Mecanica De Fluidos

Mechanical Flowmeters
Discussed in this chapter are various types of mechanical flowmeters that measure flow using an arrangement of moving parts, either by passing isolated, known volumes of a fluid through a series of gears or chambers (positive displacement, or PD) or by means of a spinning turbine or rotor.
All positive displacement flowmeters operate by isolating and counting knownvolumes of a fluid (gas or liquid) while feeding it through the meter. By counting the number of passed isolated volumes, a flow measurement is obtained. Each PD design uses a different means of isolating and counting these volumes. The frequency of the resulting pulse train is a measure of flow rate, while the total number of pulses gives the size of the batch. While PD meters are operated by thekinetic energy of the flowing fluid, metering pumps (described only briefly in this article) determine the flow rate

Figure 3-1: Click on figure to enlarge.

while also adding kinetic energy to the fluid.
The turbine flowmeter consists of a multi-bladed rotor mounted at right angles to the flow, suspended in the fluid stream on a free-running bearing. The diameter of the rotor is very closeto the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. Turbine rotation can be detected by solid state devices or by mechanical sensors. Other types of rotary element flowmeters include the propeller (impeller), shunt, and paddlewheel designs.
Positive Displacement Flowmeters
Positive displacement meters provide high accuracy(±0.1% of actual flow rate in some cases) and good repeatability (as high as 0.05% of reading). Accuracy is not affected by pulsating flow unless it entrains air or gas in the fluid. PD meters do not require a power supply for their operation and do not require straight upstream and downstream pipe runs for their installation. PD meters are available in sizes from in to 12 in and can operate withturndowns as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flowmeter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.
The process fluid must be clean. Particles greater than 100 microns in size must be removed by filtering. PD meters operate with small clearances between theirprecision-machined parts; wear rapidly destroys their accuracy. For this reason, PD meters are generally not recommended for measuring slurries or abrasive fluids. In clean fluid services, however, their precision and wide rangeability make them ideal for custody transfer and batch charging. They are most widely used as household water meters. Millions of such units are produced annually at a unit cost of lessthan $50 U.S. In industrial and petrochemical applications, PD meters are commonly used for batch charging of both liquids and gases.
Although slippage through the PD meter decreases (that is, accuracy increases) as fluid viscosity increases, pressure drop through the meter also rises. Consequently, the maximum (and minimum) flow capacity of the flowmeter is decreased as viscosity increases. Thehigher the viscosity, the less slippage and the lower the measurable flow rate becomes. As viscosity decreases, the low flow

Figure 3-2: Click on figure to enlarge.

performance of the meter deteriorates. The maximum allowable pressure drop across the meter constrains the maximum operating flow in high viscosity services.
Liquid PD Meters
Nutating disc meters are the most common PDmeters. They are used as residential water meters around the world. As water flows through the metering chamber, it causes a disc to wobble (nutate), turning a spindle, which rotates a magnet. This magnet is coupled to a mechanical register or a pulse transmitter. Because the flowmeter entraps a fixed quantity of fluid each time the spindle is rotated, the rate of flow is proportional to the...