The Eighth in a Series
Our attempts to normalize or classify water turbine hydraulic performance culminated in 1915 with the development of the specific speed concept, which was later applied to centrifugal pumps (2).
Because NPSH is not included in specific speed, however, attempts continued to understand and normalize this elusive characteristic. In 1922 (1), the Thoma‑Moody parameter, Sigma, was introduced and defined as:
Sigma = σ = NPSH/H (13‑1)
where H is the total head of the first stage impeller. Sigma was widely accepted and used for a number of years, but it had a significant shortcoming, since the NPSHR of a pump is relatively independent of the head produced by the pump.
In 1937, three engineers at Worthington-Karassik, Wislicenus and Watson-were assigned the task of developing a better concept than Sigma. Initially working independently, they joined forces. While discussing their problem over Saturday morning coffee in Karassik's kitchen, they developed the concept of Suction Specific Speed (3).
Definition of Suction Specific Speed
Suction 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:
S = (13-2)
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 Speed
Calculate 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.)
Figure A. Typical published performance curve. Single-line NPSH curve.
Importance of Suction Specific Speed
Establishing 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.
Figure 1. To reduce NPSHR, the impeller eye diameter is increased.
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.