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 :

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 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 same number.

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

SUCTION 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.

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 using.

We use this number to predict cavitation problems with your impeller selection.

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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