One recent example of what Standard Alloys calls a Custom Hydraulic Solution (CHS) involved a large circulating water system pump for a utility company in the Northeast. Cavitation damage to the impeller dictated that at least one of the six pumps at the power station be overhauled each year. Additionally, during warm summer months the power station would periodically become load limited based on condenser backpressure.
While eliminating cavitation was the primary objective of the CHS study, additional capacity was also requested. Fortunately, Standard Alloys had reversed engineered this impeller years earlier so the current impeller design was known. A redesign effort was initiated which resulted in a change in the number of vanes and a change in the vane shape. The redesign was double checked by an outside consultant and verified.
Next, a new pattern and core boxes were made for the foundry. The new impeller was cast and machined, then verified against the design prior to shipment. Once installed, the performance was checked and the improvements were verified against pre-CHS tests utilizing ultrasonic flowmeters and other plant instrumentation.
The redesigned 46" impeller, custom designed and built for this particular application, has increased the capacity of the Worthington pump from 88,000 gpm at 100 ft Total Developed Head (TDH) to 94,000 gpm. It has also decreased the NPSHR from 22 ft at the rated 88,000 gpm to 17 ft at the new 94,000 gpm rating. A byproduct of the pump operating without cavitation was a significant reduction in the noise levels measured around the pump with the new impeller.
Finally, the energy requirement for the 2500 hp motor was reduced from 329 amps to 319 amps. Just for good measure, Standard Alloys also upgraded the material to CD-4MCu, which means it will be years before a replacement is needed. Subsequent to the first impeller being installed, a second impeller was delivered this summer. Four more pumps remain to be upgraded in the upcoming years.
Elimination of the cavitation will increase the life of the impeller, bearings and seal, thus extending the Mean Time Between Repairs. Adding the value of the increased pump output to the energy reduction savings and the savings due to the longer life of the part and pump system makes this an easy project to cost justify.
Looking for opportunities to make significant gains to extend Mean Time Between Repairs while decreasing costs and increasing performance could be referred to as Reliability Engineering Plus. When looking at cost/benefit evaluations, it is important to look at the total costs and benefits over the life of the part, not just the initial cost comparison.
