Valve stem movement during control valve pressure testing
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Today, I accompanied a third party to oversee several control valves. These valves are 8”-900LB in size; during the hydrostatic testing of the valve housings, the valves were left in a semi-open position as pressure was applied； As the pressure increases, the valve stem moves upward gradually; at around 8 MPa, the valve is no longer under the control of the positioner and remains fully open. I’m asking experts for advice – is this normal? Additionally, this is the control valve for the pressure relief valve; when the pressure drops to 11 MPa, the valve remains in its highest position and will not close. Could any expert advise me on what’s going on? I am extremely grateful!
I don’t know what kind of valve that is yours – is it ANSI standard? If 150 lb is converted to 1 MPa, then it is normal for the valve to become uncontrollable when the pressure exceeds 8 MPa, as this represents an overload. Even during pressure testing, the pressure is usually applied at 1.1 times the value indicated on the valve body.
The valve is a US standard control valve; with a nominal pressure of 8\"-900\" LB, it should be around 15 MPa. Shouldn’t the shell test be conducted at 1.5 times the nominal pressure? The operating pressure is between 8.5 and 11 MPa; if the valve cannot be closed in this pressure range, how can flow be regulated then?
150 lb is equal to 1 MPa; the conversion value should be 6. According to national standards, it should be 6.4. That’s right – the valve body has a strength of 1.5, while the sealing element has a strength of 1.1
The operating pressure is significantly higher than the valve body pressure; the valve body pressure should be one level higher, and then an actuator should be selected.
The pressure test of the valve body requires the valve to be fully opened in order to check for any external leaks at 1.5 times the pressure rating. In the case of a valve being in a semi-open state, as you mentioned, it is necessary to push the valve back, and that means the actuator has to overcome that pressure; it’s a matter of overcoming a pressure difference. It doesn’t seem to mean the same thing, man
This valve can no longer meet your requirements. I guess it’s either a selection issue or a problem with the design of the valve itself.
Full shut-down pressure test: at the required pressure, the valve should not move; if it does, there is a problem with the valve.