HEAD OR PRESSURE 1

[visitor]

MAYBE A SILLY QUESTION:

IF THE REST OF THE PAREMETERS ARE KEPT CONSTANT, A CENTRIFUGAL PUMP GIVING 1O FEET WHEN PUMPING WATER, WHICH HEIGTH WILL GIVE WITH A FLUID OF A STANDARD GRAVITY OF 0.5?

IN OTHER WORDS DOES THE PUMP GIVE THE SAME HEAD (THEN THE RESPONSE WOULD BE 10 FEET OF THAT FLUID, THE PRESSURE WOULD BE HALF OF THE WATER CASE) OR THE SAME PRESSURE (THEN THE RESPONSE WOULD BE 20 FEET)

 

[jhelm]

The pressure is what stays the same. Head is expressed in feet of water, so it is corrected for specific gravity, as you noted. The Cameron Hydraulic Data book has a detailed explanation of this and other concepts. Also see the Hydraulic Institute website at

http://www.pumps.org.

 

[butelja]

jhelm is incorrect. A centrifugal pump develops its head from the peripheral velocity of the impeller. Since velocity is unchanged, head is unchanged. Power required and pressure developed are directly proportional to the fluid density.

 

Can some I get some feed back on air separators and expansion tanks for a steam boiler system, ie. does a system always require one or both.

 

[tstead]

I second butelja's response. A centrifugal pump will put out the same head regardless of the fluid (assuming inviscid flow). If you have a viscous fluid, then all bets are off. Also, if your motor does not have enough juice, then the pump will not put out the flow and head that is required becuase the motor will keep tripping out.

Remember, head is always unchanged (again, assuming non-viscous fluids) and pressure is a result of the specific gravity by the equation: PSI = TDH (in feet) * S. G. / 2.31

One of the asumptions with the Bernoulli equation is that you have a non-viscous fluid. When you are pumping crude oil or molasses, then the pump performance changes because now you have viscous forces acting on the impeller that reduce the kinetic energy and thus the kinetic energy/head trasnfer phenonema at the pump casing.

So, as long as you have enough motor and the fluid is like water, that same pump will still pump the new fluid to ten feet, but your power draw will be cut in half and so will the pressure.

Regards,
Tim

 

[packdad]

I found this thread while searching for an answer as to whether a pump TDH curve must be adjusted for changes in specific gravity (SG). For example, if a vendor develops a TDH curve for (pressurized) water at 350 deg F, what adjustments must be made, if any, to know what curve to use for pumping water at 60 deg F?

tstead gives a good answer, above. However, for a more "official" (published) response, note that you can also refer to Question 4.25 from Karassik's Centrifugal Pump Clinic. Here, Karassik states that "for a given pump operating at a given speed and handling a definite volume, the energy applied to the fluids is the same for any fluid regardless of density. Therefore, we interpret this to mean that even though our test temperatures are different from the pump curve test temperature, we do not have to make corrections to enter the pump curves...".

Recall from your fluids textbook that pump head, expressed in units of length (feet), is really energy per unit weight of fluid. So, if the energy applied is constant, the TDH is constant (assuming inviscid fluids).

Just thought I'd pass some of this along. It's a seemingly simple question with an answer that may not be so straightforward for many (including me!).

 

[Artisi]

To put it very simply, if a pump is capable of generating 100ft head when pumping water it will also generate 100ft of head when pumping petrol and 100ft of head when pumping a slurry mix.
What will change is the pressure at the pump discharge as does the power input, both change in accordance with the specific gravity of the product.

However, as already pointed out by others, if the liquor is viscous then a different set of rules apply.

Naresuan University
Phitsanulok
Thailand

 

[ChickenSoop]

10 feet of head is constant, independent of SG. If SG is 0.5 or 2, head will always be 10 feet.

PSI will change, dependent on SG. PSI = feet x SG / 2.31