Head, Pressure and Specific Gravity with a Centrifugal Pump

[tmartin125]

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.

 

[BigInch]

Not exactly synonymous, unless in a fluid world.
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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[Compositepro]

A centrifugal pump operates by spinning fluid with an impeller, which gives fluid a certain velocity. The head of a pump in terms of the height of a pumped fluid column is basically constant. If the fluid density increases the head stays the same but the pressure increases. One point of confusion is that head, in feet of water, is a pressure unit equivalent to psi and will increase. But head, in feet of pumped fluid, does not.
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.

 

[Artisi]

When discussing a centrifugal pump, head developed at any point on the curve remains the same irrespective of SG, what changes is the pressure at the pump discharge by the SG -- head x SG.
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.)

 

[lhwingking]

I totally agree with BigInch. i actually want to use the same words. Pressure and head are NOT synonymous.

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.

 

[Artisi]

Putting it into simple understandable terms.

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.)

 

[tmartin125]

Thank you all for your detailed responses. I have so frequently converted ft of head to psi by dividing by 2.31 which only is true for fresh water. Need to revise that factor for differing fluids using 144/Specific Weight. Thanks again.