Plumbing Connection 4-2015

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TION SUMMER 2015 61 1300 123 451
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OTHER SPEEDS AND EXAMPLES
I have chosen the particular example of selecting a pump
for an 8 pole running speed, but the principle is universal.
Using the Affinity Laws, you can ‘adjust’ any proposed
pump duty to allow initial selection from available standard
range charts and curves. Other examples include selection
for engine drives or using 50Hz curves to select for 60Hz
service. A few more examples are shown in the table above.
BE CAREFUL!
Using the Affinity Laws in this way is great for the initial
selection process but these laws assume that pump
efficiency does not change with speed. Efficiency correction
factors may apply and the NPSHR prediction may not always
be that accurate.
The pump manufacturer should ultimately be consulted to
confirm pump performance at the final running speed and to
confirm the impeller diameter.
When selecting pumps for a speed higher than the
published curve, it is critical that the pump design be
checked as being suitable for the higher power and pressure
ratings that will result.
The above is a useful tool that allows us to select
centrifugal pumps using available range charts and curves
when published curves are not available at the intended
running speed. However, care must to be taken to confirm
actual performance and suitability once the initial selection
has been made.
*This article originally appeared in Pump Industry
magazine
EXAMPLES:
Scenario
Affinity Speed Ratio
= Selection Curve
Speed
Proposed Duty
Speed
‘Adjusted Duty’
to be used for
selection
Multiply actual flow
and head by
Flow Head
Using 50Hz speed curves to
select for 60Hz operation x 0.833 x (0.833) 2
Using 1450 rpm curves to
select for a Diesel engine
drive running at 1800 rpm
x 0.806 x (0.806) 2
Using 1450 rpm curves to
select for a Diesel engine
drive running at 1200 rpm
x 1.208 x (1.208) 2
Using 2980 rpm curves to
select for 4000 rpm turbine
or variable speed drive
x 0.745 x (0.745) 2
Using 60Hz speed curves to
select for 50Hz operation x 1.2 x (1.2)2
Use the ‘adjusted’ flow and head to do an initial selection using the available curves.
Once a pump has been selected, use the Affinity Laws in reverse to calculate the
expected performance at the actual operating speeds.
0.833 — 5060
0.806 — 14501800
1.208 — 14501200
0.745 — 29804000
— 6050