19 replies to this topic

[ProcessEng_12]

I have two identical pumps A and B.

The fluids used in both pumps are different.

 

The properties of fluid used in Pump A are:

Density: 1154 kg/m3

Kinematic viscosity: 0.91 * 10^(-6)

Flow: 25m3/hr

Dia of discharge: 3 inches

 

The properties of fluid used in Pump B are:

Density: 1066 kg/m3

Kinematic viscosity: 0.94 * 10^(-6)

Flow: 25m3/hr

Dia of discharge: 3 inches

 

The discharge pressure in Pressure gauge of Pump A is 1.2 kg/cm2g and that of Pump B is 2kg/cm2g.

 

The density of fluid used in Pump A is more than that in B still the discharge pressure is viceversa. This really contradicts what i was taught in class as discharge pressure is always proportional to the density of fluid. Pls help.

 

And One more thing since the pumps are identical, will both the pumps give same head?

 

Thanks in advance.

 

 

 

 

 

 

[Chemitofreak]

Nishant,

 

Discharge Pressure = Differential Head + Suction Pressure

 

Therefore, check the suction side of the system as well, are both the system identical?

 

Please draw a schematic for better understanding of the system.

[ProcessEng_12]

Chemitofreak:

 

Yes both the system are identical. 

 

I have described as two different pumps to avoid the confusion. In reality there is only single pump but the composition of mixture changes as the process conditions change.

[ProcessEng_12]

Thanks Gerardo for replying.

 

I think the pump handling mercury will have the highest discharge pressure since it has greater density. Isn't it?

But in my case the pump A (1.2kg/cm2g) has less discharge pressure than that of pump B (2 kg/cm2g), even though A handles higher density (1154 kg/m3) liquid than in liquid (1066kg/m3) pumped in B.

 

This really contradicts the whole idea if the head generated is constant (pumps are identical).

[Three]

Nishant, I'll start with your last question first - I think it's worth clarifying the difference between head and developed presssure, you haven't specified what kind of pumps you are working with, which would be useful, but I'll assume it's centrifugal.

 

(Theoretically) these pumps will always supply a given head regardless what type of fluid they are pumping, if you are running oil through it you should get the same head as if you ran water through it (again theoretically assuming the pump is designed to handle both of these fluids optimally and with minimal losses etc.). The difference however is the power required to achieve this head, for an oil with a lower density than water less power will be required to achieve the same same head as would be with a column of water e.g. raising the oil column by 20m consumes less power than raising the water column 20m.

 

So yes, both of your pumps being identical they should provide the same head, but as you correctly observed the higher density liquid will undergo a greater increase in pressure, as others have pointed out here there are a number of reasons why this discharge pressure may be less. I'd hazard a guess at a lower suction pressure; whilst Gerardo is correct in stating that liquid at a higher pressure (travelling at higher velocity) will induce more losses than a lower pressure fluid I don't think these losses would ever be sufficient to lower it below the outlet pressure of the lower pressure liquid.

 

As for the pressure it may be worth observing the mass flow rate: since your volumetric flow is the same the mass flow will be higher for the denser fluid, just running the numbers in my head it looks to me that the power you will be putting into liquid B is much higher than the power supplied to liquid A - with more energy and more head, you are generating a higher output pressure, overcoming the discrepency that would be caused by the density. If this is the case then your pump is either much better suited to handling this fluid or not designed for handling Liquid A at all. The more information you can provide the better - can you measure the pump RPM/power whilst these two fluids are being pumped and does this agree with my calculation/observations?

 

3.

Edited by Three, 11 January 2016 - 02:29 PM.

[breizh]

nishant2712,

Consider reading the Book attached , all you need is there .

Good luck

Breizh

[S.AHMAD]

Dear all

The only logical explanation of Nishant data is the suction pressure for pump A is lower than that of pump-B.

Assuming centrifugal pump, the calculated pump head is about 10.5 m at 25 m³/h flow rate and the suction pressure of pump A is 0 kg/cm²g and for pump-B is about 1 kg/cm²g. I am using Excell Solver to determine the pump head at 25m³/h and the respective suction pressure.

 

Good day

 

S.Ahmad

Edited by S.AHMAD, 13 January 2016 - 10:13 PM.

[ProcessEng_12]

Gerardo:

 

Since the pumps are identical, both will have constant head. right?

 

Suction pressure is 0.3 kg/cm2

Head is 12 m.

[ProcessEng_12]

S.Ahmad:

 

Thanks for replying Sir.

 

The head calculated by you is approximately the same. (11-12 m). The suction pressure is nearly 0.3 kg/cm2g in both the cases.

How have you calculated the suction pressure?

[ProcessEng_12]

Breizh:

 

Thank you Sir.

 

I will definitely go through it.

[ProcessEng_12]

Three:

 

Yeah the pump is centrifugal.

 

The pump is suited for both the fluids as per pump data sheet.

The power data for the pump is not available.

RPM 1450

[Chemitofreak]

Nishant,

 

Just a tip...!

 

Check the reliability and repeatability of the pressure gauge.

 

It is always better to ensure that our instruments are foolproof, before utilizing their data.

[ProcessEng_12]

Chemitofreak:

 

Thanks for Replying Sir.

 

We previously thought of that and even checked the Pressure gauge a lot of times. There is no problem.

[Three]

Nishant, if RPM is the same then head produced will be the same. Consequently the only reason I can see for a lower discharge pressure of the denser fluid is that it's suction pressure is lower - indeed the quick calc's I've run are in good agreement with S. Ahmad's. When you said you checked your gauges were these both at suction and discharge?

[ProcessEng_12]

Three:

 

Yes head produced will be same due to constant RPM.

 

No sir the pressure gauge is only at the discharge.

Will you please help me with the calculations that helped you get to the conclusion that low suction pressure may be the reason for denser fluid?

[ProcessEng_12]

Three:

 

Sorry guys but that's not the reason. Rpm have nothing to do with head.

Thanks Gerardo for your prompt replies

But according to affinity laws,

Head is directly proportional to the square of the impeller speed.

[breizh]

Hi Gerardo ,

There is a Typo in your first answer : 1kg/cm2 = 10 m head of Water !

 

Breizh

Edited by breizh, 18 January 2016 - 07:16 PM.

[S.AHMAD]

Dear Nnishant

 

Suction pressure = discharge pressure - pumphead

 

see attached excel spreadsheet

 

Attached Files

•  pumrev1.xls   27KB   42 downloads

[ProcessEng_12]

Dear Nnishant

 

Suction pressure = discharge pressure - pumphead

 

see attached excel spreadsheet

Thank you Sir.

[ProcessEng_12]

Nishant,

That's true but only when you want to analyze what happens with a pump when you change variables sych rpm, head if changing the ipeller, for instance.

So, those concepts don't apply to your questions.

Regards,

Right Gerardo 

I was a bit confused.

Thanks.