Fluid flow valve characteristics

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

How remarkable is life The answer is very. Those of us who deal in networks of chemical reactions know of nothing like it. We understand some - but only some -of the characteristics of the network that make it so remarkable. One key to its behavior is catalysis. The rates of essentially all cellular reactions - the processes that convert one molecule into another - are controlled by other molecules (usually by a class of protein catalysts called enzymes). The catalysts are (in some sense) like valves in a chemical plant (which, in some sense, is what a cell is) they control the rate at which one kind of molecule becomes another in a way loosely analogous to that in which a valve controls the rate at which fluid flows through a pipe. The complexity of the network becomes clear when one realizes that the catalysts - the valves - are themselves controlled by the molecules they produce the products of one reaction can control the rate at which another reaction takes place. 

The aerodynamic valves have a specially designed inlet (e.g., a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct) in which the fluid flow characteristics act as a physical barrier (fluid diode) to the backflow of combustion products. Such pulse combustors are termed the valveless combustors. Example of valveless pulse combustor is shown in Figure 23.4 (Putnam et al., 1986). 

A compressor system typically includes process piping, valves, a compressor, a receiver, heat exchangers, dryers, back pressure regulators, gauges, and moisture removal equipment. The sequence and equipment arrangement is illustrated in Figure 9-2. Additional information about compressor systems can be found in Chapter 5. Scientific principles associated with compressors include fluid flow characteristics, pressure, temperature, heat transfer, electricity, rotation, and kinetic energy. 

Here q is the flow rate, f ) is the valve characteristic, APy is the pressure drop across the valve, and gg is the specific gravity of the fluid. This relation is valid for nonflashing liquids. See Edgar et al. (2008) for other cases such as flashing liquids. 

When manipulating a stream whose flow is independently determined, such as flow of a product or a heat-transfer fluid from a fired heater, a three-way valve is used to divert the required flow to the heat exchanger. This does not alter the linearity of the process or its sensitivity to supply variations and even adds the possibility of independent flow variations. The three-way valve shomd have equal-percentage characteristics, and heat-flow control may be even more beneficial. 

The flow coefficient Cv is determined by calibration with water, and it is not entirely satisfactory for predicting the flow rate of compressible fluids under choked flow conditions. This has to do with the fact that different valves exhibit different pressure recovery characteristics with gases and hence will choke at different pressure ratios, which does not apply to liquids. For this reason, another flow coefficient, Cg, is often used for gases. Cg is determined by calibration with air under critical flow conditions (Fisher Controls, 1977). The corresponding flow equation for gas flow is... 

You want to control the flow rate of a liquid in a transfer line at 350 gpm. The pump in the line has the characteristics shown in Fig. 8-2, with an 5 in. impeller. The line contains 150 ft of 3 in. sch 40 pipe, 10 flanged elbows, four gate valves, and a 3 x 3 control valve. The pressure and elevation at the entrance and exit of the line are the same. The valve has an equal percentage trim with the characteristics given in Table 10-3. What should the valve opening be to achieve the desired flow rate (in terms of percent of total stem travel) The fluid has a viscosity of 5cP and a SG of 0.85. 

Control valves have orifices that can be adjusted to regulate the flow of fluids through them. Four features important to their use are capacity, characteristic, rangeability and recoveiy. 

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