Page 407 - Reciprocating Compressors Operation Maintenance

392 Reciprocating Compressors: Operation and Maintenance

— (A-4b)
A

For heavier mol weight gases (M = 20), API valve velocities are
selected about 3,580 fpm or 18.2m/sec. and lighter mol weight gases, (M
= 7), 7,000 fpm (35.6 m/sec.).
Manufacturers often have interchangeable suction and discharge
valves. This can lead to putting valves in the wrong port, which can
result in massive valve breakage or broken rod or cylinder. It would be
well to specify that valves must not be interchangeable. However, a pro-
truding non-reversal feature can be lost or broken off; correct valve
placement should thus always be checked.
CALCULATIONS

General

Solving compressor problems by the use of the ideal gas laws has been
reduced to a relatively simple sequence of applying a few basic formulae
and obtaining values from some basic curves.
In order to help in the understanding of these terms and equations, the
first chapter of this text covers some basic thermodynamics. Also, fol-
lowing the sequence of events as they occur in a compressor cylinder
may help develop an understanding of the terms encountered in most re-
ciprocating compressor problems (see Figure A-2).

Piston Displacement

Piston displacement is the actual volume displaced by the piston as it
travels the length of its stroke from Position 1, bottom dead center, to
Position 3, top dead center. Piston displacement is normally expressed as
the volume displaced per minute or cubic feet per minute. In the case of
the double-acting cylinder, the displacement of the crank end of the
cylinder is also included. The crank end displacement is, of course, less
than the head end displacement by the amount that the piston rod dis-
places. For a single-acting cylinder: