Pneumatic valve

Double-acting actuators depend on the safety design; valves such as those used in heating furnaces should be designed to shut down in case of a failure... So single-acting cylinders seemingly don’t require double-acting actuators... I really don’t think there’s any need for them... It’s just a shut-off valve – is there also a control valve connected in series to regulate it?

It’s just this one valve, which also has a regulating function

This is a double-acting positioner equipped with a hold valve. So the question is, is the cylinder single-acting or double-acting? In the case of a single-acting cylinder, a position maintaining valve can ensure that it remains in its original position in the event of a failure; then why is a position maintaining valve still needed for a double-acting cylinder? The double-acting cylinder already remains in place when the air supply is cut off. Judging from the connection method shown in the picture, it seems to be a double-acting cylinder; I’m not quite sure. The original poster should provide more detailed information about the operating conditions so that a proper understanding can be achieved.

That’s right, just as you said – it’s a double-acting cylinder with a position control valve. I can’t understand why we need to use this when a single-acting cylinder together with a positioner would suffice; it was the experts in our factory’s technical department who chose this option

The intention of the selector was likely to have a double safety layer: firstly, a double-acting cylinder combined with a double-acting valve positioner is sufficient to ensure position retention. Secondly, the pressure holding valve can also ensure pressure retention in the event of a gas supply failure. It offers high safety, but this comes at the cost of availability; if the position holding valve fails, the actuator regulation may no longer function. So, in terms of the overall design, which aspect should be given priority? Furthermore, this valve does not involve interlock issues.

I disagree with the view above; using a double-acting cylinder along with a safety valve is completely unnecessary – it doesn’t constitute any form of double protection. It should be remembered that every additional component to a control valve represents an extra point of failure, so adding a safety valve when a double-acting cylinder and a positioner are sufficient is nothing but redundant.

Based on the image posted by the original poster, it seems that two levels of protection are intended to be implemented; in that case, the relief valve is somewhat redundant. If it is three-stage protection, even with a relief valve in place, another solenoid valve should also be added.

We are also a valve manufacturer, and our valves – including those with actuators for the inlet vanes of distribution compressors – come with this kind of configuration: double-acting actuators, double-acting positioners, and hold-down valves. Why is this choice made? If we consider the valve in isolation, then what you say is correct; but when the valve is placed in a specific operating environment, without a hold-down valve, will the double-acting actuator really remain in its original position in the event of a loss of air pressure? For structures such as eccentric butterfly valves and compressor inlet vanes, please take into account the forces exerted by the medium; in such cases, is the actuator in a free state? Will the valve disc move under dynamic hydrodynamic torque? But by adding a check valve to hold back the gas, there’s no problem.

【Reply to Floor 4】 We are also a valve manufacturer. Our valves, including those with actuators for the inlet vanes of distribution compressors, come with this type of configuration: double-acting actuators, double-acting positioners, and hold-down valves. Why is this choice made? If we consider the valve in isolation, then what you say is correct. But when the valve is used in a specific application scenario, without a hold-down valve, will the double-acting actuator really remain in its original position in the event of a loss of air pressure? For structures such as eccentric butterfly valves and compressor inlet vanes, please take into account the forces exerted by the medium; in such cases, is the actuator in a free state? Will the valve disc move under dynamic hydrodynamic torque? But by adding a check valve to hold back the gas, there’s no problem.

One of these is a pressure relief valve with a positioner for regulating purposes, while the other is a shut-off valve that comes equipped with a solenoid valve for switching. There’s no issue with dual locators, right? It only achieves dual functionality, but the adjustment function definitely doesn’t work well!