it is one of the most important steps in natural gas industry, it compresses natural gas and raise its pressure to make it continue flowing and reach further distances, they are located at natural gas pipeline, and they are essential to guarantee a continuous flow of gas, this is done by gas compressor mainly driven by a gas turbine, and sometimes by an electrical motor.
what is the main function of gas compressor station?
to compress and dehydrate natural gas and make it reach to the downstream process “either it is a LPG plant or a LNG plant”.
what are the main components of gas compressor station?
the gas compressor station consists of the following units:
- a suction scrubber.
- a gas manifold or a distribution header.
- gas compressor “single or multiples stages” according to the required discharge pressure.
- gas cooler fan.
- discharge scrubber.
- condensate gathering system.
- corrosion inhibitor skid.
- a dehydration unit.
- metering station.
- blow down flares.
- Utilities:
Gas Compressor Station components:
1. suction scrubber:
it is the first component of the gas compressor station, its is a 3-phase separator used to separate liquids and condensate from natural gas, the existence of any liquid in the natural gas stream will cause a compressor vibration.
the liquids will be disposed to the burn bit to be burned, while the condensate will go to the condensate gathering header.
2. gas manifold or a distribution header.
after leaving the scrubber, natural gas will enter a manifold or a distribution header, it is used to distribute the gas to the compressor station trains ” in large compressor station and if there is a big amount of gas is compressed”, before entering the compressing train it goes through a strainer to eliminate any liquid droplets.
3. gas compressor
single or multiples stages according to the required pressure. its is either driven by a gas turbine or an electric motor, single stage or multiple stages “2 or 3” according to the required discharge pressure.
centrifugal compressors are the most preferred in gas compressor station, and it is equipped with an anti-surge system.
compressors are equipped with seal oil system that seals the sour gas from the lubricating oil, which in turn lubricates the bearings, it is also equipped with vibration, speed and temperature sensors.
4. cooler fan:
after being compressed, the temperature of natural gas is greatly increased, sometimes reach to 170 – 180 ºC , this requires cooling the gas, it is done by air cooler fans, which draw air beneath, and cools the gas.
5. discharge scrubber:
after being compressed and cooled, vapors in natural gas will condense to liquids, these liquids can be separated and disposed in the discharge scrubber, as mentioned in suction scrubber; liquids will be disposed to the burn bit to be burned, while the condensate will go to the condensate gathering header, each compressing train is supplied with a suction and discharge scrubber.
6. condensate gathering system:
the condensates are gathered from all the scrubbers in the compressor station, there are many options to make use of this amount, but the most common one is to inject this condensate in the gas discharge pipeline, there is a special technology to do this, it is injected by a sparger to guarantee the homogeneous propagation in the pipeline.
7. corrosion inhibitor skid:
because of compression and cooling for the natural gas, water vapor will be converted to liquid water, it may react with hydrogen sulfide to form sulfuric acid, this will lead to serious corrosion problems to the pipelines and equipment, so corrosion inhibitor is injected before the air cooler to prevent corrosion. this is done by special type injectors which receive the chemical from a chemical injection skid consists of a dosing pump and a tank, injection pressure must be higher than the gas pipeline pressure, otherwise there will be no chemical injection.
8. dehydration unit:
each gas compressor station is equipped with a gas dehydration unit, it is used to remove the water vapor from natural gas.
9. metering station:
it is used to measure the quantity of natural gas, gas volumes are expressed in Standard Cubic Foot and the Standard Cubic Meter.
10. blow down flares:
it is a kind of flare used to dispose the gas from the compressor station when shut down happens, Emergency Shut-Down Valves “ESDVs” that are equipped in each scrubber will do this.
11. Utilities:
such as: instrument air system and fire-fighting system.
Purging & Pressurizing
this process is done before starting the compressor station, purging will expel oxygen from pipelines and equipment, while pressuring means raising the pressure in the compressor train to be prepared to receive gas to get stable operation conditions, pressuring will also protect compressor blades from sudden increase in gas pressure.
purging and pressurizing can be made automatically or manually.
How Gas Compressor Station Works?
the natural gas enters the station and goes directly to the suction scrubber, where liquids and condensates are separated from gas and each one goes to a specific location; liquids are collected from all the scrubbers and directed to the burn bit to be buried, while condensates as mentioned earlier are collected and injected to the main outlet gas line.
after leaving the suction scrubber, gas is divided to the compressor trains through an inlet header, where it is compressed by a compressor “one stage or multi-stage depending on the required pressure”; because of compression, gas temperature increases so it have to be cooled by a gas cooler “using air fans” where it is cooled to a suitable temperature, it is to be mentioned that compressing and cooling the gas causes the water vapor to be condensed and may react with H2S and form H2SO4 which causes corrosion to the pipes and equipment, so a corrosion inhibitor skid is equipped to inject a corrosion inhibitor chemical before cooling the gas.
Compression and cooling the gas will cause all the vapors to be converted into liquids, so installing a discharge scrubber is essential to separate any liquids and send them to certain equipment as mentioned above.
After being compressed to the required pressure, gas enters the gas dehydration unit, where it is dehydrated then goes to the downstream facility.
finally, here is a sketch of gas compressor station “Exclusive for AONG website” :
