8 replies to this topic

[axl456]

Hello again..

am having some doubts about a couple of problems in the crane book (1982 version)..

The doubt is with the 4-5 example (page 4-3)they use the follow equation to calculate the minimum velocity to fully lift the disc on a check valve:


But actually in the page A-27 the equation is as follows:


I know that if beta is 1 it doesnt matter, but the thing is that in the problem beta is not equal to 1..

sorry if the question is dumb but I rather ask than staying with the doubt..

[Art Montemayor]

Axl:

In the free download that our Forums give you of my worked examples of the Crane Tech Paper #410, you will find Example 4-5 (18th printing) worked out. The location is:
http://www.cheresour...__0

I used the 1979 Edition of Crane and the equation for the minimum velocity is different than the one you cite.

I had no problem working out the solution for this printing as well as that for the 1957 (6th printing).

I recommend you work out the 1982 version of the problem in the same manner and format that I used for the previous two versions. The Crane paper does have some errors in it. I have submitted my list to them in 1996 and they may – or may not – have incorporated them. However, by using my examples you may find the answer you are looking for.

Good luck calculating.

[axl456]

Thanks for replying

I can see that you use the follow equation, but I cant find it in the Crane Book:



Could you please tell me where that equation came from, am quoting the equations that am using for purpose of future references..

Edited by axl456, 04 May 2010 - 06:01 PM.

[Profe]

Hi Axl

I review the example 4-5 at Crane´s Metric edition and the diferences with the Art's worksheet are that Mr. Montemayor uses the American system edition.
If you review the nomenclature at Crane's Metric edition, the units for specific volume of fluid are in cubic metres per kilogram. Instead in the Crane's American system edition, the units for specific volume of fluid are in this case in cubic feet per pound. But in both cases the results of calculations will be equivalents with the correct units transformations if you work with the same pipe system.

I Think this clarified your doubts.

Good luck.

Edited by Profe, 04 May 2010 - 09:16 PM.

[axl456]

Thanks so much for your answer profe!!
Well this actually clarifies one part of the doubt but not all

Sorry for being so hardheaded and I can see now that the differences in this two equation is the system:



But the thing is that the Crane book (both SI and English system) says that the equations are as follow (Page A-27 for english and A-47 for SI):


Am confused because, although the book said to use the above equations in those cases, in the problem 4-5 they use the equation without the "beta" coefficient, and Mr Montemayor also use it without the beta coefficient..

So I will like to know why they dont use the equations with the beta coefficient..

Again sorry if am being stubborn or am not explaining myself well..

[Profe]

Hi again Axe.

Regarding your concerns about how the example is resolved, I think the following:
To resolve, should be done by trial and error with respect to internal diameter of the accessory to use.
In this case starts with an initial diameter equal to the pipe which will fit (A globe type, check valve lift with a wing-guided disc is required in a 3-inch horizontal Schedule 40 pipe).
Then proceed to the next smaller diameter available to meet the conditions of the problem (I think 2 1/2 inch).

What do you think Mr. Montemayor about this?

This is my small contribution in this case.

Good luck.

Edited by Profe, 05 May 2010 - 08:46 AM.

[shan]

Don’t be stubborn axl456. Just follow somebody with 43 years experience and you will have good chance to survive next 43 years, although you may not quite understand what you are doing at this moment. Actually, your question is quite straight forward.

If you satisfy


you will have no problem to meet



Therefore, it is unnecessary for you do not have to recalculate Vmin for 2.5 valve.

[katmar]

To blindly follow a recipe that does not make sense, and which you do not understand, is not a good way to prepare for the next 43 years (or even next week). I applaud axl456 for raising this problem and for looking for the correct solution. This is clearly an error in the Crane manual. Art Montemayor has mentioned above that he has found other errors in Crane and I have shouted about them from my soapbox as well. Crane 410 remains a very valuable resource, but it is good to understand what you are doing and to be aware that authors of well known books are not infallible gods. I see that a new edition of Crane was issued this year. Has anyone got a copy yet and able to check if it repreats this error?

But the real answer to this problem is that if you want good accuracy for valves then a generic method like the one offered in Crane 410 is the wrong way to go. The valves for each manufacturer will have slightly different characteristics and it would be better to download (or request from the supplier) the specifications for the actual valve you are using and get the correct answer.

Edited by katmar, 07 May 2010 - 03:33 AM.

[axl456]

Thank you all again for your answers!!

@shan: again sorry am not trying to doubt the experience of Art in any way, is just that am having a doubt and am trying to understand the logic behind the results of that problem..

profe in his second post, explain perfectly the logic behind the problem. if "beta" is equal to 1 (like it will be with the initial conditions), then Vm (1.58) is greater than V (1.07), and you need to use a smaller valve diameter to meet the conditions..

Using the next smaller diameter, beta is equal to 0.7 (2.5/3), Vm is equal to 1.1, and V is equal to 1.62.

In this case Vm is smaller than V, and the condition is satisfy..

So the problem is either that the crane book is not solving the problem explicitly enough, and they are omitting one step (the second calculation of beta an Vm), or they made a mistake that gave them a similar result, yielding to the same conclusion by not calculating Vm again, because in that case Vm is still 1.58, which is smaller than the second calculation of V (1.62)..

@katmar: thanks you so much for your words!! they actually motivate me to keep analyzing and trying to find the logic behind the problems!!

thank you all for helping me!!