Written by Arie Bregman

Water hammer is a shock wave transmitted through fluid contained in a piping system. The most basic explanation is that water hammer occurs when a fluid in motion is suddenly forced to stop moving. The momentum of the fluid abruptly stopping creates a pressure wave that travels through the media within the pipe system, subjecting everything in that closed system to significant forces.

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.

The fluid then wants to snap back to its normal, unstressed condition, much like an extended spring that has been released. This causes the liquid to travel back through the pipe. The back-flowing fluid then encounters the closed valve, potentially with significant destructive force. The reflection of this fluid pressure wave is the loud bang (and there could be more than one pressure pulse) (Figure 1).

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.