Quote:
Originally Posted by Homsar66
So I'm confused as to the need for an additional burst disc.
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A burst disk is a completely different type of pressure relief device than what we're discussing. It's not really a valve. It's a membrane (usually metallic) that is designed to rupture at a precise pressure (although in practice there can be a large range of variability between discs) and allow the working fluid to pass, reducing the entrained pressure of the rest of the system. Once the disk bursts, it has to be replaced in order to seal the system again.
A relief valve will [theoretically] re-seat and re-seal once the pressure has been adequately relieved. In practice, there are usually some interesting dynamics that occur when a relief valve opens. If improperly selected, they can actually hammer themselves apart as a result. Often, they don't seal fully again once unseated. When you throw combustible cryogenic fluids into the mix, things can get even more exciting (not applicable to FRC).