SUBJECT: The difference between specific
speed and suction specific speed 9-12
The best way to describe the shape of
an impeller is to use its specific speed number. This is a
dimensionless number that was generated by the formula :
- Capacity in GPM. Largest impeller at the BEP.
- Head. use the larget impeller at the bep. To get the 3/4
power, cube the number and take the square root of the resultant
number, two times
The following chart gives you a graphic picture of the impeller
shape represented by this number:
The major use of the specific speed number is to help you specify
pumps that are more efficient.
- The maximum pump efficiency is obtained in the specific speed
range of 2000 to 3000.
- Pumps for high head low capacity occupy the range 500 to 1000.
While low head high capacity pumps may have a specific speed of
15,000 or larger.
- For a given head and capacity the good news is that the pump
having the highest specific speed, that will meet the
requirements, probably will be the smallest size and the least
expensive. The bad news it that it will run at the highest speed
where abrasive wear and cavitation damage become a problem.
- Efficiencies start dropping drastically at specific speeds
below 1000. Also smaller capacities exhibit lower efficiencies
than higher capacities at all specific speeds.
- In propeller and other high specific speed impellers (axial
flow) it is not practical to use a volute casing. Instead, the
impeller is enclosed in a pipe like casing.
- The lower the specific speed number, the higher the power loss
you get with wear ring clearance.
The clearance between the impeller and the tongue of the volute
has a bearing on efficiency, pressure pulsations and cavitation. For
high efficiency you would want a small clearance, but this produces
larger pressure pulsations and the increased flow in this area can
reduce the fluid pressure enough to cause flashing of the product and
a type of cavitation known as " The vane passing syndrome".
For impellers up to fourteen inches in diameter (355 mm) this
clearance should be a minimum of four percent of the impeller
diameter. If you are using greater than fourteen inch diameter
impellers the clearance should be at least six percent of the
impeller diameter. Also remember that as this clearance increases the
impeller experiences some slippage. That is the major reason that we
do not like to remove more than ten percent of the impeller diameter
when trimming is called for.
If you work in both metric and imperial units as I do, the subject
of specific speed becomes very confusing because both systems use the
same specific speed numbers to describe the impeller shape. They do
this even though they use a different set of units to arrive at the
In the metric system the capacity is calculated in liters/ minute
and the head in meters. Knowledgeable people in this area feel that
if the calculations are done in imperial or other metric units the
final number should be reduced by the following amount:
U.S. Gallons/ minute and feet divide the result by 1.63
U.K. Imperial gallons and feet divide the result by 1.93
M3/hour and meters divide the result by 1.50
SPECIFIC SPEED is another number that we use in pump
selection. The formula looks the same as the regular specific speed
formula, but in this formula we use the NPSH required number rather
than the total head produced by the pump.
- Capacity in GPM. Largest impeller at the BEP.
- NPSHR Larget impeller at the bep. To get the 3/4 power: cube
the number and take the square root of the resultant number, two
NPSHR = Net positive suction head required to prevent cavitation.
Remember that this number is for sixty eight degree F. (20°C)
fresh water. You are going to have to add the vapor pressure of you
product to this number to get the real number that you will be
We use this number to predict cavitation problems with your
- The flow angle of the inlet vanes and the number of vanes
affect this number.
- A desired value would be below 8500 with impellers having a
flow angle of about seventeen degrees and five to seven vanes. The
higher the flow angle number, the faster the liquid will travel
and the lower suction head (pressure) we will get.
- Boiler feed and condensate pumps often require suction
specific speed numbers as high as 12,000 to 18,000 because of the
temperature and pressure of the water. To get to these values the
impeller inlet flow angle is reduced to a low as ten degrees and
the number of vanes reduced to as little as four . Fewer and
thinner vanes help to reduce the blockage in the impeller inlet. A
disadvantage to these low flow angles is that the pump will
probably run very rough at below fifty percent of capacity.
- Water applications can run at these higher numbers because the
amount of fluid expansion is very low for hot water. Mixed
hydro-carbons have this same advantage because unlike a single
product, the flashing of the mixed hydro-carbons does not take
place all at the same time.
- The higher the suction specific speed number the narrower the
stable window of operation.
- Inducers have been used successfully with suction specific
speed numbers of approximately 24, 000
- Should the available NPSH be so low that a suction specific
speed number of more than 18,000 is required, then a separate
axial flow impeller (an inducer) can be used ahead of the
centrifugal impeller to prevent cavitation. Its flow angle is some
where between five and ten degrees with typically two vanes and no
more than four. In other instances a booster pump can be installed
between the pump and the source.
- In their desire to quote a low NPSH required some
manufacturers will cut away the impeller inlet vanes to reduce
fluid drag and thereby lower the NPSH required. If this has been
done with your application, you must insure that the impeller to
volute clearance is adjusted correctly with open impeller designs
and the wear ring clearance meets the manufacturers specifications
with closed impeller designs, or you will experience internal
recirculation problems and cavitation at the impeller outlet vane
tips. Keep the suction specific speed number below 8500 and this
problem should never comes up.
In the metric system we calculate the capacity in liters/sec and
the NPSH in meters. You should try to keep the final SSS number below
5200. Above 7800 you're going to have trouble with internal
recirculation and cavitation.
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