Does a double-acting cylinder on a valve that maintains the fault position also require a positioning (air lock) valve?
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I saw on a project that the double-acting cylinders were equipped with check valves, on-off valves, as well as large-diameter control butterfly valves – all of which were designed to maintain their position in case of a failure. An double-acting cylinder doesn’t it have no spring? Even in the absence of air, there is no spring to act on the valve spool; then what is the purpose of adding a retaining valve (air lock valve)?
Think about it the other way around: a double-acting cylinder doesn’t have springs or holding valves, so the medium will push the valve element (valve plate) to move, as there is no force present in the cylinder
Thank you for the answer! Well, if a ball valve is used as a switch, this kind of situation shouldn’t occur; when it is fully open or fully closed, the flow of the medium does not affect the valve ball. But in the case of regulating ball valves (V-ball valves), when there is no pressure, the valve ball is in an intermediate position; the medium then pushes the valve ball. The same principle applies to regulating butterfly valves and on/off butterfly valves, as well – the medium pushes the valve disc in these cases as well.
I can only say that this is to give you a starting point; specifically, you need to use your process parameters to consult the valve manufacturers. Two-stage cylinders equipped with air storage tanks are quite common; as for lock valves, it might be due to my limited experience – I’ve never seen two of them. I’ve experienced before a double-acting switch valve losing air suddenly; the valve would flutter and stay at a certain opening degree. So, in this case, an air reservoir is indeed the more reliable option