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).
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