Problems with control valves
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I have a disc valve here; it doesn’t operate when the opening degree is below 20, and once the signal exceeds 20, it opens with a loud noise to reach the desired degree of opening. Pressure fluctuations often cause sequential shutdowns. Please analyze the possible reasons for this
I encounter such problems often; there are quite a few valves of this type in our facility. Based on my analysis, the possible reasons are as follows: 1. The medium is at high temperature or pressure, which causes the valve stem to not operate properly. 2. The positioner is not functioning correctly; to resolve this issue completely, it’s necessary to try using a new positioner. 3. Try adjusting the pre-tightening force of the valve
The technician upstairs said it very well; I’d like to add a few points: 1. The packing was compressed too tightly, which caused this issue with the control valve. 2. The O-ring of the cylinder piston is damaged. 3. A foreign object is stuck at 20% of the valve core, causing the control valve to operate inflexibly.
1. The actuator of a butterfly valve is generally piston-type, and it has a certain dead zone of its own. 2. The actuator needs to overcome the frictional force of the packing, the force exerted by the fluid, and the pre-tensioning force of the spring itself. 3. For the reasons mentioned above, great care must be taken when adjusting the positioner; the zero point must not be set too low, as the valve will move as long as there is a signal output. 4. Butterfly valves are not suitable as control valves. ’
The valve core is stuck; it is recommended to remove it for inspection
1. Is there a stuck part in the valve itself, causing it to not operate smoothly? 2. Remove the actuator and try to operate the valve manually; if the valve moves smoothly, then the problem lies with the positioner. Try a different locator. But remember to make marks on the valve stem; otherwise, after the testing is done you won’t know the position of the valve core, which will be quite troublesome and will require disassembling the valve. Remember this! 3. Then remove the actuator and test it separately; try turning it on and off with a wrench first. Based on my experience, in this case, the problem is most likely to lie with the valve itself
Is it membrane-type or cylinder-type? What about the temperature of the process medium? 1. The pneumatic actuation mechanism has air leaks ； 2. The pressure difference before and after the butterfly valve is high ； 3. The valve disc of the butterfly valve is stuck, resulting in high resistance; it needs to be repaired or replaced ； 4. For large-diameter butterfly valves, the flow capacity of the positioner is inadequate (too small); it is necessary to add a pneumatic amplifier after the positioner.
Reasons: 1. Large pressure difference before and after the valve; 2. Fault in the valve itself; 3. Issues with the air supply pressure; 4. Problems with the positioner adjustment
The valve itself and the positioner both need to be checked
1. Re-adjust the valve positioner. 2. Check the configuration settings to determine whether a 20% start value is defined
On one hand, there is a problem with the spring inside the cylinder; on the other hand, the tolerance fit between the disc and the valve seat is poor. There may also be foreign objects attached to the disc or the valve seat, and it’s possible that there is sticking in the transmission and sealing parts.