Taylor’s plot in Figure 1 places the Q = 100 percent intersect at an NPSHr = 100 percent of the manufacturer’s stated value. Unfortunately, whenever this 3 percent fluctuation occurs, some pump damage may already be in progress. For this reason, assuming a more likely and realistic NPSHr is often prudent. Alternatively and perhaps additionally, providing an NPSHa in excess of this assumed, more realistic NPSHr may be reasonable. Doing so builds a certain margin of safety into the pump and reduces the risk of frequent and sometimes catastrophic failure events—at a higher cost.
Taylor acknowledged that his curves were not completely accurate and mentioned that a reasonable demarcation line between low and high suction specific speeds (Nss) probably existed somewhere between 8,000 and 12,000. The allowable Nss number is a function of the suction energy of a pump and may be as high as (or even higher than) 12,000.3, 4 Because the determination of the allowable Nss number seemed imprecise in the 1980s, several field surveys conducted before 1999 led to picking 8,500 or 9,000 as the highest “safe” number. From 1970 to 1999, impeller configurations with Nss numbers in excess of approximately 9,000 were not to be operated at flow rates much higher or lower than the best efficiency point (BEP). Operating certain process pumps at partial flow would reduce their life expectancy or their repair-free run time capability.
PreDefining the Operating Range
The constraints of parallel operation of pumps do not always receive proper emphasis. However, the process pump standard issued by the American Petroleum Institute (API-610) provides important guidelines. Unless pumps operated in parallel have identical performance curves, they will not share the load equally. Pumps operating in parallel should have relatively steep performance curves. Pumps with flat performance curves often have marginally higher efficiencies but will risk unequal load sharing. Operating flat-curve pumps in parallel might, in some instances, cause one pump to briefly run at zero flow. Pumps operated at zero flow will fail—some after one minute, others after one hour.
While operating-range constraint questions generally apply to hydraulic parameters, pump hydraulics can, in turn, affect the life of mechanical components. Problems with short elbows near the suction nozzle of certain pumps can exist, and the resulting flow stratification and friction losses are sometimes overlooked. Other issues can be purely mechanical and are often related to the design or maintenance oversights. These are best addressed by enhanced work procedures and better quality control. Many good recommendations may be found in solidly experience-based references. Experience-based recommendations may not always please the occasional vendor (or purchaser) whose primary goal is low initial cost and who does not share a reliability focus. Some purchasers or owners will mention an isolated example in which a pump survived despite receiving no attention regarding reliability recommendations. These notions are similar to anecdotes about a driver whose life was saved because he did not wear a seat belt. Such arguments do not add value.
1. Taylor, Irving, “The Most Persistent Pump-Application Problems for Petroleum and Power Engineers,” ASME Publication 77-Pet-5 (Presented at Energy Technology Conference and Exhibit, Houston, Texas, September 18 – 22, 1977).
2. Bloch, Heinz P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley & Sons, Hoboken, N.J., 2011.
3. Bloch, Heinz P. and Alan R. Budris, Pump User’s Handbook, 4th Edition, Fairmont Press, Lilburn, Ga., 2013.
4. ANSI/HI9.6.3-1997, “Allowable Operating Region,” Hydraulic Institute, Parsippany, N.J.
5. SKF USA, Inc., Publication 100 – 955, “Bearings in centrifugal pumps,” Version 4, p. 20, Kulpsville, Pa., 2008.
6. Bloch, Heinz P., Practical Lubrication for Industrial Facilities, 2nd Edition, Fairmont Press, p. 179, 2009.,; “Mechanical Seals in Medium-Pressure Steam Turbines,” presented at the ASLE 40th Annual Meeting in Las Vegas, Nev., May 1985 (later reprinted in Lubrication Engineering, November 1985).