Water hammer can also occur in multiphase fluids, which are liquid media that also entrain solids. Examples include mortar or liquid pulp (essentially water transporting pulp fibres). The key factor is that water is the main transport medium in pipework and water can transport shock waves very effectively.

Flashing and Water Hammer

Flash evaporation is a different kind of pressure spike event. Flash evaporation occurs when steam condensate (liquid water) from a steam system accumulates in the piping system. This liquid water can suddenly convert from liquid to vapour with a subsequent volume expansion factor of 400-600 times. Flash vapour needs to be handled in a completely different way. Whilst control is equally important, for the purposes of this paper we will limit our discussion to liquid media and water hammer noise.

Causes of Water Hammer

Water hammer can be caused by improper valve selection, improper valve location, and sometimes improper maintenance. Certain valves, such as swing check valves, tilting flap check valves, and double-door check valves, can also cause water hammer problems. These check valves are susceptible to thumping because they rely on reverse flow and back pressure to push the valve flap back into the seat, which closes the valve. If the reverse flow is strong, such as when a vertical line flows normally upwards, the valve flap is likely to be subjected to a high impact force. The resulting shock will damage the alignment of the valve flap so that it is no longer in full 360 degree contact with the seat. This can lead to leakage and, in the best case scenario, destroy the efficiency of the system. In the worst case, this can cause serious damage to other pipework components.

A localised, sudden pressure drop is at least an annoyance, and one that can cause serious problems. Certain steps can prevent or mitigate water hammer. The first priority is to examine the causes, consequences and solutions.

Hydraulic shock
The most common cause of water hammer is valves closing too quickly or pumps shutting down suddenly. In fact, hydraulic shock is a transient rise in fluid pressure in a pipework system when the fluid suddenly stops. As Sir Isaac Newton observed, objects in motion tend to remain in motion unless acted upon by another force. The momentum of the fluid moving forward will keep the fluid moving in that direction. When a valve is suddenly closed or a pump is suddenly stopped, the fluid in the pipework downstream of the valve or pump will be elastically stretched until the momentum of the fluid is stopped.

Sudden valve closure is usually associated with quarter-turn type valves, more specifically quarter-turn automatic valves. A simple solution is to close these quarter-turn automatic valves more slowly. This works in many situations, but not all. For example, emergency shut-off valves need to close quickly, so other solutions may be needed for these types of applications. More information on valve closing time calculations will be covered later in this article.

Another of the most common causes of water hammer is sudden pump shutdown. In cooling tower applications or mine dewatering, where multiple pumps feed into a common manifold, either a slow shutdown is required, or a in-line silent check valve needs to be installed immediately after the pump. Silent check valves can be very effective in reducing or even eliminating water hammer.

Predicting water hammer pressure peaks
The magnitude of the water hammer pressure peak can be calculated based on a detailed knowledge of the pipework and the medium being conveyed. The actual pressure of water hammer depends on the flow rate of the fluid when it stops and the length of time it takes to stop flowing. For example, assume that 100 gallons of water is flowing through a 2-inch pipe at a rate of 10 feet per second. When the flow is quickly stopped by a rapidly closing valve, the effect is equivalent to an 835-pound hammer striking an obstacle. If the flow is stopped within half a second (which may be the closing speed of the valve), a pressure spike greater than 100 psi of system operating pressure is generated.

The equation to calculate the spike potential amplitude is as follows:

∆H=a/g*∆V

∆H is the change in head pressure

∆V is the change in fluid flow rate

a = speed of sound in the medium

g = the gravitational constant

Example:

a = 4864 feet per second

g = 32.2 feet per second2

∆V = 5 feet per second

∆H is 756 ft (328 psi)

This value assumes the presence of instantaneous valve closure.

Valve Closing Time Calculations
Water hammer is obviously a serious problem in industrial environments, such as wastewater treatment plants or municipal water systems. Contrary to the above example, the average nominal pipe diameter of a bathroom faucet is typically ½ inch, with water pressures ranging from 60-80 psi, delivering about 8-10 gallons per minute. A 6-inch line from a water treatment plant can deliver 900 gallons of water per minute at a rate of 10 feet per second. A 24-inch main can deliver over 12,000 gallons of water per minute, enough to fill a backyard swimming pool in less than two minutes.

