Difference between pressure tapping before and after a self-acting control valve:
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The operating modes of the two types of valves differ: pressure-reduction valves regulate pressure after the valve (Type B), while pressure-relief valves regulate pressure before the valve (Type K). Pressure measurement is taken after the valve in the case of pressure-reduction valves; gas-operated control valves are used for this purpose. Under normal conditions, the valve remains open, but when the pressure behind it rises, the valve closes to reduce that pressure. Pressure tapping before the valve is used for pressure relief; a air-operated control valve is selected, with the valve remaining closed under normal conditions. When the pressure in front of the valve rises, the valve opens to release pressure. Therefore, pressure sampling before the valve applies to air-operated open valves, while pressure sampling after the valve applies to air-operated closed valves; their structures are different and they cannot be used interchangeably.
Pressure measurement before the valve stabilizes the pressure at that point ahead of the valve, while pressure measurement behind the valve stabilizes the pressure at that point behind the valve. There is a big difference between the two.
In other words, the pressure measurement taken behind the valve results in stable pressure there, while the pressure measurement taken in front of the valve functions as a safety relief valve, keeping the pressure in front of the valve at a set value?