For continuously operating devices, what should be the selected mode in the event of a failure in the control valve?

It depends on your process requirements; for example, if pressure buildup is not allowed and the valve needs to be vented in case of a failure, then a gas shut-off valve should be used

Based on safety considerations, generally, if the equipment itself has safety measures in place, the emergency inlet and outlet valves will close in case of an accident; if such measures are not available, then the outlet valve will close sequentially downstream. Different processes may require different approaches

Selection is made in accordance with process requirements; the general principles are: (1) Production safety must be given top priority. That is, when the air supply is interrupted, or the controller fails to produce any output, or the diaphragm of the control valve ruptures and causes air leakage, resulting in the control valve being unable to function properly and the valve element returning to its initial state without power (an air-operated valve returning to fully closed position, and a air-locked valve returning to fully open position), it is necessary to ensure the safety of the production equipment and prevent accidents from occurring. For example, in the case of boiler feed water control valves, to prevent the boiler from being damaged in such situations, air-operated shut-off type control valves should be used. (2) Starting from ensuring product quality. When the control valve returns to its initial position due to a loss of power, the quality of the product should not be compromised. For example, the reflux flow control valve in distillation columns is often of the air-operated type; in the event of an accident, the control valve opens fully, putting the process into a full-reflux mode. This prevents the evaporation of defective products, thereby ensuring the quality of the product at the top of the column. (3) From the perspective of reducing losses in raw materials, finished products, and energy. Control valves used to regulate the feed to a distillation tower are often of the air-operated type; once the control valve loses power, it remains closed and stops feeding material to the tower, thereby preventing waste. (4) Considering the characteristics of the medium. The heating steam control valve at the bottom of a distillation tower is usually selected to be of the air-operated type, to ensure that it remains fully closed in the event that the control valve loses power, thereby preventing waste of steam. However, if the kettle liquid is a material that tends to solidify, crystallize, or polymerize easily, a pneumatically operated valve should be used to ensure that the valve closes in the event of a loss of power, thereby preventing steam from entering and causing the liquid in the kettle to crystallize and solidify.

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