Pressure drop different pipe and valve size 2

[dragnjj]

I'm trying to determine the pressure drop across a 6 inch line that goes to a 4 inch butterfly valve and continues back as a 6 inch pipe. The idea is to put in a 6 inch butterfly valve to reduce the pressure drop. I'm using Cameron's Hydraulic Data book and I can find the Equivalent length of a 4 inch butterfly to be 15.1. But do I multiply that by the head loss ft/100 ft for a 4 inch pipe or a 6 inch pipe? My flow rate is 1200 GPM currently. So I end up getting either currently getting a 9 ft drop or a 2 ft drop across that valve depending on how I calculate it. Basically, do I treat it as a 4 inch section of pipe with a 4 inch butterfly or a 6 inch section with a 4 inch butterfly? Thanks in advance.

Jeff

 

[pleckner]

Drop Cameron and get a copy of CRANE TP410. You will find an expample similar to your situation and the more proper way to do the calculation.

You should be considering using K values and not equivalent lengths, see my article (link below) for the reason why:

http://www.cheresources.com/eqlength.shtml

What you can do is convert the frictional loss of the valve in terms of 6" (Crane tells you how) and use that with your 6" pipe. By the way, you also need to account for reducers upstream and downstream of this 4" butterfly valve.

 

[vzeos]

dragnjj,

If you are working with fully turbulent flow, the equivalent footage of 15.1’ is the footage of 4” pipe that will cause the same pressure drop as the valve. If you chose to do the calculation on a 6” basis you need to convert 15.1’ to the equivalent footage of 6” pipe... One foot of 4” pipe is equivalent to (will cause the same pressure drop as) about 8.3’ of 6” pipe. So 15.1’ of 4” pipe is the equivalent of 125.213’ of 6” pipe. You would add 125.213’ to your length of straight 6" pipe to do your calculation. You would also have to add the equivalent length of two 6” x 4” reducers.

 

[katmar]

Good post by vzeos, but don't forget that the pressure drop (or equivalent length) of a reducer is generally much higher in the small to big mode (i.e. expander) than it is in the big to small mode (i.e. reducer).

Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[proinwv2]

The ISA S75 publication deals with this commonplace occurence.

Do you have a copy of it?

Paul

http://www.ostand.com

 

[rongyang504]

hi ,i am newer!!

 

[Boun05ChE]

I think pleckner showed you the right way to do it. Crane is the best source you should use for pressure drop calculation.

It is not important whether you use Equivalent length or not.

If you first try to obtain total K value, all K values sould be with respect to same pipe size. I mean that if you take 6" pipe as reference pipe, you should convert the K value of 4" bautterfly valve w.r.t. 6" pipe size, because of the formula head=K*v^2/2/g. So you should use the velocity of 6" pipe in order to calculate pressure head.

Another way is for each part of piping use the formula head=K*v^2/2/g, thenn add all head lost to find the pressure drop. But the trick is to know the K value of reducer w.r.t. bigger or smallaer diamter. At this point Crane will lead you to the right way.

(sorry my poor english)

 

[CountOlaf]

A little off the track here, but 1200 gpm thru a 6" pipe is 13+ fps: pretty high velocity. I can imagine there was significant pressure drop through that 4" valve! What would the velocity here be? Off the charts within CRANE TP410.

 

[sheiko]

See also:
thread378-173164
thread378-214445

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