First, let's take a look at the consequences of not installing a check valve on the water pump:
The water flow in the outlet pipeline of the water pump suddenly changes due to valve opening, valve closing, and pump stopping, causing a sudden change in flow velocity in the pipeline, which in turn causes a change in momentum per unit time, inevitably generating corresponding inertial forces, resulting in alternating changes in pressure rise and fall in the pipeline. The phenomenon where the flow velocity and pressure of water change with time and location is called water hammer (or water hammer).
Pump station water hammer includes startup water hammer, valve closing water hammer, and pump shutdown water hammer (caused by sudden power outages, etc.). The first two types of water hammer will not cause any safety hazards to the unit under normal operating procedures. The water hammer pressure generated by the latter is often very high, leading to accidents.
The purpose of studying and calculating water hammer is to:
① Develop protective measures when the maximum water hammer pressure causes damage to pipelines and units;
② Propose the minimum water hammer pressure that causes unacceptable negative pressure inside the pipeline, as well as protective measures in case of pipeline damage;
③ Prevent damage caused by unit reversal.
In a pumping system, the characteristics of the pump serve as the boundary condition at the starting end of the pipeline. If a check valve is not installed at the outlet of the water pump and a flap valve is not installed at the outlet of the pipeline, when the pump stops in an accident and loses its driving force, the gate valve on the outlet side of the pump cannot be closed in time, and the water hammer process (or hydraulic transition process) occurs when water flows back in the pipeline.
1. Pump operating conditions
After the water pump loses power, its speed drops sharply. The water flow in the pump and outlet pipeline continues to move in the original direction due to inertia, but its velocity rapidly decreases and pressure drops until the water flow stops flowing forward and the flow rate is zero. At this moment, the water pump is still rotating in the forward direction, and the water flow is in the forward direction (the movement of water flow from the water pump towards the outlet pool is called forward flow), which is called the water pump operating condition.
2. Braking conditions
In a transient stationary water body with zero flow rate in the outlet pipe, due to the action of the static water head in the outlet tank, a reverse water flow is generated in the outlet pipe from the outlet tank to the pump. The reverse water flow acts as a brake on the pump impeller that is still rotating in the forward direction, forcing the speed of the unit rotor to accelerate and decrease until it reaches zero speed. At this moment, due to the resistance of the forward rotating impeller against the reverse flow, the outlet pressure of the pump gradually increases. The transient operating condition of the water pump at this moment is called the braking condition.
3. Turbine operating conditions
As the reverse flow rate increases, the speed of the water pump starts to reverse from zero and rapidly increases. At the same time, the centrifugal force of the rotating impeller on the water flow also increases, which hinders the reverse flow.
This resistance increases with the acceleration of impeller reversal, causing the pressure behind the pump to rapidly rise and reach its maximum value at a certain moment, and the corresponding reverse speed also reaches its maximum value. At the same time as the resistance increases, it also resists the continued increase of the reverse flow rate, and gradually decreases after reaching a certain maximum value.
The energy acting on the impeller also decreases accordingly, causing the reverse speed to gradually decrease until the torque of the water flow acting on the impeller and the resistance torque of the rotating part of the unit are balanced under the stable static water head. The unit works like a water turbine with a constant reverse flow rate and speed under no load, and the water hammer pressure disappears accordingly. This transient operating condition is called the turbine operating condition.
The above are the consequences of not installing a check valve on the water pump.
