Recirculation loop to boost NPSH

[jimdvfd]

I have a water based product that I need to pump from a tank that will result in a NPSH of 3 ft. All the pumps I can locate need at least 5 ft to operate. I have heard the a Recirculation loop can be installed to increase the NPSH. The velocity pressure from the loop directed into the suction will increase the NPSH. Where can I find info on how much to recirculate?

 

[Ashereng]

Hmmm. I have not heard of this solutio before.

Off the top of my head, just quickly thinking about it, it doesn't sound right.

Have you tried talking to the source that you heard this solution from?

Typcially, if we are only out 2 feet on NPSHa, we dig deeper.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[MMcLean]

To only have 3 ft of NPSHa (seems very low) you must have a very high vapour pressure, or no relative suction level, or very high sucion line losses, or a combination of these. A recycle into the suction line is only going to increase your suction line losses, making the NPSHa worse.

Can the layout of the pump relative to the minimum liquid level in the tank be changed?

Mark McLean

WorleyParsons

 

[waross]

Jet pumps use a similar principle. The water is returned through a venturi and the venturi develops the suction. The added volume of fluid entering the ventury suction raises the loop pressure and forces the fluid out the discharge port.
Shallow well pumps may have the venturi mounted on the pump or inside the pump casing.
respectfully

 

[Artisi]

What are the flow rates for this pump?

Naresuan University
Phitsanulok
Thailand

 

[25362]

Visit thread407-130418.

 

[waross]

Hi Artisi;
I don't know the flow rate. The pumps are generally manufactured in sizes from 1/3 hp. to 2 hp. The control is typically but not always a pressure switch on the discharge or a connection to the pump housing; on at 30 psig., falling and off at 50 psig., rising.
The shallow well versions will easily lift water 20 or 25 feet.
Some models are constructed with the venturi in a removable housing bolted to the front of the pump housing. The venturi housing may be removed from the pump and lowered down the well on two pipes for deep well applications.
respectfully

 

[Artisi]

Hi Waross
Thnx, however, my question was to "jimdvfd" - is the flow rate 1gpm or 10000 as it will make a big difference to the recommendations.



Naresuan University
Phitsanulok
Thailand

 

[jimdvfd]

To answer some of your questions. The flow rate is 160 GPM. I’m drawing the fluid from a tank that is at 15 torr. The pump is mounted below the tank and the fluid level in the tank is only 3 ft. without major reconstruction I can’t lower the pump. What I’m looking for is what recirculation rate do I need in a venturi to get the NPSH up to at least 5 ft. Yes this is similar to how jet pumps work. I just can’t find any design information on jetpump recirculation rate

 

[jimdvfd]

By the way 760 torr is 14.7 psi so 15 torr in the tank is .29 psia

 

[25362]

A similar query was treated in thread407-130418. Have you considered inducers ?

 

[d23]

jimdvfd

Before you get nervous about the NPSHa you need to account for the atmospheric pressure. Talk with your manufacture.

D23

 

[JJPellin]

Unless your product is at the boiling point in the tank, you have the advantage of atmospheric pressure. This gives you an extra 32 feet of NPSH(a). You probably need to be more concerned with minimum submergence in the tank. In order to avoid ingesting air, you need to have the suction nozzle submerged below the surface of the liquid by at least 1 foot for each foot/second of velocity in the inlet nozzle. The use of internal baffles or vortex breakers can reduce this required submergence considerably.

 

[thewellguy]

Waross was correct about jet pumps, however he left out one detail. Jet pumps are extremely ineffecient. The operating cost of this pump would be high in conparison to other types of pumps due to the recirculation. Also, I am unaware of a jet pump that would meet you volume requirements.

 

[timbones]

jimdvfd,

I would think that recirculating more flow back to the pump suction would only make the situation worse, as it pushes the pump performance farther right on the curve which generally has a higher NPSHr.

Are you working with an existing pump? Or are you buying new? If you have the option of buying new, you could select a vertical canned pump to get some additional NPSH. This option keeps the pump out of a pit and saves major piping modifications. It does require a hole to sink the can into however. I've seen it used fairly regularly for pumping out tanks that are near full vacuum.