Operating check valves should avoid the occurrence of excessively high impact pressure and rapid oscillation of the closing part of the valve due to the closing of the valve. In order to avoid the excessively high impact pressure caused by the closing of the check valve, the valve must be closed quickly to prevent the formation of a fast reverse flow velocity. The reverse flow velocity is the root cause of the impact pressure when the valve is suddenly closed. Therefore, the closing speed of the valve should correctly match the velocity reduction of the downstream medium.
However, the velocity reduction of downstream media can vary greatly in liquid systems. For example, if two parallel pumps are used in a liquid system and one of the pumps suddenly fails, the check valve at the outlet of the failed pump must be closed almost simultaneously. However, when there is only one pump in the liquid system, the pump suddenly fails. The pipeline is long; the backpressure and pumping pressure at the outlet end is low; it is better to use a check valve with a lower closing speed.
The rapid oscillating movement of the valve's closing part must be avoided to prevent excessive wear of the valve's moving parts, resulting in early failure. The valve diameter is determined by calculating the flow rate that generates the force to move the valve's closing part to avoid the occurrence of rapid oscillating movement. If the medium is a pulsating flow, the check valve should be placed as far away as possible from the source of the pulsating source. The rapid oscillation of the closing member may also be caused by severe media disturbance. When this situation exists, the check valve should be placed at the place where the media disturbance is minimal. Therefore, the first step of selecting a check valve is to determine the operating conditions of the valve.
Evaluation of check valves with quick closing
In most practical applications, check valves can only be used for quick closing qualitatively, and the following can be used as a basis for judgment:
- The stroke of the closing member from fully open positions to close positions should be as short as possible. Therefore, from the perspective of the closing speed, the closing speed of a small check valve is faster than a large check valve of a similar structure.
- The check valve should be closed from the fully open position at the maximum speeds of downstream media before the reverse flow to obtain the longest closing time.
- The inertia of the closing member should be as small as possible, but the closing force should be appropriately increased to ensure the fastest response to the deceleration of downstream media. From the point of low inertia, the closing member should be made from lightweight materials, such as aluminum or titanium. In order to take into account the lightweight structure and great closing force, the closing force generated by the weight of the closing member can be enhanced by spring force.
- Around the closing member, the limiting factors that delay the free closing action of the closing member should be eliminated.
The mathematical application of operation of check valves
The application of mathematical methods to the normal operation of check valves has been developed in recent years. For check valves with hinged discs, some foreign check valve manufacturers provide a calculation method that involves the establishment of the motion equation of the disc and the application of the deceleration characteristics of the fluid medium in the system. Before establishing the motion equation of the valve disc, some physical constants of the valve must be known. The reverse flow speed of the valve can be determined by calculation when the valve is suddenly closed, and the impact pressure of the reverse medium caused by the sudden closing of the valve can also be calculated.
Valve manufacturers can use mathematical methods to design and predict the impact pressure according to the important application of the check valve. Knowing this is very important for valve users.
