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The Practical Pumping Handbook ~ ................................................................ - ---- ................................ =.

unacceptable and can be ignored. Also, the extent of the temperature
change needed to provide an appreciable difference in NPSHA will
usually render this method ineffective.

4.4.2.4 Friction losses (Hf)
As pump inlet piping is notoriously bad in the vast majority of
installations throughout the world, this is the area where significant
improvements can often be realized. However, the tendency to shorten
the length of suction piping simply to reduce friction losses should be
resisted as it could deny the liquid the opportunity of a smooth flow
path to the eye of the impeller. This, in turn, could cause turbulence
and result in air entrainment difficulties that create the same symptoms
as cavitation. To avoid this, the pump should be provided with a
straight run of suction line in a length equivalent to 5 to 10 times the
diameter of the pipe. The smaller multiplier should be used on the
larger pipe diameters and vice versa.
The most effective way of reducing the friction losses on the suction
side is to increase the size of the line. For example, friction losses can be
reduced by more than 50% by replacing a 12 inch line with a 14 inch
line. Exchanging a 6 inch line for an 8 inch line can reduce the friction
losses by as much as 75%. Reduction in friction losses can be achieved
even with the same line size by incorporating long sweep elbows,
changing valve types and reducing their number.
Suction Strainers that arc left over from the commissioning stage of a
new plant can also be a problem. The blockage in the strainer basket
steadily increases the friction loss to an unacceptable level. Even when a
strainer is considered necessary in the process, it can frequently be
located downstream of the pump and the pump selected to handle the
solid sizes expected.

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