Specific Gravity and Pump Selection

The density of a substance is defined as its mass per unit volume. Water has a density of 997 kg/m3 or 62.43 pounds per cubic foot. The term specific gravity refers to the density of a substance, such as another fluid, relative to the density of water. 

Because water is the standard scientists use to determine specific gravity, it has been assigned a specific gravity of 1. Thus, a liquid having a specific gravity of 0.4 will be 60% lighter than water, whereas a liquid having a specific gravity of 1.4 will be 40% higher than water.

Specific gravity is important when sizing a centrifugal pump because it’s the determining factor in terms of a fluid’s weight. Fluid weight has a direct impact on the amount of work that your pump can perform which will, in turn, determine factors such as the amount of horsepower needed and the pressure that the pump can develop. 

The presence of solids increases the Specific Gravity (SG) and weight of the fluid being moved. The higher the Specific Gravity, the more motor power is required. For example, wastewater with an SG of 1 will have little impact on motor power, while slurries or sludges with a higher specific density and weight will require more motor power. In fact, the increase or decrease in power consumed is directly proportional to the specific gravity of the liquid being moved.

The pump’s pressure will change in direct proportion to the specific gravity of the fluid to be moved. While a pump may develop about 50 PSI (Pounds per Square Inch) while pumping water, when pumping a fluid with a SG of 1.2, it will only develop pressure of 41.7 PSI. We get this figure by dividing 50 by 1.2. Specific gravity is a relevant consideration when you are selecting a pump to develop a given pressure.

What’s the best advice we can give, all of the above considered? Seek professional help when choosing a dewatering pump. That way, you can be certain that the equipment you are using aligns with the needs of your application. The professionals at Atlas Copco are experts in pumps and the myriad number of applications to which they can be assigned. Hopefully, you now have a better idea of what questions you should ask yourself. You can get the answers you need from an Atlas Copco pump equipment expert.