Graph of Minimum Continuous Flows
In 1982, Richard Dubner of Chevron developed a graph for use in establishing minimum continuous flow rates for centrifugal pumps (see Figure 1). It allows a lower flow rate for pumps handling hydrocarbons than for those pumping water, and it requires larger pumps to operate closer to the BEP than smaller pumps.
Figure 1. Dubner's (Chevron) Chart for Minimum Capacity
Note that Figure 1 cautions against S values greater than 11,000, and prohibits operation of any pump larger than 100 gpm (BEP) from operating continuously at less than 20 percent of BEP.
This graph may be used to establish operating guidelines for existing pumps, and in the selection process for new pumps to eliminate offerings that have a minimum flow above that anticipated for the intended service.
Problem No. 1. Minimum Continuous Capacity
Using Dubner's graph (Figure 1), determine the recommended minimum continuous capacity for the pump in Problem No. 2 ).
Qeye = 1,250 gpm
S = 12,000
Pumpage is water. From Dubner's graph, we read a factor of 0.76.
Qmincont = (0.76)(2450) = 1,860 gpm
"Stable" Window of Operation
In 1985, Lobanoff and Ross (1) provided the graph shown in Figure 2. It provides not only a minimum capacity, but also a maximum capacity for "stable" pump operation. It represents test results of a 4 in (discharge) pump with eight different impellers. The onset of instability was determined by measuring pump vibration. The minimum flow values obtained from this graph compare favorably with Dubner's water values (Figure 1).
Figure 2. "Stable" Flow Range vs. Suction Specific Speed for a 4 in Pump
Figure 3 is an adaptation of the Lobanoff‑Ross graph. Dick Allen developed the graph in 1990 to assist in evaluating pump bids. It provides a method for evaluating the size of the stable window by penalizing pumps that have smaller stable windows (higher suction specific speeds).
Figure 3. Centrifugal Pump Stable Window of Operation Based on Suction Specific Speed (Courtesy Dick Allen)
The penalty values are arbitrary, based on the judgment of the graph's author, and may be revised to suit the experience and philosophy of a given company.
Key points made by this graph:
1. Do not buy a pump to operate continuously at a capacity within the "unacceptable" area (roughly any capacity less than 50 percent of Qbep).
2. To operate in the "excellent" hydraulic range, capacity must be higher than the "penalty" range (roughly 75 percent of Qbep).
3. Do not buy a pump to operate continuously at a capacity higher than Qbep.
4. Do not buy a pump rated at a capacity higher than about 115 percent of Qbep.
Reliability of existing pumps can be increased by maintaining flow rates within the guidelines established by Figures 1 and 2.
Reliability of future pumps can be increased by using these graphs to establish a maximum suction specific speed acceptable for the intended service.
Effect of Speed on Suction Specific Speed
Although specific speed does not change as a pump's speed is changed, suction specific speed does change. Because of the 1.5 exponent relationship between NPSHR values (Understanding NPSH, August 2009), a pump will have a higher S when it runs at a higher speed (because the NPSHR is based on the 3 percent head drop).
To establish the effect of speed on S, substitution of NPSH2 = NPSH1 (N2/N1)1.5 into a ratio of S equations,
yields the following relationship:
S2 = S1 (13-3)
S2 = Suction specific speed at second speed
S1= Suction specific speed at first (base) speed
N2 = RPM of pump at second speed
N1 = RPM of pump at first (base) speed
Suction specific speed, based on the 3 percent head drop criterion for the same pump, varies as speed to the 3/8th power. (Doubling the speed increases S by 30 percent.)