6 replies to this topic

[Sharoon.Gill]

Hello everyone..

Currently we are pumping HCl with an available NPSH of almost 7 ft at -25 deg C with a vapour pressure of 12.4 bar a and the pressure on the top of the vessel is 12.6 bar a. We can assume the static height to be zero although the pipe is 1.5 m long. The vessel is 3.3 m high but this head is ignored in the calculations too. 

Now the fluid is to be changed to VCl which has a vapour pressure of just .6725 bar a at -25 deg C. now according to the same calculations the NPSH available increases drastically to some 400 ft. 

I am to check at what height of the tank will the pump cavitate? NPSH req is 5 ft. Now as the vapour pressure is decreasing, can we use this pump for the service. Assuming the viscosity remains the same and the fluid will have no effect on the pump metallurgy. Aren't the effects of the heights irrelevent? 

Please inform me if I am working on the correct lines. I append the excel sheet of my calculations.

 

Thank you.

Attached Files

•  pump.xlsx   13.84KB   24 downloads

[breizh]

Sharoon.Gill

 

Consider this paper to support your work.

 

A sketch will save words ! refer to the document attached .

 

 

Are you sure about the properties ( Vapor pressure ,etc ) and pressure above the liquid?

 

Breizh

Edited by breizh, 28 June 2014 - 11:22 PM.

[farid.k]

Hello everyone..

Currently we are pumping HCl with an available NPSH of almost 7 ft at -25 deg C with a vapour pressure of 12.4 bar a and the pressure on the top of the vessel is 12.6 bar a. We can assume the static height to be zero although the pipe is 1.5 m long. The vessel is 3.3 m high but this head is ignored in the calculations too. 

Now the fluid is to be changed to VCl which has a vapour pressure of just .6725 bar a at -25 deg C. now according to the same calculations the NPSH available increases drastically to some 400 ft. 

I am to check at what height of the tank will the pump cavitate? NPSH req is 5 ft. Now as the vapour pressure is decreasing, can we use this pump for the service. Assuming the viscosity remains the same and the fluid will have no effect on the pump metallurgy. Aren't the effects of the heights irrelevent? 

Please inform me if I am working on the correct lines. I append the excel sheet of my calculations.

 

Thank you.

Sharoon.Gill

 

If you change the fluid, does it suction vessel operating pressure still 12.4 bara? I don't think so. For me the suction vessel operating pressure would be slightly higher than vapour pressure (just to remain the fluid at liquid state) probably 1 bara (1atm) for the new liquid service. Double check for the suction vessel operating pressure. Appreciate others to comment.

[Ajay S. Satpute]

Sharoon,

 

1. I agree with Farid's comment.

2. I wonder why subject the pump with very different kind of fluid? Are you talking to the pump vendor as well?

3. Calculate power consumption post modification and check if existing motor can provide the same.

4. Make sure the new flow is within pump Allowable Operating Region (AOR).

5. Review the pump downstream control valve/orifice perormance post modification. It should not be affected adversely.

 

Regards.

 

Ajay S. Satpute

[fallah]

Sharoon.Gill,

 

In the situations such as yours in which the liquid is in equilibrium with the relevant vapor and near the boiling point, the vapor pressure at the surface of the liquid inside the vessel counterbalances with that of the liquid in the pump eye.Then, the NPSHA is normally limited to the additional pressure above the vapor pressure at the top of the vessel plus the liquid static head at the pump centerline minus the friction loss.

Therefore, you cannot keep high vapor pressure in HCL case for new liquid case. You should renew your calculations based on this fact and check if the pump can handle different fluids in motor power limitation standpoint...

[Sharoon.Gill]

Thank you very much to everyone for their time to reply to this post..i will have to check the operating pressure in teh other case...if the vapour pressure balances the operating pressure as you all point out..still we get an NPSH of 5 feet...now i have a NPSH req of about the same head..will the pump operate satisfactorily if we lower the pump, say by one feet..it will cavitate though while emptying the vessel..

[Bobby Strain]

While you are about your chores, you should learn more about pump behavior. If you do that, you will find answers to most of your questions.

"Give a man a fish and he will eat for a day. Teach a man to fish and he will never go hungry."

 

Bobby