Measuring mass flow, temperature and density with the same instrument

Accurate measurement of liquids is important for all oil and gas industry production or consumption sites. This is especially true for bulk transfer devices where large volumes are being moved, and need to be monitored. In the past, mass transfer was measured in batches with weigh scales or load cells. However, installation, calibration and maintenance of a scale or load cell are expensive, time consuming and do not work when the process is continuous.

Orifice plates and magnetic flow tubes can measure volumetric flow, but additional instruments are needed to record temperature and pressure to compensate for fluid density changes. Coriolis mass flow meters, on the other hand, can measure mass flow, temperature and density at the same time. Transfer measurement by mass is the most accurate method, since mass is independent of, and unaffected by, changing process fluid characteristics such as pressure, temperature, viscosity, conductivity and gravity.

The Coriolis Effect

The Coriolis Effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a clockwise rotation, the deflection is to the left; in anti-clockwise direction, it is to the right. The mathematical expression for the Coriolis force appeared in an 1835 paper by the French scientist Gaspard Gustave Coriolis.

Coriolis mass flow meters

Measurement of petroleum fluids have so far been done with the use of mechanical meters. However, they have to be calibrated on a single grade, and recalibrated each time a product is measured. For example, if a facility is handling crude oil from Venezuela one day and from Texas on the second day, the meter would have to be re-proved each time.  

Mass flow measurement with Coriolis meters means that a single point of measurement can obtain multiple measured values, including mass flow and mass total, density and concentration, volume flow and volume total, and temperature.Recent advances in manufacturing technology have led to larger Coriolis meters for oil and gas applications. Such meters have no moving parts, and measure product mass directly, independent of pressure and temperature.

Reducing pressure drop

Pushing fluids past a mechanical meter creates a pressure drop. In most Coriolis meters, though, there are twin tubes, and the only pressure drop is caused by splitting the flow into the two pipes. Additionally, using a straight pipe Coriolis meter design creates lower pressure drop than the bent tube variety.

Accuracy, too, must be considered. The majority of mechanical flow meter users run separate pressure and temperature meters. Other benefits include ease of installation/maintenance and electronic controls, which enable operators to view everything that is going on with the fluid, including volumetric flow, mass flow, density and temperature.

Refinery chooses Coriolis meter

A petroleum refinery site in Ontario, Canada produces refined gasoline, jet fuel and other petroleum products, and ships its product through a 5-kilometer 8-inch pipeline that connects to a pipeline serving the Toronto area. The company did not wish to install additional structural support for meters and looked at metering technologies, including ultrasonic, vortex shedding, and orifice plate meters, before selecting a Coriolis mass flow meter.

The Coriolis mass flow meter enables flow and density measurements from a single device, made it possible for the company to fit it into a tight space. This narrowed the choice further down to straight tube or bent tube Coriolis meters. Though the costs were equal, installation costs for the straight tube meter were lower. Therefore, facility operators chose a straight tube Optimass 2000 Coriolis meter from Krohne.

Andre Verdone is business manager for KROHNE Ltd., the global centre of excellence for Coriolis Mass flowmeter technology. KROHNE Ltd is responsible for research and development, manufacturing and product support of mass flowmeters that are sold worldwide.