Page 235 - Compression Machinery for Oil and Gas

224 SECTION II Types of Equipment

maximize nozzle diameter, reduce the compressor speed, select a cylinder
with larger inlet and discharge flange size, use a higher power rating
compressor, etc.)

Cylinder Lateral Gas Pulsation Effects
A basic assumption in compressor performance is that pressure everywhere
inside a cylinder is equal. In fact, this is often not the case. The worst point
is when the discharge valve opens, then gas next to the valve starts to flow
out, but gas remote from the valve can only begin to move toward the valve
when a pressure wave traveling at the speed of sound reaches the gas. This
results in an over pressure in the cylinder causing a pressure wave pulsation
which travels laterally across the face of the piston from discharge to inlet valve.
This pressure wave goes back and forth across the piston face and gradually
decays as the piston reaches the end of the stroke.
The strength of the pressure wave is function of three things:

l The relative cylinder diameter as indicated by the degrees of crankshaft rota-
tion it takes for a sound wave to traverse the cylinder. (This factor includes
cylinder diameter, rotative speed, gas sonic velocity.)
l Volume in the cylinder when the discharge valve opens, that is, the lower the
fixed volume and closer the piston is to the end of stroke the worse the pul-
1/n
sation. Approximated as 1/R where R is the compression ratio and n is the
volume exponent.
l Instantaneous piston velocity at the time the discharge valve opens. (Piston
velocity is a maximum at midstroke but starts to drop significantly as it
reaches 75% stroke.)
The degrees of crank rotation for a sound wave to traverse the cylinder can be
calculated from the formula A ¼6ND/u where A is the angle in degrees, N is the
rotational speed in rpm, D is the cylinder diameter in meters, and u is the sonic
velocity in m/s. The low medium and high level correspond to approximately
5%, 10%, and 15% p-p pulsation as a percent of the discharge pressure. Effect of
high lateral pulsation; high pulsation will have an adverse effect on discharge
valve operation and will impose a large bending moment on the piston and cyl-
inder. This can result in piston rocking and can result in piston reliability prob-
lems in sensitive pistons (large diameter compared to piston length and rod
diameter), cylinder vibration (rocking mode about horizontal plane at 90
degrees to piston rod axis).

Rotordynamics
Torsional Rotordynamics

Torsional failures are sometimes referred to as “silent killers,” because the
events are rarely preceded by an increase in frequently monitored vibration