NPSHa vs NPSHr 1

[YoYo2000]

Hello:

I have an application where the vapour pressure (in gauge) is slightly higher than the pump suction pressure. This information was retrieved from the pump datasheet. However there is a postive difference between the NPSHa and NPSHr. How does this work? Shouldn't the pump be cavitating in the suction pipe before it comes into the pump? How can the NPSHa be higher than NPSHr? Just trying to understand as pumps is not my cup of tea.

Thanks!

 

[LittleInch]

NPSHa means Net pump suction head AVAILABLE. NPSHr means NPSH REQUIRED. Hence NPSHa needs to be greater than NPSHr, normally be a least 2-3m to prevent cavitation. NPSHr does not always mean that the pump won't cavitate, only that at that inlet head the pump head difference drops by 3%.

Please supply figures or attach scan of the data sheet and units so we can see where you're going wrong. If vapour pressure is in guage, not bara or psia then your product is boiling....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

I wouldn't say he is going wrong, I would say it's more likely that there is just bad info on the datasheet.

 

[LittleInch]

Probably, but without seeing it who knows?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[YoYo2000]

Thanks for the response and my apologies as the datasheets were seen at customer site and I dont have a copy of it. I just remembered see this and having a discussion later on stating the pump will cavitate and my senior stating their is a margin in the NPSH, hence it will not cavitate. He said the NPSHa is a function of vapour pressure and since this is a positive value and the the margin between NPSHa and r is present, there should be no cavitation. I dont understand this at all! Oh..and I converted the vapour pressure from absolute to gauge to see the margin between the suction and VP. The VP was indeed higher..I thought this was a mistake in the datasheet.

Sorry for the lack of info.

 

[1gibson]

There should be max/min/rated values for vapor pressure and suction pressure. Maybe some of it was blank, wrong, or maybe you just happened to grab the max vapor pressure and compare it to the rated suction pressure.

 

[LittleInch]

The VP is not the same thing inlet or suction pressure/head. If the liquid level is above the pump inlet pressure this is a positive sum in the NPSH calculation whereas friction head loss and Vapour pressure are negative. Also if the liquid is in a pressurised tank, then this also is a positive number (always do this in absolute numbers). If your vapour pressure at the pumping temperature was indeed above 1 bara / 14.5 psia, then I suspect the liquid would be in a pressurised tank to stop it boiling?

Hence the VP could indeed be higher than the pump suction pressure, but when you add it all together the end pump suction head is still above NPSHr

I suspect what you saw was something like Vapour pressure / head say 7m and NPSHR 3m / NPSHA 5m

If your liquid is say 5m above pump suction and friction losses 4m and your tank is at atmospheric pressure at sea level (say equivalent to 10m), then NPSHA is

5 + 10(atmosphere in liquid terms -7 - 3 = 5m NPSHa

At the pump suction NPSHa always needs to be greater than NPSHr

As vapour pressure rises you run the risk of cavitation unless your other elements (tank pressure / liquid height) more than compensate for that plus friction losses.

Makes more sense now?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DubMac]

Once you are at the entrance to the impeller, the NPSHA is the available energy in the liquid over and above the vapor pressure; that's it, period.

That available energy is made up of two components: the pressure head due to elevation difference, and what is left of the pressure originally applied to the liquid (either atmospheric or from another source); nothing else. Calculating that final component is where all the trouble comes from.

Sometimes working backwards from the impeller entrance helps to "get it".

That is all I will add to this discussion on NPSH, which by the way has been going on as long as there have been impellers.

 

[Artisi]

YoYo2000
NPSHa/r has been done to death in eng-tips and other forums, suggest you do a bit of research on the subject for a complete understanding.

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.)