Selecting a Valve for Natural Gas Flow

[cjm181]

Homework Statement

A valve is required to regulate the flow of natural gas. Details of the gas are given in table below. It can be assumed that the pipe size and the valve size are the same [i.e. piping geometry does not have to be allowed for].

The chosen valve type is the V250 rotary ball valve by Fisher Controls International I have this data sheet, it the chart it gives values for Cv, Kv, Fd, Fl, Xt

1) Determine the required valve size in inches from the range of sizes given in the data sheet [the data sheet gives a range of nominal pipe sizes (NPS) in inches].

2) Estimate the percentage the chosen valve would have to be open to carry the required flow.

required volumetric flow rate = 2.8x10^6 litre per min
Density (@STP) = 0.724 kg/m^3
Specific heat ratio (y) = 1.31
inlet pressure = 20barg
outlet pressure 6barg
inlet temp 20degC

Homework Equations

K_v=q_m/31.6YSQRT(xp₁P₁) ...1
where kv is valve coefficient
qm is mass flow in kg/h
Y is expansion factor
p1 density
P1 inlet pressure

Y=1-(x/(3F_yX_t)) ...2

F_y = y/1.4

The Attempt at a Solution

So, I think here I have to find the value of Kv, then use that to size the valve on the sheet. So to satisfy equation 1, I must find Y

the issue is I am not given Xt. Xt features on the valve chart, so how can I specify this? Or am I off in the wrong direction?

 

[JChemP]

Did you get given the equation for x?
x = dp / P1
x limit = xT * Fk

If dp / P1 > xT * Fk then your flow is choked and x = xT * Fk
If dp / P1 < xT * Fk then your flow is not choked and x = dp / P1

This determines what x you use in the sizing equation. (NOTE make sure you use bara in all your equations) 21.01325 bara.

For preliminary sizing you could;
1. Use a range of xT factors from table (maybe at 80 / 90% open and estimate required Kv)
2. Select appropriate valve from datasheet
3. Calculate flow of selected valve at range of opening % using provided kv / xT and plot curve
4. From curve read the % open based on your flow.
5. Typically good control range is between 20 - 80% open. If your valve is > 80% open maybe do the same procedure for the next valve size up and check % open.

Hope this helps.