Differential Head 2

[Milkboy]

Does Differential Head differ to Head (produced by centrif pump)

Pump Data Sheet states

Del Press = 33.23 BarG
Diff Head = 341m
Density = 957 kg/m3


Im looking to find the Box Pressure using Rho.G.h
but I get an answer of 33 BarG
This made me think Diff Head was different to Head

Is this so


TIA+

-
Milkboy

 

[Fizzhead]

Milkboy:

Differential Head is the pressure differential created by the pump under a given set of conditions with a reference fluid and is independent of the absolute pressure of the environment in which the pump operates. That is, the pump could be extracting fluid from a pressurized source (relative to the environment, could be the bottom of a reservoir or a pressurized tank ), or from a source at a negative pressure relative to the local environment and would be capable of imparting the same net increase in internal energy in either situation.

This differs from "Head" depending upon your usage of this term. "Head" is often used to refer to the conditions under a particular service (i.e., for a particular fluid). Thus, for pumping crude oil, the pump "Head" would be different than the "Differential Head" you quoted due to variability in density and viscosity between the two fluids. As seen in discussions of "Normal" or "Standard" conditions, one must be certain what the basis of the author is. That is, the author should be clear about this.

 

[TD2K]

The head produced by a centrifugal pump should not vary as a function of the fluid's density. The pump produces the same head whether it's handling propane (SG about 0.5) or water (SG = 1). The differential pressure will change as a function of the SG however.

Viscosity will affect the pump curve but typically isn't a significant factor until you begin to exceed 50 cP.

For SI units, my copy of the GPSA data book has Head (m) = 0.102 * dP (kPa) / SG. dP for your numbers should be then 3199 kPa or 32 bar. Is the SG at pumping temperature (which it should be) or at standard conditions? What is the suction pressure? The delivery pressure will be the dP across your pump plus the suction pressure.

 

[25362]

One -probably irrelevant- small addition. Since these pumps, being rotodynamic, impart velocity "heads" to the pumped fluids, differential heads developed by centrifugal pumps, as well as other characteristics, change with flow rates.

 

[Fizzhead]

Td2K:

I am a little confused by your statements. First you say that Head should not be affected by density and then quote the GPSA data book for a Head equation that shows an inverse relationship between head and S.G. Have I forgotten something from school? Please clarify.

 

[25362]

Although head and pressure are about synonyms, they really aren't.
Head, ft or m, is converted into pressure, psi or kg/m², when multiplied by the density, lb/ft³ or kg/m³, respectively.

The "pressure head" of a column of water 2.31 ft high equals 1 psi. A column 4 ft high of a naphtha with a sp.gr. (SG) of 0.59, would also exert a pressure of 2.31 x 1/0.59 = 1 psi. In summary, differential pressure is related to differential head as follows:

[Δ]P_psi = (SG)[Δ]H_ft/2.31​

Centrifugal pumps impart kinetic energy (velocity) to the fluid, which when measured in units of head, would be about equal for all -Newtonian- fluids, barring influences of viscosity. However, when expressed in pressure units, the density, as seen, plays a role.

Thus for a centrifugal pump discharging water at 110 psig with a suction pressure of 10 psig, the differential pressure when pumping the above naphtha (same suction pressure) would be:

[Δ]P = (110 - 10) x 0.59/1.0 = 59 psi

and the discharge pressure would be 10 + 59 = 69 psig.

Although the head will not change, the power demand would. Work is measured in foot-pounds. The feet of head is not affected by switching from water to naphtha, but the mass of liquid pumped is proportional to the specific gravity (SG). Thus, if the SG drops by 41% and the liquid flow rate (GPM) stays constant, the work done by a typical centrifugal pump would drop by 41%, and so would the motor's electrical work.

As for temperature effects. If the viscosity doesn't change much on cooling the fluid say, a light hydrocarbon, by 100^oF, and only its SG increases by 5%, the discharge pressure (with constant suction pressure) would rise by 5%.
So, if the pump developed 100 psi of differential pressure at 200 deg F, the differential pressure would increase to 105 psi upon cooling the fluid to 100 deg F.

Opening a control valve to keep the discharge pressure constant will result in a flow (GPM) increase. The discharge pressure drops because a typical centrifugal pump develops less feet of head at higher flow rates. It also means that the driver delivers more work (as seen in the motor amperage) to accomodate this maneuver.

Moreover, even for the same liquid, if one installs pressure gages on two close points on a pump's horizontal delivery line having two different diameters, the readings would differ.
The reading on the larger diameter would be higher, because the resulting reduction in kinetic energy (velocity) is converted into pressure energy following the famous Bernoulli equation. This will happen even when the flow runs -contrary to common sense- from the lower to the higher pressure.

 

[TD2K]

25362 has outlined it very nicely.