Installation of drain valves before and after the instrument control valve
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A project under design requires the installation of drain valves between the control valve and the isolation shut-off valve; in other words, drain valves are to be placed before and after the control valve. Is it necessary to install drain valves before and after the control valve on the instrument? Are there any regulatory standards that explicitly require this design?
Previously, vent valves were usually installed only in front of control valves, and those control valves were mostly of the type with low inlet and high outlet. Nowadays, more control valves of the type with high inlet and low outlet are being used, but design conventions have not changed. For hazardous media that require complete displacement, especially those for which the displacement medium has too low pressure to enter the system, we install vent valves before and after the control valve.
It is not necessary to install drain valves separately before and after the control valve
For FC valves, they are installed on both the front and back; for FO valves, only the front one is installed. That’s what UOP stipulates.
Assume that the drainage method is used in practice for the convenience of valve maintenance, under the following circumstances: 1. Pressure relief is required； 2. Hazardous gases or liquids ； 3. Rinse.
SH/T 3012-2011 Code for Design of Metal Piping Layout in Petrochemical Industries
Could you explain it in more detail? Is there a standard basis?
Is it necessary to install a drain valve at the inlet of the safety valve?