Option 3: Install one large pump with a VFD to replace the existing four pumps. This option provided the most energy effective and reliable solution, however the high equipment and installation costs resulted in an unacceptable payback period.
Option 4: Continue with the existing pumps — this was the baseline.
Option 5: Replace the existing 4×3×10 pumps with 6×4×10 pumps. Due to differences in hydraulics of these pumps they would produce significant energy and reliability savings. The operator rejected this option because the performance curve of the pump is very flat, creating the risk of running one of more of the pumps at shutoff during various operating scenarios.
Figure 8. Operating near shut-off shortens the MTBF of a pump.
As the example demonstrates, a thorough knowledge and understanding of pumping systems is required to effectively undertake pumping system analyses. Recognizing a potential value to its customers, Flowserve has established a team that provides LCC optimizing services for existing installations. In one example, the team conducted analyses of over 140 pumping systems and identified $2.3 million in annual savings.
The future is bright
As leading equipment users in the process industries continue to see the economic benefits of optimizing existing pumping systems, there will be a growing demand to use this strategy to drive out costs and improve equipment reliability. The continual improvement of modeling software will aid in implementing a total cost of ownership perspective. CP For more information on pump lifecycle costs, please refer to www.flowserve.com.
NPSH Margin Reliability Ratio is a safety factor for pump NPSHrequired.
S = N(Q)0.5/NPSHR0.75
The Hydraulic Institute recommends a suction speed of less than 8,500 at low flow, however, speeds up to 11,000 have been used. A quick fix for a high suction speed with an existing pump installation may be a recirculation line or VFD. Unfortunately, it may be necessary to consider a pump change such as multiple pumps or departing from centrifugal pumps altogether. Suction speed fixes the impeller type; and, changing the impeller may very well change the pump characteristics and power draw requiring a new pump. Suction Energy (S.E.) is a relatively new term related to suction speed. It is easily calculated.
For example, with S = 9,000, shaft speed (N) = 3,550 rpm, an impeller diameter of 6 in. (De), and a specific gravity of 1.0 (Sg) suction energy can be calculated according to the following equation:
S.E. = De × N × S × Sg = 6 × 3,550 × 9,000 × 1.00 = 192×106
A value above 160×106, for a typical pump is too high.
For additional information on suction energy and suction speed and how they relate to NPSH and speed, refer to http://www.gouldspumps.com/cpf_0008.html, http://www.lawrencepumps.com/newsletter/news_v01_i5_oct.html, http://www.gouldspumps.com/cpf_0005.html, and http://www.chemicalprocessing.com/articles/2004/187.html.
Tom Carsten is the director of alliance development for Flowserve in Dayton, Ohio, and Barry Erickson is the key account manager for Flowserve serving Eastman Kodak in Rochester, N.Y.; e-mail them at TCarsten@flowserve.com and BErickson@flowserve.com.