe, from the outlet (atmospheric) side.

The Arma-Gard graphite disc is the black area in the centre of the picture, with a lighter coloured holder surrounding it. In the sequence of images, it is very easy to see the rupture point and subsequent fragmentation.

Using the set of images (below, left), it is possible to measure the response time of the bursting disc to the over-pressure event. Frame 1 is a fully closed, integral disc and frame 6 shows that the disc has completely opened and is relieving the excess pressure via the full bore of the pipe. The time interval, or disc response time, is 5/2000 seconds or just 2.5ms.

Modern bursting-disc designs (both forward- and reverse-acting) use scoring (the addition of lines of known weakness to the disc) and similar methods to 'retain' the disc membrane and deliver nonfragmenting discs.

Burst pressure accuracy The coarse control of the burst pressure is typically the choice of thickness and material type used for the disc membrane. However, as disc membrane materials ('foils') do not come in an infinite range of thickness in each of the material types required for chemical compatibility, manufacturers of bursting discs have developed techniques to fine-tune the burst pressure of a particular disc. For example, developments in reverse-acting disc design aim to trigger a burst due to a collapse of shape, rather than reaching a limiting stress value of the foil. Coupled with the introduction of computer-controlled production equipment, this has enabled bursting discs to be offered with a burstpressure tolerance of ±3%, an improvement on historical values of ±10-15%.

During the filming of the high-speed video, a pressure transducer and data logger (set at 20 scans per second) were used to record the pressure profile. The results obtained from three reverse-acting, non-fragmenting Opti-Gard discs that featured a rated burst pressure of 200 psi (15.2 barg) were 15.2, 14.7 and 15.6 barg, all within the specified 3% tolerance for this disc.

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 one or two spare bursting discs in stores, waiting for an over-pressure event or a planned maintenance interval, at which point the inservice disc is removed and one of the spare discs used as a replacement. A process plant with two separate bursting discs specified at differing nominal burst pressure (Discs A & B). If a new Disc C is specified with a tighter burst tolerance, it may be possible to replace both Discs A and B, thus offering the possibility of rationalising stock holding requirements with the attendant operational and financial benefits.

Burst detection Plant control systems and staff increasingly demand remote indication of a disc burst event (not least due to the size of modern processing plants!). A non-invasive burst detection system, based on magnetic fieldsensing, has been introduced which is also ATEX approved for use in Zone 0 area (Figure 4). This replaces the older style of in-line membranes featuring wires / electrical circuits within the process pipe and the related problems with reliability.

Several benefits accrue from this advancement in burst detection. The known fragility of the membrane burst detectors has been eliminated with a mechanically robust detector. The operation of the detector can be checked while the process is running and simply requires the sensor to be unscrewed from the disc holder to increase the distance between sensor and magnet.

Conclusion Through a combination of different disc designs, materials and sizes, virtually any burst pressure can now be catered for. As a leading manufacturer of bursting discs, explosion panels and detection systems, Elfab is actively advancing bursting disc and ancillary component design and manufacturing techniques. Used in tandem, these developments have resulted in a truly modern bursting disc range which offers:-

Reliable safety protection (with very fast response times) against over-pressure events Highly accurate burst pressures and tight tolerances Built-in burst detection Short manufacturing lead times Stock rationalisation at user facilities

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