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?
Please, how can I calculate the NPSHa for a pump?
And, What is the deference between NPSHa and NPSHr?





RE: NPSH Calculation
http://www.mcnallyinstitute.com/11-html/11-12.html
"a" => NPSH available
"r" => NPSH required is that NPSH required by the pump at a given flowrate.
What would you be doing, if you knew that you could not fail?
RE: NPSH Calculation
In addition to the McNally Site, you may find this site useful:
http://www.pdhonline.org/courses/m124/npsh.pdf
RE: NPSH Calculation
Here I am sending the document "NPSH Calculations", by Art Montemayor.
I hope this tutorial can be of help for you.
El que no puede andar, se sienta.
RE: NPSH Calculation
I'll read what u sent
RE: NPSH Calculation
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
What would you be doing, if you knew that you could not fail?
RE: NPSH Calculation
RE: NPSH Calculation
RE: NPSH Calculation
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
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
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
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
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
What would you be doing, if you knew that you could not fail?