Page 196 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 185

temperature. The piston returns to Position 1, comes to rest, and the process
repeats. Segment 4-4A-1 is called the suction event.
This completes the basic reciprocating compression process. One cycle
around the P-V diagram represents one revolution of the crankshaft and two
stroke lengths of the piston, one from Position 1 to 3 and another from Position
3 to 1. Four events make up the P-V diagram—compression, discharge, expan-
sion, and suction.
The basic governing equation for the entire process is the first law of ther-
modynamics. Typically, the operation of the piston can be considered slow
enough that the cylinder thermodynamic properties (i.e., pressures, tempera-
tures, etc.) are treated as quasi steady and without any spatial variation. Neglect-
ing kinetic and potential energy effects, the first law of thermodynamics can be
written for the control volume encompassing the cylinder as shown in Eq. (5.1).

_
_
Here, Q is the heat transfer rate from the environment to the fluid; W ¼ PV is
the rate of change of work done by the fluid on the boundary; _ m s h s and _ m d h d are
the rates of change of flow energy entering the suction and leaving the discharge
valves, respectively; _ m l h l is the rate of change of flow energy lost by leakage
_
through the piston seal; and E ¼ d muÞ=dt ¼ _ mu + m _ u is the rate of change of
ð
internal energy within the control volume. Also, recognize that mass continuity
is imposed on the flow rates by _ m ¼ _ m s _ m d _ m l . Discussion of the mecha-
nisms for mass exchange through the valves is presented in a later section.
_
_
Q W + _ m s h s _ m d h d _ m l h l ¼ E _ (5.1)

Valves

Types
Compressor valves are simple spring-loaded check valves. They are not
mechanically actuated in any way and open and close due to differential pres-
sure. The fundamental purpose of the spring is to provide a “push” to help the
seal element move from the open to the closed position.
There are four basic designs of compressor valves, delineated by the form of
the seal element:
– ported plate;
– individual ring;
– poppet; and
– reed.
Every compressor valve consists of a seal element, seat, guard, spring, and a
method to hold the seat and guard together as the valve is installed and removed
from the cylinder
The ported plate design, as shown in Fig. 5.3, is the most common. The seal
element is a disc (a “plate”) with slots that allow gas to pass through the disc.