Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#30 CNG station

7 min read

Copyrights 2008 Mario R. Duran ( under Creative Commons Atribution 3.0 license (free to share and modify it but attribution is required).

  1. CNG Refuelling Station is a facility set-up for supplying CNG mainly as fuel to vehicles. The system receives odorized gas at a certain pressure (19 Bar), through the transmission line and supplies the same to the customers/vehicles after filtration, compression (up to 250 Bar) and check metering.
  2. CNG is stored at compression stations which are directly connected with the gas pipeline. Here the gas is compressed to the required pressure and aids fuelling. CNG can also be transported to other retail outlets by cylinder trucks. These trucks carry a number of cylinders that provide CNG to fuel stations that are not connected by pipelines.
  3. The system has major assets like Odoriser Unit, CNG Compressor, Storage facility, Dispenser, LCV/HCV Filing Point, Mobile cascade vehicle, Electrical Installations, DG set, and Air Compressor. The size, rating, and type of the major and sub-assets are selected based on the pressure rating, capacity and other applicable criteria considered for refueling station for meeting the current and future customer potential, health and safety of employees, customers and public at large, environmental protection and complying the statutory, legal requirements, national and international technical and safety standards/ practices.
  4. Various operation and maintenance activities, as per annual operation and maintenance plans are carried out for ensuring safe and uninterrupted gas supply.

Types of CNG Refuelling Stations:

  1. Mother Station: A CNG station provided with whole set up (compressors, dispensers, cascade, etc.) along with an LCV filling point is known as a mother a CNG station.
  2. Online Station: This CNG station has the same set up as a mother station but LCV connection is not provided for filling.
  3. Daughter Booster Station: Daughter station provided with the compressor (known as boosters) to compress the gas we are getting from the mother station are known as daughter booster station.
  4. Daughter Station: Daughter stations are established in those areas where laying a pipeline is not possible. In that case, gas is delivered from mother station to daughter station; via mobile cascade van. The gas from the mother station is filled in a mobile cascade by an LCV filling point.

A COMPRESSOR is a mechanical device that increases the pressure of air/gas by reducing its volume. Compressors are used to increase the pressure of air from low pressure to high pressure by using some external energy.

Compression ratio: It is defined as the ratio of the volume of air before compression to the volume of air after compression.

Compressor capacity: It is the quantity of air actually delivered by a compressor in meter cube per minute. 

Pump vs Compressor:

In case of a pump the fluid (either liquid or gas) is moved from one place to another. A compressor squeezes the volume of a gas and (commonly) pumps it elsewhere. While pumps can use liquids or gases, compressors for the most part work only with gas. That is because liquids are extremely hard to compress.

Increase the kinetic energy of the fluid which further increases the pressure energyIncrease the potential energy by pressuring in smaller volume
Fluid can be liquid or gasUses only gas
The volume form inlet to outlet is not changedThere is a volume change
There is not necessarily a pressure changeThere must be a pressure change
No storageHas storage capacity
CheaperMore expensive

Reciprocating Compressor: In a reciprocating compressor, a volume of air is drawn into a cylinder, it is trapped, and compressed by piston and then discharge into the discharge line. The cylinder valves control the flow of air through the cylinder, these valves act as check valves.

Two types of Reciprocating compressor are :

  1. Single-acting compressor: It is a compressor that has one discharge per revolution of the crankshaft.
  2. Double-acting compressor: It is a compressor that completes two discharge strokes per revolution of the crankshaft. Most heavy-duty compressors are double-acting.

Compressor can be single stage or multi stage:

Single stage compressorMulti stage compressor
Only one cylinder for the compression processMore than one cylinder are connected in series
used in low-pressure ratio applicationAchieve a very high pressure ratio
The size of cylinder is very high when compared to the cylinders in the multistage compressorIndividual cylinders are small when compared to single cylinder compression
The temperature of fluid due to compression is very high. No intercooler.The temperature is low. Intercooling is more efficient than cooling with a cylinder wall surface. It also reduces thermal stresses.
The high-temperature damages cylinder head and burns lubricating oillower temperature facilitate effective lubrication
Volumetric efficiency is low for a given pressure ratioVolumetric efficiency is high for given pressure ratio
HIgh leakage loss due to high pressure ratioChance of leakage loss is low
The large size of flywheel required due to high torque fluctuationprovide more uniform torque, it needs light flywheel
suitable for light tasksuitable for heavy task. It can manage larger load.

