November 05, 2008

Find out Pump Head without curve

You are handling lot many centrifugal pumps & is always struggling for design data which may or may not be available specially in case of vintage units its very difficult to recollect them.

Even in newer plants if you do not have all procedures in place for documents handling you might face the same problem. What to do in such a case? Boss is not ready to listen any excuse.....You know that there is something wrong which can be improved and therefore you get a good recognition.......But

Here is the answer....

Few rules are there based on data analysed by experts in this field which are generally true for conventional systems. Such rules can be utilized when you do not have any other source to know least an initial guess is must.

Shut-off head can be calculated by obtaining the square of impeller dia in inches. This gives you shut off head in feet. That means if your dia is 15" then 15 x 15 = 225 feet will be the shut off head for water.

There are other conditions also in this rule.
1. This rule is applicable at 1800 RPM.
2. This rule is applicable for water so apply density correction for other fluids.
3. Its generally accurate by ~95%.
4. Its a guide line only.
5. Its applicable to 85% population with 15% being exceptions in design.
6. The rule do not apply to PD pumps.

What is the shutoff head? The shutoff head is the beginning of the pump curve. It represents maximum elevation (in feet or meters) at zero flow. The performance curve proceeds to and ends at a point called maximum flow at zero elevation.

Now how can you use this information. Say you measure the performance of a pump you have collected all the data but you dont have pump curve. But you know from maintenance person that impeller dia is 15". So you know that this can develop 225 feet of water head.

Let us say fluid is also different say alcohol where SG = 0.8
So you know that 225 feet is = 225 * .3048 meters of water = 68.6 meter
Now fluid is alcohol so new head shall be = 68.6 * 0.8 = 54.9 meter

So the differential pressure must be 5.49 Kg/cm2.

No....No....No you are ......wrong....This is shut off differential head i.e. zero flow. So as a thumb rule it must be ~85% of shut off head i.e. 5.49 * 0.85 = 4.66 Kg/cm2. So if your actual differential is significantly lower than you can say that the pump is having something wrong with it.

What is that something......Well you yourself have to identify it.????

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