Plumbing Connection 2-2015

58 / 132

5 8 PLUMBING CONNECTION WINTER 2015
This recirculation can produce high vibration levels,
surging, axial shuttling of the shaft and mechanical damage
to the impeller and casing. When severe at low flows,
recirculation cavitation like damage will be evident on the
pressure (non visible) side of the impeller eye vanes.
This should not be confused with classic cavitation
damage due to inadequate NPSHA which will appear on the
visible low pressure side of the impeller vanes. The noise
from recirculation will be similar to cavitation noise but is
more random in character. Typically, recirculation noise will
reduce as the flow is increased, whilst classic cavitation
noise will normally increase with higher flows. These
problems will impact on other areas of the pump affecting
the reliability of seals and bearings in particular.
Research by J. L. Hallam, studying 480 pumps and 1881
failures over a five year period in the refining industry
concluded that pumps with high values of NSS (above
12,800 in metric units or 11,000 in US units) had a failure
rate approximately double that of lower NSS pumps.
This has resulted in many Oil Industry specifications
applying arbitrary limits on NSS.
Overall, this is not a bad thing, but it is important to
remember that it was a statistical survey and such an
arbitrary approach may reject a few perfectly good pumps
that have tested out as smooth units despite having a
high NSS. It is also important to remember that a well
selected pump with a higher value of NSS will still be a
better proposition than a pump with low NSS selected near
minimum flow.
The benefit of this knowledge is as a tool to predict the
likely NSS requirement for the pumps at the system design
stage. This gives the designer the option to raise vessels
etc. if it looks like the original system NPSHA will result in
the need for pumps with a narrow hydraulic stability range.
IMPACT ON MINIMUM FLOW
Most commercially available pumps now comply with
the upper limit of NSS as a matter of routine, but there
may be some pumps in the field with high numbers for this
parameter due to suction constraints. Pumps with a High
PUMP SCHOOL
RON ASTALL
vale of NSS will probably be much more sensitive to low
flow operation and additional care will be required, such as
setting higher minimum flow values for these units.
*This article originally appeared in Pump Industry
magazine. For a limited time, readers of Plumbing
Connection can subscribe for free to Pump Industry
magazine. Go to now to take
up this offer and get more information on pumps.
IT IS ALSO IMPORTANT TO
REMEMBER THAT A WELL SELECTED
PUMP WITH A HIGHER VALUE OF NSS
WILL STILL BE A BETTER PROPOSITION
THAN A PUMP WITH LOW NSS
SELECTED NEAR MINIMUM FLOW.
FIG 4: STABLE OPERATING
WINDOW VS SUCTION SPECIFIC SPEED
CAPACITY % OF BEST EFFICIENCY
FIG 3: SUCTION SPECIFIC SPEED
LOW Nss
(HIGH NPSH)
HIGH Nss
(LOW NPSH)
N SS=
NPSHR3/4
Q N.
NPSH - ft
60
50
40
30
20
10
0
0 20 40 60 80 100 120 140
NSS =
NPSHR 3/4
Q N.
ROSS/LOBANOFF CURVES
(USGPM. FT. RPM)
STABLE WINDOW
7000
8000
9000
10000
11000
12000
20000
Ron Astall is a Mechanical Engineer with extensive
pump industry experience including Engineering, Contract
Management, Applications, Sales, Marketing and Quality
Assurance; particularly in the Process and Petrochemical
Industries in his role at United Pumps Australia.
With his consultancy, Astech Pumping Services and through
Strategic Achievement, he has also been an active provider of
pump industry training. Ron is the current President of Pump
Industry Australia (PIA) and Contracts Manager at United
Pumps Australia.