Effect of Liquid Properties on Pumps

Specific Gravity

Have you ever wondered why oil floats above the water if you mix them together, whereas the sand sinks to the bottom, if you mix sand and the water .

This phenomenon of liquids/solids floating / sinking under the water due to the difference in the density is called the relative density.

As you see in the formula,

Ratio of Density of liquid/solids to the density of water is called relative density.

If the SG > 1, the liquids tend to be heavier than water

If the SG < 1, the liquid tends to be lighter than water.

SG & its uses

How does it impact the pump system then.

Well, it does in two ways.

It impacts the pressure head calculations and the shaft power calculations.

As you see in the formula,if the head generated by the pump is constant , then the pressure is directly proportional to the specific gravity.

i.e, if you run a pump that gives the discharge head of 10m, let’s say the pressure is 1.2 barg for SG 1.2 liquid. The same pump would give the same head, but the discharge pressure value could be 0.8 barg for SG 0.8.

Hence, don’t panic, if the pressure goes up or goes down when you change the liquids.

What it usually affects is the motor.

i.e., Workdone by the pump to produce the head is directly proportional to the specific gravity of the liquid.

So higher the SG, higher the power consumption. This is being reflected in this formula as shown in the screen.

Viscosity

If you carefully observe this animation, the water is running faster and honey is coming down slower from the container.

This variation of running speed among liquids is due to the intermolecular friction between them that induces a resistance to motion.

This is defined as viscosity and it  is a property of a liquid that is dependant on temperature.

Let’s compare the three liquids again,

Water having low viscosity. i.e., around 1 cP flows faster due to poor intermolecular bond between them. Molecules of water, sort of hate each other.

Whereas honey on the other end of the spectrum , i.e, typically around 10,000 cps at 21 deg C. seems to have a stronger intermolecular bonds.

Performance vs Viscosity

If you observe this pump performance curve closely, the flow and head of viscous fluid dips down significantly when compared with its performance against water. 

In general, it reduces the efficiency, increases the power consumption and also increases the Net Positive Suction Head Requirement of a pump.

The Viscosity correction factor ( the indicator of how much reduction that you will experience on each criterion such as flow, head, efficiency etc) could be shown on most of the pump performance curves .

The shaft power calculation will be revised to reflect the correction factors as follows.

As you may have observed in the pump performance curve, that the efficiency gets affected a lot due to the viscosity.

A typical comparison of efficiency vs viscosity is as follows :

As you see for a centrifugal pump, the efficiency decreases for the same pump when the liquid becomes viscous/ thicker.

Whereas when you observe a positive displacement pump, the efficiency is slightly lower for the thinner fluids, probably due to the slips which occurs on close clearance parts.

However, when the liquid is viscous, the PD pump tends to perform well and yield a better volumetric efficiency.

That’s why for viscosities less than 150 cP , we tend to prefer Centrifugal pumps and for viscosities above 150 cP , we prefer PD pumps.

In the next segment, we are going to discuss more about Vapour Pressure & Net Positive Suction Head.

If you would like to listen to this segment, the above link is for you to understand the concept a bit better. 

Image Credits :

Getty Images - Vapour Pressure, Specific Gravity and Viscosity

Pump Operating Principle - Licensed under CC by NC 2.0