Regarding the isolation valves and bypasses before and after the control valve
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I would like to ask: 1. When it is necessary to service or adjust the control valve, should one \"first shut off the rear valve, open the bypass, and then shut off the front valve for servicing\"? Or some other order, what should be taken into consideration? What is the purpose of the shut-off valve behind the control valve? 2. If there is no bypass, only a control valve and upstream and downstream isolation valves are available, what is the sequence of operation, and what factors need to be taken into account? Please explain. Thank you so much.
1. Turn on the bypass first to enable continuous production without stopping. 2. Without a bypass, there is no need to cut off the supply; thus, there is no such thing as an operation sequence
Regarding the second question: What is the purpose of having two shut-off valves in a configuration of “→shut-off valve — control valve — shut-off valve→” without a bypass?
If the valve needs to be repaired or replaced, only this section can be isolated while leaving the rest unchanged, resulting in minimal impact.
For the first question, the order of disconnection doesn’t matter; either sequence is fine. When servicing the control valve, it is necessary to relieve pressure first through the drain valve. For the second question, refer to the sequence for the first question.
Without a bypass valve, the significance of the isolation valves before and after the control valve is not very great
In continuous production systems with bypasses, it is generally done by first slowly opening the bypass, then slowly closing the control valve, and subsequently isolating the control valve for maintenance. Those without bypasses are usually intermittent or have little impact on main production, and they don’t significantly affect the sequencing of isolation using upstream and downstream valves
Are bypass and shut-off valves installed so that maintenance doesn’t disrupt production? The sequence must be to activate the bypass first and shut off the upstream valve at the same time, in order to minimize the impact on flow rate; after making the adjustments, then deal with the downstream valve, and finally address any issues that arise!
Well said; the original poster deserves to adopt their own advice.