Drop Valve

it is possible to determine the flow through the valve.

Suppose that the diameter of the pipe is known and we have readings of pressure from a pressure gauge before and after the valve.

If the problem is underspecified, the parameterized others must be known to determine this.

Unless you know the flow coefficient Cv value of the valve that is only available from the curves of different postions valve trim valve, you can not calculate the flow with the pressure drop only. Anything you can get the pressure drop just a difference in the square of the velocity before and after the valve according to the Bernoulli equation.

Here is the real formual for flow
Cv = q (SG / dp) ^ 1 / 2

Cv is the flow coefficient determined by the valve manufacturer provided partial open.
SG is the specific weight
q is the flow

you need the CV curve of the valve with an opening provided by the manufacturer of valve

Yes, you can ... but wth more info.
Refer to the Bernoulli equation: http://home.earthlink.net/ ~ mmc1919/img/v ...

Taking this as A + B + C = D + E + F;
C & F cancel. ur on the same plane.

PS You know, you know the Rhos,
For the V at a given moment, you need the V to the other.

Yes, you can almost report a problem

V2 / V1 = P2 / P1
lets put a number to that

you know that the pressure P1 = 40 P2 = 10 and to calculate the size you need crossectional area and speed if the crossection 10gpm and is 2 "there are approximately 7.5 cubic feet in a gallon so the speed is 10 / 7.5 = 1.33 cfm this is so if you need the gpm CFM ..
then solving
V2/10 = 10/40
Cross multiplying
40v2 = 100
v2 = 100/40v2
2.5 gallons per minute = v2 ...
I send you a link that has the PC gamma classified if the heat is a factor, then you must use a modified form of the .. above

http://www.grc.nasa.gov/WWW/K-12/airplan ...

there is the E

Manufacturers data sheet may have curves of pressure drop for the valve.
Then, as long as you know the opening angle, you should be able to determine the value from the graph