Ask The Experts...

We have covered some interesting topics with our email letters this year, following is a brief recount on some, please contact us if you wish to receive the whole letter on a certain subject.

THERMAL EXPANSION
Metals have a thermal expansion coefficient. This means the metal will expand a certain amount with every increase in temperature. The expansion is often considered too small to include in the calculations. However, in certain applications, this could affect the accuracy of the flow meter.

The V-Cone is a differential pressure flow meter. When the meter is calibrated, the temperature is ambient, around 70 degrees F. If that meter were placed in an application where the flowing temperature was 500 degrees F, both the pipe and the cone inside would be slightly larger than when the meter was at 70 degrees F. The geometry of the meter would be different than when calibrated. The change in geometry is corrected with a term called the thermal expansion factor. By changing the meter temperature from 70 to 500 degrees F, the thermal expansion factor goes from 1 to 1.008. If the thermal expansion factor was not included in the flow calculation, the measurement would be low by 0.8%.

FLOWRATE UNITS
There are several basic categories of flowrate units. In this newsletter we concentrated on VOLUME, MASS, and STANDARD. Volume flowrate refers to flowrate in terms of volume per unit of time, for instance, gallons per minute or liters per second. Mass flowrate refers to flowrate in terms of mass per unit time, such as pounds per second or kilograms per second. Standard units can be related to both volume and mass units. The units a flowmeter will use to display flowrate depends mostly on the fluid being measured. In general, water is usually measured in volume units, steam in mass units, and natural gas in standard units.

DENSITY
Density is an important fluid property in flow measurement. In this newsletter we covered the three measurement applications for density.

1. Measure the density directly. Densitometers accomplish this and can directly measure either liquid or gas density.

2. Measure the components of the fluid. A good example of this is measuring the components of natural gas. A gas chromatograph will analyze a sample of natural gas and show the amounts of each major component in the sample. A density for the sample is then calculated using the densities of each component.

3. Infer the density of a fluid from the measurements of pressure and temperature. This is the most common method because of its simplicity. The cost of pressure and temperature measurements is much smaller than a densitometer or chromatograph. Since most liquids are non-compressible, pressure does not affect them significantly and therefore temperature is only required for density compensation.

Differential pressure meters, such as orifice plates and V-Cones, are dependent on density because of the Bernoulli theorem on which they are based. This is true whether the meter is measuring volume or mass flowrates.

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Copyright 1999 McCrometer

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