Issues with the positioner of the control butterfly valve
Thread Content
There are two installation methods for the positioners of our on-site butterfly valves. 1: Install directly; after multiple tests, 1% do not respond, while those above 1.5% start to respond. 2 ； Mechanical contact installation; it will operate at 0.5% or higher. The precision of the positioners is very high nowadays, so they should be able to meet the requirements. I would like to ask the experts: is this a problem with the valves themselves or with the positioners?
It could be a problem with the valve, or it could be a problem with the positioner. It is necessary to check the specific models and parameters of the butterfly valve and the actuator, verify that the installation is correct, and confirm whether there are any areas of non-functioning due to mechanical clearance or friction. If the valve and actuator parameters are matched and installed correctly, it may be necessary to adjust the actuator’s zero point and range to improve accuracy. If there are still problems, it may be necessary to replace the valve or actuator. .
This is actually quite normal. Firstly, direct installation is inevitable; there will definitely be a gap at the groove where the feedback rod connects to the valve, and a 1% valve position is considered normal. It is connected by mechanical links, with each connection being fixed; in particular, there are spring-connected devices at the feedback arm, so the error is very small. As for the failure of this small valve position to function, it is most likely due to the butterfly valve; when the butterfly valve is fully closed and sealed, the contact area between the sealing surfaces is large, and whether it is a soft seat or a hard-sealed seat, this will cause the small valve position to get stuck and fail to operate. If opening this small valve is really a concern, it is possible to replace the positioner – either one from Metso or Fisher will work – or increase the air supply pressure. Another option is to operate the valve several times beforehand.
First of all, it should be clear that, based on the owner’s description and accompanying drawings, what the owner means is that \"1% results in no reaction, while a level of 1.5% or higher triggers some reaction.\" Mechanical contact installation: it will operate at 0.5% or more – this refers not to control precision but rather to dead zone and hysteresis ； In valve control, a dead zone is inevitable, because even if mechanical transmissions are designed without any gaps, the elasticity and creep of materials can still cause a dead zone to form ； The dead zone affects the control accuracy of the valve, but it does not affect the system’s control accuracy when the system has an integral action. The function of a valve positioner is to eliminate the effect of the pressure difference before and after the valve on its opening degree. When the valve is removed for inspection, there is often little difference in the signal-position relationship of the valve with or without a positioner (for pneumatic valves equipped with pneumatic positioners).
If such high precision is required, butterfly valves are not suitable!