There is a sentence in API 521 that I’m not sure how to interpret
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To determine the relief loads, it may be assumed that manually operated and remotely controlled valves that are normally open and functioning at the time of inadvertent closure or failure, and that are not affected by the primary cause of failure, remain in operation at their normal operating positions.
It is recommended that you rephrase this sentence by splitting it into two sentences, which will make it easier to understand: 1. To determine the relief loads, it can be assumed that the manual and remotely operated valves remain in their normal operating positions. 2. The manual and remotely operated valves are normally open and functioning at the time of accidental closure or failure, and they are not affected by the underlying cause of that failure.