Errors In a Computerized Gas Production Measurement System
Authors: Caragannis C. , Stehwien W. , Kenny R. 
Published: March 2010

In any metering system where material balance calculations are applied, a metering difference or error will become apparent. Operating any such system requires the identification of 'en-or sources, error propagalion and minimum achievable metering error. A natural gas metering system using a digital computer has a number of significant sources of error. These are the limitations of the theory of orifice flow metering, the meter run design, the data transmission medium and the flow calculations. Once identified, these errors can be minimized and their effect on the material balance can be ascertained.

The purpose of this paper is to describe a method of calculating the over-all error in an automated gas measurement system. Computerized data gathering systems were introduced into the Canadian natural gas industry in the early 1970's, their primary purpose being production data acquisition and performance monitoring. At present there is no standard method of calculating the over-all error in flow volumes obtained from such a system.

The method used to calculate the over-all error involves locating the source of error and deriving a method to estimate its magnitude, and identifying how the error propagates through the data transmission system and through the flow calculations. This method is demonstrated on a sample system for the purpose of explanation. It may be adapted to any gas measurement system when understood and applied specifically to that system.

For a given measurement system under given operating conditions the calculated over-all error is the minimum that may be achieved. This error bas been named the 'minimum achievable error'. Frequently, the material balance between the inlet and outlet gas volumes is calculated as a check On the gas system for leaks. There will always be a difference between these two volumes due to measurement errors, but this does not necessarily indicate that the system is performing poorly. This difference, however, should be less than the minimum achievable error.

The over-all error in the calculated value of flow volume will be careful to both operators and designers of automated measurement systems. Designers can use the method presented for ensuring the whole system meets the user's requirements. Operators will find it useful in distinguishing between proper and poor system performance.

System Description

Flow Element

The device used to measure gas flow rate is the orifice meter. It consists of a meter tube, orifice changer, orifice plate, upstream and downstream pressure taps and a thermowell.

Flow Calculation

The formula used to calculate the gas flow rate is the one recommended by the American Gas Association (A.G.A.) in A.G.A. Report No.3. The equation is of the form:

Mechanical Measurements

Pressure, differential pressure and temperature arc all me3sured by commercially available transmitters which generate a 4–20 mA current signal.

Analog Data Transmission

The current signals are carried by means of twisted multipair buried cable from individual meter runs to a remote terminal unit.

Remote Terminal Unit (RTU)

The current signals are multipkxed, conditioned, digitized, and transmitted to the master terminal unit using Frequency Shift Keying (FSK) over a UHF radio link.