SAFETY IN THE PLANT
relief by allowing air to pass through a unique membrane to relieve minor variations in pressure.
“Rupture discs are traditionally installed for their excellent leak-tight performance under normal plant operating
conditions. However, to improve reliability and longevity, the new requirement is for a rupture disc that will allow air fl ow in and out of a sealed container when a small pressure diff erential arises due to temperature change without the disc bursting,” explains Brazier.
ENGINEERING A NOVEL SOLUTION
To meet the development
challenge, BS&B’s engineering team focused on the unique properties of sintered PTFE fi lm, which is produced in sizes, quantities and thicknesses suited to the construction of low-pressure
rupture discs. T e PTFE fi lm can produce rupture discs in sizes from under 1in burst diameter to over 6in burst diameter (DN25 to DN150) using plastic fi lm down to 0.1mm in thickness. T e unique fi lm is processed to achieve a specifi c level of porosity that allows air to pass through at a pressure diff erential of a few inches of water column/millibar but prevents passage for fl uids like water until much higher diff erential pressures of around 0.5 bar are reached.
Constructing a rupture disc from
only breathable PTFE fi lm solves the low-pressure fl uctuation challenge due to
temperature. However, adding a second level of safety by having such a rupture disc burst due to a greater over or under pressure condition is a further challenge since the mechanical properties of the fi lm are too strong to meet the low burst pressure requirements of atmospherically stored equipment; typically below 7.5psi (0.5 bar). According to Brazier, variations in ambient temperature can also lead to unacceptably large tolerances in burst pressure from a plastic fi lm acting alone as the pressure relief member. A simple, fl at PTFE membrane does not make a good rupture disk due to wide burst pressure spread, sensitivity to temperature, and potential fatigue. However, the fact that the porous material could allow air fl ow in either direction – without allowing liquid through – remained a desirable characteristic. T e BS&B engineering team resolved the second level of safety issue by combining the structural and thermal stability of a traditional metal domed rupture disk with the PTFE fi lm. Under normal operating conditions, the metal support arch holds the PTFE dome, and air is free to pass in either direction. In the event of an overpressure event, the metal support arch collapses and the dome is cut by a blade on the downstream side to relieve the pressure. Brazier notes that this is possible with burst pressure capability as low as 2in water column/5 millibar. “By combining the proven reliability and accuracy of metallic rupture disc components with the calibrated air/gas transmission performance of the PTFE
fi lm, a dual function rupture disc device can be constructed. Normally, this permits air fl ow to maintain atmospheric pressure due to daily temperature changes. For safety, the disc activates at a predetermined set pressure in one or two directions [positive pressure or vacuum relief ],” says Brazier. T e dual function pressure equalisation
and relief device is ideal for use on low- pressure vessels that are often below the design limits of the PED and ASME Standards. To accommodate the size and weight limitations of many light duty atmospheric storage containers, the rupture disc is designed in an integral assembly that includes the disc as well as housing. T e integrated assembly is certifi ed to perform at the desired set pressure and allows for easier installation and quick removal if the rupture disc is activated. T e device is designed for installation between sanitary/ aseptic fi ttings or traditional pipe fl anges, or a custom installation confi guration can be accommodated. Although process industry operators and OEMs have long relied on rupture discs in their hydraulic and pneumatic equipment, low-pressure environments susceptible to ambient temperature changes have been a challenge. Fortunately, with the availability of rupture discs that can “breathe” to provide pressure relief while still bursting open when a true overpressure condition arises, users can signifi cantly enhance equipment safety, reliability and product quality, while reducing maintenance expense.
An innovative “breathable” rupture disc technology has been developed
The unique fi lm is processed to achieve a specifi c level of porosity that allows air to pass through
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