How does a single-acting shut-off valve achieve fail-safe operation?
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As the title suggests, to achieve fail-safe operation for a single-acting shut-off valve, is it sufficient to add a holding valve?
It is difficult to achieve the FL function with a single-acting design, because even if a check valve is used to stop air intake, the cylinder of the actuator cannot be sealed tightly enough and will still leak air gradually. Once leakage occurs, the pressure on the air side becomes lower than that on the spring side, causing the spring to compress the cylinder and accelerate the exhaust of air. As a result, the air is discharged very quickly, and it’s difficult to determine exactly how fast this happens; one can only find out through experimentation. The FL function is generally implemented in a double-acting manner.
A fault retention buffer tank is used, and the volume of this fault tank is calculated based on the gas volume
If the original design requires that the fault position remain at FL, then why not simply install a double-acting cylinder? After all, the original function of a double-acting cylinder is FL, but if it is already a single-acting cylinder, the simplest and most effective way to turn it into a double-acting cylinder is to remove the spring inside the cylinder.
1. The air cut-off fault can be maintained by installing a safety valve, but this mechanism will fail if the air chamber leaks (as mentioned on the 2nd floor; however, normal operation does not involve any leakage). 2. Even when using double-acting cylinders, safety valves must be installed on the air inlets of both chambers, as in practical applications, the action of the fluid and the pressure imbalance between the two chambers of the cylinder make it difficult to maintain the valve position. The claims regarding the 3rd and 4th floors are definitely incorrect; the spring in the actuator is used both to provide pre-tensioning force and to balance the pressure in the air chamber, and it cannot be removed.
As a supplementary note, the method of removing the spring is only applicable to piston-ram type single-acting cylinders; springs cannot be removed from diaphragm-type cylinders, and protective valves must be installed instead.
Applicable only to piston-type rack cylinders, with springs at both end caps that can be removed when there is no air supply.