An Automation Process Application Measurement Reminder

Trends May Be Your Friend, but They Can Also Get You in Trouble If That's All You Look at Without Understanding the Fundamentals

By Jim Montague

Good process application management begins with good process measurement. But, are you sure what you're measuring and how? Ted Porter, process control supervisor at Enterprise Products, reports quantity and quality of oil, natural gas, natural gas liquids (NGLs) and other products are based on molecular characteristics, and so quantities must be inferred by weight and volume, while quality can vary among initial gas sources and as they're separated into products. Likewise, differing temperatures, pressures, locations, measurement definitions, standards, instrument accuracies and other factors can have big impacts on how well many substances are measured.

"Gasoline may contain all kinds of cats and dogs as long as specifications such as vapor and pressure are met," says Porter, who presented "Fundamentals of Oil and Gas Measurement" at the recent Honeywell Users Group Americas 2012 in Phoenix, Ariz. "Even standard temperature and pressure (STP) isn't really standard because it depends on what one is measuring, what standards organization is involved and where one lives. Likewise, accuracy is defined as the degree of closeness to the true value. However, it may be quoted by the vendor as a percentage of range or a percentage of value. So the same accuracy quoted as a percentage of value means one is more accurate at lower values than one that's only accurate to a certain percentage of range." 

Porter uses the case of a mixing, salt-dome cavern/well storing 2 million barrels of NGLs. "A turbine meter is measuring what's going in with a range of 100,000 barrels per day (bbl/day) at ±0.25%, and another going out with a range of 80,000 bbl/day at ±0.25%," he states. "Subtracting these gives the amount accumulated in the well each day. The uncertainty of these combined measurements is the square root of the sum of the squares of the absolute uncertainties of 250 and 200, respectively, or 320 bbl/day. However, it may be a year before this well is emptied. In that period, averaging 100,000 bbl/day, the uncertainty of the accumulation in the well has grown to 320 bbl/day multiplied by 360 days/yr, which is 116,800 barrels.

This represents 5.84% measurement uncertainty if there are 2 million barrels in the well. Many purity gas liquids are sold only by net volume, but a Coriolis doesn't measure this directly. Comparing this with a turbine meter, one would determine the combined uncertainties of the mass and density. Say the mass was ±0.2% and the density was ±0.1%. Taking the sum of the squares of the fractional uncertainties, we get ±0.224% overall uncertainty."

Porter adds users must know their measurements and how to convert them. "Measurement definitions can change over time—even for molecular weights. Y-grade, demethanized NGL is sold by weight converted to pure component volumes with chromatograph analysis. It's based on component densities and molecular weights as defined by the Gas Producers Association, (GPA) that changed every few years."    

Meanwhile, a thorough understanding of density helps when Enterprise separates NGLs using fractionators, which separate liquids by differences in vapor pressure/boiling point. The raw NGLs, or Y-grade, are fed into the first fractionator, and then ethane—the lightest or least dense of the hydrocarbons in NGL—comes out the top. Similarly, when Enterprise receives mixtures of propane and propylene that need to be separated, it uses a many-trayed fractionator in which the propylene goes out the top. Propylene has a slightly lower boiling point than propane, so mixtures of the vapor in equilibrium with liquid will have higher concentrations of propylene in it than the liquid.

"Don't assume how things work," explains Porter. "In NGLs, lighter or less-dense hydrocarbon from ethane to butane just so happens to have a lower boiling point. Trends may be your friend, but they can get you in trouble if that's all you look at without understanding the fundamentals." And don't you forget it.