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Selection Of Type Of Pumps For "negative" Npsha And Solution T


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#1 linda_pro

linda_pro

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Posted 29 May 2024 - 05:53 AM

Hi all,

I have a difficult question regarding to select a suitable pump for closed drain vessel service in gas distribution center.

The closed drain vessel was installed at 3m below the ground. In Feed design, 2 diaphragm pumps were designed to pump the hydrocarbon to the burn pit. There was not any calculation for these pumps available in Feed.

 

Because of serving hydrocarbon, client requires Pumps must be API 676 type, hence diaphragm is not accepted.

We are doing calculation and provide data for Mechanical, and because the pump is installed at ground, and the vessel is at lower position. The calculated NPSHa is "-" 2.8 m. Mechanical could not find any suitable pump for this service.

 

We have tried to bring down the pumps to the same level with the vessel however some safety concerns have been raised and client refused that option.

 

Another idea is to choose the vertical pumps and installed on top of vessel. I do not agree because it impact the instrumentation in the vessel as well as easy to cause fire if there is a leakage gas. (the low-pressure gas is connected to LP flare header and flare stack).

 

some questions have been raised what if we bring the closed drain vessel to ground floor, the pump can be a typical centrifugal, and if drained liquid from other process equipment can flow by gravity during maintenance or not? if the liquid flows by different pressure, then why not just install at ground level?  Closed drain receiving liquid from filter Seperators, Fuel gas vessel, pipe and HP Knock-out drum. (=> this will impact piping and civil and structure because other equipment needs to be lifted to higher level)

 

PIDs is attached for your reference.

 

It would be great if i can have your opinion in this case. Thanks in advance for any discussion and opinion.

 

Cheers,

Linda

 

 

 

 

Attached Files

  • Attached File  PID.pdf   354.78KB   10 downloads


#2 breizh

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Posted 29 May 2024 - 06:01 AM

Hi Linda,

Can you share your NPSHa calculation? weird to me. How high is the vapor pressure of the liquid? What is the pressure within the vessel?

Can you pressurize the vessel and/or cool down the liquid prior to enter the vessel to reduce the Vapor pressure?

 

https://www.eng-tips...d.cfm?qid=59805

 

Most of the underground pits I've seen were equipped with vertical pumps.

 

Note: Diaphragm pumps require positive NPSH!

 

Breizh 

Attached Files



#3 Pilesar

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Posted 29 May 2024 - 08:41 AM

A sealed submersible pump should meet safety and performance requirements if an API 676 progressive cavity pump is unsuitable. A pump on top of the vessel is not necessarily bad with a safe housing so don't rule that out too quickly. Raising the vessel would be my last resort.



#4 linda_pro

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Posted 29 May 2024 - 09:04 PM

Hi Breizh,

The closed drain vessel operates at 0.9 barg which is the same value of vapor pressure we have used for NPSHa calculation.  We have thought about decreasing the vapor pressure value, but this could be a risk for wrong pump selected, so we decided to keep both operating and vapor pressure are the same.

 

That means, only friction losses of pipe and fittings at the pump suction and static head.

 

Cheers,

Linda

Hi Linda,

Can you share your NPSHa calculation? weird to me. How high is the vapor pressure of the liquid? What is the pressure within the vessel?

Can you pressurize the vessel and/or cool down the liquid prior to enter the vessel to reduce the Vapor pressure?

 

https://www.eng-tips...d.cfm?qid=59805

 

Most of the underground pits I've seen were equipped with vertical pumps.

 

Note: Diaphragm pumps require positive NPSH!

 

Breizh 



#5 linda_pro

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Posted 29 May 2024 - 09:26 PM

Hi Pileser,

 

We are trying to avoid buying a fancy pump for this service :)).

 

Linda

 

A sealed submersible pump should meet safety and performance requirements if an API 676 progressive cavity pump is unsuitable. A pump on top of the vessel is not necessarily bad with a safe housing so don't rule that out too quickly. Raising the vessel would be my last resort.



#6 shvet1

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Posted 29 May 2024 - 11:51 PM

@linda

You are able to pressurize the liquid in the vessel above vapor pressure during pumping out procedure. For this case you should add a N2 line from bottle to a valve N9.

This will help to avoid cavitation.

 

The question is - do you really need the diaphragm pump to operate within NPSH limits or a minor damage/defficiency of this pump is tolerable.

 

The liquid will cavitate inside of the pump during pumping out - and what...? What is wrong?


Edited by shvet1, 30 May 2024 - 12:09 AM.


#7 seuenergy

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Posted 03 June 2024 - 06:38 AM

My simple idea:

As the liquid hydrocarbon is sent to pit, whether you could just use the pressure of N2 for transfer? 

I assume the drainage are typically done during maintenace and TAR. And the vessel is relative big enough to store for a long period. So the drain could be stopped for a short time for N2 (pressure ) transfer. 






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