Checking pump performance curves is one of the first steps in troubleshooting a centrifugal pump problem or seeing if a pump can handle new service conditions. Figure 1 shows vendor-supplied curves for a 2,220-gpm pump that ran into problems; they depict total dynamic head versus capacity, net positive suction head required (NPSHR) versus capacity, and pump efficiency versus capacity. Pump power (at a fixed fluid density) versus flow rate also is often charted.
Figure 1. Manufacturer provides various data about the performance of a 2,200-gpm centrifugal pump.
This pump was in hydrocracker stabilizer bottoms-circulation service. Upstream operating changes temporarily reduced flow rate to the stabilizer to approximately 25% normal operating rate. Immediately after this change, operators reported hearing gravelly sounds at the pump suction a typical symptom of cavitation. Cavitation damages both the pump impeller and seals.
Hearing from operations that a pump has a cavitation problem, a process engineer invariably first focuses on the net positive suction head available (NPSHA). Here, a check of the system hydraulics showed that NPSH shouldnt have been a problem. The NPSHA was 24 ft. While the vendor NPSH curve stopped at 1,250 gpm, extrapolating for the circulation rate of 500 gpm gave an NPSHR of 7.8 ft.
Other, less-appreciated pump hydraulic limits can create the same symptoms as insufficient NPSH. For instance, a high suction-specific-speed pump can suffer from inlet eye recirculation at operation far from the pumps best efficiency point (BEP). Equation 1 shows the usual calculation for suction specific speed:
NSS = N(Q)1/2/NPSHR3/4 (1)
where N is impeller speed in rpm, Q is flow rate in gpm, and NPSHR is in ft. NSS is evaluated at the flow rate and NPSHR at the best efficiency point.
The BEP for this pump is 2,220 gpm, as shown by the peak in the efficiency curve in Figure 1. So, this pump has a suction specific speed of:
NSS = 3,570 (2,220)1/2/11.73/4 = 26,600 (2)
Whats the significance of this value? The Hydraulic Institute recommends pump suction-specific-speed values below 8,500 for maximum operating flexibility and reliability. Typically, values of 18,000 limit the prudent operating range to 50% of BEP flow or greater. Yet, this pump has an even higher NSS and a flow of only 500 gpm, just 22% of the BEP flow. So, inlet eye recirculation very likely is causing this pumps problems.
The effect of pump recirculation issues didnt become widely known among pump users until the mid-1980s. By the end of that decade, most major operating companies had added suction-specific-speed recommendations to their engineering standards. However, this pump had been purchased in the early 1970s.
Upstream control changes allowed the plant to increase the flow rate to more than 1,300 gpm. All signs of gravelly suction noise disappeared. A field experiment of varying upstream level and flow rates generated an apparent NPSH curve (Figure 2). While this curve is not as precise as from a true hydraulic test, it does give a good idea of how much head is required to suppress inlet eye problems at low flow rates. Whats especially fascinating is that the point where the vendors NPSH curve stopped is nearly exactly the point where the suction head required to suppress inlet eye problems takes off.
Figure 2. Plant data led to curves that show how much NPSH is required to suppress inlet eye problems.
By Andrew Sloley, contributing editor