Page 98 - Reciprocating Compressors Operation Maintenance
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Design and Materials for Reciprocating Compressor Components 85
compressor builder. This value is different for each model of compressor,
but generally crosshead pin bushing pressures are limiting.
Rod loads can be calculated by simple arithmetic, but in operation,
suction and discharge pressures can change so fast that the operator does
not have time to calculate.
ROD LOAD CALCULATIONS
Force = Pressures x Area
Two methods of calculation are used, an average method and an exact
method, which requires calculation of compression and tension loads.
The following formulas are used:
Average Method
F = P x A= (P d - P s) x net cylinder area
Where
1
Net cylinder area = cylinder area - A rod area
Exact or Compression and Tension Method
Piston moving toward head end or rod in compression
(a)Fc = (P d xA he )-(P s xA ce )
Piston moving toward crank end or rod in tension
(b)Ft = (P s xA he )-(P d xA ce )
Where
P s = suction pressure, psia
P d = discharge pressure, psia
A he = cylinder head end area, sq. in.
A ce = cylinder crank end area, sq. in. = A he - rod area
The average method can be used for quick checks, but if above or very
close to maximum allowable, or if the cylinder is relatively small, 10
inches diameter and below, then the exact method must be used.
