Page 251 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-93
In Illustrative Example 5.6b it was found that the actual shaft horsepower
required by the compressor to compress air to 284.1 psia was 193.2. The derated
horsepower available from the prime mover was 340.4. The prime mover for this
rotary compressor is also used to operate the hydraulic pump that operates the top
drive system. Thus, to obtain the fuel consumption for this prime mover (using fuel
consumption calculations outlined in Chapter 4) the power usage to operate the top
drive system must be estimated and added to the above compressor usage. For this
example, it is estimated that the torque applied to the top of the drill string is 2,000
ft-lb at a rotary speed of 60 rpm. For this example, Equation 5-11 becomes
(, 2 000 ) (60 )
W ˙ td =
5252
W ˙ td = 22 .9
Assuming an overall mechanical efficiency for the hydraulic top drive system to
be, ε = 0.8, the actual shaft horsepower needed for the hydraulic pump which
m
operated the top drive system is estimated to be

W ˙ td = 22 .9
. 080
W ˙ td = 28 .6

Thus, the total actual shaft horsepower required by both the compressor and the
˙
hydraulic pump operating the top drive, W , is
t
W = W ˙ as + W td
˙
˙
t
˙
W = 193 .2 + 28 .6
t
˙
W = 221 .8
t
The prime mover power ratio is
.
PR = 221 8 ( 100) = 65 1
.
340 8
.
From Figure 4-17 the approximate diesel fuel consumption rate at this power level is
0.558 lb/hp-hr. The total weight rate of diesel fuel consumption per hour is
˙ w = . 0 558 (221 . ) =8 123 .8 lb/hr
f