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. You cannot use a LPG cylinder for testing CNG.
2. You cannot 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.
- Model Approval
sign
- Manufacturer's mark
- Name of the Instrument
- Serial no. and year of manufacture.
- Minimum
measured quantity.
- Minimum and Maximum Flow Rate (Qmin and Qmax)
- Maximum
pressure of the gas, Pmax
- Minimum and Maximum temperature of gas(Tmin and Tmax
which should be +10 to +40° C )
- Environmental Class
Maximum Permissible Error, MPE
The MPE, for verification and re-verification will be ±2% of the measured quantity.
Preliminary Checking
- Whether the instrument comply with the Certificate of Model Approval
- Whether all mandatory descriptive markings are clearly
indicated on the fixed plate for stamping.
- Do the indication of weight, unit price and total price corresponds with each other and
related to the selected nozzle.
- Are all the indications are clearly visible under any condition of day and night.
- Are all
the hoses and the nozzles are in good condition
- Is there any leak any where
Checking Indicating Devices
The checking of Indicating devices consists of visual checking of the entire display.
- Displaying all the elements ("eights"
test)
- Blanking all the elements
- Displaying the Zeros
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
- The Zero Setting Devices of Price Indicating and Weight Indicating Device will be such that when one is set to zero, the other
will automatically reverted to Zero.
- The Zero-Setting Device shall not be capable of changing the result except entirely removing it
and setting to zero.
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.
- If the dispenser issues a printed receipt, the printed values of weight, unit price and total price must be same
as shown on the display panels.
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
- A verified Working Standard Weighing Instrument of sufficient capacity to weigh
the CNG test cylinder and gas.
The verification scale interval should be 10 g or less. - Verified Test Weight of at least 10
kg.
- Two CNG test cylinders to hold at least 10 kg of gas.
- Necessary valves, hoses and couplings to be able to fill
and empty the cylinders.
Verification Procedure
- Set up the weighing instrument on a flat surface in a wind free environment. Switch on the instrument and allow warm up time and
set the instrument at zero after levelling it.
- Weigh an empty cylinder or tare it off. Add the 10 kg test weight to check accuracy of
weighing. If the error is in excess of MPE allowed for the instrument, adjustment should be made.
- Place the cylinder on the center of
the weighing platform and weigh the cylinder. If the cylinder is not tared off, note the weight in kg
- Remove the cylinder from the
platform and set up the dispenser.
- Connect the cylinder to the dispenser through the hose.
- Note the Totalizer
Reading.
- Start filling the cylinder and wait till the delivery is 10 kg approximately.
- Close the cylinder valve, remove the
hose and return the nozzle to the dispenser.
- Take the cylinder back to the weighing instrument and record the weight of the gas. If
the cylinder was not tared off, then weight of the gas will be equal to gross weight minus weight of the empty cylinder.
- Record the
weight displayed on the dispenser.
- Calculate error by using the formula:
Error =
(weight displayed by the dispenser - weight
measured) / weight measured X 100 - Repeat the process with the second cylinder.
- Check if the Totalizer has increased
accordingly.
Important !
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
Components requiring Sealing
- 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.
