What’s In A Unit? Flow Measurement – Part 5


“Counting the cars on the New Jersey Turnpike, they’ve all come to look for America” (Paul Simon). I just went under the New Jersey Turnpike which was tied up with mid-afternoon traffic. If there were no barriers, things would be so simple. Why should standand conditions be any different? They may be conditions, but they are far from standard.

When performing flow measurements, it is often necessary to select seemingly arbitrary conditions that are typically near atmospheric conditions. Depending upon purpose and the individual’s background, these conditions may be called base conditions, reference conditions, Normal conditions, or standard conditions. The actual fluid to be measured may not operate even close to these conditions, but the measurement reflects fluid flow as if the gas were flowing at these conditions. As such, volumetric flow units that incorporate standard conditions can be considered mass flow units. It should be clearly understood that the density of the gas at different standard conditions will be different. Therefore, measurements referenced to different standard conditions will be different. An incomplete list of examples of standard conditions include water at 70 degF, 75 degF, and 20 degC, and gas at 59 degF and 14.696 psia, 60 degF and 14.696 psia, 68 degF and 14.696 psia, 60 degF and 14.73 psia, … I seem to recall that Standard Temperature and Pressure (STP) is 0 degC and 1.0325 bar absolute. The point of this list is to illustrate that standard conditions vary.

From an industry or plant perspective, it might be confusing, but it matters little what standard conditions are — as long as they are understood (and preferably consistent across the plant or industry). In other words, if all natural gas flowmeters in a state use the same standard conditions — fine. If a plant specifies all flowmeters to measure using the same standard conditions — fine. However, when the standard conditions are not the same, factors should be applied to convert the flow measurement at one set of standard conditions to a flow at another. It should be clear that problems can occur when different standard conditions are used, and corrections are not made.

Manufacturers generally feel the brunt of this problem. Many users do not know what their plant standard conditions are. Often, they do not know what standard conditons are. In this case, the manufacturers simply calibrate to their own (arbitrarily seleced) standard conditions. The result is a plant where the measurements may be inconsistent because they are not referred to a common set of standard conditions. This situation can cause errors with material balances. In light of the above, it is suggested that each plant set standard conditions that are applied to all measurements with few exceptions, such as those measurements defined by tariffs (for example, natural gas).

So you wanted to measure flow, did you? It seemed so easy at the time…

ISSN 1538-5280

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