In this chapter you will learn what natural gas compression is, why and where
it's being, used who's using it, and how the process works. We will also cover the
main types of compressors that are used in our industry and look at some of the
differences between high and low pressure compressors.
Natural gas compression is the process of squeezing gas molecules in order to increase gas pressure.
Natural gas is compressed to enable it to be moved from one place to another.
This may need to be done for a variety
of reasons such as to help lift resources to the surface, to create a
pressure drop to condense and remove liquids from the gas, or to help achieve gas processing requirements.
Compression is used in every sector of the industry
whenever conditions do not normally exist for various processes to take place.
Upstream producers use compression to inject gas back into a well to aid in
lifting liquids to the surface.
They also use it to compress low-pressure gas from tanks
control devices and other equipment to help dispose of fugitive gas emissions
In the midstream, compression is used to move gas from one location to the next through miles of pipeline.
In the downstream sector it aids in the removal
of liquids to meet consumer and safety requirements.
Gas is compressed from zero
to two thousand psi in the upstream sector. Midstream and downstream can
compress an excess of 2,000 psi. Compression is done by increasing gas
pressure in stages until reaching the desired delivery point. The starting
pressure, desired ending pressure, and the volume will help determine how many stages a compressor will have.
A compressor may be as small as this one shown here.
This single stage model is used to gather fugitive gas and send it
to the combustor or flare. It is known as a vapor recovery unit or VRU.
Compressors aren't only for natural gas. Here is a small air compressor housed
inside a shed at a tank battery. Many producers are compressing air to be used
for instrument supply. They do this to cut emissions and lower their environmental impact.
This medium-size compressor can handle larger volumes and
higher pressures. This may be found at the wellhead for gas injection or in a
small gathering system. The largest compressors are found at
compressor stations. These are the engines that power a pipeline. The size
and number of compressors vary based on the pressures and volumes of gas to be moved.
There are two main types of compressors used in our industry:
reciprocating and screw. Reciprocating uses pistons and positive displacement
to compress the gas. Gas enters the manifold, flows into the compression
cylinder, and then is discharged to add a higher pressure.
Screw compressors use
two meshing helical screws or rotors to compress the gas. Gas enters the suction
side and moves into the threads of the screws. As it does this, it is compressed
and then exit the discharge at a higher pressure. This video shows the inner
workings and the flow path of a three-stage reciprocating compressor.
The inlet flow or suction side of the compressor starts at 30 psi and 80
degrees. It enters the inlet scrubber and any free liquids fall out. At the first
stage of compression, the pistons will compress the gas to 155 psi and the
temperature will increase to 260 degrees. As it exits the first stage it goes into
the intercooler. This cools the gas down to 120 degrees.
The heating and cooling of gas along with the compression makes more liquids
fall out of the gas. From here it enters into another scrubber so that the
liquids can fall out.
The second stage of compression increases the pressure to 490 psi and
the temperature also heats up to 270 degrees.
From there it's back through the cooler to get the temperature back down to 120 degrees.
With more pressure and cooling there's more liquids that will fall out in the last scrubber.
The third stage of compression gets the up to 1,200 psi and 240 degrees. Once
again the hot gas will go through the cooler and exit at the discharge at 120 degrees.
Some producers will run the gas through one last scrubber to give any remaining liquids a place to drop out.
This is a simplified rendering of a low-pressure
single stage compressor. The red components are Kimray products that are
used on a system operating less than 300 psi.
This is a similar rendering but of a
high pressure, single stage compressor. The products all have the same function
but all of them have a higher working pressure of above 300 psi. You will need
to know the basic flow and connections of a compressor when talking with a
customer. Gas flow comes in through the suction control valve and into the
scrubber. From the scrubber it goes to the compressor then through the cooling
fan and out the discharge. If the suction pressure drops below
setpoint the low suction recycle valve will open and pressure will go to the
scrubber. If the discharge pressure goes above setpoint the high discharge
recirculating valve will open and send pressure to the upstream of the suction control valve.
Thank you for participating in this training