Screw Compressor Package Components
With the exception of the oil separator and the oil cooling system, most of the components on a screw compressor package are similar to a reciprocating machine. Figure 14 provides an illustration of a typical process flow diagram for a natural gas screw compressor package. The inlet gas is fed through a suction scrubber or filter to remove any free liquids and particulate. Liquids are drained from the vessel to skid edge and the gas is taken off the top and fed into the compressor where it mixes with the lube oil. The high pressure gas is fed through the oil separator where the oil is removed to accumulate in the bottom of the vessel. The gas is then sent to the after cooler where it is cooled from normal discharge temperatures of 170-220°F down to 120°F. These temperatures will vary a little depending on different requirements and ambient conditions. The cooled gas is then taken off skid for connection to the field piping.
The lube oil will flow from the bottom of the oil separator through an oil cooler into the main oil header. The header splits into two main feed points for the compressor. Some of the oil is filtered down to roughly 10 microns and then injected into the bearings and shaft seal. The remaining oil is injected directly into the rotors.
We have labeled the process gas, compressor lube oil, water/glycol for compressor oil cooling and water/condensate drain lines.
CONCLUSION
In recent years, declining pressures in natural gas fields have made rotary screw compressors a very attractive alternative and supplement to reciprocating machines. We know that screw compressors are not designed to compete against the reciprocating market in all applications. Screws are low pressure machines, designed to operate up to roughly 350 psig discharge pressure. They are not designed to operate at the same high pressures that piston machines can achieve.
Even with their growing popularity, very few people understand how screw compressors actually operate. We have looked at a comparison of the compression process of a screw machine to a reciprocating machine. We have seen that screw compressors do not have any suction and discharge valves that open and close automatically based on pressure differential across them. Screws require proper positioning of the radial discharge port to control volume ratio, which is ultimately required to match the internal compression ratio of the machine to the system requirements.
We reviewed the concept of capacity control using the slide valve, and the power savings associated with the reduced capacity. We have looked at the lube oil system, why it is required, and how it operates. We also reviewed the overall screw compressor package to see what major components were required to make the system operational.
Screw compressors are not designed to replace reciprocating machines. They are designed to provide a very flexible, highly reliable alternative for low pressure applications.
ACKNOWLEDGMENTS
We would like to thank Frick and Ariel Corporation for their co-operation in providing some of the graphics and information presented in this paper. Their help was greatly appreciated.
