Protect your system from this damaging phenomenon.
by Joe Evans
September 18, 2017

Pump Starts & Stops

Water hammer’s effects can be more significant in low pressure systems. The additional pressure generated by a shock wave is proportional to the length of the pipe and velocity of the water flowing in it and is completely independent of its operating pressure. Therefore, the shock wave created in a 1,000 foot pipe flowing at 5 ft/sec will be the same whether the operating pressure is 50 psi or 200 psi. The difference is the ratio of shock pressure to design pressure can be significantly higher in the low pressure system, therefore the potential for damage is greater.

In many large systems, it is normal procedure to start a pump against a closed discharge valve. Once the pump is running at full speed, the valve is opened slowly. Flow is initiated and then increases to its maximum as the valve continues to open. This procedure is reversed when a pump is stopped. Starting and stopping against a valve that is opened or closed slowly will inhibit the initiation of water hammer.

The discharge valve may be operated manually or by some automatic mechanism. One shortcoming of manually operated valves occurs during a power outage. When a pump motor loses power, the reduction in pump speed and flow occurs rapidly. The resulting change of kinetic energy to that of pressure can produce water hammer waves in the discharge line. As the water column reverses direction, the impeller will accelerate backward. When it reaches its maximum reverse speed, backward flow is reduced and an additional pressure surge is created.

In most pressure boost applications, a “spring loaded” check valve is installed at or near the pump discharge and remains closed when the pump is idle. When the pump starts, flow does not begin until the pressure it generates exceeds the pressure on the downstream side of the closed valve. If the downstream pressure is not allowed to decrease below a certain level, flow increases slowly and water hammer inception is avoided or reduced.

When the pump stops, an unexpected event occurs—a quick closing valve actually inhibits, rather than initiates, water hammer. In this particular instance, the spring provides quick closure of the valve, which prevents the water column from changing direction due to the higher downstream pressure. Even though flow changes abruptly, pressure remains relatively constant throughout the downstream column. If a standard check valve was installed, the water column would accelerate backward, slam the check closed and initiate a shock wave.

Today, variable frequency drive (VFD) control is used in many applications to eliminate the inception of water hammer during pump starts and stops. This technique, known as soft start and stop, is accomplished by ramping the motor speed up or down over a period of seconds. This allows the flow velocity to increase or decrease much more slowly than it would during across the line starts and stops.

So far, our discussion of water hammer has dealt with single phase systems. In these systems, water remains in a single state (liquid in our examples) regardless of the changes in the hydraulic conditions. The shock waves generated by single phase systems are due to an abrupt change in flow and the resulting transformation of kinetic energy.

The water hammer generated by water column separation and closure is a two phase process. In a two phase system, water changes state and can exist both as a liquid and a vapor within the same confined volume. This change can take place whenever the pressure in a pipeline is reduced to that of the vapor pressure of the water. When a pressure drop occurs, the water column can become separated, in one or more locations, by a pocket of water vapor. When the pressure rises above the vapor pressure, the column rejoins or closes and can create a high pressure wave. Water column separation, by itself, can cause problems in very large diameter or thin wall pipes (which can collapse), but water hammer during closure is the more common problem.

Water column separation can occur when a pump is stopped and the water column reverses direction or in condensate lines where high temperatures can mitigate the need for a large pressure drop. Although both forms can be extremely damaging, condensate lines tend to be far more dangerous. The shock waves generated by column closure can travel in opposite directions, and if they hit secondary barriers they can be redirected back toward one another. It would not be unusual for these reflected waves to increase in intensity when they collide. This is certainly the case with water and voltage waves and may account for the often greater damage resulting from closure-initiated water hammer.

In the coming months, we will publish articles that fill a gap in our industry: basic information for new pump users. We’ll pull these from classic Pumps & Systems articles, as well as new content. If you have a go-to article for training new employees or refreshing yourself on the basics, please tell us about it at pumpeditors@cahabamedia.com.

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