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|By David W. Spitzer, P.E.
Flowmeters used for custody-transfer applications, for instance, need to be as accurate as possible. These flowmeters determine the amount of money that changes hands for the sale of the fluid that flows through the flowmeter. In petroleum and natural gas applications, transferring fluids worth millions of dollars between parties in the space of a few hours is common.
Suppliers and users pay careful attention to detail in custody-transfer applications. Consider an orifice plate installation used for custody transfer. Methods to ensure accurate flowmeter operation include the machining of the entire straight-run pipe to exacting tolerances and wet calibrating the orifice plate in the machined straight run. The amount of straight run upstream and downstream of the orifice plates should conform to the applicable standard, noting that different standards have different requirements. The required straight run is typically longer than the “rule-of-thumb” of 10 diameters upstream and five diameters downstream commonly used in industrial applications.
Regular inspection of orifice plates can often determine if their geometry has changed. Special fittings to extract the orifice plate for inspection and to reinstall it in the same position can significantly increase the cost of orifice-plate installations. Depending on pipe size, the cost of these flowmeter installations can run into the tens of thousands of dollars. In some applications, the installation of multiple flowmeters in parallel helps ensure accurate measurement of low, medium, and high flow rates. This further increases the cost of installation.
What value do these investments represent? Consider an utility boiler that can operate on coal, oil, or natural gas. If the boiler uses, for example, $500 million of natural gas per year, an uncertainty of 0.2 percent of the actual flow rate results in a billing uncertainty of one million dollars per year. Spending tens of thousands of dollars to improve and maintain flowmeter measurement uncertainty can be justified by the magnitude of the bill and potentially large negative consequences that could occur if the flow measurement is in error.
Operating the natural gas pilots on the boiler may be necessary to stabilize the flame when firing coal under certain conditions. During coal firing, the pilot gas flow may be only a few percent of the full firing rate of the boiler. Still, the natural gas may cost the utility a few million dollars per year. Therefore, the flowmeter system must accurately measure natural gas flows over a range of approximately 50-to-1 to the boiler. Add another few more boilers to the power plant and the potential turndown can become even greater.
In this boiler application, the flowmeter system must accurately measure natural gas flows ranging from essentially one (or a portion of one) set of pilots to full-load operation of multiple boilers. Measuring this wide range of flows is often accomplished by installing parallel orifice plate flowmeters, usually of different sizes.
Installing parallel differential pressure transmitters on the orifice plates allows accurate measurement of their differential pressures over a wider range of flows. Custody-transfer applications in certain industries often require the ultimate in flow measurement performance. Naturally, implementing and achieving this performance over time is relatively expensive. Such installations are generally not economically feasible in industrial applications. Regardless, installing industrial flowmeters using the practices commonly employed for custody-transfer applications can be feasible in some instances. One technique in particular that can be efficiently employed is purchasing smooth straight pipe and installing it with a minimum of welds so that the inside pipe walls remain smooth and straight. Another technique is to provide long straight runs that conform to standards when feasible.
About the Author
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 25 years of experience in specifying, building, installing, start-up, and troubleshooting process control instrumentation. He has developed and taught seminars for almost 20 years and is a member of ISA and belongs to ASME MFC and ISO TC30 committees. Mr. Spitzer has published a number of books concerning the application and use of fluid handling technology, including the popular The Consumer Guide to… series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
For More Information: www.spitzerandboyes.com