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Fundamentals of Densitometers and Pycnometers

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October 10, 2019

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This paper will discuss the use of pycnometers for densitometer provings. The primary objective will be to provide guidance in the operation and maintenance of a densitometer and pycnometer (pyc), as well as common issues found during normal operation.

Densitometer Operation/Installation

Several different types of densitometers are available today. The most common used in the oil and gas industry rely on vibration to calculate fluid density. Whether a tuning fork, straight tube, or Coriolis design, each of these use the same basic principle of vibration. They all vibrate at a resonant frequency. As a fluid is introduced into the system, that frequency changes. The instrument senses the frequency change and then calculates the fluid density.

Typically, densitometers are installed using a slip-stream method which involves placing the densitometer on a line parallel to the main line. Usually this is a smaller diameter pipe as compared to the main line piping. This method relies on a means to generate differential pressure to flow through the densitometer. Sometimes the process conditions will not allow enough differential pressure to generate sufficient flow through the density meter. This normally results in an inaccurate density indication because the measured fluid is no longer representative of the fluid in the main line. Though not exactly a prevalent problem, the potential always exists and should be monitored. Low flow rates through the density meter normally lengthen the time it takes to equalize fluid temperatures to the main line.

Coriolis meters can provide an alternative solution to slip streaming. Their larger sizes can match the main line size. This allows density measurement of the full stream, along with volume and/or mass flow measurement. The added benefit is that 100% of the fluid is being measured by one device. In most cases, the only deterrent of this type of configuration might be the added cost of a larger Coriolis meter versus that of the smaller densitometer. Ultimately, the deciding factor as to which densitometer to use comes down to fluid type, pipeline configuration, and contractual agreement.

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