Procedure adopted in CNG compressor;

  1. Steel line of 2 inches diameter coming to compressor as a suction line of compressor which contain gas at a pressure of 25 bars.
  2. Suction line is provided with isolation valve over it followed by a strainer for removing of foreign particles.
  3. After the strainer, suction filter is placed of size 5 micron.
  4. From here, the suction line is divided into two lines, one is inlet to 1st stage of compressor and another one is instrument line for operating actuator valves in the compressor.
  5. Instrument line having gas at pressure about 25 bar, it goes to PRV which reduces pressure upto 6-8 bar for the SOV which provide functioning of the Actuator.
  6. Now, the main suction line goes in Non-Returning valve which provides only forward movement of gas.
  7. From NRV, it goes to the blow-down vessel. We have two interconnected blow down vessels. In these gas is stored which is mainly used during the start of the compressor.
  8. Now, from NRV, it comes to main PRV & then first suction volume bottle from here enter the compressor.
  9. In other compressor, valve & piston are provided separatly. but herevalve is provided in piston only, so function of both is performed by piston only.
  10. The first stage is double acting stage, here compression is affected fro both the ends of the cylinder of the compressor. Then gas is sent to the intercooler for the cooling of the gas.
  11. Finally it reaches to the second stage for final compression from intercooler for the desired final pressure of gas.
  12. At every stage gas pressure rises as: First stage = <70 bar & second stage(final discharge) = <240 bar
  13. After second stage compression, it comes in discharge line.
  14. Lubrication oil is provided for piston and crank case is servo press220 normally.
  15. Final discharge line goes to separator first and then filter of size of 1 micron.
  16. Now, gas enters to mass flow meter for measurement.
  17. From mass flow meter, gas comes to priority panel.

Discharge line coming out from the compressor is splitting into five lines in sequences as,

  1. The direct line which is directly going to dispenser high-pressure bank.
  2. The high-pressure line directly going to cascade high-pressure bank.
  3. Followed by medium pressure line divided into two lines one goes to cascade and other to dispenser medium pressure bank.
  4. Followed by a low-pressure line divided into two lines one goes to cascade and other to dispenser low-pressure bank.
  5. The Last line is provided for the mobile cascade filling.
  6. Each line is provided with a Nonreturning valve and pressure transmitter respectively.
  7. Priority system having the following set points: High Bank – 220 Bar, Medium Bank – 210 Bar & Low Bank – 200 Bar
  8. Each Bank diverts compressor discharge to the lower side in sequence
  9. Auto Start 190 Bar at High Bank priority & Auto Stop at 245 Bar at all 3-Bank priority

Priority panel is designed to fulfill the priority requirement of gas to different consuming ends according to pressure availability. It gives first priority to the dispenser, LCV/HCV filling point, and then stationary cascade. It operated by PLC panel. Logical working of priority panel is as below:

  1. When High bank pressure of cascade decreased to set point, PLC will give command to start compressor.
  2. When compressor starts priority panel will decide the priority either to Dispenser or Cascade through High bank/direct line.
  3. If there is no any load / Vehicles at dispensing area, Priority panel will start filling gas in Stationary cascade.
  4. Cascade fill in a sequence of High, Medium & Low bank respectively and dispenser will dispense the gas to vehicles in a sequence of Low, Medium & High.
  5. When High bank pressure reaches up to set point (215 bar), priority panel will open medium Bank and gas will starts fill in both Medium bank and high bank simultaneously.
  6. When Medium bank pressure reaches up to set point (225 bar), priority panel will open low Bank and gas will starts fill in all three bank Low, medium and high simultaneously.
  7. When Low Bank pressure reached up to set point (240 bar), PLC gives command compressor to stop.

Cascades: A bank of cylinders used for buffer stock of compressed natural gas is known as cascade. Stationary cascade is used to store the gas when vehicle is not there at the dispenser then the gas from compressor flows into a cascade.

Stationary Cascade: These are fixed cascades established on CNG station for the storage of CNG. It has a direct inlet from the compressor and outlet to a line connected to the dispenser.

Mobile Cascade: Cascade is mounted over an LCV and it carries CNG from Mother Station to Daughter station or Daughter booster station as per the requirement.

Cylinders in a cascade are divided accordingly as:

  • High Pressure Cylinders (HP) – 5 cylinders
  • Medium Pressure Cylinders (MP) – 15 cylinders
  • Low Pressure Cylinders (LP) – 20 cylinders

Dispenser: Main components of dispensor are: High , medium ,low bank lines, Gas filter, SOV, Actuator, Mother board, NRV, Ball Valve, PRV, Mass flow meter, Pressure transmitter, hose pipe, three way valve, filling probe , O-ring.

Working of Dispensor: A dispenser has three banks connection namely:-

  • Low-pressure Bank (LP)
  • Medium pressure Bank (MP)
  • High pressure Bank (HP)

Gas from dispenser will dispense in the sequence: – low bank, medium bank and high bank.

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