Dual valves on the high-pressure to low-pressure pipelines for ammonia synthesis
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In ammonia synthesis plants, the pipelines that carry fluid from the high-pressure system to the low-pressure system are usually equipped with double valves, such as those for venting, draining, and oil removal. There are two opinions regarding what kind of valve should be used for dual valves. Theory 1: A high-pressure stop valve is used for the first one, while a high-pressure throttle valve is used for the second, as the throttle valve can better control the flow rate and pressure downstream of the valve. The second theory is that high-pressure shut-off valves are used for both valves; since a dual-valve system is employed here to prevent high pressure from leaking into the low-pressure system and causing accidents, shut-off valves are used in both cases. Throttle valves are prone to wear and leakage, so only globe valves can be used. Both of these explanations seem to make sense on their own; which one is more reasonable when compared?
A high-pressure stop valve can be used; it can also serve as a throttle. Since flow control is not a primary concern here, using a stop valve is sufficient, and there is no need for a special throttle valve.
A high-pressure shut-off valve will suffice; during normal production, to vent air it is sufficient to close the first valve – only if the first valve is not airtight does it become necessary to close the second valve as well. The same is true for the oil drain valve ； During normal operation, the ammonia discharge valve remains fully open; when shutting down the plant, it is sufficient to close the first valve. Only if the first valve is not airtight does it become necessary to close the second valve as well. Using a stop valve is fully sufficient to meet the requirements. There is no need to use more expensive stop valves.
The dual valves here also need to take pressure drop into account. The pressure drop before and after the valve can reach as high as 21 MPa. If only globe valves are used, what could happen during actual operation if workers make careless mistakes? Could this lead to high-pressure gas or liquid entering the low-pressure pipeline system and causing damage? Although venting leads to the atmosphere, evacuation occurs in a low-pressure sealed system; inadequate pressure relief can have disastrous consequences.