8 replies to this topic

[sgsuresh]

Hello

The pump manufacturer issues performance curve tested with water.
How to interpret these curves for the operating liquid?

Regards
S.Suresh

[ankur2061]

suresh,

Multiply the head at any given capacity with the specific gravity of the operating liquid to arrive at the new head on the performance curve.

Regards,
Ankur.

[DB Shah]

Head (meter) v/s volumetric flow rate for cf pump will be same for various liquids.
for eg at a given point if Q = 50 m3/hr & head = 100 mtr, for water (SG=1) & any other fluid SG=0.5 for eg the performance will be -

Water (SG=1)
Q = 50 m3/hr , Head = 100 mtr & discharge pressure = suction press + 100*1/10.2 bar

Whereas for fluid with sg =0.5
Q = 50 m3/hr, head = 100 mtr (will be same for any fluid-ignore viscosity effect), discharge press= suc press +100*0.5/10.2

To summarize-
Head for all fluids will remain same
Pressure & Power will be a function of SG

[breizh]

Hi Suresh,

Consider this resource :

http://www.pumpfunda... total head.pdf

hope this helps
Breizh

[P K Dwivedi]

suresh,

Multiply the head at any given capacity with the specific gravity of the operating liquid to arrive at the new head on the performance curve.

Regards,
Ankur.

Dear Ankur Srivastava Ji,

I think, you meant the pressure for the new specific gravity.

Use of the term "head in m or feet" has caused/is causing considerable confusion to the new users and students. They often confuse the head with the pressure, as the head can always be converted to pressure by multiplying with the specific gravity.

Because the operating pump is capable of raising/delivering the liquid to a particular height (reported as head and measured in m), it would be the same for any liquid, irrespective of its sp. gravity. However, this is subject to the condition that the pump driver is capable of adding that much energy to the pumped liquid through the mechanical device, i.e. pump.

The height to which the liquid could be raised by a pump would progressively go down, as more and more liquid is being involved in the pumping process. This is reflected by reducing head with the increasing capacity. The head on pump performance curve corresponding to a particular volumetric flow is merely an indication of energy added by the pump to the liquid. To estimate the ability of liquid to move from one point to the other, use of discharge pressure is more meaningful, which involves the use of parameter for liquid density. Also, the pump would develop the pressure at its discharge (i.e. differential head) only euqal to the resistance avaialble it its discharge circuit between delivery point to pump discharge flange. Use of the term head becomes nearly meaningless and often confusing at this point.

On the face of it, it appers to common user, how a pump would raise the pumped liquid to same height, if it was half as heavy compared to the orignal test liquid i.e. water? The point often not highlighted enough is that though the energy added remains the same, the dischrage pressure for lighter liquid is lower and is dictated by the pressure drop required for the lighter liquid to move from pump discharge to the point of delivery. For higher density fluid, the pump would have to develop more discharge pressure, dictated by the pressure drop required for the heavier liquid to move from pump discharge to the point of delivery.

The issue is in pumping Industry Practices. One still finds it hard to understand the difference in the term "head" used in pumping industry (which is independent of the parameter density) with that used in the rest of engineering and science world that teaches that Head = Pressure/Density.

I am also curious to find out, how to prevent the confusion in using the term head/differential head with dischrage pressure/differential pressure?

Kind Regards,

P K Dwivedi

[ankur2061]

All,

My apologies for any confusion that I have created due to the half-answer that I have provided . When I do pump hydraulic calcualtions, the following relations are used:

Head to Pressure Conversion

P = H*SG / 10 ...................................................(1)

where:

P = Pressure in kg/cm²

H = Head in meters

SG = Speific gravity of the liquid


P = H*SG / 10.198 ..........................................(2)

where:

P = Pressure in bar

H = Head in m

SG = specific gravity of the liquid


P = H*SG / 2.31 .................................(3)

where:

P = pressure in psi

H = Head in ft

SG = specific Gravity of Liquid

Regards,
Ankur.

[flash]

Dwivedi,

<On the face of it, it appers to common user, how a pump would raise the pumped liquid to same height, if it was half as heavy compared to the orignal test liquid i.e. water? The point often not highlighted enough is that though the energy added remains the same, the dischrage pressure for lighter liquid is lower>

Agreed, the pump will lift the different SG liquids to the same height provided they are not highly viscous. However, the power consumption will be different as it has a factor SG in calculation. I guess when you said energy added is nothing but the energy added by the pump (hydraulic power) and it will not be the same. Is that right or I am missing something?

[kkala]

A few notes that may be useful.
1. Power P supplied to the pump by its drive is P=V*ΔP/ef=(M/ρ)*(h*ρ*g)/ef=M*h*g/ef. Thus P/M=h*g/ef, meaning that energy per unit mass of pumped liquid remains same if the liquid changes (volumetric flow remains same).
V=volumetric flow rate, ΔP=differential pump pressure, ef=pump efficiency, M=mass flow rate, ρ=pumped liquid density, g=gravity acceleration, h=pump head for the specific flow rate V.
2. The term "head" alone corresponds to pump differential pressure (discharge-suction pressure in operation). Perry uses it in this sense, alternatively calling it "total head". Perry also reports "total suction head" and "total discharge head"; the difference of the two is the "total dynamic head", identical to "total head" or "head" according to my understanding. Probably there is a popular terminology beside an official one established by some institute.
"Head" is used here in the mentioned sense. In lieu of it "differential head" is seldom used, which may not be a successful term (contrary to differential pressure).
Note: See Perry's Chemical Engineers' handbook, 7th edition (1997), Section 10 - Transport and Storage of fluids, Chapter "Pumping of liquids and Gases" - Terminology - Centrifugal Pumps.
3. Curve of NPSHr versus volumetric flow rate remains same. Actually a few liquids have lower NPSH requirements than water (see Perry, ibid, Fig. 10-25 for hydrocarbons), but (at least here) we conservatively neglect this NPSHr reduction. Advice on relevant practices elsewhere would be appreciated.
Note: For an installed centrifugal pump, NPSHa is generally different to that of water, due to different vapor pressures (e.g. taking from an atmospheric tank).
4. Power has to be multiplied by (liquid density/water density at about 4 oC). This means multiplication by liquid specific gravity as used in USA. Water density at 4 oC is practically same as water density at ambient temperature.
5. Above is referring to a specific RPM value and to one pump impeller.

[sgsuresh]

Dear ALL

Thanks for your answers & references.

The correlation for head w.r.t. water & the operating fluid is clarified. How about the power calculation? Please clarify.

Regards
S.Suresh.