June 23, 2008

Impact of Specific Gravity on Pumps

The pump performance or characteristic curve is often referred to as a water performance curve (see Figure 1). Water is naturally the fluid of choice for testing pumps because of its availability. The question is: can the water performance curve be used for other fluids? The answer is YES with care.



The pump manufacturers will sometimes identify the unit of the total head axis as feet of water, or sometimes there will be a notice somewhere on the chart saying: tested with water at 70ºF. This is the manufacturer’s way of telling the user that the pump was tested with water and that the power curves are ONLY valid for water, but careful there are other pitfalls.

Related References


Here is the question that you need to think about. Say that you measure the shut-off head of a pump with water, will you get a different value if the fluid is a dense salt solution (assuming the viscosity is the same)? The answer is NO. Why....Because pump is going to develop the same head.



Head is what?.... Height of Liquid.....So Height of liquid column will remain same whether it is water or salt solution but the pressure exerted by the head will be different. Head remains same is the typical property of a centrifugal pump....Keep this always in mind whenever you are dealing with pumps. They are constant head machines.

Head = Pressure / SG

So,

Pressure / SG = Constant

So,

P1/SG1 = P2/SG2

Thus developed pressure will be = curve pressure x SG / SG of water (1.0)

Here the curve pressure is the developed pressure for water, SG is the specific gravity of fluid in question & SG of water is 1.0 at test conditions or curve conditions.

Impact on Power

Power = Q (M3/Hr) x H (Head in meters) x SG / 367.2 / Efficiency of pump

So the power will be different because of SG term in the equation. So in case of different fluid do not consider power from pump curve it is better to calculate from above formula...OR if you consider curve power correct it for SG change.

Next I am going to cover the impact of viscosity on pumping.

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