Page 223 - Compression Machinery for Oil and Gas

212 SECTION II Types of Equipment

FIG. 5.29 Example of capacity control using recycle methods. (Courtesy of Ariel Corporation.)

Variations in Rotating Speed
Varying the rotating speed is a simple and efficient method of capacity control.
When the speed is reduced the capacity and power are reduced. If the driver is an
electric motor then there is substantial cost in the VFD and the VFD draws
approximately 2% of the motor power. However with an engine driver the var-
iable speed comes basically for free. A drawback of varying speed is that the
compressor valves now have to function reliably over a range of speed which
can be difficult. Also the varying speed might cause the compressor to operate
closer to frequencies that can excite pulsation or torsional-related issues.

Adding/Subtracting Fixed Clearance
Adding and subtracting fixed clearance is one of the most common methods
used to control the capacity of a reciprocating compressor. When fixed clear-
ance is added to the compression chamber of a compressor cylinder the VE
is reduced thus reducing capacity. This is an efficient method of capacity
control.
There are many mechanical devices used to add clearance. Some of the most
common as follows:
– head-end fixed volume clearance pocket;
– head-end variable volume clearance pocket;
– valve cap fixed volume clearance pocket; and
– valve clearance spacer.

Head-End Fixed Volume Clearance Pocket
This device replaces the standard head-end head. When the plug valve opens,
fixed clearance is added to the compression chamber and the capacity
decreases.
This device also replaces the standard head-end head. As opposed to open-
ing and closing a fixed volume, the volume is variable, as shown in Fig. 5.30.
Most common is a manually operated device, as shown in Fig. 5.31).