Issue with the installation of manual valves before and after the control valve.
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Dear seniors, I would like to ask why some control valves have manual valves on both the front and back, while others have them only on the front or back, and yet there are those without manual valves on either side? Are there any specifications or standards regarding the installation of manual valves before and after the control valve?
If there are no manual valves, how can the valve be repaired if it breaks?
If you look closely at the PID diagram, there are still many control valves that do not have manual valves.
I have only seen two-piece valves, none of the other types
Hand valves are available at both the front and back: In a continuous production system, operations must not be halted. If the control valve fails, a bypass can be used to maintain production; the valves located before and after the control valve should be closed, and a drain valve should be opened to remove the material, allowing for the repair of the valve； Some have hand valves only at the front or the back: only at the front, for the same reason as mentioned earlier; there is no pressure behind the control valve, and under normal conditions there is no fluid present. Only afterward can it be possible to shut off the valve before the control valve; this way one valve can be saved. There is a risk of backflow of substances from downstream side after the valve ； Some systems do not have manual valves at either end: they operate in a batch mode, allowing for shutdown at any time; or, if the quality of the valves is reliable, no bypass valves are needed (most foreign projects do not use bypass valves).
There seems to be no standard for this; it depends on the maintenance requirements set according to the manufacturing process. If the control valve is damaged, it does not affect production, so you can set it however you like. If there is a high frequency of use, then maintenance settings are necessary, including those for the drain valve!
Okay, thank you, it was explained very clearly. :handshake
Okay, thank you. :handshake
One more thing: foreign designs do not include bypasses for control valves; this is part of the normal operation approach, and it reflects foreign design concepts. However, we later added bypasses to those devices that didn’t have them, as we felt it was more practical. Not having manual valves before or after the control valves is indeed intended to reduce costs and the risk of leaks, but if such considerations are not a concern, it’s still advisable to install manual valves, with some exceptions.