Problems with SIS valves
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This post was last edited by sdwfsgyykai on 2019-6-26 07:49. Does a SIS valve need to be in a safe state only when the gas supply is lost? Is it still necessary to use it in conjunction with a power source? One of the SIS valves on site is a gas-actuated valve that remains fully open during normal operation; however, it stays open even when the gas supply is lost. It only closes when there is a power outage. Is such a design reasonable? How does it manage to do that? Please share your experience. Thank you
It’s unreasonable. In the event of a gas supply failure and/or an electrical signal failure, the valve should move to a safe position.
The SIS valve must remain in a safe state whether it loses its gas supply, power supply, or electrical signal. The safe condition of the valve is determined by the process flow of the production facility, and it may be reasonable or unreasonable.
This post was last edited by sdwfsgyykai on 2019-6-26 07:48. Please don’t be so irresponsible – everyone knows there are only two possibilities. What does it mean to say something is plausible or not? What the original poster needs is a definite judgment! Under normal circumstances, in the event of a power outage or loss of gas supply, one should be in a safe state; what the original poster is doing is clearly unreasonable
Check what the design requires; it’s not necessarily the case that the shut-off valve must be opened or closed – it’s possible that a retaining valve has been installed. It is also a design requirement that the valve closes in case of power loss; the interlock mechanism comes into action.
Correct judgment is predicated on certain premises. Pneumatic valves can be in three states—open, closed, or holding position—when there is no air supply ； When the solenoid valve is powered off, it can be in three states: intake, exhaust, and closed. Among the various combinations of the two, which one is appropriate depends on the circumstances; it’s not the case that only one particular combination is suitable in all situations. For example, it is desired that the pneumatic valve maintain its position when there is a loss of air but not power, and open when both air and power are lost; the condition described by the original poster is reasonable ； But it would be unreasonable to set it this way even in the absence of such a requirement.
Whether this is reasonable or not depends on considerations related to process safety. If, after analyzing the process, it can be deemed acceptable for the valve to remain in its original state when there is no gas supply, then it is reasonable. In your case, however, I personally think that it’s more likely to be unreasonable; after all, the gas supply has been cut off, which constitutes a type of fault, and the valve should be in a state that ensures process safety
In my opinion, the design is reasonable; the actuation mechanism following interlock of this control valve is carried out by the solenoid valve in the instrument air line, and it is this solenoid valve that enables the operation of the SIS system. When the instrument air pipeline is interrupted, it does not mean that there is no air pressure inside the valve; the valve will remain open for a certain period of time, and it will gradually close once the pressure is depleted. After the solenoid valve operates, it quickly releases the air pressure inside the valve diaphragm, triggering the interlock action. Instrument air interruption and emergency start/stop are different concepts.
Both power loss and air loss should occur in a safe state
A gas supply failure refers to the loss of instrument air, with its pressure dropping to 0. The gas supply failure you mentioned probably means that there is still pressure in the pipeline network of the instrument air system.
For example, if you close the air supply ball valve connected to the valve, this is not considered an air supply failure; since there is still pressure in the instrument air between the air supply ball valve and the valve, the valve will gradually move as a result of the leakage of that instrument air.