pressure drops in valves

[Cniper]

I am currently trying to use Hevacomp for general pipesizing. My client prefers the output. Manually I have a clear approach but tedious. When using the software it requests for a K value, fairly ok with this, dimensionless and there are lists from manufacturers. It also requires an acceptable static pressure drop? Both are required for the progression of the software. From experience what would be likely figures for fittings?

 

[zekeman]

Well, if you know the K values then the pressure drop for a particular fitting would be
K*Rho*V*V/2g
where
rho= density in lb/Ft^3or Newtons/M^3
V fluid velocity Ft/sec or M/sec
g= gravitational constant 32ft/sec^2 or 9.8M/sec^2
The answer is in lb/ft^2 or Newtons/M^2 which can be converted to psi or whatever you Brits use.
For example,
if a globe valve has a K factor of 1.5 , then for a 1 inch pipe size handling 8 gpm of water which corrresponds to 2.5 ft/sec, the
V^2/2g called the velocity head is
.097 feet and at water's density 62.4 and K=1.5 the pressure drop across the valve is
1.5*62.4*.097=9lb/ft^2=.06 psi
I imagine that if you use the mfr K value, they want to make sure that you are accepting that value translated to psi drop as a check. So enter any reasonable acceptable value, say .1 psi and see what the program asks. It may say there is a discrepancy and tell you why. I just don't know.

 

[Cniper]

Many thanks. I will try this. But remember the question rom the software is K? and static pressure? before the pipesizing has been achieved. Once I have pipesized, I will then have to size my control valves for AHUs cooler coils.

 

[katmar]

I can only imagine that a static pressure drop across a valve would apply to a non-return valve which required some minimum pressure drop before there is any flow at all. Once that minimum pressure drop has been overcome a normal K-value would model the pressure drop vs flow behavior. For a normal shut off or regulating valve (eg gate, globe, ball etc) I would enter 0 as the static pressure drop.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JStephen]

The way it's worded, it sounds like the software is going to check pressure drop and then if it exceeds your specified static drop, it will upsize the line or the valve. In that case, if you enter zero, it won't work.

 

[Cniper]

And if so what value to enter, or where to look for acceptable values rather than judgement