I What is a water hammer?

Water hammer is called water hammer because of the inertia of pressure water flow when the power is suddenly cut off or the valve is closed too fast, which produces water flow shock wave, just like a hammer hitting.

The water hammer of pump station includes starting water hammer, valve closing water hammer and pump stopping water hammer.

Under normal operating procedures, the first two water hammers will not cause problems endangering the safety of the unit.

The water hammer pressure formed by the latter is often very large, resulting in accidents.

II Stop pump water hammer

Stop pump water hammer refers to the hydraulic impact phenomenon of pressure rise and fall caused by the sudden change of flow velocity in the pump and penstock when the valve is opened and stopped due to sudden power failure or other reasons.

For example, due to the failure of power system or electrical equipment, accidental failure of water pump unit and other reasons, valve opening and shutdown may occur, resulting in pump shutdown water hammer.

The maximum pressure of the water hammer when the pump is stopped can reach 200% of the normal working pressure, or even higher, which can destroy the pipeline and equipment, and cause "water running" and water stop in general accidents;

The serious accident caused the pump house to be flooded, the equipment to be damaged, the facilities to be destroyed, and even the personal casualty accident.

III Elimination measures

There are many protective measures for water hammer, and different measures shall be taken according to the possible causes of water hammer.

1. Reduce the flow rate of water transmission pipeline

It can reduce the water hammer pressure to a certain extent, but it will increase the pipe diameter and increase the project investment. During the layout of water transmission pipeline, hump or sharp change of slope shall be avoided as far as possible.

After emergency pump shutdown, the pump shall be started after the pipeline behind the check valve is filled with water.   

When starting the pump, do not fully open the outlet valve of the pump, otherwise it will produce great water impact.

2. Set water hammer elimination device

(1) Constant pressure control technology is adopted:

PLC automatic control system is adopted to control the frequency conversion speed regulation of the pump.

As the pressure of water supply pipe network changes with the change of working conditions, PLC is used to feed back the start and stop of water pump and speed regulation through the detection of pipe network pressure, so as to control the flow, so as to maintain the pressure at a certain level, avoid excessive pressure fluctuation and reduce the probability of water hammer.

(2) Install the water hammer eliminator

The equipment is mainly used to prevent water hammer from stopping the pump. It is generally installed in the outlet pipeline of the water pump and uses the pressure of the pipeline itself as the power to realize low-pressure automatic action.

That is, when the pressure in the pipeline is lower than the set protection value, the drain will automatically open to release water and pressure, so as to balance the pressure of local pipeline and prevent the impact of water hammer on equipment and pipeline.

(3) Install slow closing check valve on the outlet pipe of large diameter water pump

It can effectively eliminate the water hammer of pump shutdown, but because there is a certain amount of water backflow when the valve acts, the suction well must have an overflow pipe.

Generally, the valve is closed 70% ~ 80% within 3 ~ 7S after power failure, and the remaining 20% ~ 30% closing time is adjusted according to the conditions of water pump and pipeline, generally in the range of 10 ~ 30s.

(4) Bypass pipe (valve) is set in the pump station

During the normal operation of the pump system, the check valve is closed because the water pressure at the water pressure side of the pump is higher than that at the water suction side.

When the pump is suddenly stopped due to power failure, the pressure at the outlet of the water pump station decreases sharply, while the pressure at the suction side rises sharply. Under this differential pressure, the transient high-pressure water in the suction main pipe is the transient low-pressure water that pushes the check valve plate to flow to the pressurized water main pipe, and increases the low water pressure there;

On the other hand, the pressure rise of water hammer on the suction side of the water pump is also reduced. In this way, the water hammer rise and pressure drop on both sides of the water pump station are controlled, so as to effectively reduce and prevent the harm of water hammer.  

(5) Set multi-stage check valve

In the longer water transmission pipeline, multiple check valves are added to divide the water transmission pipeline into several sections, and check valves are set on each section.

When the water flows back in the water transmission pipe during the water hammer process, each check valve is closed one after another to divide the backwash flow into several sections. Because the hydrostatic head in each section of the water transmission pipe is quite small, the water hammer pressure rise is reduced.