The switching valve (or forced valve) is a pneumatic control valve installed at the hot end of a switching heat exchanger (or cold storage unit). The valve is actuated by compressed air controlled via a solenoid valve. Depending on its function in the process, there are air/nitrogen switching valves, pure nitrogen suppression valves, and nitrogen three-way valves. Structurally, switching valves are either vertical or horizontal. Vertical valves can be further divided into normally-open and normally-closed types. Since the valve action is controlled by a solenoid valve, attention should be paid to selecting the correct type and coordinating the solenoid state with the valve to prevent hazards in case of power failure.

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1. Nitrogen Switching Valve: Closes when the four-way solenoid valve is energized and opens when de-energized. In case of power or air source failure, it opens by gravity to ensure venting through the cold storage unit, preventing overpressure.

2. Air Switching Valve: Opens when the four-way solenoid valve is energized and closes when de-energized. In case of control power failure, the valve closes; if the control air source fails, it closes by gravity to prevent raw air from entering the air separation tower.

3. Nitrogen (or Pure Nitrogen) Three-Way Valve: When the four-way solenoid valve is energized, the three-way valve is in the delivery position; when de-energized, it goes to the venting position. In case of power or air source failure, the three-way valve defaults to venting.

Vacuum Valves are valves operating below atmospheric pressure. Two common connection types exist:

1. Loose Flange Connection: The most common. According to mating surface shapes, there are:

  1) Smooth Type: For low-pressure valves, easy to manufacture.

  2) Raised/Concave Type: For higher pressure, uses medium-hard gaskets.

  3) Tongue-and-Groove Type: Uses deformable gaskets, widely used with corrosive media, providing good sealing.

  4) Trapezoidal Groove: Uses elliptical metal ring gaskets, suitable for working pressure ≥64 kg/cm² or high-temperature valves.

  5) Lens Type: Lens-shaped metal gaskets, suitable for working pressure ≥100 kg/cm² or high-temperature valves.

  6) O-Ring Type: Developed with various rubber O-rings, offering more reliable sealing than standard flat gaskets.

2. Threaded Connection: Simple method, used for small valves, with two sealing methods:

1) Direct Seal: The inner and outer threads themselves seal. Sealing is often enhanced with lead paste, hemp, or PTFE tape, which is highly corrosion-resistant and easy to use and remove.

2) Indirect Seal: The tightening force of the threads is transmitted to a gasket between two flat surfaces to achieve sealing.