Article #3: Suction Specific
Speed (NSS)
Suction Specific
Speed is another dimensionless parameter used in centrifugal pumps. Much of the
discussion on specific speed (NS) applies to suction specific speed (NSS). (See
topic "Specific Speed (NS)- why is it dimensionless, or - is it?").
While specific speed
(NS) is mostly related to the discharge side of the pump, the suction specific
speed deals primarily with its suction (inlet) side. The head (H) term in the
denominator of the defining formula for the NS is substituted by the NPSHR:
NSS = RPM x Q0.5 / NPSHR0.75, where
flow
is in gpm, and NPSHR is in feet.
Values of NSS vary
from about 6,000 to 15,000, and sometimes even higher for the specialized
designs.
From the discussion
of a pump suction performance (see topic "How does pump suction limit the
flow?"), we know that conflicting demands are imposed on a pump system by
the pump user and a pump manufacturer.
A user would prefer
to provide as low NPSHA as possible, as it often relates to a system cost: for
example, higher level of liquid in the basin of the cooling water pumps
requires taller basin walls, or deeper excavation to lower a pump centerline
below the liquid level. A pump manufacturer, on the other hand, wants to have
more NPSHA, with an ample margin above the pump NPSHR, to avoid cavitation,
damage, and similar problems.
In other words, a
wider margin (M) can be achieved either by increasing the NPSHA, or decreasing
the NPSHR, since
M
= NPSHA - NPSHR
Thus it may appear
that a lower NPSHR design is preferential, and a competing pump manufacturer
might present a lower NPSHR design as automatically translated into
construction cost savings - because of not having to increase the NPSHA. Since
a lower NPSHR design means a higher value of NSS (according to the definition
above), the highest NSS design might seem to look best. In reality, however,
this is not so.
In a topic "How
does pump suction limit the flow?" it was explained that higher flow
velocities result in reduction of the static pressure, which may then become
dangerously close to the fluid vapor pressure and cavitation. Thus, lower
velocities result in higher localized static pressure, i.e. a safer margin from
the cavitating (i.e. vaporization) regime. Since the velocity is equal to flow
divided by the area, a larger area (for the given flow) reduces the velocity, -
a desirable trend.
This is why a larger
suction pipe is beneficial at the pump inlet. Cavitation usually occurs in the
eye region of the impeller, and if the eye area is increased - velocities are
decreased, and the resulting higher static pressure provides a better safeguard
against vaporization (cavitation). So, a larger impeller eye seems like a way
to lower the NPSHR:
Figure 3-1
Larger impeller eye results in lower NPSHR at BEP, but certain problems arise
at off-peak operation
Unfortunately, the
flow of liquid at the impeller eye region is not as simple and uniform as it is
in a straight run of a suction pipe. Impeller eye has a curvature, which guides
the turning fluid, like a car along the sharp curves of the road, into the
blades and towards the discharge. If a pump operates very close to its BEP, the
inlet velocity profile becomes proportionally smaller, but the fluid particles
stay within the same paths:
If, however, a pump
operates below its BEP, the velocity profile changes, and no longer can
maintained its uniformity and order. Fluid particles then begin to separate from
the path of the sharpest curvature (which is the impeller shroud area), and the
resulting mixing and wakes produce a turbulent, disorderly flow regime, which
makes matters difficult from the NPSHR standpoint.
Figure 3-2:
Even thought large eye impeller has better NPSHR at BEP, it has flow separation
problems at low flow
The upshot of all
this is that a larger impeller eye does decrease the NPSHR at the BEP point,
but causes flow separation problems at the off-peak low-flow conditions. In
other words, a high Suction Specific Speed (NSS) design is better only if a
pump does not operate significantly below its BEP point.
Interestingly, with
very few exceptions, there is hardly a case where a centrifugal pump operates
strictly at the BEP. The flow demands at the plants change constantly, and
operators throttle the pump flow via the discharge valve. High NSS designs are
known to result in reliability problems because of such frequent operation in
the undesirable low flow region. Actual plant studies have shown, that above
NSS of 8500 - 9000, pumps reliability begins to suffer - exponentially:
Figure 3-3
Plant experience shows that impellers designed with NSS greater than 9000 have
poor reliability record
Realizing this,
around mid-80s, users started to limit the value of the NSS, and a Hydraulic
Institute uses NSS = 8500 as a typical guiding value. It might be of interest
for you to calculate the values of NSS of your pumps, and find out if a
correlation between those and reliability exists at your plant.
To
learn more about this topic, e-mail your comments to us at: