#LinkedInsights features the latest and in-trend discussions around the instrumentation industry on social media platforms. A flowmeter measures the rate at which a fluid or gas passes through it and today’s discussion focuses on defining a limit for the uncertainty of flowmeters. 

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Regarding this subject, an instrumentation and control specialist from Brazil wanted to define the limits of a flowmeter’s accuracy. Specifically, he wanted to know about these standards. He says, “What defines a limit of the uncertainty of flowmeters? As a standard, I thought of defining it as AGA-3.1, but what’s the uncertainty of this system? I also must check with calibrations of orifice place, multivariable transmitters, etc. This way, I know if this system works well with a limit.”

Image for Orifice Plate Flowmeters
Image courtesy of automationforum.in

An analyst specialist from California believes that the uncertainty refers to the error introduced by the use of an orifice plate versus another type of flow device. “The orifice plate is probably the least accurate. At the same time, it is the most cost-effective tool. The DP transmitter is by far much more accurate than the plate so the error introduced by that is minimal.”

Further, he explains that there are tables you can use to determine the error introduced in any given plate. This can be minimized by setting them to tight tolerances. “In this case, generally, +/- 2 mils is good,” he suggests.

Evaluate the Fluid Measured

A former instrumentation and electric mentor from France says that the fluid being measured needs to be evaluated. It makes a difference if it is liquid, gas, or steam. According to him, one must consider whether the flow is steady or pulsating, single or multi-phase. You must also determine if the physical installation of the orifice plate is flow horizontal or vertical.

Further, you can observe what straight run lengths are available upstream and downstream of the orifice plate. Also check to see if there are piping bends in one or multiple planes. You should evaluate the pipe quality for internal roundness and roughness.

He adds that it is necessary to know the density and the viscosity of the fluid at working conditions. In the case of a compressible fluid, it is also necessary to know the isentropic exponent of the fluid at working conditions. “You can measure the density of the fluid at the upstream pressure tapping directly. Or, you can calculate it from an appropriate equation of state, knowing the absolute static pressure, absolute temperature and composition of the fluid at that location,” he says.

Image for Orifice Plate Flowmeters
Image courtesy of emerson.com

Furthermore, he adds that “The temperature of the fluid should preferably be measured downstream of the primary device. Temperature measurement requires particular care. The thermometer well or pocket should take up as little space as possible. The distance between it and the primary device should be at least equal to 5D (and at most 15D when the fluid is a gas) if the pocket is located downstream.

Ideal Beta Ratio is around 0.6 (acceptable range 0.3 to 0.7) Reynolds Number Rd > than 100,000. Your flow downturn – max flow/min flow – should ideally not be more than 3:1 sizing flow for orifice d use max flow * 1.1 normal flow should then be around 70%. Only now are you ready to establish expected accuracy of your measurement. For this, refer to ISO/TR 5168:1998, measurement of fluid flow, evaluation of uncertainties.”

Final Tips

In the same vein, the instrumentation and control specialist from Brazil suggests that it has been vital to know exactly what one is measuring and the state in which it is flowing for the most accurate readings.

“Overall, though the orifice plate is adequate for many applications, we install a majority of orifice plates in our refinery and use venturi measuring equipment on our critical products like final product output or gasoline diesel.”

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