Degrees of density

Iain Gordon of Solartron Mobrey looks at a variety of uses for densitometers and discovers what factors determine a good density measurement application.

Three variables affect the density of a fluid: temperature, pressure and composition. The degree of influence of these variables on the density measurement is fluid dependent. For example, the density of crude oil, changes typically from 860 kg/m3 at 30 degrees C to 880 kg/m3 at 15 degrees C.

Densitometers can measure most single phase fluids including light to heavy crudes, refined products, supercritical fluids like ethylene and carbon dioxide, and a range of fluids used in the general processing field. Accuracy depends heavily upon suitability of the instrument to the application and the method of installation.

Vibrating element densitometers, which measure the frequency of vibration of a tube containing the process fluid, are the most accurate and are widely used in the hydrocarbon industry. The vibration of the tube is sensed up by electromagnetic coils and transmitted to an amplifier. The amplified signal is fed back to the drive coils which achieve a resonant vibration at a natural frequency determined by the mass of the tube plus the mass of the fluid inside it. The mass of the tube does not vary and therefore the frequency output varies only when the mass of the fluid changes. Vibrating tuning forks are also commonly used in the process industry. These operate in a similar manner, but can be characterised for parameters such as viscosity as well as density.

To achieve high accuracy at any operating condition, secondary effects of temperature and pressure acting on the vibrating element must be determined and compensated. The manufacturer of the transducer will usually provide additional coefficients to correct for expansion and contraction of the instrument in the field.

If the densitometer has not been calibrated in a laboratory, the true accuracy can only be determined by field calibration and proving. Such is the case with nuclear densitometers and ultrasonic type meters where output is related to the installed environment. If the meter has been calibrated only on a single fluid, the quoted accuracy is likely to be based on a single density value. This may not be the same for fluids with densities other than the calibration fluid.

Field proving of a densitometer can be difficult and time consuming or in some cases, impossible. If the application depends on the ability to field-prove a densitometer, many issues such as reference fluids, pycnometers, hydrometers, and sampling must be addressed.

A common application for densitometers is to measure the relative density of fluids in multiple product pipelines or tanks. The requirement is to identify the fluid interface in order to direct each product to the appropriate storage vessel or customer.

If a densitometer were only measuring line density, a change in temperature might be seen as a product change when the product composition fluids are similar. To eliminate this, temperature is measured and equations are used to determine the density at a base (or reference) condition – for example 15°C. Some densitometers currently available allow the input of a user programmable matrix which refers the line density to a temperature or concentration that is more meaningful to the user.

A densitometer may be used in the hydrocarbon industry to determine the quality of a finished product by measuring fluid composition. Pure products such as propane or ethane have a published relative density. Any mixing of another product will change the fluid composition and change the relative density, which would then be identified by a densitometer to monitor the mix or purity of the product. Crude oil, if purchased based on a contract that specifies its relative density range, is often checked for its quality by a densitometer.

Densitometers are not limited to use in mass flow applications. Relative density is also used in the calculation of volumetric flow.

A densitometer measures line or observed density, and from this value CTL and CPL are calculated. These factors are used to convert the measurement from gross volume to net volume using API published standards.

Since a densitometer always requires a representative sample of the fluid, all applications should be reviewed for potential installation problems at the point the measurements needs to be made.

The applications for densitometers are numerous and varied. As installed costs reduce, and the capability of electronics is enhanced, more applications are becoming viable. Furthermore, requirements continue to increase for closer process control, tighter cost control and better process profitability.

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