AN INTRODUCTION TO SCREW GAS COMPRESSION
By Trent Bruce
When it comes to selecting a compressor for natural gas compression, rotary screw compressors offer many advantages. This article offers an excellent introduction to rotary screw gas compression.
Abstract
Screw compressors have been used on air and various refrigeration and process applications for a great number of years. In recent years the machines have become very popular in the natural gas industry for natural gas compression; specifically, for booster and gas gathering applications. Declining field pressures in the USA and Canada are forcing the industry to look at more flexible gas compression alternatives to the conventional reciprocating compressor.
This paper will discuss specific applications where screw gas compressors are used and the advantages of the rotary screw for natural gas compression to conventional reciprocating machines. We will look at some specific features of the screw compressor, which make it the machine of choice for many gas compression applications requiring high reliability, low maintenance costs and a very wide overall operating range.
We will look at a graphic model of a typical process flow diagram and review the components required to make up a rotary screw gas compressor package. We will also take a detailed look at the internals of the machine to better understand the overall operation, capacity control system, and the associated power savings that go with it when using screw compressors for natural gas compression.
It is extremely important to optimize adiabatic efficiencies in order to provide reduced power costs. On rotary screw gas compressors this is done using a feature called Vi, or internal volume ratio. The Vi can be changed on different machines using a couple of different methods. We will compare both these methods and their associated advantages.
Project economics always play an important role in any equipment selection. With the proper flexibility built into the initial package, rotary screw gas compression units can be used on numerous applications.
In review, this paper will provide a detailed look at screw compressors, where they are applied and exactly how they operate. We will compare the rotary screw gas compressor to other types of machines for similar natural gas compression applications and show the benefits in using this type of machine.
INTRODUCTION TO SCREW COMPRESSORS FOR NATURAL GAS COMRPESSION
Oil flooded rotary screw compressors have been widely used on various air and refrigeration applications for over forty years. These machines did not make a significant presence in the natural gas compression industry until the early 1990’s. Until this time, reciprocating compressors had been used almost exclusively for natural gas compression. As gas fields have matured and field pressures have dropped, screw compressors have become a very attractive alternative and supplement to reciprocating gas compressors.
This paper will discuss some of the applications and features of screw compressors, basic operating principles and the advantages of the rotary screw compressor over conventional reciprocating compressors for the natural gas compression industry. We will look at the machine itself, as well as the overall compression system and the components required in a screw gas compressor package. The screw compressors we will focus on are the oil flooded, heavy duty process gas machines rather than the air derivative types. We will provide numerous illustrations to help better understand the screw machine in natural gas compression applications.
Figure 1 shows the basic geometry of the rotary screw compressor. View (A) is a simple representation of the actual rotors. We have labeled the male rotor lobe and the female rotor flute. As the rotors turn in an outward direction, the male flute will unmesh from the female flute forming an area for the gas to enter. The gas becomes trapped in the machine and compression occurs when the lobes of the rotors begin to mesh together again. The shaded area represents the pocket of gas that occurs within a specific flute. View (B) is a representation of the side view of the machine. The same flute is shaded for comparison. Here we see the suction port in the upper left corner and the discharge port in the lower right corner. The rotors will turn in an outward direction forcing the male and female flutes to unmesh, allowing process gas to enter the top of the machine. The gas will travel around the outside of the rotors until it reaches the bottom where the gas compression actually occurs. Gas will be discharged in the lower right corner of the picture.
Natural Gas Compression Process in Screw Compressor
APPLICATIONS FOR SCREW GAS COMPRESSORS
The rotary screw compressor is designed for low pressure applications with inlet pressures ranging from vacuum pressure up to 100 psig and discharge pressures up to 350 psig. These pressure ranges are typical for most process style machines and can vary depending on manufacturer, frame size and operating speed. There are some screw machines available capable of operating at higher pressures by using cast steel casings but these are not yet commonly used in the natural gas compression industry due to capital cost and availability.
Screw compressors are commonly used in a variety of air compression, process gas compression, process refrigeration compression and natural gas compression applications, including individual wellhead boosters, low pressure gathering systems, low stage boosters to existing reciprocating gas compressors, solution gas compression and flare gas compression. They have been used on sweet and sour gas as well as acid gas applications with H²S concentrations of 35% and CO² concentrations of 65%. Although most natural gas compression applications are based on a specific gravity of 0.57 – 0.65, screw gas compressors can be used on very light gases such as hydrogen and very heavy mole weight gases where specific gravities exceed 2.0.
The most common applications for screw compressors in natural gas service range in horsepower from roughly 90 to 1,500 and are available in both engine and electric drive. Screw compressors were originally developed to operate with electric drive two pole motors at 3,550 rpm. As they have become more popular for natural gas compression, engine drive applications have become much more common. On most of these applications, the screw compressor is operating direct drive at 1800 rpm, or half the rated speed.
