Page 345 - Compression Machinery for Oil and Gas

330 SECTION II Types of Equipment

six-throw) the pressure in cylinder area of the piston rod crank radius
algebraically added for all cylinders, this reaches a max at 90 degrees of
crank rotation. After start of rotation, the bearing friction drops but windage
loss through the inlet unloaders build as the speed increases. Torques
reaches a max at 100% speed but is usually in range 5%–10%
3. At first-stage inlet pressure with an overall bypass. If required there is fre-
quently also a first-stage bypass. The breakaway torque is as before. This
time as the compressor starts to rotate gas flows through the bypass and
depending on the bypass sizing the torque due to cylinder compression to
drive the gas flow can be significant. With proper bypass design and sizing
the torque at 100% can be limited to the range 10%–20%
Fig. 7.18 shows the first revolution torque versus crank effort curve appropriate
for a two-, four-, and six-throw machine starting out with 800psi pressure in the
cylinders so typical of a gas transmission application. Notice how the torque is
initially low then after 90 degrees of crank rotation reaches a peak then between
180 and 360 degrees of crank rotation goes negative for the two- and four-throw.
The six-throw has 120 degrees crank angles and so the gas torque due the pres-
sure times area of rod times crank angle cancels out and so remains relatively
flat. Friction is high at breakaway when the crank is metal to metal on the bear-
ings, but quickly drops to a low value (usually 2%) as it starts to move and lifts
onto the oil film.
Fig. 7.19 shows a traditional speed-torque curve that would typically be sup-
plied by the original equipment manufacturer (OEM) or packager and is pro-
duced by the OEM performance program.
The zero speed or breakaway torque is the summation of the breakaway-fric-
tion-torque and the compression power and pressure times the rod-area-torque,

1st revolution % torque vs crank angle for
2, 4,6 throw at 800 psi unloaded using bypass
25.00%
20.00%
15.00%
10.00%
5.00%
0.00%
0 50 100 150 200 250 300 350 400
–5.00%
–10.00%
–15.00%
–20.00%

2 throw 4 throw 6 throw
FIG. 7.18 First revolution torque versus crank effort curve for a two-, four-, and six-throw
machine.