Page 177 - Air and gas Drilling Field Guide 3rd Edition

168 CHAPTER 7 Reverse Circulation Models

and average annual temperatures for sea level and various elevations above sea
level can be approximated for most of North America by midlatitudes data given
in Tables 5-1a and 5-1b. These reference pressures are denoted as p r and t r . For
engineering calculations, the actual atmospheric pressure and temperature of the
air entering a primary compressor(s) are p at and t at and can be approximated as
(7-2)
p at p r
t at t r ; (7-3)
2
where p at is atmospheric pressure (psia, N/cm abs), p r is the reference atmo-
2
spheric pressure (psia, N/cm abs), t at is atmospheric pressure (psia, N/cm 2

abs), and t r is the reference atmospheric temperature ( F, C). Also see Appendix
B for more detailed plots for atmospheric conditions for North America midlati-
tudes. Similar data as that given in Tables 5-1a and 5-1b and Appendix B for North
America midlatitudes can be obtained for most other continents and latitudes
around the world.
The aforementioned pressures and temperatures in field units must be con-
verted to units consistent with Equations (5-11) and (7-1). These are

P g ¼ P r (7-4)
T g ¼ T r ; (7-5)
2
2
where P r is the reference atmospheric pressure (lb/ft abs, N/m abs) and T r is

the absolute atmospheric reference temperature ( R, K).
Substituting Equations (7-2) and (7-3) into Equation (5-11), the specific weight
of the gas (air) g g entering the compressor(s) is
P g S g P r S g
g ¼ ¼ ; (7-6)
g
R e T g R e T r
3
3
where g g is specific weight (lb/ft , N/m ), R e is the engineering gas constant

(53.36 ft-lb/lb- R, 29.31 N-m/N-K), and S g is the specific gravity of the gas (S g ¼
1.0 for air at standard conditions). The weight rate of flow of the gas, _ w g through
the compressor is
_ w g ¼ g Q g ; (7-7)
g
where _ w g is the weight rate of flow of gas (lb/sec, N/sec) and Q g is the volumet-
3 3
ric flow rate of air into the circulation system (actual ft /sec, actual m /sec).
If the gas for the drilling operation is natural gas from a pipeline and the pres-
sure and temperature of the gas in the pipeline are p pl and t pl , then the pressure
and temperature of the gas entering either directly into the circulation system or
to the booster compressor are

P g ¼ P pl (7-8)
T g ¼ T pl ; (7-9)