Converting a on/off valve into a control valve
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Currently, there is a pneumatic on/off valve (with a butterfly valve body) and a pneumatic control valve (also with a butterfly valve body). The on/off valve has a larger valve body as well as a larger cylinder; the air supply output from the actuator of the control valve is now connected to the input of the cylinder of the on/off valve (mainly because the actuator is too small to be installed directly on the cylinder of the on/off valve). As a result, regardless of the set opening degree, the butterfly valve remains fully open； When the opening degree is 0, the butterfly valve is also in the fully open position; it will only close after a power outage, presumably after more than half a minute. What could be the reason for this? Is there something wrong with the plan to replace the on/off valve with a control valve?
High, really high – you can think of anything. Can the cylinders of on/off valves and control valves be equivalent? Disassemble both types of cylinders and study them carefully.
Experiments are being conducted inside the factory, and there aren’t large enough control valves available; therefore, temporary modifications had to be made. It turns out that these modifications don’t work, and it is suspected that the difference in cylinder design is the cause. Are there any experts with experience in this area?
The principles of cylinders and diaphragm heads are different; a cylinder can be modified into a diaphragm head by adding a spring inside it.
This post was last edited by Benbang Solenoid Valve – Engineer Li on 2019-5-18 at 11:48. Both valves are pneumatic butterfly valves with angular stroke. It’s just that the control valve has an additional valve positioner; you connect the output pipe of the positioner to the switch butterfly valve and then try to make adjustments – but if it actually works, that would be amazing. The positioner controls the amount of air intake in order to determine the opening degree of the valve. If the cylinders have different sizes, how does the positioner regulate the air volume? It’s through feedback! Simply put, you send a signal, such as to open it to half position; the signal transmitter supplies air to the cylinder, causing it to move. Air is supplied continuously during this process until the sensor in the transmitter detects that the valve has reached the desired position, at which point the transmitter automatically stops supplying air to the cylinder, and the valve is properly positioned. Right now, you’re simply connecting the outlet of the positioner directly to the on-off type cylinder. When you send a signal, the positioner doesn’t receive any feedback regarding the valve’s position, so it keeps supplying air to the cylinder, resulting in the valve being either fully open or fully closed! The correct approach is to remove the actuator and install it on a switch-type pneumatic butterfly valve, or purchase a new actuator and install that instead (the old one cannot be used with the air cylinder; it’s not because the actuator is too small or incompatible, but rather because the mounting bracket is not suitable. Using the appropriate bracket will do; actuators are not designed to match specific cylinder sizes – they are universal, and each cylinder requires its own corresponding bracket). But you need to learn how to adjust the locator. For intelligent locators, you can figure it out by reading the instructions; as for mechanical ones, it’s better to ask the manufacturer for remote assistance, as they are not user-friendly for beginners! PS: It takes over half a minute after power is cut before the valve shuts down automatically. Perhaps the pneumatic switch butterfly valve is of the single-acting type; even though power is off, the air remains in the cylinder and does not escape, continuing to push against the spring. Eventually, the air leaks out slowly and the spring resets, causing the valve to close!
I don’t know what device is being used. So it’s hard to say. But based on the description, there are two possible reasons for the failure of the modification: one is using the locator as a converter ； Second, the feedback direction of the locator is reversed.
All that’s needed to change a on/off valve into a control valve is to install a positioner; it’s definitely a problem with the installation of the positioner. Pay attention to the matching between the air inlet and the cylinder
Take a picture of how you installed it and send it; those with experience will understand at a glance
This post was last edited by hhk9801 on 2019-5-18 at 11:21. The reply on floor 5 is quite comprehensive. If the valve body of the on/off valve is larger, it cannot be connected simply; the corresponding cylinder must have sufficient torque. It’s better to choose a adjustable pneumatic actuator with matching connection dimensions and replace it, otherwise it simply won’t work.
The old one won’t fit on the large cylinder; it’s not that the locator is too small or incompatible, but rather that the bracket is not suitable – just use the appropriate bracket. Positioners do not come in different sizes for cylinders; they are universal in nature, and the corresponding cylinders use the appropriate brackets