The basic formula for valve closing time is: T = 2L/a

T = minimum time in seconds

L = length of straight pipe between the closing valve and the next elbow, tee, or other change piece

For 70°F (21°C) water with 100 feet of straight pipe:

T = 41 milliseconds minimum shutoff time

Consequences of Water Hammer
The consequences of water hammer can range from mild to severe. A common sign is a loud thumping or hammering sound from the pipe, especially after a quick shutdown of the water pressure source. This is the sound made when a pressure shockwave strikes a closed valve, fitting or other blockage with great force. This sometimes loud noise can cause great distress and concern, especially if people are working nearby.

However, the recurring occurrence of water hammer is not just an annoyance. Water hammer can also seriously damage pipes, pipe joints, gaskets and all other components of the system (flow meters, pressure gauges, etc.). During a shock, pressure spikes can easily exceed 5-10 times the working pressure of the system, thus putting a lot of stress on the system. Water hammer causes system joints to leak. It also causes cracking of pipe walls and deformation of pipe support systems. Repairing or replacing damaged pipe components and equipment can be costly. If the leak causes environmental problems, the costs can be staggering.

In most cases, water hammer is considered a safety hazard. The extreme pressure of water hammer can cause gaskets to burst, resulting in the sudden rupture of a pipe. People in the vicinity of such an event can be seriously injured.

Water Hammer Solutions
Depending on the cause of water hammer, there are a number of ways to mitigate the effects of water hammer. One of the easiest ways to reduce water hammer caused by hydraulic shock is to train and educate operators. Operators who understand the importance of properly opening and closing manual or actuated valves can take precautions to minimise the effects. This is especially true for quarter-turn valves such as ball, butterfly and plug valves.

Piping Design Considerations
Water hammer eliminators provide a relief point for pressure spikes caused by water hammer. These piping system components reduce the characteristic noise and synthetic stresses of piping systems by acting as shock absorbers. When properly sized and installed, water hammer eliminators may be an effective solution.

On the other hand, pump output media to long lengths of vertical piping should be avoided. Vertical pipe sections should be minimised or quiet check valves installed as close to the pump as possible should be used.

Another aspect of reducing water hammer is to install check valves in vertical piping. Swing check valves, tilting flaps and double gate valves can operate vertically. However, they will not stop reverse flow in the vertical direction. Only silent check valves will work in this direction.

Hydraulic shocks caused by the sudden closing of the swing check, tilt flap and double gate check valves can be resolved by replacing the swing check, tilt flap and double gate check valves with silent or non-silent check valves. Silent check valves close when the differential pressure through the valve closure decreases, rather than from counterflow. As a result, they are less likely to close suddenly and thus trigger water hammer. The valve is fully closed when the differential pressure across the valve flap approaches the rupture pressure of the valve. This decelerates the fluid flow so that the fluid momentum is reduced before the valve closes completely, while ensuring that the fluid flow is not reversed.

System designers must be familiar with best practices and industry standards for minimising water hammer, such as the use of slow-closing valves where appropriate, an understanding of optimal valve locations in piping systems, and special piping design considerations for high operating pressure systems.

If the piping system is properly designed, the likelihood of water hammer occurring will be greatly reduced, if not eliminated. In systems already in place, the damaging effects of water hammer can be limited in a number of important ways, such as installing water hammer eliminators, moving check valves out of the vertical piping, installing silent check valves as a primary line of defence and ensuring that quarter-turn valves are operated in a procedure that has a slow closing speed. Note that the closing time in an automated system should be at least 10 times the value calculated in the T=2L/a formula.

Conclusion
Water hammer has been studied for many years. Some of the founding research dates back to the late 1800s. Research is still continuing. Many major universities in the United States, the United Kingdom, and the Netherlands, as well as highly respected valve companies, have written articles comparing the dynamic characteristics of various types of check valves and their installations.

This article only scratches the surface of the subject of fluid transients by exploring some of the causes and solutions to what we commonly refer to as water hammer. Solutions to deal with water hammer can be quite expensive and, as always, an ounce of prevention is worth a pound of cure. Pumps and quick valve closures for delivery to vertical lines or common manifolds can all be designed in at the outset. Once the piping is in place and the plant process is underway, the challenge is to find a solution within the specific constraints.

Most manufacturers of pipeline quiet check valves are well aware of the water hammer phenomenon and have engineers on their staff who can help. They may be the best source of knowledge when it comes to the right solution.