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.

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.

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