Pressure drop across a valve with air going thru ! Help ME!! 2

[shup0739]

Ok I am really stuck here and I need your help!

here is the information I have so far :

I am pulling facuum on a vessle using a jet, and I know the pressure at the jet and the pressure at the vessle. I have a valve mid-way and I need to know the pressure going into this valve and the pressure leaving the valve. I have measured and accounted for fittings from the vessel to the valve and after the valve up to the jet. I have calculated the pressure drops associated with the piping and fittings but I still do not know how to find the pressure drop across this valve.

inlet pressure : 49687 Pa
Size : 2 inch
Type : Ball Valve
Cv : 104
Type of FLow : Air ( this is a gas not water)

What is the correct formula to find the outlet pressure, this formula must take into account the Cv.

Also, what I want to do is see if this pressure drop will be different if I replace the valve with a 2 inch GLOBE valve with a Cv or about 50.

Please help me out on this one guyz...I would really appreciate it.


shup

 

[Montemayor]

shup0739:

You're not pulling much of what I call a vacuum with the vessel being at 0.5 atm.

Go to Crane's Tech Paper 410, page 3-4.

Pressure Drop = (3.62)(K * D *q^2)/d^4

where,

Pressure Drop = psi
K = for a 2" ball valve
D = gas density, lb/ft3
q = actual ft3/sec
d = internal diameter, inches

From my compiled database for valves:

a 2" Velan ball valve (split body) has a Cv of 130 for a reduced port and 430 for full port.

Cv = 29.9 * d^2/(K)^0.5

The above will give you the answer. You should be able to handle the math.


Art Montemayor
Spring, TX

 

[shup0739]

Thank you I think that helped a lot. I actually have cranes 410 but I was not able to find your pressure drop formula that you have sugested I ened up using another formula that correlates the head loss to pressure drop which is simple multiplying your head loss by the density and gravity.

However, when you find the flow coefficent, Cv the formula asks for the d (diameter in inches) I was just wondering if the number value before the formula was put there to balance the units or not?

but thank you I think your reply helped a lot.

shup