Orifice type flow elements

Orifice plates are the simplest and least expensive flow elements within the head-type and d/p-type sensor family. The total installed cost is relatively independent of pipe diameter because only the cost of the plate is a function of pipe size—the d/p manifold and the d/p readout or transmitter are relatively constant. Consequently the orifice-type installations become more and more cost effective as the pipe size increases. 

The variable-area flowmeter is a head-type flow sensor, but it does not measure the pressure drop across a fixed orifice instead, the pressure drop is held relatively constant, and the orifice area is varied to match the flow (Figure 3.98). In gravity-type variable-area flowmeters, increase in flow lifts the float, piston, or vane, and it is the weight of these flow elements that has... 

It is important to examine all the elements in the loop when determining what type of response is expected. For example, a differential pressure transmitter, also known as a d/p cell, has a linear response. However, when the head flow element, i.e. orifice, and the d/p cell are connected together the response is nonlinear, as shown in Figure 7.2. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Attempts to use these kinds of devices on low-viscosity materials showed that the flows did not twist and curl in quite that same fashion. In the low-viscosity region, pressure drop is a key factor. The second type of static mixer gets pressure drop through controlled channels, different types of static elements, as well as random placement of baffles, blades, orifices, or other devices inside the pipeline. 

Flow transmitters. Flow measurements are made in high-pressure lines by sensing the pressure drop across a calibrated orifice or venturi, or by the transmitting variable-area type of flowmeter. The latter meter resembles a Rotameter with float position transmitted electrically. It has the advantage of being an in-line element but is not readily applicable to large flows. 

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