Head, Pressure and Specific Gravity with a Centrifugal Pump
Head, Pressure and Specific Gravity with a Centrifugal Pump
(OP)
How does specific gravity affect the head and pressure of a centrifugal pump? I was under the assumption that pressure and head were synonymous? But appears that head is constant with changes in specific gravity while pressure is directly related to specific gravity. As is hp.





RE: Head, Pressure and Specific Gravity with a Centrifugal Pump
Pressure in the solid world is a measure of Force / Unit Area. As it is not related to a datum, no potential energy is inferred, whereas Head in the fluid world is dimensionally a distance of 1 unit mass of that fluid above a datum, inferring it is a measure of potential energy. If pressure is of a fluid, it is presumed to be describing the potential energy of 1 unit of that fluid held a certain distance above a datum plane, the distance = head, and hence it also becomes a unit of potential energy, thus relating the two... in a fluid world.
Specific gravity directly affects the amount of potential energy of the system. A fluid with high specific gravity requires more power to pump, as there is more mass to move, and will have a higher measure of potential energy (in terms of pressure related to a datum) than a less dense fluid at any given value of head.
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RE: Head, Pressure and Specific Gravity with a Centrifugal Pump
I did not figure this out till long after college. Think of a pump as a rotating disc with two balanced buckets of water attached at the periphery. If you suddenly release the buckets so the one goes straight up, it will reach a certain height based on the speed of the disc. This height is the "head". If you increase the density of the fluid it will still go to this same head. But, there is more potential energy put into the denser fluid. The pressure that a column of denser fluid exerts will also be greater.
This is the reason that centrifugal pumps are rated and discussed in terms of head rather than pressure. Head is not feet of water unless that is the fluid that is being pumped.
RE: Head, Pressure and Specific Gravity with a Centrifugal Pump
The actual produced being pumped could have an effect the pumps performance in terms of flow / head but that becomes another discussion.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Head, Pressure and Specific Gravity with a Centrifugal Pump
Pressure is a measure of force per unit area, irrespective of height of liquid from datum.
Head is a measure of potential energy. It is a measure of height of the liquid from a datum. Though this head can be converted to pressure in terms of numerical values. Head * 0.1 * specific gravity = pressure (kg/sq cm).
Specific gravity of a product is numerically represented as density of product / density of water. It is a measure of how heavy the product is. If the product weighs more, its density is more. If its density is more, its specific gravity is more. If specific gravity is more, you will have to pump more which means your pump will require more energy.
Therefore, specific gravity effects pressure and not head.
To sum up, pressure and head aren't synonymous.
RE: Head, Pressure and Specific Gravity with a Centrifugal Pump
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Head, Pressure and Specific Gravity with a Centrifugal Pump