Page 136 - Air and Gas Drilling Manual

4-22 Air and Gas Drilling Manual
3
where Q i is the input volumetric flow rate (m /sec),
2
P i is the input pressure (N/m , abs),
2
P o is the output pressure (N/m , abs).
An alternate expression which is convenient for booster compressor calculations is
 k−1 
nk  R T    P  nk 
˙
W = s ˙ w g  i    o  s − 1  (4-36b)
s
k −1  S g   P   

i

where ˙ w is the gas weight rate of flow (N/sec),
g
T i is the input gas temperature (K).
The compression ratio for each stage, r s, is given by
1 1
 P   p 
r =  o  n s =  o  n s (4-37)
s
 P   p 
i
i
Equations 4-35 through 4-37 are valid for both reciprocating and rotary
compressors (together with the other pertinent equations in this chapter).
4.6 Prime Mover Input Power Requirements
The above section discusses the theoretical shaft power required to compress the
gas in a compressor. In order to obtain the complete picture of compressors, it is
necessary to ascertain the prime mover input power requirement to operate the
compressor shaft and the actual power available from the prime mover. The
application of the above equations will be slightly different depending upon whether
reciprocation piston compressors or rotary compressors are being analyzed.
The actual power available from a prime mover is a function of elevation above
sea level, and whether the prime mover is naturally aspirated or turbocharged.
Figure 4-15 gives the power percentage reduction in power necessary for naturally
aspirated and turbocharged prime movers as a function of elevation above sea level
[1].
4.6.1 Compressor System Units
Compressors used for air and gas drilling operations are driven by stationary
prime movers. For small drilling rigs (i.e., drilling depth limits less than 1,000 ft),
the compressor is often integrated into the rig design and the compressor is driven
by a shared stationary prime mover that also drives the rig drawworks and rotating
table (or hydraulic pump and motor rotating system). Larger drilling operations
have separate compressor system units that are fabricated with their own dedicated
stationary prime movers. These separate compressor system units are usually
provided to the operator by a contractor other than the drilling rig contractor.
Primary Compressor System Unit
Primary compressor system units take air directly from the atmosphere. These
compressor systems can be rotary or reciprocating compressors. The compressors in