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Jet Pump NPSHr Data
4

Jet Pump NPSHr Data

RE: Jet Pump NPSHr Data

Why do you need to know?

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.)

RE: Jet Pump NPSHr Data

The first link says max lift 8m. So NPSHR is 2m.

But these are self priming so accept a lot of air and water and still operate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

My take would be that NPSH is not that relevant for a jet pump since it wont be damaged by insufficient NPSH - and thats normally what is the reason for mentioning NPSH for centrifugal pumps.

Of corse there is a limit where the pump will not pump anymore at LittleInch mentions.

--- Best regards, Morten Andersen

RE: Jet Pump NPSHr Data

As MortenA has pointed out, NPSHa or NPSHr is irrelevant for jet pumps, the reason I asked who do you need to know.

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.)

RE: Jet Pump NPSHr Data

(OP)
I'm sure cavitation can happen in any system, NPSH is not an issue for jet pumps that have the ejector down the well, however these models all have the ejector inbuilt.

I want to install a jet pump with a flow rate of 90L/min @ 48m Head. The suction lift is 7.5m including friction and the max suction lift of the pump is 8m.

However, I believe the manufacturer when they say max suction lift of 9m means the maximum height for re-priming. It may not be healthy for the pump to have a suction lift of this amount at all duty points.

The height it can reprime is a different characteristic of jet pumps that still will require NPSH. It would not however be 8m across the entire performance curve.

Jet because it has an ejector before the inlet, this would not stop cavitation would it?

RE: Jet Pump NPSHr Data

I've not managed to find a drawing or picture of what's inside those pumps. Do you have any?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

Have you checked the net for researched on cavitation in ejector nozzle / diffuser. I don't see any problem with NPSHa/r relating to the pump impeller operating via an ejector.

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.)

RE: Jet Pump NPSHr Data

These pumps look fairly solid. The pump seems to work by internal recirculation of water through the ejector before blasting that jet at the center of what looks like an open impellor pump.

So yes you look as though you're operating at the outer edge of the envelope but the extra pressure just beyond the end of the jet would look to me like it would create more pressure. Efficiency must be pretty terrible and you need to have the pump at least 40% full to work and create "suction".

Basically if the vendor doesn't warn you about something then I think it's all good to go!
 

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

Efficiency of an ejector system is a meaningless consideration.
I was involved in research / design over 50 years ago for ground dewatering beyond suction lift of a de-watering pump - this was for deep below ground construction sites with high water table low permeability. We designed an inpipe ejector that was a driven by smaller diameter pipe located within and 1 1/2" dia. pipe terminating with a wellpoint spear - the driving supply was 10-12 GPM with a total return flow of 11 to 14 GPM, ie., a recovery of 1-3 GPM of groundwater - now, as the ejector performed to what it was designed to do, we considered it to be 100% efficient.
Never bothered investigating the overall eff. as it wasn't important, but as you can imagine a 40/50 hp diesel or electric motor driving the pump for 50 - 100 wellpoints to give a groundwater yield of 50 to 100 gpm is a pretty low eff.

Is the OP over thinking the problem, but as we don't know the application - assuming there is one we can only guess what is going on.

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.)

RE: Jet Pump NPSHr Data

(OP)
Thanks for your help,

I may well be overthinking the problem.

I don't have high experience with this type of pump and could not find any information regarding them cavitating at low NPSHa.

The main point I wanted to clarify is why a manufacturer would not provide information on this and is there something special about them that prevents cavitation.

I asked two manufacturers of pumps and they could not provide an answer for me which is why I asked here.

It seems to me that nobody has a definite answer and I will need to purchase a pump I think will work and see how we go.

RE: Jet Pump NPSHr Data

What is the application?

The "pump" ie, the impeller that suffers from cavitation will not be affected for this unit as it is only the driving force for the operation by supplying water to the ejector, the ejector is a seperate portion of the unit and any cavitation (if any) will be within the nozzle / diffuser, however, wear of the ejector is more of a concern than cavitation.

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.)

RE: Jet Pump NPSHr Data

It may also be because these pumps are all small pumps for domestic applications. So most buyers would not understand npsh.

--- Best regards, Morten Andersen

RE: Jet Pump NPSHr Data

Artisi, Far from me to question your knowledge of these, but they seem to be some sort of hybrid pump,where there is only one source of water in and one out.

Hence the impellor performs two purposes - recirculation through the ejector and also flow out of the discharge at a reasonable head. See the picture below from Pierres links. A more "traditional" jet pump uses a separate pump and water flow as you describe. The jet pumps mentioned so far though does need to be water filled or at least 40-50% in order to start generating recirculating flow through the inbuilt ejector and start "sucking" air and water in to prime the unit until gradually more water comes in and the pump starts flowing water or liquid from wherever it is picking it up from.

But in this case where it seems flow out at a reasonable head (Harltey mentions a head of 48m on the discharge)the efficiency is power in versus flow and head out.

No idea how much flow goes around through the ejector but maybe 50%?

Hence the impellor is essentially fed by higher pressure water coming out of the ejector so NPSH doesn't really come into it. I suspect whoever you are talking to in the vendor doesn't really understand themselves....

At least that's my may of looking at it.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

(OP)
Ok, thanks for your response LittleInch. That makes perfect sense. So would you say if cavitation was to occur it would be where the lower and higher velocity fluids combine just after the jet nozzle?

RE: Jet Pump NPSHr Data

I think so yes. Somewhere in the ejector is the lowest pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

"However, I believe the manufacturer when they say max suction lift of 9m means the maximum height for re-priming."

Jet pumps are not self priming. You should prime manually and use a foot valve to keep from losing prime. Most shallow well jet pumps will suction lift from as deep as 25', but they will not self prime. Also, there is usually a valve or some kind of restriction on the discharge of the pump so you can adjust how much pressure/flow you are sending back to the ejector.

RE: Jet Pump NPSHr Data

Well all the links on the OP state "self priming". I can't see why they wouldn't be?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

Just adding to Valvecrazy's comments.
Self prime is a misleading term.
Only vacuum assisted "self-priming" pumps will self prime, all other so called self-priming pumps need to be primed initially and will re-prime each time the pump is restarted, provided the system remains primed.

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.)

RE: Jet Pump NPSHr Data

If you look at the drawing in an earlier post you will se that normally a volume of water would be left in the pump house. So in that case i think that it would ne enough to get the ejector effect going had "suck" more water to the pump. Remember, this is a pump for domestic application so frequent start/stop but with relatively small volume.

--- Best regards, Morten Andersen

RE: Jet Pump NPSHr Data

In practice once the pump is filled it should stay more or less filled as the inlet and outlet are both at the top.

The operating manual does say that you need min 50% full for the self priming to work.

So yes, the pumps are not inherently self priming, but if the pump body is partly full of water then it effectively acts like a vacuum pump.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Jet Pump NPSHr Data

On a domestic application a foot valve is mandatory and keeps the pump from losing prime. Even if/when a jet pump will self prime, it would take a minute or so. With frequent starts/stops of a domestic application, you don't want to be waiting for water as the pump re-primes after each start. Also, there are ways to keep a jet pump from cycling on and off so much on domestic applications. bigsmile

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