On Site Proving Of Gas Meters

Author: Daniel J. Rudroff

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With the increased use of Natural Gas as a fuel, and higher natural gas prices buyers and sellers of natural gas are seriously looking at ways to improve their natural gas measurement and reduce the error in natural gas measurement.

A 6” Turbine or Ultrasonic meter operating at 1,000 Psi will move 100 MMSCF/Day. An error in measurement of only one tenth of one percent (0.1%) on 100 Million Standard Cubic Feet (MMSCF) of Natural Gas selling at $4.00 per Thousand Standard Cubic Feet (MSCF) will cause an over or under billing of $400.00. Therefore the error in a year is ($400 X 365) $146,000.00 This will more than pay for a proving or verifying system.

The Btu in one barrel of oil is equivalent to the Btu in approximately 5,600 cubic feet of natural gas. At $4.00 per thousand cubic feet, the natural gas equivalent of one barrel of oil is $22.40 which is much less than a barrel of oil so natural gas is becoming the fuel of choice.

In the petroleum liquid industry, no custody transfer liquid measurement system would be complete without a method to prove the meter, either as part of the equipment, or through connections provided to connect a portable prover.

Under billing causes loss of revenue and over billing can cause a future correction that can cost the company millions of dollars to correct over billing.

For this and many reasons gas meter proving is important and necessary to insure precise measurement of natural gas that both the buyer and seller can agree upon.

With the publication of AGA Report No. 6 “Field Proving of Gas Meters Using Transfer Methods” there is now guidance on how to prove Gas Meters in the field.

Unfortunately there are no volume devices like in liquid proving currently on the market, but there are other methods such a sonic nozzle proving and master meter proving that can be used.

In the ASME standard MFC-7M-1987 Reaffirmed 2001 “Measurement of Gas Flow by Means of Critical Flow Venturi Nozzles” it is stated “The Venturi nozzles specified in this Standard are called primary devices.” Nozzles have been used for many years to prove natural gas meters. Sonic Nozzles have a long history, are well documented and the formulas, although a bit complicated, are in most flow computers.