By Diane Meyer
An essential element in the design of water and wastewater pumping systems is the proper selection of the pump control valve, the primary purpose of which is to prevent reverse flow when the pump is not in operation. A pump control valve must also be able to carefully and slowly control changes in fluid velocity to prevent water hammer or surges, especially in long pipelines.
Another function that is often overlooked is the valve’s ability to minimize energy consumption. It is estimated that water and wastewater plants consume nearly 80 percent of their costs pumping water and overcoming pressure and friction losses. With proper valve selection to minimize valve headloss, significant energy savings can be achieved.
There are several types of pump control valves, including butterfly, ball, and eccentric plug valves, which are electrically wired to the pump control circuit to provide synchronized functions with the pump to systematically control the changes in pipeline fluid velocity over a long period of time (i.e., 60 to 300 seconds) to prevent surges in pipelines, force mains, and distribution systems. When selecting a pump control valve, you should consider performance, low headloss, and flow characteristics.
When pumping systems are part of very long piping systems (i.e., 20,000 feet), pump control valves are required to control water hammer or pressure surges. These are typically quarter-turn valves equipped with slow opening and closing electric motor or hydraulic cylinder actuators. The motion of the closure member in these valves is controlled by the power actuator so they are not subject to fluttering or slamming. Further, because the actuator rigidly restrains the closure member, there is no need for three to five straight diameters of pipe upstream of the valve as with swing check valves.
Battery systems or pressurized accumulator systems can be used to enable the pump control valve to close after a power failure. Alternatively, an automatic check valve such as an AWWA C508 Check Valve with low headloss can be used to prevent reverse flow after power failure. Finally, quarter-turn valves are designed to handle high fluid velocities (up to 16 ft/sec) so are often sized to be smaller than the pump discharge to provide improved flow characteristics.
Valve body geometry dictates the general flow area through the valve. Some control valves restrict the flow area to below 80 percent of the pipe area. Also, the internal contours of the body and seat should be smooth to avoid creating excessive turbulence.
The design of the closure member is also important in reducing headloss. The lowest headloss will be achieved if the closure member rotates out of the flow path. There are many flow coefficients and headloss formulas in general use today for rating various valves on the basis of headloss.
The headloss from valves can be converted into an annual energy cost related to the electrical power needed by the pump to overcome the additional headloss. By using various factors such as Kv, Cv and headloss values of various valves, an annual energy cost savings can be determined.
Remember, pumping costs can be more significant than the installed costs of the valves. Further, the larger the valve, the greater the impact from the energy costs. Before designing your pumping system, consider evaluating the various types of pump control valves to determine which will provide the greatest in energy savings. WW
About the Author: Diane Meyer is the marketing manager for Val-Matic Valve & Manufacturing Corporation. She is also a member of the Water and Wastewater Equipment Manufacturers (WWEMA) Board of Directors. WWEMA is a Washington, D.C.-based non-profit trade association representing water and wastewater technology and service providers since 1908. For more information about WWEMA, visit www.wwema.org.
Circle No. 255 on Reader Service Card