Page 621 - Air and Gas Drilling Manual

where G is the gear box reduction ratio. Using Equation 11-3, the runaway speed
r
of the drive shaft (drill bit) is
55 642 Chapter 11: Specialized Drilling Equipment 11-59
,
N rb
.
112 5
.
N 494 6 rpm
rb
The torque developed by the motor at any drive shaft (drill bit) speed is
N b
Torque b Torque sb 1 (11-4)
N rb
where N is the drive shaft (drill bit) speed (rpm). Using Equation 11-4, the torque
b
at the drive shaft (drill bit) speed of 40 rpm is
40
Torque (,980 ) 1
3
b
494 .6
,
Torque 3 658 ft lb
b
The drive shaft (drill bit) horsepower developed at any drive shaft (drill bit) speed is
Torque N b
b
HP b
,
5 252
3
(,658 ) (40 )
HP b
, 5 252
HP b 27 86
.
Maintaining Sonic Velocity in Nozzle Throat
In order to maintain sonic gas flow in the throat in each active nozzle, the ratio
of the pressure below the turbine device to the pressure above the turbine device
must be less than the critical ratio as defined by Equation 6-62. To obtain this
condition, several of the 23 nozzles in the stator must be plugged. Plugging these
nozzles restricts the flow of gas to only a few nozzles and, thus, increases the
pressure above the turbine stator.
The nozzle throat opening, L , is
t
l t 0 136 inches
.