What is water hammer? How to prevent water hammer from damaging pumps?

 

This article will describe the situations that are most likely to cause water hammer problems and the issues that pump and piping designers and operators may face. It will also outline some methods for addressing these issues.

 

 

What is water hammer?

Water hammer occurs when the flow rate of a fluid in a pipe changes rapidly. It is also called "shock flow". It can cause extremely high pressure in the pipe, huge pressure on the pipe supports, and even sudden backflow. It can cause pipe bursts, damage to supports and pipe racks, and leaks at joints.

Water hammer can occur in any fluid, in any pipe, but its severity depends on the specific conditions of the fluid and the pipe. Water hammer usually occurs in liquids, but it can also occur in gases. It can cause pipe ruptures and building collapses.

 

How does water hammer occur?

Water hammer is a shock wave caused by a sudden change in flow that is transmitted down the pipe. The most common causes are valves that close too quickly or pumps that trip or start suddenly. This creates a shock wave that starts at the valve or pump, travels down the pipe, and changes the velocity of the fluid. This is what causes high pressure. If the shock wave is large and passes through a pipe elbow, the step change in pressure causes an unbalanced force that moves the pipe. This can cause the pipe to break free of its supports or transmit forces to the pipe anchors. The pressure wave can travel through the pump and damage the impeller and drive.

 

If I have water hammer, how do I fix it?

A quick fix is ​​to slow down the valve speed a lot if it is a valve problem! A rule of thumb is "1 second per inch of pipe diameter" - but I think this is not a good approach and recommend 5 seconds per inch of pipe diameter, or even longer. If it is a pump problem, you can install a multi-turn valve (such as a gearbox for butterfly valves and ball valves) and let the operator operate it slowly.

However, a better approach is to model the system using suitable software so that all potential solutions can be tested in the model, allowing designers to choose the best and most cost-effective solution for their piping system. This should be mandatory for long pipelines (such as more than 500 meters in length) that carry toxic or flammable substances.

 

Solutions depend on the specific situation. They include the following:

1. Eliminate the cause of the hammering.

Some causes can be resolved by eliminating or controlling the equipment in question. In addition to the equipment discussed previously, this may include vibration relief valves, rapid emergency shutoff valves, and some manual valve closing devices, such as butterfly valves. Soft starters can help resolve some water hammer problems caused by pumps.

2. Reduce pumping speed.

This can be accomplished by using a larger pipe diameter or a lower flow rate.

3 Make the pipes stronger.

This can be expensive, but it may be a solution if the pipe specifications are only slightly exceeded.

4. Slow down valve speeds, or use valves with better discharge characteristics in the pipe system.

5. Use surge tanks. Surge tanks allow liquid to leave or enter the pipe when water hammer occurs, and are usually only used in water supply systems.

6. Use surge relievers. These are similar to pulsation dampeners commonly installed on positive displacement pumps, but are larger.

7. Use a pump flywheel. A flywheel can be used when the pump is spinning too fast after a water hammer trip.

8. Use pressure relief valves. These valves are not suitable for toxic substances unless a collection system is provided.

9. Use air inlet valves. Air inlet valves are not suitable if air or other possible foreign substances are not allowed to enter.