Water Hammer

Normally the pressure wave is dampened or dissipated in a very short amount of time, but the pressure spikes can do enormous damage during that brief period.

Water hammer is evidenced by a thumping or banging sound that, in extreme cases, can indicate that extensive and costly damage is occurring to expansion joints, pressure sensors, flow meters and pipe walls.

Water hammer also can occur in a multiphase fluid, which is a liquid media that also has entrained solids. An example would be sand slurry or liquid pulp (which is basically water transporting the pulp fibers). The key factor is that water is the main transport medium in the piping system, and water can transmit shock waves very effectively.

FLASHING VS. WATER HAMMER

Flashing is a different kind of pressure spike event. Flashing occurs in steam systems where steam condensate (liquid water) has accumulated within the piping system. This liquid water can suddenly convert from a liquid to a steam with a subsequent volumetric expansion factor of 400-600 times. Flashing needs to be dealt with in totally different ways. While it’s equally important to control, for purposes of this article, we confine our discussions to liquid mediums and water hammer noises only.

CAUSES OF WATER HAMMER

Water hammer can result from improper valve selection, improper valve location and sometimes poor maintenance practices. Certain valves, such as swing check valves, tilting disc checks and double door check valves also can contribute to water hammer problems. These check valves are prone to slamming because they rely on reversing flow and backpressure to push the disc back onto the seat so that the valve closes. If the reverse flow is forceful, as in the case of a vertical line with normal flow upwards, the disc is likely to slam with a great deal of force. The resulting shock can damage the alignment of the disc so that it no longer makes full, 360-degree contact with the seat. This leads to leaks that, in the best case, undermine the efficiency of the system. In the worst case, this could do serious damage to other piping system components.

Localized, abrupt pressure drops are an annoyance at the least and a serious problem at the most. Certain steps can prevent or mitigate water hammer. The first is to study causes, consequences and solutions.

HYDRAULIC SHOCK

The most common cause of water hammer is either a valve closing too quickly or a pump shutting down suddenly. Hydraulic shock is, in fact, the momentary rise in fluid pressure in a piping system when the fluid is suddenly stopped. As Sir Isaac Newton observed, an object in motion tends to stay in motion unless acted upon by another force. The momentum of the fluid traveling in its forward direction will work to keep the fluid moving in that direction. When a valve suddenly closes or a pump suddenly stops, the fluid in the piping system downstream of the valve or pump will be elastically stretched until the momentum of the fluid is arrested.

Sudden valve closure is most often associated with quarter-turn types of valves and more specifically, automated quarter-turn valves. A simple solution is to close those automated quarter-turn valves more slowly. This works in many cases but not all of them. For example, emergency shutdown valves need to close quickly, so other solutions may be necessary for these types of applications. More on valve closure time calculations is included later in this article.

The other most common cause of water hammer is sudden pump shutdown. Multiple pumps feeding into a common header, as in cooling tower applications or mine dewatering, either need to be shut down slowly, or they need to have in-line silent check valves installed immediately after the pump. Silent check valves can be extremely effective in reducing and sometimes eliminating water hammer.

PREDICTING WATER HAMMER PRESSURE SPIKES

It is possible to calculate the magnitude of water hammer pressure spikes based on detailed knowledge of the piping system and the media transported. The actual force of water hammer depends on the flow rate of fluid when it is stopped and the length of time over which that flow is stopped. For example, consider 100 gallons of water flowing in a 2-inch pipe at a velocity of 10 feet per second. When the flow is quickly brought to a halt by a fast-closing valve, the effect is equivalent to that of an 835-pound hammer slamming into a barrier. If the flow is stopped in less than a half second (which might be the closing speed of the valve), then a pressure spike over 100 psi greater than the system operating pressure can be generated.

The equation for calculating the potential magnitude of the spike is as follows:

∆H = a/g * ∆V

∆H is the change in head pressure

∆V is the change in fluid flow velocity

a = acoustic velocity in the media

g = gravitational constant

An example is:

a = 4864 feet per second

g = 32.2 feet per second2

∆V = 5 feet per second

∆H would be 756 feet (328 psi)

This value is assuming instantaneous valve closure exists.

VALVE CLOSURE TIME CALCULATIONS

Water hammer is obviously a serious issue in industrial settings, such as at a wastewater plant or municipal water system. In contrast to the example above, the average bathroom faucet is usually based on a half-inch nominal line size and has a water pressure that ranges between 60-80 psi and delivers about 8-10 gallons per minute. A 6-inch line in a water treatment plant would deliver 900 gallons per minute with a velocity of 10 feet per second. A 24-inch water main could be delivering over 12,000 gallons of water per minute, enough to fill the average backyard swimming pool in less than two minutes.

