Suction Specific Speed
Definition of Suction Specific SpeedSuction specific speed, like specific speed, is not a speed at all. It is an index number, or "yardstick." It is based on the NPSHR of a centrifugal pump, normally the 3 percent head drop NPSHR and normally at its best efficiency point (BEP). The equation for suction specific speed is the same as specific speed, except that NPSHR is substituted for head, as follows:
Where (in U.S. units):
S = Suction Specific Speed
N = RPM of Pump
Q = Pump Capacity*†, GPM
NPSHR = NPSH required by pump†, feet
*If the impeller is double suction, Q in the above equation is one-half the BEP capacity of the pump. This is a major difference from calculating specific speed, in which we use total pump capacity, whether the impeller is single suction or double suction.†Normally calculated at the BEP
The symbol Nss is often used in place of S for suction specific speed.
The value of S for most pumps is typically between 7,000 and 15,000. The higher values are more common in higher speed, higher capacity units. (See next month's article for additional discussion of the effect of speed and capacity on S.)
Problem No. 1: Suction Specific SpeedCalculate the suction specific speed for the pump represented by the performance curve in Figure A (Figure 2 from the May column).
N = 3,550 rpm
Qbep = 450 gpm
NPSHRbep = 14 ft
S = = =10,400 (RPM-GPM-FT)10,400 (RPM‑GPM‑FT)(Our answer is different than the 9,000 stated on the curve.)
Importance of Suction Specific SpeedEstablishing a Maximum Value for S
For a number of years, the push from users and competitors required pump manufacturers to continually strive for lower values of NPSHR. The philosophy was that "The lower the NPSHR, the better the pump." (NPSHR in centrifugal pumps is normally reduced by increasing the diameter of the impeller eye, as shown in Figure 1.) That philosophy has now changed. Due to problems that have been attributed to oversized impeller eyes, pump users have established maximum values for S, which establishes minimum values for NPSHR.
Establishing a "Stable" Window of Operation
Every centrifugal pump would like to run at its BEP-always. All pump components would experience maximum life at that capacity.
Seldom does a pump run at its BEP, but component life will be significantly extended if it operates within its "stable" window of capacities.
To a large extent, suction specific speed indicates the size of that window. Pumps with lower values of S have larger windows.
Suction Recirculation, or the "Big Eye Syndrome" (Monster or Myth?)For decades, industry recognized that centrifugal compressors would "surge" if operated below a certain capacity, but only more recently have we recognized that centrifugal pumps have a comparable characteristic.
We now know that any centrifugal pump will experience recirculation in the impeller eye if the capacity is below a certain value. Larger impeller eyes and higher speeds (i.e., higher peripheral velocity of the eye-U1) produce higher energy recirculation.
The large eye required to obtain low NPSHR leads to the problem of (higher energy) "eye recirculation" or "suction recirculation" (4). As shown in Figure 2, flow through the eye is proper at the BEP, but at some reduced capacity, recirculation starts in the large eye. As pump capacity is further reduced, the intensity of the circulation increases, sometimes resulting in a reversal of flow at the i.d. of the suction pipe, near the pump. If strong enough, this vortex causes cavitation, noise and pulsations. The capacity, at which this recirculation starts, increases as the eye diameter is increased.
When the vortices are strong enough to cause cavitation, the vapor bubbles collapse on the driving side, or pressure side, of the impeller vane, near the eye. If the vane twists as it enters the eye from the larger diameter, this part of the vane cannot be seen by looking directly into the eye, but must be viewed with the assistance of a small mirror.
Study by HallamNumerous technical papers and articles reported problems caused by suction recirculation, but none quantified the phenomenon until Hallam (5) reported in 1982 the results of a 5 year study of 480 centrifugal pumps in Amoco's Texas City refinery. Most of these pumps were in hydrocarbon services. The remainder pumped water. The average power requirement was about 150 hp and the maximum was 1,000 hp.
Figure 3 shows the results of the study. The pumps were divided in groups according to their suction specific speed. The average numbers of failures/year/pump were plotted for each group. A failure was defined as any problem with the pump that required service in the refinery repair shop. The graph shows an increase in failure frequency of almost 100 percent above an S of about 11,000.
The logic contained in some of the papers on suction recirculation is not totally rigorous. Most of the symptoms attributed to recirculation can be attributed to other centrifugal pump phenomena. The high S attributed to some centrifugal pumps has also been found to have been obtained by improper methods of NPSH testing. Ross (6) claims that the problem of suction recirculation has been distorted-that it simply boils down to inadequate NPSHA.
Regardless of the reason for the problem-whether it is suction recirculation, improper methods of testing or just inadequate NPSHA-Hallam's paper clearly shows that a pump with a suction specific speed greater than 11,000 should be selected with caution.
This finding caused a number of companies to prohibit the purchase of a pump with a suction specific speed in excess of 11,000.
Problem No. 2. Suction Specific Speed for a High Speed Multistage Waterflood PumpA four-stage pump is operating on an offshore platform in waterflooding (secondary recovery) service. Figure 4 is a redrawn copy of the performance curve provided by the pump manufacturer. The pump is equipped with a double suction, first stage impeller.
Calculate the suction specific speed:
S = == 12,000 (RPM-GPM-FT)
Would you suspect that this pump could be a problem? Yes, and it was.
The first stage impeller would periodically experience cavitation-erosion on the pressure side of the vanes, throwing the rotor out of balance and causing excessive vibration.
Figure 4. The vendor performance curve for the water flood pump