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NPSH Calculation

NPSH Calculation

(OP)
Hi....
Please, how can I calculate the NPSHa for a pump?
And, What is the deference between NPSHa and NPSHr?

RE: NPSH Calculation

NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer.

In addition to the McNally Site, you may find this site useful:

http://www.pdhonline.org/courses/m124/npsh.pdf

 

RE: NPSH Calculation

(OP)
Thanx for every body...
I'll read what u sent

RE: NPSH Calculation

my review of Mr Montemayor's document:  The drawings to the right of the calculations are misleading.  Pump elevation appears to be subtracted twice based on drawing dimensions.  Assuming friction loss can be problematic and should be verified any time possible. Atmospheric pressure, or "pressure above liquid" is only 14.7psia at sea level (I would assume everyone who posts knows this but I was always taught not to assume).  I don't believe the tutorial provided above is as beneficial as others I have come across.  I tend to reference the Cameron Hydraulic Data book, but if not available this has also proven to be a good source
http://www.mcnallyinstitute.com/11-html/11-12.html.  GPSA books are also a great source of information.


Some times it is necessary to take credit for liquid levels if you can ensure a minimum liquid level will always be present.  In many plants raising a vessel is not an option so in some cases the only way to gain necessary NPSH is to raise the liquid level.  

RE: NPSH Calculation

nope, I missed it and couldn't go back to edit.   

RE: NPSH Calculation

I see no problems with Mr Montemayor's document. Perhaps the format was unclear with the extra line below the vinculum instead of the total. I thought the examples were very good.

RE: NPSH Calculation

I agreed with nfinit's finding.
The NPSHA is calculated based on the liquid level above or below the impeller eye. If the liquid level changes in the course of pumping out, the NPSHA should be based on the lowest level= LLL. In most case the pump is stopped when reaching this LLL. Most end-user will still specified a certain margin is required between the NPSHA and NPSHR of the pump at this LLL condition which is really not necessary and only lead to more expensive pump being purchased if the NPSHA is very low.

The tutorial also give out wrong message in the recommendation to
specify a much lower NPSHA when in fact the calculated value id higher.

RE: NPSH Calculation

So, you are agreeing with nfinit's mistake? Good for you.

Regarding "If the liquid level changes in the course of pumping out, the NPSHA should be based on the lowest level= LLL."

That his what Mr Montemayor's has done. He is setting the LLL to be 3 ft off the tank bottom.

Regarding "The tutorial also give out wrong message in the recommendation to specify a much lower NPSHA when in fact the calculated value id higher."

Do you think that an NPSH of 20 ft is low? Mr Montemayor's "NOTE: Very few pumps need more than 20 ft of NPSHa."

Mr Montemayor is an engineer with considerable experience and one should hesitate before questioning his posts.

RE: NPSH Calculation

my mistake?  His drawings suggest the static height is 13ft for the first two examples, then adds 10ft from the bottom to centerline, and again subtracts 3ft for pump elevation?  Also suggest posting that the NPSHa is only 20ft, when it is actually 38ft in example 2.  The NPSHa would be 38ft, the acceptable design condition can be based on whatever is determined acceptable.

And what Pumpsonly was agreeing with was only that the statement
"Do NOT account for Liquid Level" is not always a solution.  I have several specific instances where not counting the liquid level would suggest I had to raise a tower 20 feet.  If I am designing a new system, I would gladly take his advice and start at the bottom of the tower.  On existing retrofits, you can install protection devices to ensure the liquid level maintains the desired elevation.  A viable alternative in many cases.

Montemayor wrote that document in 1997.  To expect that an engineer would agree with everything they have ever done in their lives would be quite the accomplishment.  He may still stick by his posts, I am sure there is a valid point to it.  But there are several instances where even the smartest of people are not correct.  

I have a lot of respect for those that post on here, but Montemayor's tutorial leaves questions that I feel are answer in places (like BigInch pointed out) like the McNally Institute.


 

RE: NPSH Calculation

After reviewing the drawings, I see your issue. On the drawing, the tank elevation arrows end at the pump centerline which should indicate that the tank bottom is 13 ft off the ground.

However, the text states that the bottom tank elevation is 10 ft.

Anyway, his calculations are correct.

He also provides some insight:

Assume tanks are empty;
assume pump suctions are 2-3 ft off grade;
etc.

The fact that the Montemayor document was made in 1997 is of no relevance to the discussion of NPSH. NPSH calculations precede this discussion by decades.

I fail to understand your issue with the statement to use a maximum of 20 ft NPSHa. If you had a water tower, would you state the NPSHa as 200 ft?

RE: NPSH Calculation

To save all the problems of who's right / wrong etc - just refer to Cameron's Hydraulic Data - Net Positive Suction Head, starting on page 1-10 in my 16th Edition.  Clear, easy to follow and covers all situations.

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: NPSH Calculation

McNalley's is correct.  Don't care about the others.

What would you be doing, if you knew that you could not fail?

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