Rupture Discs For The Modern Engineer
In recent years, requirements for an efficient, flexible process plant have forced plant engineers to look beyond the simple rupture 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, and so 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 increase focus on a broader range of selection criteria than might have been previously considered.
(Photo caption: Elfab’s Opti-Gard™ rupture disc in a holder)
The two main types of rupture discs are categorised as forward- or reverse-acting, depending on whether the pressure forces are acting on the concave (forward acting) or convex (reverse acting) faces. The ratio of maximum (plant) operating pressure divided by the rated burst pressure is called the operating ratio of the rupture disc. Discs have historically been forward-acting, typically having operating ratios of 80 to 85%, but as application demands have increased, manufacturers have started switching to reverse-acting designs, with operating ratios as high as 95%. This is because, during normal operation, a reverse-acting design is able to support pressures much closer to its rated burst pressure than a forward-acting design. This can be utilised by plant operators to increase the pressure of their process steps, which in turn could deliver an improved yield without the need to purchase a completely new reactor.
In 2003, a new series of ISO standards (ISO 4126 entitled ‘Safety Devices for Protection against Excessive Pressure’ parts 1 to 7) were introduced. The standards are recognised product standards which may be used to demonstrate compliance with the essential safety requirements of the Pressure Equipment Directive (PED) (97/23/EC). Under the PED, rupture discs are classed as safety equipment and fall into Category IV, requiring a government-notified body to review areas such as design and quality systems.
Parts 2 and 6 are directly applicable to rupture discs and present methods for calculating discharge flow rates through the disc. This figure can then be compared with the required discharge rate evaluated in a risk assessment. The calculations are separated into simple and complex systems. The definition of a simple system is as follows:
1) Discharges directly to atmosphere
2) Upstream pipework less than 8 pipe diameters from vessel; downstream pipework less than 5 pipe diameters
3) Rupture disc has an open area at least 50% of pipe area
4) Nominal upstream & downstream pipework diameters are greater or equal to the rupture disc nominal diameter
5) Flow is single phase (solid, liquid or gas)
Calculating discharge-flow-rate capacity of a rupture disc in a simple system is a relatively straightforward process that, due to the definitions of a simple system, assumes that the rupture disc is the controlling restriction to discharge. In a complex system, an iterative process is required in which a full analysis of the discharge pipework is undertaken to determine the pressure drops and flow resistances through the discharge pipework system.
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, there have been limits to the tolerance being achieved, especially for forward-acting discs. 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 rupture discs to be offered with a burst-pressure tolerance of ± 3%, a vast improvement on historical values of ± 10-15%.
This increased accuracy can be used to deliver commercial benefits in addition to the obvious performance benefits. When a rupture disc operates after an over-pressure event, eliminating the cause and replacing the disc quickly is of the utmost importance. The traditional approach to coping with these events is to purchase spare discs for each installation and retain them in stock in an ongoing basis. As commercial pressures on process plants increase, operating expenses must be cut at every opportunity and the cost of stock is becoming significant. Not only does stock tie up cash, it incurs as stock has to be stored, physically protected and managed. Releasing this cash by managing down the value of stock is becoming a useful technique to increase financial efficiency.
With tighter tolerance discs and the use of advanced manufacturing methods bringing significant reliable lead time reduction, it has been found that a typical facility can reduce the variety of spares by up to 65% while maintaining the service levels required of a modern operation.
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 field-sensing, is also becoming standard; this is also ATEX approved for use in Zone 0 areas. This system replaces the older style of in-line membranes featuring wires/electrical circuits within the process pipe, and combats the reliability issues suffered by these traditional technologies. The latest improvement to this technology has seen the introduction of wireless communication to further improve efficiencies by reducing the need for expensive electrical wiring and thermal sensors located with the disc.
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