Page 199 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Comprmors 181

Method of preparing compressor cylinder rod-load check graph such
as Figure 5-5A for any compressor cylinder.
1
P. = -[CRL + PI(A - a)]
A
Where:
A = cylinder area,
a = rod area,
CRL = compression rod load assigned to unit,
PI = suction pressure (psig),
P2 = discharge pressure (psig).
Compression Rod Load. Known data for cylinder covered by
Figure 6-5A:
Cyl. Dia. = 34 in.
Compressor Rod Dia. = 4 in.
CRL = 125,000 lbs
Choose any suction pressure (say 10 psig) and solve for Pz:

P2 = - [125,000 + lO(907.9 - 12.56)] = 147.5 psi.
907.9
The PI of 10 psi and Pz of 147.5 psi will locate one point on a
graph.
Choose another suction pressure higher than design, say, 40
pig, and substitute in above formula, Pz then is 177 psi. The PI
of 40 psi and P, of 177 psi established a second point for the com-
pression-rod load line.

Tension Rod Load. The formula for tension rod load is

1
Pz = -
[TRL + PI 9 A]
A-a
Known data are same as compression rod load except the as-
signed tension rod load, TRL, is 115,000. To determine load line
for it, substitute PI of 10 psi. P2 would be 138.5 psi, which would
give one point.
Next choose a P, of 40 psig. For above formula, P2 would be
169 psi. Insert this point on the graph and draw the tension rod
load line. Since the compresson rod load line is above the tension
rod line it should be disregarded. Use the graph as described in
the text.
-

Figure 5-5B. Example of rod load calculation.