NPSH (static height) piping question

[WWSME]

We have a piping situation that is not ideal, but can't be avoided. We have a tank with roughly 10 feet of liquid which can drain down to roughly 1 ft of liquid. A vertical pipe draws liquid out of this tank, up and over the wall of the tank and back down the other side to a self priming pump at the same level as the floor of the tank. The pipe that goes over the wall is roughly 1 ft above the 10 ft high level of the tank.

My question has to do with what static height to use in calculating the NPSHA. It seems to me that I have to consider two situations: The first would be the NPSH at startup to lift the liquid to the height of the over-the-wall pipe so basically suction lift is as though the pump was at the same elevation as the over-the-wall pipe. The other situation is the NPSH after the system is primed and running, so becoming a flooded suction system so that the static height is now the distance from the over-the-wall pipe to the pump centerline.

Am I correct?

 

[sspeare]

You might check out the NPSH discussion in the first chapter of the Cameron Hydraulic Data Book. The book is a classic. Still available for approx $60.

 

[WWSME]

Thanks. I own a couple of different versions of Cameron's but it does not get into that particular level of detail on how to handle my application.

 

[zdas04]

I think you pretty well have it figured out. With a drained line you have to get the water to the top of the wall (I'm assuming this is the tank rim) and then it will fall over. Once you've established a siphon then the pump will see the head of the height of the wall above its inlet.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MJCronin]

Consider adding a small "priming tank" atop the tank wall. The tank will assure a prime and cut startup times for your self-priming pump. The tank is operated briefly to flood the suction during pump startup

The priming tank is described in the Goulds Pump Manual.

I have never heard of an NPSHa evalation of a self-priming tank during startup.

See chapter 2.4 of the "Pump Handbook" for a more detailed discussion of priming tanks

 

[SNORGY]

To answer the immediate question, I think in the worst case the NPSHA calculation needs to treat the tank height plus one foot less the low tank level as suction lift.

Then I would prepare a work order to cut a nozzle into the bottom of the tank at the first opportunity.

 

[WWSME]

Thanks. I've already allowed for a priming system to aid the priming process. Also have a copy of the "Pump Handbook" right here.

 

[WWSME]

Snorgy: Thanks, but unfortunately, stuck with the system piping as is, so no wall penetrations can be done. I know it is not ideal, but its what has been thrown at me to make work, so hence my question.

 

[zdas04]

And submersible pump is not an option?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[SNORGY]

WWSME,

What pipe diameter and pump mass are involved and what is the fluid being pumped?

Just wondering if it is feasible to cut a tee into the top of the pipe and drop in a SMR-TEK or Flygt or some such thing via a mounting flange at the top of the riser coming out of the tank. I do it for knockouts and buried tanks frequently. Might eliminate the need for the priming system or self-priming pump.

 

[itdepends]

If it's a self priming pump that has the ability to pull negative suction pressures then you only need to ensure than;
a) It has sufficient self priming ability to lift the water 10ft (your 1 ft minimum level to the top of the pipe 1ft over the tank.
and
b) That during normal operation the NPSH available with the tank liquid level 1ft above the pump is sufficient. The height the pipe goes up over the tank wall is largely irrelevant once the piping is flooded as long as the vapour pressure isn't too high (which would lead to flashing in the high points) or the length/diameter of the piping does not add excess pressure drop to the suction side of the pump.

Keep in mind that the NPSH required by the pump increases with flow, being a self priming pump it's designed to operate with insufficient head on startup put should have a rated lift capacity. Note that if your suction piping is too large for the flow (i.e. velocity is too low) then you may not fully push all the air out of the suction pipe which can lead to extra head losses.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[SNORGY]

itdepends,

That's one of my concerns: the system as described...isn't it going to be prone to vapor lock? Once the bubble sets up at the top of that riser...

 

[WWSME]

zdas04: No, submersibles are not an option. The tank interior is comprised of biological membrane cartridges.

Snorgy: The pipe diameters range from 5" SS to 2.5" SS. The fluid is a treated wastewater so basically water. See my response to zdas04. The design is based on particular guidelines set by the membrane supplier with recommendations of using a self primer or PD pump. Due to cost, a self primer is being used. Accommodations have been made to vent the header and prime the system.

Unfortunately we are straying a bit off topic as I stated in my initial post that the piping/pumping configuration could not be avoided, so I'm not looking for recommendations on pipe/pumping changes. I'm merely looking for answers on how to apply the NPSH calculation given my situation.

itdepends: Thanks, that is what I thought as well.

Muleshoe: WTF?

 

[Artisi]

Just attach the self-priming pump (I assume a wet-self priming pump - ie, one you fill with water prior to starting) to the pipework, fill it with water and push the start button - presto - water out of the discharge pipe. If the tank is 10 feet high forget about the academics of NPSHa/r.
Might be worthwhile fitting a NRV on the inlet pipe - make it easier to re-prime every time you start up. It might also be worthwhile fitting a external permanent priming water supply to the pump - if the tank has only 1ft of water in it and the NRV is a bit suspect you might well need to reprime again.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)