The basic formula for valve closure 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

For water at 70°F (21°C) where you have 100 feet of straight pipe:

T= 41 milliseconds minimum closure time

CONSEQUENCES OF WATER HAMMER

The consequences of water hammer can range from mild to severe. A common sign is a loud banging or hammering sound emanating from the pipes, especially after a water pressure source is shut off quickly. This is the sound of the pressure shock wave hitting a closed valve, joint or other blockage at high force. This sometimes-deafening noise can be a source of great distress and concern, especially if people are working close by.

Repeated occurrences of water hammer aren’t just an annoyance, however. Water hammer also seriously damages pipelines, pipe joints, gaskets and all the other components of the system (flow meters, pressure gauges, etc.). The pressure spikes can easily exceed 5 to 10 times the working pressure of the system upon impact, thereby placing a great deal of stress on the system. Water hammer causes leaks at the joints in the system. It also causes pipe wall cracks and deformation of piping support systems. Repairing or replacing damaged pipeline components and equipment can involve steep costs. If the spill results in an environmental issue, the costs can be staggering.

In most situations, water hammer is considered a safety hazard. The extreme pressure of water hammer can blow out gaskets and cause pipes to rupture suddenly. People in the vicinity of such an event can be seriously injured.

SOLUTIONS TO WATER HAMMER

There are many ways to mitigate the effects of water hammer, depending on its cause. One of the simplest methods of minimizing water hammer caused by hydraulic shock is to train and educate operators. Operators who learn the importance of opening and closing manual or actuated valves properly can take precautions to minimize the effects. This is particularly true for quarter-turn valves such as ball valves, butterfly valves and plug valves.

PIPING DESIGN CONSIDERATIONS

Water hammer arrestors provide a point of relief for pressure spikes caused by water hammer. These piping system components reduce the characteristic noise and resultant stress on the pipeline system by acting like a shock absorber. When sized and installed properly, water hammer arrestors can be an effective solution.

On the other hand, pumps that output into a long run of vertical pipe should be avoided. The vertical leg should either be minimized, or silent check valves installed as close to the pump as possible should be used.

Another area to look at in minimizing water hammer is installing check valves in vertical pipe lines. Swing checks, tilting discs and double-door valves can be made to operate in a vertical line. However, they will not prevent reversing flow in this orientation. Only a silent check valve can work in this orientation.

Hydraulic shock resulting from the sudden closure of swing check, tilting disc and double-door check valves can be remedied by exchanging these valves with silent or non-slam check valves. Silent check valves close upon the decrease of the differential pressure across the closure member of the valve, rather than closing from reverse flow. Thus, they are far less likely to slam shut, which induces water hammer. When the differential pressure across the disc approaches the cracking pressure of the valve, the valve has fully closed. This allows the fluid flow to decelerate, which allows the momentum of the fluid to decrease before the valve is fully shut while still ensuring that the fluid flow does not reverse direction.

System designers must be familiar with the best practices and industry standards for minimizing water hammer, such as using slow-closing valves when appropriate, knowing optimal valve locations within a piping system and giving special piping design considerations for high-operating pressure systems.

When piping systems are properly engineered, the likelihood of water hammer occurring is greatly reduced or even eliminated. In systems that already are in place, the damaging effects of water hammer can be limited in a number of significant ways, such as installing water hammer arrestors, relocating check valves out of vertical lines, installing silent check valves as a primary line of defense and ensuring operating procedures for quarter-turn valves have a slow closing rate. Note that the closure time in automated systems should be at east 10 times what is 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 19th century. Research continues today. Many major universities in the United States, U.K. and the Netherlands as well as well-respected valve companies have authored articles on the comparison of various styles of check valves and their installed dynamic characteristics.

This article only scratches the surface of the subject of fluid transients by exploring some of the causes and solutions of what we commonly call water hammer. Solutions to deal with water hammer problems can be quite costly, and, as always, an ounce of prevention is worth a pound of cure. Pumps feeding into vertical lines or common headers and rapid valve closures can all be designed out of a process at the beginning. Once the piping is in place and the plant processes are underway, the challenge is to find solutions given the specific constraints.

Most manufacturers of in-line silent check valves understand water hammer very well and have engineers on staff that can help. They can be the best source of knowledge when it comes to the right solution.

ARIE BREGMAN is vice president and general manager at DFT Valves. Reach him at abregman@dftvalves.com.