Pump flow rate with zero counter pressure

[cap1948]

How to evaluate the flow rate of a centrifugal pump (without inverter) when the suction pressure is the same as the outlet pressure and we are out of the pump curve?

 

[LittleInch]

Inverter?

You can't is the short answer and this is virtually impossible in practice.
Hence it is a purely academic exercise.

Depends a lot on the size of the unit. Smaller one will probably survive, but bigger units will just vibrate and you will probably have cavitation going on,

A bit more than one line would help to understand what is going on and why you want to know.

It's a bit like saying how many RPM can I get if I floor the accelerator when I'm not moving forward and there is no limiter on the engine speed? Why ??

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

 

[MortenA]

Look at the pump curve. Most actually go to dH=0 (or close), then check with your npsh-r. If you dosnt then i would use the right most value (and assume its somewhat higher).

--- Best regards, Morten Andersen

 

[EdStainless]

Most pumps won't reach that flow unless the inlet head is high. As in well over 1 atm.
The cavitation and vibration will at least destroy the seal and likely the bearings also.
And maybe the coupling and motor bearings as well.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Just bypass the pump completely as it's not doing anything in your example.

Which just highlights how ridiculous this question is.

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

 

[Rogue909]

What application would a centrifugal pump be in where inlet pressure = outlet pressure? Where does the energy go? If volume in = volume out and pressure in = pressure out. I admit to not being intimately familiar with pumps.

 

[Rputvin]

What application would a centrifugal pump be in where inlet pressure = outlet pressure? Where does the energy go? If volume in = volume out and pressure in = pressure out. I admit to not being intimately familiar with pumps.

That's the point - there is no application where this is true, as the pump will most likely be trying to self-destruct. It happens when people who aren't familiar with pumps and piping systems try and hook up a pump that's grossly undersized for the application and it's allowed to flow off the end of the curve from lack of system resistance/backpressure.

50% and 25% of nominal flow backup pumps can suffer from this if they're not throttled, or systems designed for multiple pumps in parallel that aren't running enough pumps to intersect the desired system curve and end up flowing off the curve. Both instances of not enough pump for the pipe they're flowing into, but where NPSH is still likely to be satisfied, and you'll see a ΔP of 0 (or close enough to not register on your gauges) across the pump.

 

[horacio Torres]

It is a no sense question.

Horacio

https://www.linkedin.com/company/lagotuy/

 

[cap1948]

I better formulate my question. I transfer water from tank A (at atmospheric pressure) to tank B with a difference in height of 50 m.
Tank B has a design pressure of 6 bar and a PSV capable of discharging the pump flow rate with DP = 50 m. If, by mistake, the pressure in tank A becomes 5 bar, what is the flow rate of the pump that must be discharged from the PSV?
Sorry for the delay with which I reply, I have been abroad

 

[LittleInch]

A diagram would help....

A few things - it won't be a tank with a DP of 6 bar, it's a Pressure vessel.

PSVs are safety devices and shouldn't be used for other purposes

Which way is the 50m? Tank A higher or lower than vessel B

Has the pump got a differential pressure of 50m?

How far is tank A from tank B? It seems the pressure drop when flowing from A to b is about 5 bar / 50m??

Sorry, but this still doesn't make sense.

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

 

[Rputvin]

The easy answer; size your tank relief valve to the maximum flow your pump is rated for.

I'm assuming Tank B has one inlet, and is otherwise closed save for the PSV? If your pump's closeoff/deadhead/max head rating is greater than the 6bar rating of Tank B you will need to relieve some of that flow.

If Tank A becomes pressurized at 5bar I'm assuming the entire line between Tanks A & B, and thus Tank B will be at 5bar? If so your pump will only need to add 1bar to exceed Tank B's rating and you're going to need to relieve most of the pump's flow.

But as Little Inch said, this is speculation based on assumptions. If you provide a pump curve and a diagram we can take a look at what you're trying to accomplish and make sure you're not missing additional information like elevation head that might influence the answer.

 

[cap1948]

Sorry littleInch, I am talking about pressure vessel.
This is a generic question: how can I estimate the flow rate of a pump which, due to a malfunction of the upstream regulation systems, is operating with a very low head and outside its curve (on the right side)? Which flow rate should I have to consider to calculate the downstream PSV on vessel B?

 

[LittleInch]

Sorry cap1948, please answer the questions above and provide a diagram as I just can't understand what your question is.

PSV are, in my head, pressure safety valves - you need to consider any number of scenarios to determine the max flow needed.

But in essence you can't calculate these things once you go beyond the end of the plotted curve. You can guess / estimate, by extrapolating the curve to the zero diff head line, but add 20%.

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

 

[cap1948]

Thanks for the reply. Indeed it is not possible to estimate the flow rate with zero diff. head, extrapolating is unrealistic and often ends up outside the sheet of paper that contains the curve. The problem is to convince the Authority' inspector that the pressure safety valve guarantees the design pressure of the downstream vessel is not exceeded

 

[Rputvin]

The pump will not be contributing toward or increasing the pressure in the vessel if it's running off the end of the curve.

You should only need to demonstrate the flow at the lowest pump discharge pressure that will force the PSV to open. If it's flowing more than that value you don't have sufficient pressure to open the PSV. Trying to accommodate the absolute maximum flow the pump can produce, unless the pump's curve scale starts above 1bar and it could produce 1bar of pressure off the end of the curve, seems to be an exercise that's not required.

Or maybe not, we're back to needing a diagram and a pump curve to make any real determinations about how your system needs to be setup for the behavior you're looking to guard against.

 

[Artisi]

Round and round like the bloody great wheel etc -- so far a complete waste of everyone's time, you have been asked a couple of times for a sketch and a better description as to what you want - how about helping yourself as people are just spinning wheels on your behalf.

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.)

 

[LittleInch]

I missed the bit about "tank" A suddenly going to 5bar. Actually it is a pressure vessel also, but now the pressure in vessel B could be 10 bar.

Flow you need to calculate for the Pressure Safety valve based on your new conditions 5bar, 50m head and 6 bar at the end. I would ignore the pump and assume it isn't there or add something like 20m diff head.

But you're drip feeding info here and no one understands your constantly changing story.

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