Use PLCs to Measure Natural Gas Flow

Siemens has implemented seven AGA Calculations in their flagship S7 PLC platform. The implementation is a pre-built and pre-tested library of function blocks that make using a standard PLC practical for measuring the flow of natural gas.  Since it is compressible, natural gas is difficult to measure with conventional uncompensated flow devices. To remedy this, the American Gas Association (AGA) has released a series of reports, or standards, that define the measurement equations for the flow of natural gas. They include orifice plates, turbine and positive displacement meters, ultrasonic multipath meters and Coriolis mass flowmeters—as well as a whole suite of calculations designed to improve the accuracy and repeatability of flow measurement of natural gas.

Much of natural gas transmission is for custody transfer, so it's essential to accurately measure the flow of natural gas in applications, such as check meters adjacent to custody transfer meters, and in OEM skid applications where a complete wellhead control system or a set of packaged compressor controls are provided.

Up to now, customers were limited to using expensive, purpose-built flow computers that had the AGA standards programmed in. Many of these flow computers are difficult to use and increase the number and type of controllers that operators, engineers and maintenance technicians must be familiar with.

In these days of limited resources and reduced personnel, many customers have requested the ability to use a standard controller in this application. Also, flow PCs need costly shelf spares, whereas a PLC with commercial off-the-shelf (COTS) software would be useful for many applications.

"The AGA Calculation Library allows customers to use standard Siemens hardware in a commercial, off-the-shelf platform that their people will be able to recognize and be able to implement," says Paul Ruland, marketing manager for Siemens Industry Inc. "The library includes most of the required calculations and several versions and calculation methods for AGA 3 and AGA 8 to allow consistency with the user's existing calculations."

The library includes PLC blocks that implement:

• AGA Report No. 3, Orifice Plate Metering of Natural Gas. Both the 1985 and 1992 versions are included to match the customer's existing local standard.
• AGA Report No. 5, Natural Gas Energy Measurement. This stand-alone calculation has methods of calculating heat values of natural gas with different compositions.
• AGA Report No. 7, Flow Calculation for Turbine and Positive Displacement Meters.
• AGA Report No. 8, Compressibility Factor of Natural Gas and Related Hydrocarbon Gases. Three methods are available for calculating the compressibility factor using mole weights of constituent—basic, iterative and complete. The library also includes a simplified compressibility calculation, NX19, which is a version of the calculation using only specific gravity, temperature, pressure, and nitrogen and carbon dioxide components.
• AGA Report No. 9: Flow Calculation for Measurement of Natural Gas by Multipath Ultrasonic Flowmeters.
• AGA Report No. 10: Flow Calculation for Speed of Sound in Natural Gas and Other Related Hydrocarbon Gases. This function block calculates speed of sound and also entropy, enthalpy and C* coefficient for sonic nozzles.
• AGA Report No. 11: Flow Calculation for Measurement of Natural Gas by Coriolis Meters.

These blocks combined within S7 controllers are not an API 21.1-certified system, so they can't be used for custody transfer, but they are applicable for check metering up/downstream from a custody transfer meter. The S7-1200 compact controller will support these blocks in the first quarter of 2011.

According to Siemens, the S7 platform (ET200S, S7-300, S7-400 and S7-1200 in Q1-2011) can now be used instead of a dedicated flow computer, which will save customers time and money, and allow them to use the same equipment they are using to control other facets of gas transmission lines. The library includes HMI example screens to operate on a standard 6-in. WinCC flexible display.

For more information call 800/241-4453 or log on to www.usa.siemens.com/Industry.