It is very important when working with centrifugal pumps that there is a clear understanding of how specific gravity, commonly abbreviated to just SG, affects or doesn’t affect pump output in terms of head and or pressure.
Let’s first define SG. The dictionary definition is simply “ the ratio of the density of a substance to that of water”. As shown below one cubic meter of water weighs 1000 kg, while an equal volume of granite weighs 2700 Kg. , 2.7 times as much. Hence granite is said to have an SG of 2.7.
A word of caution! Whatever the sample size, there must be no voids. Materials such as sand or gravel must be adjusted by a factor to allow for the air space between the particles. This factor is referred to as the “bulk ratio”. A subject for another day!
Turning attention to the head. It is sometimes defined as “the vertical distance (in feet or meters) from the elevation of the energy grade line on the suction side of the pump to the elevation of the energy grade line on the discharge side of the pump.”
When applied to a centrifugal pump’s output, it is more simply described as “the height a pump can lift a liquid to”. For example, if a centrifugal pump tries to pump straight up a 300 ft pipe, but flow stops as the water in the pipe reaches 231 ft, the pump is said to have a shut-off head of 231 ft.
You may ask, why not express the output of a centrifugal pump in PSI? Well, the answer to that question is based on the operating physics of a centrifugal pump.
Centrifugal pumps use centrifugal force generated as a liquid is spun down the vanes of the impeller to create pressure within the pump’s casing/volute. The heavier the liquid, the more force or pressure is created. Some pump manufacturers rate performance at a stated output pressure based on water, usually in terms of PSI, but the majority of pump manufacturers use the term head, as it does not vary with liquid SG.
The basis of the statement in bold above is illustrated to the right. The pumps are identical, but the blue column represents water being lifted while the orange column represents a liquid with an SG of 1.5 being lifted. Notice the output pressures vary in proportion to the SG while the rating for lift remains constant.
The centrifugal pump handling a liquid that has an SG of 1.5 is accelerating a liquid weighing 1.5 times that of water down its impeller vanes, creating a force (pressure) that is 1.5 times that of an identical pump pumping water. The column of liquid, however, also weighs 1.5 times more than water. Hence it needs a pressure equal to the pressure on water times 1.5 to lift the heavier liquid to the same height.
The obvious question then becomes, if pressure gauges measure in PSI, how do we relate that to a pump’s output in ft or meters of the head? The full explanation of that is again a subject for another day, but the equation below and the diagram to the right will get you by for now.
I hope today’s blog helps explain the relationship between SG, Head, and Pressure. As mentioned in today’s blog, I will in future blogs address the “Bulk Ratio” and fully explain the 2.31 ratio of head to PSI.
Stay tuned, and bye for now.