Relationship between TDH and NPSHA in pump 1

[Ashjanj]

Hi,

What is the effect of having a very high Total Differential Head. The reason for my question is:

NPSHA for the pump is 18 m
TDH is 18m

Is there any relationship with the two?

thanks for your responces.

 

[EmmanuelTop]

NPSHA is immanent to your specific system (service fluid conditions, flowrate, elevation profile etc.) while TDH is characteristic of the pump and it is a function of actual flowrate. There isn't any relatioship between these two parameters (NPSHA/TDH), as far as I know about pumps.

There is dependancy between pump flowrate and NPSHR (due to higher friction losses when liquid accelerates at the eye of impeler at higher flowrates), and it is usually given as chart-type data NPSHR vs Q.

There are many free internet resources for centrifugal pump design and operation, some of them are:

http://www.cheresources.com/centrifugalpumps1.shtml

http://www.mcnallyinstitute.com/home-html/Technical_paper_index.html

Regards

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[MortenA]

For the pump du you mean NPSHR not NPSHA?

R is requires - that is the minimum for the pump
A is avaiable - thats a calculated value and should be higher than R. There is no relationsship as such between TDH and NPSHA. NPSHA will be dependant on piping and flowrate.

Best regards

Morten

 

[Artisi]

Have you asked the right question?

" What is the effect of having a very high Total Differential Head. The reason for my question is:

NPSHA for the pump is 18 m
TDH is 18m "

In this case I don't see that you have a very high differential head irrespective of what NPSHa you have available. A bit more information or a clearer description of what you have would help with our advice.

 

[djack77494]

The simple answer is that there is no relationship between NPSHr and TDH. Both are largely independent characteristics of the pump. NPSHa is a characteristic of your system and piping. It also has no relationship with TDH.

 

[GL431]

There might be one relationship. If NPSHA is less than NPSHR you are outside the predictable operating envelope of the pump and you might have no TDH. If NPSHA is equal to or larger than NPSHR you are in the predictable operating envelope and TDH will follow its curve. Very mathematical and thought around the corner, but there is a conditional relationship.

 

[Shimasu]

the TDH is a difference basically so the sign can tell you something, normally it is something like this

TDH = Against - Aid

this means that you are going to sum all the elements against your pump and substract all the elements that help your pump (height, pressure etc)
So if you are having a very very big TDH means you need a big pump roughly speaking. If TDH is negative it means that your pump probably is not needed (maybe you want to increase pressure and the pump is then justified). However this is all about convention of signs.
Now the relation between TDH and NPSHA is that NPSHA is your "Aid- vapor pressure"

 

[pumpking]

Doesnt sound quite right to me, NPSHr figures are normally very small (if your Impeller design in good !!) it doesnt say 1.8 metres does it ? That sounds more like it espeially for a single stage centrifugal pump which i think would be the selection for the head you are talking about.

18m NPSHr for such a pump is a schoolboy design,most of our units are down to 0.8-0.9 metres, then we can look at Inducers on the Impellers if we need to go even lower than that.

Have a look at some curves from

http://www.cdrpumps.co.uk

to see some NPSH figures.

Ash Fenn

http://www.cdrpumps.co.uk

 

[itdepends]

He quoted NPSHA at 18m, not NPSHR