Sun. Jun 20th, 2021

City gas distribution

Transporting future fuel

#86 CNG Station

5 min read

There are generally four types of CNG Stations viz. Mother station, Online station, Daughter Booster station, Daughter station. The salient features of each station are described in the following sections.

1. Mother CNG Station:

  1. A mother station is connected to natural gas pipelines & has preferably more than one compressor.
  2. The mother station compressor is of capacity of around 1200 SCMH & it compresses natural gas from a pressure of 15-19 bar to 200-250 bar. The compressor is of high capacity.
  3. The mother CNG station fills mobile cascades for daughter stations and also fills stationary cascades for dispensing CNG into vehicles. All categories of vehicles can be refueled at a mother station, i.e. bus, car, light motor vehicle, light commercial vehicle & autos.
  4. The mother station has a facility to feed two to three daughter stations. Mother station are connected to steel grid, through which gas is transported from CGS to mother station.
  5. Gas from the distribution pipeline is compressed using a multistage compressor into a cascade storage system. This system is maintained at a pressure higher than that in the vehicles on-board storage, so that gas flows to the vehicle under differential pressure. Typically the cascade storage operates in the range of 205 bar to 250 bar, while the vehicles maximum onboard cylinder pressure is 200 bar. In order to make the utilization of compressor and buffer storage, more efficient fast fill CNG station usually operates using a three stage cascade storage system.
  6. From the natural gas tank, gas goes through refueling connector and a non-return valve. After that it passes through a solenoid valve, pressure regulator, engine management & injection system & is finally filled to the vehicle.

2. Online CNG Station:

  1. Natural gas is transporter from CGS to Online stations via. steel pipelines.
  2. It is used to compress gas directly from the pipeline and the discharge can be used to fill the CNG vehicles at CNG retail outlet.
  3. The capacity of such a compressor is selected based on the vehicle likely to be filled in that station and with the provision for expected future expansion. It may be equipped with a 1200 SCMH capacity compressor for compressing the natural gas from 49 bar to 225 bar. All categories of vehicles are refueled i.e. Bus, Car, LMV, LCV, and autos.
  4. Cascade filling facilities will not be available at the online station as it is in Mother station. The investment of an online station is midway between daughter station and mother station.

3. Daughter CNG Station:

  1. Mother stations feed gas to daughter stations and daughter booster stations. CNG is dispensed using mobile cascades. Mobile cascades at daughter stations are replaced when pressure falls. The pressure depleted mobile cascade is refilled at mother station.
  2. In this case the investment is least among all stations. In each successive filling, there is a decrease in storage pressure. It is to be noted that once storage pressure drops, refueling time increases and quantity of CNG dispensed to vehicle also decreases.
  3. The sizing of mobile cascades and trucks is decided based on the distance between mother stations to daughter station. Number of LCV/HCV, to and fro frequency is decided based on the load.
  4. Daughter station has a facility to feed small vehicle alone.
  5. It is easy to set up daughter station and it can be set up at small towns, junctions & villages (where steel pipeline network does not exist). This facility has proved to be boon for villages, which do not have direct access to natural gas. This type of station may be converted to booster or online as per demand.

4. Daughter Booster CNG Station:

Daughter Booster station is not connected to natural gas pipeline. In this station gas is being feed through the mother station. Here, dispensing CNG to vehicles are through mobile cascades and booster compressor. Mobile cascades at daughter stations are replaced when pressure falls & pressure depleted mobile cascade is re-filled at mother station. Booster station improves the utilisation of residual CNG in mobile cascade up to 70 to 80%. Daughter station may have the facility to feed small vehicles alone. Installing a booster compressor negates the drawback of the daughter station. The mobile cascade can be connected to the dispensing system through a booster. It is designed to take variable suction pressure and discharge at a constant pressure of 200 bars to the vehicles being filled with CNG. The investment in daughter booster station is higher than that of daughter station.

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