Our interest in the measurement of air and water flow is timeless. Knowledge of the direction and velocity of air flow was essential information for all ancient navigators, and the ability to measure water flow was necessary for the fair distribution of water through the aqueducts of such early communities as the Sumerian cities of Ur, Kish, and Mari near the Tigris and Euphrates Rivers around 5,000 B.C. Even today, the distribution of water among the rice patties of Bali is the sacred duty of authorities designated the "Water Priests".
Our understanding of the behavior of liquids and gases (including hydrodynamics, pneumatics, aerodynamics) is based on the works of the ancient Greek scientists Aristotle and Archimedes. In the Aristotelian view, motion involves a medium that rushes in behind a body to prevent a vacuum. In the sixth century A.D., John Philoponos suggested that a body in motion acquired a property called impetus, and that the body came to rest when its impetus died out.
Transaction Volume Series
In 1687, the English mathematician Sir Isaac Newton discovered the law of universal gravitation. The operation of angular momentum-type mass flowmeters is based directly on Newton's second law of angular motion. In 1742, the French mathematician Rond d'Alembert proved that Newton's third law of motion applies not only to stationary bodies, but also to objects in motion.
The Flow Pioneers
A major milestone in the understanding of flow was reached in 1783 when the Swiss physicist Daniel Bernoulli published his Hydrodynamica. In it, he introduced the concept of the conservation of energy for fluid flows. Bernoulli determined that an increase in the velocity of a flowing fluid increases its kinetic energy while decreasing its static energy. It is for this reason that a flow restriction causes an increase in the flowing velocity and also causes a drop in the static pressure of the flowing fluid.
The permanent pressure loss through a flowmeter is expressed either as a percentage of the total pressure drop or in units of velocity heads, calculated as V2/2g, where V is the flowing velocity and g is the gravitational acceleration (32.2 feet/second2 or 9.8 meters/second2 at 60° latitude). For example, if the velocity of a flowing fluid is 10 ft/s, the velocity head is 100/64.4 = 1.55 ft. If the fluid is water, the velocity head corresponds to 1.55 ft of water (or 0.67 psi). If the fluid is air, then the velocity head corresponds to the weight of a 1.55-ft column of air.
The permanent pressure loss through various flow elements can be expressed as a percentage of the total pressure drop, or it can be expressed in terms of velocity heads. The permanent pressure loss through an orifice is four velocity heads; through a vortex shedding sensor, it is two; through positive displacement and turbine meters, about one; and, through flow venturis, less than 0.5 heads. Therefore, if an orifice plate with a beta ratio of 0.3 (diameter of the orifice to that of the pipe) has an unrecovered pressure loss of 100 in H2O, a venturi flow tube could reduce that pressure loss to about 12 in H2O for the same measurement.
Omega
Reviewed by Unknown
on
8:41 PM
Rating:
