Suction Specific
Speed
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.
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.
Figure 2. Although resulting in
reduced NPHR at the BEP, the large eye creates eddies and recirculation at
reduced flow rates.
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 Hallam
Numerous 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.
Figure 3. Failure frequency vs.
suction specific speed (J.L. Hallam, [5])
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 Pump
A 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
![Figure 3. Failure frequency vs. suction specific speed (J.L. Hallam, [5])](file:///C:\Users\RAVICH~1\AppData\Local\Temp\msohtmlclip1\01\clip_image008.jpg)