What are the options for selecting air-operated control valves for air-open and air-close operation?
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Today, the temperature is -7 degrees Celsius; freezing of the instrument air caused the control valve for the 3.5 MPa steam supplied by the heat power plant to close, almost leading to a shutdown of the entire plant. I would like to ask whether, for a pneumatic valve like this, an air-open valve or an air-close valve should be chosen. From a safety perspective, should an air-close valve be selected so that the valve remains fully open when there is no air supply?
It should be designed in accordance with the requirements of safe production; usually, the original design is already mature. As for the water content in the instrument air supply, it should be addressed first. There are strict regulations regarding the water content in instrument air; dry air or dry nitrogen should be used as the gas for instruments. Using its air as a source of instrument gas, apart from the risk of low-temperature freezing, it also causes corrosion and blockages in the actuators, which can lead to accidents in production. Therefore, addressing the moisture content in the instrument air supply is the top priority. It’s really because the moisture freezes the valve at a certain opening position, so no matter how it’s set, it’s pointless. For reference. Note: Whether water is present in the instrument air supply is related to faults in the drying and purification system, such as adsorbent poisoning or leaks in the regeneration switch valves.
. The faulty open or faulty closed mode of the control valve is selected as needed. The basis for selection is usually two: security requirements or availability requirements. Generally, when safety and availability cannot be balanced (when a control valve operating in fault open or fault closed mode can only meet one of these conditions), safety should be given priority. The steam inlet valve should be closed, as a safety measure – in the event of earthquakes, fires, explosions, power outages, gas shortages, or other situations that cause the valve to malfunction ； From an availability perspective, if the steam supply must be maintained at all costs, fail-open should be chosen. .
Whether the control valve operates in a fault-open or fault-close mode is determined based on process safety requirements. In your case, a position maintaining valve can be installed to maintain the valve position of the control valve in the event of a failure in the instrument air supply.