Page 199 - Standard Handbook Of Petroleum & Natural Gas Engineering

184 General Engineering and Science

Now check on the discharge coefficient.
The Reynolds number for gases can be calculated directly in terms of flowrate and
gas gravity as

(2-70)

where Q is in scfm, s is the specific gravity of the gas, d is the pipe hydraulic diameter
in inches, and c~ is in centipoise. From Figure 2-20 the viscosity of the gas is
p = 0.0123 cp

and

(28.8)( 14021)( 0.6)
Re = = 9,850,000
(0.0123)(2)

From Figure 2-24, using p = 0.5, the value of the discharge coefficient is read as
C = 0.62, and a new estimate of Q is:

Q = 0.62( 14,021) = 8,693 scfm
A further iteration produces no change in the estimated flow rate for this case.
In subcritical flow the discharge coefficient is affected by the velocity of approach
as well as the type of choke and the ratio of choke diameter to pipe diameter. Discharge
coefficients for subcritical flow are given in Figure 2-24 as a function of the diameter
ratio and the upstream Reynolds number. Since the flow rate is not initially known,
it is expedient to assume C = 1, calculate Q use this Q to calculate the Reynold's
number, and then use the charts to find a better value of C. This cycle should be
repeated until the value of C no longer changes.

Example 2-1 8
A 0.65 gravity naturally gas (K = 1.25) flows from a two-in. line through a 1.5-in.
nozzle. The upstream temperature is 90°F. The upstream pressure is 100 psia while
the downstream pressure is 80 psia. Is icing a potential problem? What will be
the flowrate?
Check for critical flow using Equation 2-67.

The flow is clearly subcritical.
Check the outlet temperature using Equation 2-65.
T, = 550(0.8°.25/'.25) = 506.86'R = 66'F

There will be no icing.
Calculate the flowrate.