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The Practical Pumping Handbook
NPSHA = Hs + Ha Hvp - Hf
= 45 + 34.474 6.775 - 0.378
= 72.321 feet.
4.5 Suction specific speed
In some industries, the concept of Suction Specific Speed (Nss) has
been introduced to compare the ideal flow rate and rotational speed
with the NPSH required at that flow rate. This renders the NPSH a
dimensionless number for convenient comparison of the hydrodynamic
conditions that exist in the eye of the impeller.
RPM x Q05
Nss =
NPSHRO 75
where RPM = Pump rotational speed
Q = Flow at BEPin GPM
NPSHR = NPSH required at BEP in feet
The suction specific speed is calculated from the information on the
manufacturer's pump performance curve and only at the Best Efficiency
Point which is usually on the maximum diameter impeller. Consequently,
a single line curve may not always be an appropriate reference, and a
composite pump curve as shown in Figure 2.8 should be used. It is also
further assumed that the Best Efficiency Point reflects the flow for which
the eye of the impeller was originally designed.
When a double suction impeller is being considered, the flow (Q) in the
above equation should be divided by two as the intent is to compare
the performance in each individual impeller eye.
In many applications, the ability to use a pump with a low NPSH
requirement would prove to be very beneficial in the physical design of
the system. However, if this is carried to the extreme in pump design it
has proved to cause recirculation problems within the impeller (see
4.6.1 below). This is particularly the case as it relates to operation of
the pump at flows which may be much lower than the BEE The use of
suction specific speed provides a convenient method of identifying
when such a condition may occur.
As the NPSH required is reduced, the value of the suction specific
speed will increase. However, it has been noted that there is a tendency
towards a decrease in pump reliability when the suction specific speed
exceeds 11,000.
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