That is, the set pressure must be sufficiently adjusted to ensure that the relieving pressure isn't greater than the tank's design presure. So, the correct statement for such applications (API 650 tank which has a tailpipe on the relief device) is to say that the tailpipe friction loss must be accounted for. In this example the relieving pressure is going to exceed the allowable limit (tank design P) if the friction loss in the outlet pipe is equal to the set pressure (80% of the tank's design P). For example, say the relief device is set at 80% of the tank's design pressure, which is perfectly OK as long as the relief device reaches the necessary relief flow at 20% overpressure. The set pressure can be any value as long as it results in preventing the relief pressure from exceeding the tank design pressure. i think you're probably assuming that the set pressure can't exceed 50% of the tank's design pressure, but that isn't true. That's because there is no prescribed set pressure for these devices. It's correct to say that API 650 tanks are not allowed any pressure accumulation (the tank's design pressure is the peak allowable pressure), but that isn't automatically accomplished by limiting the outlet pipe's friction loss to the relief device set pressure. No, that statement isn't sufficiently correct.
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