Page 150 - Reciprocating Compressors Operation Maintenance

136 Reciprocating Compressors: Operation and Maintenance

2, Lubrication and condensate further delay separation of the two con-
tact surfaces between valve plate and seat land. This results in a
sticking effect.
3. The pressure that eventually will force the valve plate to lift has an
approximate linear increase over time (Figure 3-10). As it increases
sufficiently to overcome the spring load, the valve plate will begin
to accelerate.

a (Ap)

FIGURE 3-10. Valve acceleration and pressure differences versus time.

The velocity increases gradually

According to the laws of physics, the velocity will begin at zero and
increase gradually (parabolic). Valve lift, or the distance that the valve
plate travels, lags behind valve velocity (Figure 3-11).
The combination of the above three factors creates the effect of the high
initial differential pressure between the cylinder and plenum, which can
be seen on all pV-diagrams. A pressure differential is necessary and, on

v,s
lift=distance from seat

FIGURE 3-11. Valve velocity and lift versus time.