What is the classification and function of check valves?

May. 15, 2020

Check valve plays a very important role in the pipeline, the general pipeline will use the check valve, so what is the function of the check valve in the pump?

Consequences of not installing check valve in water pump:

Due to the sudden change of water flow in the water outlet pipe of the pump, such as the opening of the valve, the closing of the valve and the stopping of the pump, the sudden change of flow rate in the pipe is caused, and the change of momentum in unit time is caused, and the corresponding inertial force is bound to be generated, thus causing the alternating change of the sharp rise and sudden drop of pressure in the pipe. This change in flow velocity and pressure with time and position is called water hammer (or water hammer).

API FLANGE CAST STEEL SWING CHECK VALVE

API FLANGE CAST STEEL SWING CHECK VALVE

The water hammer of pumping station has starting water hammer, closing valve water hammer and stopping pump water hammer (formed due to sudden power failure and other reasons). Under normal operation procedure, the first two water hammers will not cause problems endangering the safety of the unit. The water hammer pressure value formed by the latter is often very large, leading to accidents.

The purpose of studying water hammer and calculating water hammer is:

(1) to formulate the maximum water hammer pressure on the pipeline and the unit caused damage to the protective measures;

(2) to draw up the minimum water hammer pressure in the pipeline caused by the negative pressure is not allowed, and lead to the pipeline damage when the protection measures;

(3) prevent the damage caused by the reversal of the unit.

In a pumping system, the characteristic of the pump is the boundary condition at the beginning of the pipe. If the pump outlet is not equipped with a check valve and the pipeline outlet is not equipped with a tap door, the water hammer process (or hydraulic transition process) occurs when the pump stops in an accident, the pump loses its driving force, and the gate valve on the outlet side of the pump cannot be closed in time.

1. Pump working condition

After the pump loses power, the speed of the pump drops sharply, and the water flow in the pump and outlet pipe continues to move in the original direction due to inertia. However, the flow rate decreases rapidly and the pressure drops until the water flow stops flowing in the positive direction and the flow rate is zero. At this moment, the water pump is still positive, the water flow is positive (from the water pump to the direction of the water flow out of the pool known as the current), known as the pump working condition.

2. Braking condition

For the transient static water body with zero flow rate in the outlet pipe, due to the action of the static head of the outlet tank, the reverse water flow from the outlet tank to the water pump is generated in the outlet pipe. The reverse water flow ACTS as a brake on the pump impeller which is still in full rotation, forcing the rotor speed of the unit to accelerate and decline until it reaches zero. At this moment, due to the positive rotation of the impeller against the countercurrent resistance, the pump outlet pressure gradually increased. The instantaneous transient condition of the pump is called the braking condition.

3. Turbine operating conditions

With the increase of the reverse flow rate, the rotation speed of the pump starts to reverse from zero and rises rapidly. At the same time, the centrifugal force of the rotating impeller on the water flow also increases, hindering the reverse flow.

image.pngAPI FLANGE STAINLESS STEEL SWING CHECK VALVE

API FLANGE STAINLESS STEEL SWING CHECK VALVE

This resistance increases with the acceleration of the impeller reversal, so that the pressure after the pump rises rapidly and reaches the maximum at a certain time, and the corresponding reverse speed also reaches the maximum. When the resistance increases, it also resists the continuous increase of the reverse flow rate, which gradually decreases after reaching a certain maximum value.

Less effect on the impeller of energy, make the reverse speed decreases gradually, until the stable under the action of static head, the water flow in the impeller on the parts of the torque and turning resistance moment balance, the unit at a constant reverse flow rate and rotational speed under the condition of no load work like turbine, water hammer pressure disappear. This transient condition is called turbine condition.

Above is the result of the pump not installed check valve. At the same time, after the pump pressure changes will be in the form of water hammer wave along the water pipes to the pool is shot, reflected by a pool of water, the water form a complicated water hammer in pipeline water hammer Check Valve products can use API Flange Stainless Steel Swing Check Valve and API Flange Cast Steel Swing Check Valve.


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