Compressed Natural Gas (CNG) Dispensers

Part X of the Eighth Schedule of the Legal Metrology (General) Rules, 2011.

Compressed Natural Gas (CNG) is compressed to over 200 bar pressure for use in the automobile sector. It is colorless, odourless (but for an odorant for detection) and non-toxic.
Typical Composition:
Methane - 88%, Ethane - 5%, Propane - 1%, CO2 - 5% and inert gases - 1%.

The supply system

There are basically three types of CNG stations:

Mother Stations

These stations are directly connected to the source through a pipeline and have high compression capacity. They can supply gas to all types of vehicles (bus /car/ auto) and also to daughter stations (through mobile cascades).

Daughter Stations (with or without booster)

They use mobile cascades which need to be replaced when the pressure falls. With reduced pressure, the quantity supplied to a vehicle will be much less. The problem is solved through installing a booster compressor. The cascaded cylinders are re-filled at a mother station.

On-line Stations

They also receive supply of CNG through a pipeline and uses compressor of medium capacity to increase the pressure to around 250 bar. They supply gas to CNG dispensers.

CAUTION

Inflamability Comparison



Methane, the main constituent has a very low energy content. So, it is compressed at a high pressure of not less than 200 bar. It makes CNG highly inflammable compared to other vehicle fuels.

Test Cylinder

Extra caution is necessary during verification of a CNG dispenser. The test cylinder must have statutory declarations. The Pressure rating should be 200 to 250 Bar (20,685 to 24,822 kPa). A typical test cylinder to hold 30 kg gas will weigh 130 kg (with all metal body) to 85 kg (with non-metallic body) to be safe.

1. Never use a LPG cylinder for testing CNG.
2. Never use a test cylinder with a 200 bar pressure rating for a 250 bar dispenser.

Verification of a CNG Dispensing Unit

Mandatory Descriptive Markings

There must be a data plate which will bear the following information regarding the dispenser.

Maximum Permissible Error, MPE

The MPE, for verification and re-verification will be ±2% of the measured quantity.

Preliminary Checking

Checking Indicating Devices

The checking of Indicating devices consists of visual checking of the entire display.

This test is performed by looking at the indicators before and after removing the nozzle from its rest position. Ensure that the price and weight displays are on zero before any delivery is possible.

Zero-Setting

Price Computing

The Price-Indicator should show the result of weight multiplied by unit price indicated. This test may be done during a test delivery for checking accuracy.

Preset Indications

Some systems employ Weight and Price pre-setting devices. Such devices stops flow of fuel passing through the nozzle when the price or weight correspondences with the preset value.

  • Set the dispenser to zero.
  • Enter a preset value of weight.
  • Start delivery of the weight and check the weight with a measuring instrument. The delivery from the nozzle should not stop until the preset weight is reached.
  • The test will also be carried out for preset Price Indicator. The price will be divided by the unit price to calculate the weight and the weight will be compared with the actual delivery and weight indicator reading.

Checking Accuracy

Equipment

Verification Procedure

To note:

An empty cylinder may not be empty after all. If the pressure gauge shows reading below 1379 kPa (13.5 bar), the cylinder will be considered empty.

Sealing and Stamping

Sealing of Components

The following components should be sealed

  • The Metering Unit including its correction or adjustment device
  • Transmitters - Pulsar / Sensor / signal generator
  • System Controller (if it can be used for calibration of pulse count)
  • Mass Flow Meter
  • Totalizer
  • Indicating Devices - Total weight delivered and Price

Stamping

The Data Plate should be stamped.