The sudden change of the velocity of flow in the pipe caused by the sudden change of the opening, closing and stopping of the pump in the outlet pipe of the pump, which causes the change of the momentum in the unit time, inevitably produces the corresponding inertia force, thus causes the pipeline pressure sharp rise and the sudden drop alternating change phenomenon. This phenomenon of water flow velocity and pressure change with time and position is called Water Hammer (or water hammer) .
There are three types of water hammer
1. Pump condition
When the power of the pump is lost, the rotational speed of the pump drops sharply. The water flow in the pump and the outlet pipe continues to move along the original direction because of inertia, but the flow rate decreases rapidly and the pressure drops until the water flow stops and the flow rate is zero.
2. Braking conditions
In a transient stationary water body with zero flow in the outlet pipe, due to the action of the static head of the outlet pool, a reverse flow from the outlet pool to the pump is generated in the outlet pipe. The reverse flow acts as a brake on the pump impeller which is still turning, forces the unit rotor speed to accelerate down until the speed is zero.
3. Working condition of hydraulic turbine
With the increase of the reverse flow rate, the speed of the pump starts to reverse from zero and rises rapidly. Meanwhile, the centrifugal force of the rotating impeller to the flow also increases, which hinders the reverse flow. This resistance increases with the acceleration of the impeller, which makes the pressure after the pump rise rapidly and reach the maximum value at a certain moment, and the corresponding reverse speed also reaches the maximum. When the resistance increases, it also resists the continuous increase of the reverse flow, and decreases gradually after reaching a certain maximum value. The energy acting on the impeller is correspondingly reduced, causing the reverse speed to decrease gradually until the torque acting on the impeller and the resistance moment acting on the rotating part of the unit are balanced under the action of a steady static head, unit with constant reverse flow and speed in the absence of any load, like a water turbine, water hammer pressure disappeared.
The best solution to the water hammer problem is to install a check valve