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5 6
PLUMBING CONNECTION
WINTER 2015
T
o recap, Fig 1 is generalised but provides a convenient
overview of the symptoms associated with low flow
operation issues such as:
∫ Increased internal turbulence
∫ Recirculation
∫ Increased pressure fluctuations
∫ Increased vibration due to the above
∫ Increased axial thrust – depending on pump hydraulic
balance method
∫ Increased radial thrust; particularly with single volute
casings
∫ Temperature rise due to high internal energy loss
In this article we will look at the parameter “Suction
Specific Speed” (NSS) and how it influences low flow
stability in a centrifugal pump.
FLOWREVERSAL AND VORTEXING AT LOW FLOW
Again, from our last discussion, Fig 2 shows a simplified
representation of smooth flow at BEP versus the sort of flow
disturbances that occur at reduced flow.
The degree of turbulence and associated vibration and
buffeting depends on the hydraulic design and on the energy
levels in the pump. The effects are generally worse for
impellers with relatively large entry diameters. In larger
higher energy pumps, vortexing (recirculation) can be so
severe that cavitation like effects ensue.
“Suction Specific Speed” (NSS) is a design parameter
that can be used to predict how susceptible a pump may be
to recirculation and instability at low flows and also at high
flows.
NSS gives an indication of the priority given to suction
performance for a given impeller.
It is calculated as follows :
∫ Where N is the running speed in rpm, Q is the flow per
impeller eye at best efficiency and using the NPSHR at
best efficiency flow.
PUMPS DESIGNED FOR VERY LOWNPSH
For a given flow, a higher value of NSS denotes improved
suction performance (low NPSHR). Fig 3 shows the typical
variation between a low NSS impeller and a high NSS impeller.
The high NSS impeller has a much larger eye diameter, which
reduces velocities and thus entry losses for improved NPSHR.
TRADE OFFS
Unfortunately, by virtue of this larger eye diameter, this
type of impeller is much more prone to flow instability
and recirculation when operating at flows away from best
efficiency. Considerable research has shown that the range
of stable operation is dramatically reduced at higher values
of NSS. This is shown graphically in the hydraulic stability
guidelines in Fig. 4 (US units).
The primary problem created is recirculation which can
occur at the pump inlet and at the outlet of the impeller at
partial flows and, less frequently, at high flows.
UNDERSTANDING PUMP CURVES
#6: MINIMUM FLOW –
PART TWO
IN HIS LAST DISCUSSION,
RON ASTALL
LOOKED AT WHAT HAPPENS WHEN A PUMP IS OPERATED IN AN OFF DESIGN
CONDITION, IN PARTICULAR, LOW FLOWS. HERE HE ELABORATES ON THE SUBJECT.
PUMP SCHOOL
RON ASTALL
FIG 1: HEAD VS. FLOW
FIG 2: FLOWDISTRIBUTION
IDEAL FLOW
DISTRIBUTION
DISTORTED FLOW
REVERSAL VORTEXING
FLOW
HEAD
HIGH TEMP RISE
LOW FLOW CAVITATION
REDUCED BEARING AND SEAL LIFE
REDUCED IMPELLER LIFE
SUCTION RECIRCULATION
DISCHARGE RECIRCULATION
CAVITATION DUE TO
LACK OF NPSHa
BEP
PREFERRED
SELECTION
ZONE
BEP
LOWFLOW
COURTESY PIA AUSTRALIAN PUMP TECHNICAL HANDBOOK