Page 259 - Air and Gas Drilling Manual
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6-4 Air and Gas Drilling Manual
6.2.1 Weight Rate of Flow of the Gas
In order to carry out the derivation of the governing equations for direct
circulation, the weight rate of flow of air (or gas) to the well must be determined.
Assuming the compressed air is provided by compressor(s), the weight rate of flow
through the circulating system is determined from the atmospheric pressure and
temperature of the air at the compressor location on the surface of the earth, and the
characteristics of the compressor(s). For air, the atmospheric pressure for sea level
and various elevations above sea level can be approximated for most of North
America by the mid latitudes data given in Table 4-1. These reference pressures are
denoted as p r. Thus, the atmospheric pressure of the air entering the primary
compressor(s) is, p at, and this pressure can be approximated as
p at ≈ p r (6-2)
where p at is atmospheric pressure (psia),
p r is the reference atmospheric pressure (psia).
Similar data as that given in Table 4-1 for North America mid latitudes can be
obtained for the other continents and latitudes around the world.
The above approximation is used when the actual atmospheric pressure at the
drilling site has not been measured and recorded. The gas pressure, P g, is
P = P at = p 144 (6-3)
at
g
2
where P at is the atmospheric pressure (lb/ft , abs),
2
P g is the gas pressure (lb/ft , abs).
To determine the weight rate of flow through the primary compressor(s) the
actual temperature of the atmosphere, t at, must be used. The absolute gas
temperature, T g, of the air entering the compressor(s) is
T = T at = t at + 459 67 (6-4)
.
g
where t at is the atmospheric temperature (˚F),
T at is absolute atmospheric temperature (˚R),
T g is absolute gas temperature (˚R).
Equation 4-11 in Chapter 4 gives the perfect gas law. This equation relates
absolute pressure, specific weight, and absolute temperature [1]. The equation is
P = R T
γ S g (6-5)
2
where P is pressure (lb/ft , abs),
T is absolute temperature (˚R),
3
γ is specific weight (lb/ft ),
R is the gas constant (53.3 ft-lb/lb-˚R),
S g is the specific gravity of the gas (S g = 1.0 for air at standard conditions).