These are thin diaphragms held between flanges and calibrated to burst at a specified static inlet pressure. Unlike relief valves, rapture discs cannot reseal when the pressure declines. Once the disc ruptures, any flow into the vessel will exit through the disc, and the disc must be replaced before the pressure vessel, can be placed back in service. Rupture discs are manufactured in a variety of materials and with various coatings for corrosion resistance.
The most common disc materials are aluminum, monel, inconel, and stainless steel, but other materials or coatings, such as carbon, gold and plastic, are available.
Rupture discs may be used alone, but they are normally used as a backup to a relief valve set to relieve at approximately 115% MAWP, This ensures that the disc ruptures only if the relief valve fails or in the unlikely event that the pressure rises above 110% MAWP and the relief valve does not have enough capacity.
Rupture discs are also used below relief valves to protect them from corrosion due to vessel fluids. The rapture disc bursts first and the relief valve immediately opens. The relief valve reseals, limiting flow when the pressure declines. When this configuration is used, it is necessary to monitor the pressure in the space between the rupture disk and the relief valve, either with a pressure indicator or a high pressure switch. Otherwise, if a pinhole leak develops in the rapture disk, the pressure would equalize on both sides, and the rupture disk would not rapture at its set pressure because it works on differential pressure.
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