Check valves are a type of valve that make sure water flows only one way through a pipe. The main purpose of the check valve is to prevent backflow, which can occur when the water pressure inside a pipe is greater than the pressure in the surrounding area.
Check valves have many advantages over other types of valves:
1. They are able to withstand high pressures without leaking or breaking. This means they can be used in high-pressure situations where other types of valves would fail due to excessive pressure.
2. Check valves can be used with any type of liquid or gas that flows through them. They do not require electricity in order to operate and do not require any special equipment such as pumps or compressors for operation.
3. The only moving part of a check valve is its ball valve which opens when there is an increase in pressure inside the pipe so that it can allow fluid through but closes when there is a decrease in pressure so that it does not allow any more fluid through it until it has time to adjust again and open again.
Some disadvantages of check valves are mentioned below:
1. Check valves cannot be used for fluids having a high viscosity or density.
2. When there is a pressure difference between two sides, then it can cause leakage through the valve.
3. When there is a large differential pressure across a check valve, then it can cause wear and tear to the body of the valve and also cause damage to the sealing ring inside it.
The check valve should be operated in such a way as to avoid the occurrence of excessive impact pressure and rapid oscillating action of the valve closing member due to the valve closing.
To avoid the formation of excessive impact pressure due to the closure of the check valve, the valve must be closed quickly, thereby preventing the formation of a great backflow velocity, the backflow velocity is the root cause of the impact pressure when the valve is suddenly closed, so the closing speed of the valve should be correctly matched with the velocity decay of the downstream medium.
However, the velocity decay of the downstream medium may vary greatly in the liquid system. For example, if the liquid system uses a group of parallel pumps, and one of the pumps suddenly failed, the check valve at the outlet of the failed pump must be closed almost simultaneously. However, if the liquid system has only one pump, and this pump suddenly failed, and long delivery pipeline, and its outlet back pressure and pumping pressure are low, the use of a smaller closing speed check valve is better.
The rapid oscillating motion of the valve closing member must be avoided to prevent early failure due to excessive wear of the valve moving parts. The rapid oscillating motion can be avoided by determining the valve passage diameter based on the flow rate that is calculated to generate the force that makes the valve closing member move. If the medium is a pulsating flow, the check valve should be placed as far away from the source of the pulsation as possible. Rapid oscillation of the closing member may also be caused by violent media disturbance, and when this exists, the check valve should be placed where the media disturbance is minimal.
Therefore, the first step in selecting a check valve is to determine the working conditions in which the valve is located.
1. the assessment of the fast closing check valve
In most practical use, the check valve can only be used qualitatively for a fast closing, the following can be used as a basis for judgment.
1) The stroke of the closing member from fully open to the closed position should be as short as possible. Therefore, from the point of closing speed, small check valves are faster to close than large check valves of similar construction.
2) The check valve should be closed from the fully open position at the maximum possible downstream media velocity before backflow to get the longest closing time.
3) 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 downstream media velocity reduction. From this point of low inertia, the closing member should be made of lightweight materials, such as aluminum or titanium. In order to balance the lightweight structure and the large closing force, the closing force generated by the weight of the closing member can be enhanced by spring force.
4) Around the closing member, the limiting factor that delays the free closing action of the closing member should be removed.
2. check valve operation of mathematical applications
The application of mathematical methods to the normal operation of the check valve is developed in recent years. For check valves with articulated flaps, foreign check valve manufacturers such as POOL, Porwit, and Carlton provide a calculation method that involves the establishment of the equation of motion of the valve flap and the application of the deceleration characteristics of the fluid medium in the system. Before the equation of motion of the valve flap can be established, some physical constants of the valve must be known. Calculations can be made to determine the backflow velocity of the valve during sudden closure, and also to calculate the impact pressure of the backflow medium caused by the sudden closure of the valve.
Valve manufacturers can use mathematical methods to design and predict the impact pressure according to the important applications of check valves, and it is important for valve users to know this.
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