Page 217 - Compression Machinery for Oil and Gas

206 SECTION II Types of Equipment

120

100
Average piston velocity (%) 80

60

40

20

0
0 20 40 60 80 100
Valve open time, percent of stroke (%)
FIG. 5.26 Plot of piston velocity vs. compressor valve open time. (Courtesy of Ariel Corporation.)

But the velocity used in the above relationships for valve pressure drop and
VLP is the average piston speed during the time the compressor valves (suction
or discharge) are open as shown in Fig. 5.26.
For example, if the suction valve is open for 40% of the stroke, the average
piston velocity would be about 87% of the full stroke average velocity.
If the above relationships are substituted back into the VLP equation, the
following results are obtained:
3 3
ð
ð
P MWÞ VEÞ R P Þ A BORE Þ S RPMÞ VEÞ
ð
ð
ð
ð
VLP
2
ð
ZTðÞ N A VLVPKT Þ
VLP and friction represent all of the inefficiency in a reciprocating com-
pressor (not considering pressure drop in getting gas to and from the compres-
sor and possible efficiency losses due to P-V diagram distortion resulting
from gas pulsation). The magnitude of typical friction power is 5%, meaning
the majority of the inefficiency is associated with VLP. Some comments
about VLP:
3
l VLP varies with (S RPM) . This is a large number and therefore signifi-
cantly impacts VLP.
l VLP varies with stroke and rotating speed and not just rotating speed.
Comments are sometimes made that “high-speed compressors are ineffi-
cient.” This is not correct. A more accurate statement is “high piston speed
compressors are relatively inefficient.” The following table lists several
combinations of stroke and rotating speed that result in the same piston
speed: