There are many cases in gas production operations in which the pressure of a gas must be raised to a higher value. As the pressure in a gas reservoir depletes, it will eventually reach a point where it will no longer overcome all the pressure losses in the system and the pressure of the line into which the gas in being delivered. It is then necessary to add a compressor to the system to supplement the reservoir energy but we have to know the types of gas compressors to choose.
In this type of application, the suction or intake pressure, and possibly the volume compressed, will change with time even though the discharge pressure may remain constant. Compressors have been used to lower the wellhead pressure below atmospheric, that is, to pull a vacuum on the well in order to obtain maximum rates. Compressors are also used to overcome the losses incurred in the long distance transportation of natural gas through transmission lines. This may require large capacity machines operating at essentially constant conditions.
The reinjection of gas for pressure maintenance or cycling requires compression of produced gas to a high pressure to move sufficient volumes into the reservoir. A compressor capable of operating under a wide range of conditions.
The engineer is concerned with essentially two types of compressor design problems:
- Determination of the power required to compress a certain volume of gas from some given intake pressure to a given discharge pressure.
- Estimation of the capacity of an existing compressor under required pressure increase conditions. It is also frequently necessary to calculate the temperature increase occurring in the gas as it is compressed.
Types of gas compressors in general
The principal types of compressors are positive-displacement, or intermittent flow, units and continuous flow units.
- Positive-displacement units are those in which successive volumes of gas are confined within a closed space and elevated to a higher pressure.
- Continuous flow units are those in which a rapidly rotating element accelerates the gas as it passes through the element, converting the velocity head into pressure, partially in the rotating element and partially in stationary diffuser or blades.
Reciprocating compressors are positive-displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder.
Rotary positive-displacement compressors are machines in which compression and displacement are effected by the positive action of rotating elements.
Sliding-vane compressorsare rotary positive-displacement machines in which axial vanes radially in a rotor eccentrically mounted in a cylindrical casing. Gas trapped between vanes is compressed and displaced.
Liquid-piston compressors are rotary positive-displacement machines in which water or other liquid is used as the piston to compress and displace the gas-lobs handled.
Two-impeller straight-lobe compressors are rotary positive-displacement machines in which two straight mating lobed impellers trap gas and carry it from intake to discharge. There is no internal compression.
Helical-or spiral-lobe compressors are rotary positive-displacement machines in which two intermeshing rotors, each with a helical form. Compress and displace the gas.
Centrifugal Compressors are dynamic machines in which one or more rotating impellers, usually shrouded on the sides, accelerate the gas. Main gas flow is radial
Axial compressors are dynamic machines in which gas acceleration is obtained by the action of the bladed rotor shrouded on the blade ends. Main gas flow is axial.
Mixed-flow compressors are dynamic machines with an impeller form combining some characteristics of both the centrifugal and axial types.
Ejector are devices that use a high velocity gas or steam jet to entrain the inflowing gas, then convert the velocity of the mixture to pressure in a diffuser.
Every compressor is made up of one or more basic elements. A single element, or a group of elements in parallel, comprises a single-stage compressor. Many compression problems involve conditions beyond the practical capability of a single compression stage.