Page 260 - Air and Gas Drilling Manual

Substituting Equations 6-3 and 6-4 into Equation 6-5 and solving for the specific
weight of the gas (in this case air) entering the compressor(s), γ g, gives
PS g Chapter 6: Direct Circulation Models 6-5
g
γ = (6-6)
g
R T g
The weight rate of flow of the gas, ˙ w , through the circulation system is
g
˙ w = γ Q (6-7)
g g g
where ˙ w is the weight rate of flow of gas (lb/sec),
g
3
Q g is the volumetric flow rate of air into the circulation system (actual ft /sec).
This volumetric flow rate is usually the flow rate entering the primary compressor(s)
(see Chapter 4).
If the circulation system is natural gas from a pipeline and p pl and t pl are the
pressure and temperature of the gas entering directly into the circulation system from
the pipeline (or exiting the booster compressor from a pipeline), then, the absolute
pressure of the gas, P g, is
P = P pl = p pl 144 (6-8)
g
The absolute temperature of the gas, T g, is
T = T pl = t pl + 459 67 (6-9)
.
g
where p pl is the pipeline pressure (psia),
2
P pl is the pipeline pressure (lb/ft , abs),
t pl is the pipeline temperature (˚F),
T pl is the absolute pipeline temperature (˚R).
Substituting Equations 6-8 and 6-9 into Equation 6-6 the specific weight of the gas
from a pipeline can be obtained. This is
PS PS
pl
g
γ = = (6-10)
g
R T g R T pl
Substituting the result from Equation 6-10 into Equation 6-7 gives the weight rate
of flow of gas from a pipeline, where Q g is the volumetric flow rate of natural gas
from the pipeline at the pressure p pl and temperature t pl. Note that the volumetric
flow rate in a pipeline is usually given in either scfm, or in acfm at the surface
location temperature and must be converted to obtain the actual volumetric flow rate
at p pl and t pl. In order to use natural gas from a pipeline it is necessary to place a
meter run in the flow line leading from the pipeline. This meter run allows an