Instrumentación, Control y Automatización Industrial

The flow rate is the variable that is measured more frequently in the industry today. Water, natural gas, steam, oil, chemicals, wastewater and process streams are just some of the fluids that are routinely measured every day. The many periodicals, reports and books published annually on the subject are indicative of the importance attached to the flow measurement.

There is no single technology that meets all of the multiple applications, is available, however, hundreds of designs, mainly based on ten principles of operation ¬ ment. Each technology or design has its particular benefits and advantages. Taken together, these technologies cover the diverse range of accompaniment ¬ operational conditions that may occur in any process plant.

BS EN 7405 is the classification, selection and optimization rules for selection of flowmeters in closed pipes. This standard sets out 10 basic groups that classify all forms of flow measurement (see table). Lower numbers (left column) represent the older techniques. It is interesting to note that ¬ sante older techniques are based on the extraction of energy from the fluid, while the most modern techniques add energy to the fluid. Today it tends to the use of flowmeters no moving parts, they cover fields as larger values, require lower pressure drops, and - above all wear is minimal.

The table below adds two more groups representing the accountants for applications with solids and flow meters for open channel applications. Although BS EN 7405 does not include them in this manual, we will try.

Flowmeters Classification according to BS EN 7405 (Britisfa Standard)

The above table distinguishes between two basic types of counters:

- Those who extract energy from the fluid.

- Those who add something to the fluid flow rate deduced from the added property.

Energy extraction is associated with a pressure drop. Most accountants are in this category. By adding energy, pressure drop, if it occurs, is minimal (other than that produced by the effects of friction). The energy extraction methods consist of placing a stranglehold on power, a solid body or rotor. These convert potential energy into kinetic energy of the fluid that is used to determine the flow rate.

In the second type of methods (adding energy), energy can be introduced in the form of magnetic flux (magnetic flow meter), sound waves (ultrasonic flow meter) or heat (thermal flow meter mass flow). The fluid reacts to the increased energy, and this change allows us to deduce or measure the flow.

Multiple flowmeters of different measurement principles

Differential Pressure Flowmeters

Born in Groningen, the Netherlands, more than 200 years, the Swiss family scientist ¬ Da niel Bernoulli (1700-1782) established the theoretical basis for this method to develop some of the principles of fluid flow.

All differential pressure flow meters, commonly called "counters PD", using the pressure difference caused by primary elements such as orifice plates, nozzles, Venturi tubes, Pitot tubes, etc.. This pressure differential is drifting towards a reduced bore (not in the Pitot tube), where pressure is measured and from it you can determine the flow velocity for a fluid of constant density.

The design and differential calculus were improving by leaps and bounds as they were getting new results from decades of research discoveries were published ¬ fundamental procedures and continued the work of standardization. Differential pressure devices still represent a large proportion of the flow meters in sold around the world. Available in a wide variety of models and sizes, differential pressure flowmeters are generally a cheap and reliable flow measurement of gases, liquids and steam. The two main designs common in today's industrial applications are of two types:

- Reduced bore (orifice plates, nozzles, Venturi tubes)

- Pitot Tubes