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TURBULENCE AT PUMP DISCHARGE

Either a multipurpose or a check valve usually is installed on the discharge side of a pump. However, as previously mentioned, the increased use of variable-speed drives with pumps has resulted in the installation of more check valves. To decrease costs, wafer-style check valves are being chosen over globe-style check valves. However, a wafer-style check valve has a higher pressure drop than a bulbous-shaped globe-style check valve.

Valve manufacturers recommend five to 10 pipe diameters of straight pipe between a pump and valve so fluid turbulence off the pump will dissipate. However, space constraints or a poor understanding of proper installation have resulted in check valves regularly being installed directly on pump discharges without spool pipe. The effects of this can be disastrous. Photo B shows how turbulence from a pump nearly vibrated the hinge pin out of a valve body. The valve “chatter” that would have occurred as the pin continued to vibrate would have been an early warning of premature valve failure.

One solution is to install straightening vanes, which condition fluid flow yet require significantly less space than the recommended length of spool pipe. Figure 9 is a CFD image of straightening vanes installed in a pipe. The images show the vanes' effectiveness in eliminating turbulence while improving flow-meter reading accuracy.

Straightening vanes can be utilized as part of a standard flexible connector. This configuration provides the flow-straightening effectiveness of 10 pipe diameters of straight pipe in a fraction of the length. Additionally, a valve can be installed directly on the connector, eliminating the need for a spool of pipe.

Sometimes, solutions can be simple. For example, turbulent flow from a pump and an eccentric reducer caused a check valve to chatter from the day the system was put online (Photo C). Because the eccentric reducer was a source of its own turbulence, it was relocated and connected directly to the pump (Photo D). The double-sphere connector then was replaced by a flexible connector with straightening vanes installed between the eccentric reducer and check valve. As a result, the straightening vanes eliminated turbulence coming off the pump/reducer configuration, and the chattering check valve was silenced.

CONCLUSION

Ultimately, an engineer must review the entire system and develop a design that will achieve the owner's goals. CFD analyses show how designs and practices that have become industry standards can be advanced with vane technologies. Pre-engineered vanes reduce pressure drop and protect components by eliminating unwanted turbulence in ever-tightening engineered-piping installations.

For past HPAC Engineering feature articles, visit www.hpac.com.

Regional sales manager for The Metraflex Co., Jim Clauss has more than 27 years of practical piping experience, holding various positions, including technical-service-staff member, project engineer, and production manager. He has written and co-written standards and codes for pipe and fittings and been an ASTM International task-group chairman.