Verification of auto LPG Dispensers

Part IV of the Eighth Schedule, The Legal Metrology (General) Rules, 2011
Specific provision: Part 2 Rule 5(7)

The type of LPG used as a motor vehicle fuel is a liquid mixture containing at least 90 percent propane, 2.5 percent butane and higher hydrocarbons, and the balance is ethane and propylene. The mixture is genetically named as propane but commonly referred to as LPG (Liquified Petroleum Gas ). It is sold by volume and not by weight as in domestic LPG.
The LPG dispensers are classified as Class 1.0 instrument with MPE limit of ±1%

LPG as Auto-fuel
  • LPG exists as a gas at atmospheric pressure and temperature.
  • LPG is supplied at a pressure of 7 - 10 bar (700 - 1000 Kpa)
  • 1 kg of LPG normally has a volume of 1.835 litres at 30°C but the volume varies with change of temperature. The volume is 1.724 litres at 15°

Essential Components

The Metering Unit

A typical meter

A typical sensor

The metering unit consists of a meter and a transducer (sensor) device. The meter has a system for mechanical adjustment or calibration.

A Solenoid valve is also often employed to control the flow of liquid.

It also contains a calculator along with a memory device for recording quantity delivered.

Processing Unit

It controls the functions and operation of the metering, display and registering units and all peripheral ancillary devices like preset, price calculator and printers. It also includes a calibration card to control the delivery of fuel from the dispenser.

Display Unit

The display unit consists of

  • a Totalizer
  • a Volume Indicator

  • Unit Price Indicator
  • Total Price Indicator

Verification Procedure

The LM (General) Rules does not prescribe any procedure for testing and verification at user level. The following is based on practices followed internationally.

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
         (Refer to Liquid Dispensers for statutory markings.)
  • Do the indication of volume, 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 Devices

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 volume displays are on zero before any delivery is possible.

Zero-Setting

  • The Zero Setting Devices of Price Indicating and Volume 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.

Preset Indications

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

  • Set the dispenser to zero.
  • Enter a preset value of volume.
  • Start delivery of the volume and check the volume with a measuring can. The delivery from the nozzle should not stop until the preset volume 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 volume and the volume will be compared with the actual delivery and volume indicator reading.

Price Computing

The Price-Indicator should show the result of volume 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 volume, unit price and total price must be same as shown on the display panels.

Checking Accuracy

Methods employed for testing accuracy of an LPG measuring device may be Volumetric or Gravimetric.

Volumetric Method

The volumetric method uses a master meter or a mass flow-meter as the reference standard.

Advantages

  • Since LPG is sold by volume, it is a direct, the most efficient and reliable method
  • Gas disposal is not a problem
  • Fast, secure and safe

Disadvantages

  • Unavailability of Meter
  • Meter cost is high
  • Meter calibration facilities not always available.

Preliminary Steps

  • The tests should made between 0 to 30° C to minimize expansion of LPG due to temperature.
  • Connect the dispenser nozzle to the Master Meter.
  • Connect the Meter nozzle to the maintenance port or return passage of the dispenser.
  • Note the Totalizer reading.
  • Circulate at least 100 litres of LPG to balance any temperature difference between the meter and the dispenser.
  • Check if the Totalizer is functioning properly.
  • Enter a value on the Preset (if present) and observe the delivery. Towards the end of delivery, the rate of flow should slow down and stop at the setting.
  • The dispenser and the Meter may be fitted with ATC (Automatic Temperature Compensation). Ensure that ATC is either active on both the instruments or inactive in the both.
  • At this stage, turn off the valve of the Master Meter and switch off the dispenser.

Equipment

  • Certificate/s of approval.
  • A master meter or mass flow-meter (±0.2% mpe) with reference standard thermometer (±0.2°C mpe) and 2 500 kPa reference standard pressure gauge (±25 kPa mpe) fitted to the inlet of the master meter.
  • A hydrometer pressure vessel with a reference standard hydrometer (mpe of ±1 kg/m3 for master meters and ±0.5 kg/m3 for mass flow-meters), a reference standard thermometer (±0.2°C mpe) and a 2 500 kPa reference standard pressure gauge (±25 kPa mpe).
  • A reference standard thermometer (±0.2°C mpe) suitable for inserting into the thermo-well of the dispenser.

Verification Procedure

Gravimetric Method

Gravimetric test is done by using a weighing machine to determine the weight and then converting it into volume via density. Density correction is necessary for conversion of weight into volume as required during testing accuracy by Gravimetric Method.

Equipment needed

  • A Certified Weighing Instrument to measure weight up to: (Sum of test cylinder weight, weight of LPG extracted for testing) plus additional 10% thereof.
  • The weighing instrument should have an e value of 0.1 (10% of 1%) or better.
  • One Standard Thermometer with ±0.2°C accuracy.
  • One hydrometer pressure vessel with ±0.5 kg/m3 accuracy.
  • ASTM Table 53 (for density correction to 15°C
  • ASTM Table 54 (for volume reduction to 15°C)

Down load    ASTM Table 53   |    ASTM Table 54

Disadvantages

  • Density varies with Pressure and Temperature. So, maintenance of constant temperature and pressure throughout the testing period is necessary.
  • The result is not very accurate.
  • Gas disposal is a problem.

The Method

Example
The hydrometer shows a density of 0.505 kg/l, Temperature recorded is 25°C. Then using the table 53 the density of LPG at 15°C is 0.520 kg/l; obtained at the intersection of the column for observed density 0.505 and the row for temperature 25°C.

Example

The temperature of LPG is 25°C, and the density of LPG at 15°C was calculated to be 0.520 kg/l, then from the table 54 the volume conversion factor CtFD = 0.972; obtained at the intersection of the column for density 0.520 and the row for the observed temperature 25°C.

  • Check if VFD15 as shown in the Compensated Volume Indicator of the dispenser agrees with VFDc as calculated.
  • Calculate Error by using the formula:
    Error = (converted dispenser volume VFDc - volume at base condition, VREF) / volume at base condition, VREF X 100
  • Repeat the process twice more