Flow Rate and Water Hammer Curves

Valves’ Effect on Flow Rate & Water Hammer

Flow Rate and Water Hammer Curves
Excerpt from the December 2021 Pumps & Systems Article by Pete Gaydon

Not all valves are well suited for controlling system flow rate.

Can any valve be used for controlling flow rate in a pump system, and what is the effect on water hammer?

Not all valves are well suited for controlling system flow rate. Image 1 (header image) compares the valve characteristics of various valves. You can see that the gate valve, based on its position, increases flow rate the fastest out of any of the other valves. Of the valves presented here, the plug valve provides the most consistent flow increase versus its valve position, which is ideal for throttling. In this case, a gate valve would not be good for flow control because of the lack of linear control over the flow.

Overall, the performance and characteristics of each valve will affect the magnitude of any transients developed by valve closures and openings. It is important to consider rate of flow change versus valve position and control the closure rate of the valve such that the effects of system transients (water hammer) are reduced.

Upstream Valve Closure Data

In Image 2, we have a comparison of some common valves where each valve closes over 60 seconds. The middle chart shows the pressure rising immediately upstream. The globe valve has the minimum pressure rise because the characteristics of the globe valve shown in the image above are more linear than either the butterfly or ball valve.

You can see in Image 1 that the characteristic curve of the ball valve is relatively flat coming from the bottom, with anything less than 40% open having very little effect on the flow rate. Without much change in flow until about 55 seconds, there is significant decrease in flow over the last five seconds of closure, which results in a rapid change in velocity and a high pressure change upstream. Using the ball valve would result in the greatest likeliness of a water hammer effect. This sudden rise in pressure can be damaging to equipment within the system.

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