Variable frequency drives (VFDs) slowly start and stop pump motors and add the capability of pacing pump flow, but they increase project capital costs. VFDs are ineffective during power outages, and emergency power components rarely restore power quickly enough in the short times in which hydraulic transients are generated.
Air vacuum and combination air release and vacuum relief valves, although inexpensive and simple to use, are not 100% effective. They require significant maintenance and can contribute to water column separation.Best Operations
For any pipeline, and particularly for critical community infrastructure, a diligent analysis of surge and water hammer potential is required to selectthe ideal control devices.
Systems experiencing any of the following conditions warrant a detailed analysis:
- Total dynamic head (TDH) >50 feet and pump station flow (Q) >500 gpm
- Column separation potential
- Transmission main >8 inches in diameter and >1000 feet long
- Transmission mains with a long, steep upslope followed by a shallow upslope
- Transmission main velocity >4 fps
- Quick-closing valves.
Surge/water hammer analysis may not be required in these cases:
- Pump station flow (Q) <100 gpm
- Static head <40 feet
- When using positive displacement pumps with pulsation dampener
- Transmission main velocity <2 1/2 fps.
System modeling can range from simple hand calculations to computer modeling with commercially available software or modeling experts. While hand calculations suffice for small, uncomplicated systems, expert modeling is the best choice for most systems.
Infrastructure owners and operators can make the best use of capital funds by using detailed models to identify the sources of potentially significantsurge and water hammer and select the best mitigation devices.
–Steve Liming, PE, is a senior project manager with the Phoenix office of PBS&J.