Page 244 - Air and Gas Drilling Manual

5-86 Air and Gas Drilling Manual
The above pressure is the approximate injection pressure into the top of the
borehole annulus. The calculations have ignored the tool joint obstruction losses
(inside the dual wall pipe annulus) and of a blooey line type structure (most shallow
drilling operations do not have blooey lines). These losses are usually not important
in shallow drilling operations (Chapter 8 calculation examples will consider these
additional minor losses). Therefore, the above calculated pressure will only slightly
underestimate the actual pressure that is seen at the pressure gauge just downstream
of the compressor. In order for this compressor to be used for this drilling
operation, the above injection pressure must be less than the derated fixed pressure of
the rotary screw Sullair Model 840.
The fixed pressure capability of the Sullair Model 840 compressor is an output
of 340 psig (at API standard conditions). However, this output must be derated
when the compressor is placed at a surface drilling location above sea level. To
determine the derated fixed pressure capability of this compressor for the 6,000 ft
surface drilling location, the design fixed ratio of the compressor must be
determined. As demonstrated in Chapter 4, the fixed pressure ratio is referenced to
sea level conditions (usually API standard conditions). Thus, the assumed design
input pressure, p 1 (air flowing into the compressor from the atmosphere), is
p = 14 696 psia
.
1
The output pressure at API standard conditions, p 2, is
.
.
p 2 = 340 + 14 696 = 354 696 psia
The total fixed compression ratio across the two stages of this compressor, r c, is
p
r = 2
c
p
1
.
r = 354 696
c
.
14 696
r = 24 14.
c
The approximated derated output pressure, p d2, for this compressor when placed at a
6,000 ft surface drilling location is determined from
p = r p
d2 c at
where p at is the actual atmospheric pressure at the 6,000 ft surface drilling location
(i.e., 11.679 psia) (see Table 4-1 or Appendix D). Thus, the derated output pressure,
p d2, is
11 769)
p d2 = ( 24 14) ( .
.