13 replies to this topic

[Brightonnk]

I'm currently doing a project where I need to find Cv values and equivalent length values for a set of valves. I've figured out the Cv values but am having trouble finding the equivalent length. The valves are reduced bore ball valves and I am stuck on how I can go about to find the resistance coefficient K, What equations should I use?

[srfish]

You may of already seen this paper about that type of valves. But it is a way of estimating a resistance coefficient if an estimate is sufficient. The name of the paper is "Estimate Valve Pressure Drop Correctly" by Salid Rahimi.

[ankur2061]

neera,

Try using the spreadsheet provided as an attachement to the following link for equivalent length of reduced bore and full bore ball valves:

http://www.cheresour...ngs-and-valves/

Regards,
Ankur.

[Steve Hall]

Here's what I do. Knowing that Cv = gal/min water at 60F resulting in 1 psi pressure drop:

1) set up a worksheet to calculate pressure drop when given flow, pipe diameter, roughness, equivalent length, water properties at 60F
2) use Goal Seek to find out the equivalent length that results in a pressure drop of 1 psi, using flow = Cv

This gives me the equivalent feet of the valve in question that is consistent with my actual piping (diameter and roughness).

I use the same technique for equipment. For example, if the calculated pressure drop through a heat exchanger is 6 psi at 100 gpm (data provided by heat exchanger vendor), I plug those numbers into the spreadsheet - with the appropriate physical properties for the fluid - and use Goal Seek to get the equivalent length.

See attached example, found in Columns H:K

Steve

Attached Files

•  pszvba equiv L.xls   62.5KB   269 downloads

[sheiko]

I believe the method proposed by Steve Hall will give you the equivalent length of the valve at a given % opening, corresponding to the Cv considered only.

Edited by sheiko, 15 September 2012 - 08:14 PM.

[Brightonnk]

I do need calculations as opposed to estimations so I will have a look at the spreadsheets you have provided me with, Thank you

[katmar]

I'm sorry to come to the party so late. I have answered similar questions to this one many times, and being too lazy to write it all out again I thought that it was time for me to consolidate some of my previous answers into a coherent document. And then other deadlines cropped up and suddenly two weeks had flashed by.

Anyway, I have now posted the document on my web page if you still want more information. At least I will be able to answer quickly next time this topic comes up ;-)

Pressure Drop in Pipe Fittings and Valves - A Discussion of the Equivalent Length (Le/D), Resistance Coefficient (K) and Valve Flow Coefficient (Cv) Methods
http://www.katmarsof...essure-drop.htm

[Art Montemayor]

Harvey:

Hooray for you! You have accomplished documenting most – if not all - of the material and information regarding the subject of fluid pressure drop that you have been discussing in Forums for the past several years. I think I have collected most all of your prior posts, but this time I believe I have the subject completely covered with the document you have published on your website. Thank you for putting most of your knowledge on the subject in one document. Anyone interested in fluid flow (and what engineer can escape that) should visit your website and copy your document for subsequent insertion into their copy of Crane’s Tech Paper #410.

[Brightonnk]

Hey guys, Thanks for all your help - I have now derived a method to calculate Kv and CV values using all your information. I now am in the position where I need to test the valves I have to validate my calculations. I understand I need a flow loop, however am a bit unsure about what equipment is needed for this. I assume I need a pitot tube but thats as far as my understanding goes. Can anyoen provide me any help with what equipments/tools I would need to set up a flow loop testing rig that tests for cv values?

[Brightonnk]

Hey everyone, I've come across a slight problem. I've basically calculated my cv values for my different sized valves but realised I have just put in a random number for absolute roughness. The valve I am working on is a SCH40 steel pipe, butt weld ends, Reduced bore. I can find the wall thickness from online for a SCH40 pipe online but I need the wall irreularities, how do I work this out?

Ps the drawings I have for the valves do not specify roughness other than 'all unspecified surface finishes to be taken as 3.2um'

[katmar]

Have a look at my article that I referred you to before - there is a section that specifically addresses this (Section 3.4.4 Effect of the fitting roughness).

[Brightonnk]

Thank you

[Brightonnk]

Hey everyone. I just have a question/answer that I would like someone to validate. The term 'equivalent length' represents the same amount of pressure drop that occurs in a valve is equivalent to the same amount of pressure drop in a straight pipe?

Does this make any sense/is this right?

Thanks

[fallah]

neerakukadia,

It represents a specified length of the pipe on which the valve is located and produces the same amount of the pressure drop as the valve produces due to fluid passing through it.

Fallah