Flow characteristic curve, valve

K = constant which depends on the valve size p = specific gravity of the flowing liquid (relative to water) fix) = valve flow characteristic curve The valve flow characteristic curve, fix), depends on the geometrical shape of the plug s surface. Figure 13.10 shows the most common types of plugs while Figure 13.11 shows the flow capacity characteristics for the various valves. 

IV.l Consider the flow control loop shown in Figure 13.2a. The following information is also available (1) An orifice plate is used to measure the flow (2) a variable capacitance differential pressure transducer is employed (see Appendix 11 A) to sense and transmit the pressure difference developed around the orifice plate (3) the controller is PI and (4) the control valve is of equal percentage, with the valve flow characteristic curve given by... 

IV.2 Consider the liquid level control loop of Figure 13.2d. The differential pressure transducer cell exhibits second-order dynamics, the controller is proportional, and the control valve is linear with flow characteristic curve given by... 

Figure 7 Installed flow characteristic curve and installed gain curve fora butterfly and a globe control valve. [1] Other factors affecting control valve perfonnafice ...

Most centrifugal pumps are controlled by throttling the flow with a valve on the pump discharge, see Section 5.8.3. This varies the dynamic pressure loss, and so the position of the operating point on the pump characteristic curve. 

Chamcteristics. By changing the shape of the plug and the seat in the valve, different relationships between stem position and flow area can be attained. The common flow characteristics used are linear trim valves and equal-percentage trim valves as shown in Fig. 7.7. The term equal percentage comes from the slope of the /Ji, curve being a constant fraction off. 

Determine the operating point on the pump characteristic curve when the flow is such that the pressure drop across the control valve is 35 kN/m. ... 

A graph of discharge coefficient ( m) plotted against dimensionless lift L/D) is known as a flow characteristic. Fig. 9 shows an example of flow characteristics plotted for various pressure ratios relieving from an air receiver, where Pb is the total backpressure at the valve exit port and Pq the reservoir total pressure.According to ideal one-dimensional compressible flow theory, choked flow should occur at pressure ratios below 0.528. In practice, choked flow will occur at lower pressure ratios because of friction losses upstream of the curtain area. Nevertheless, the curves in Fig. 9 are close-together at pressure ratios below 0.333, and have collapsed to almost a single curve for ratios less than 0.25. Other work shows a similar effect. 

Depending on the characteristics of the device (reactor volume, conductance, pmnping speed), we e>q)eiimentally determine the flow-pressure curves when the valve is open and closed. 

Area control of air/fuel ratio, that is, linked valve control, uses one common contol motor to drive a linkage to both air and fuel valves. The air and fuel valves must have very similar characteristic curves. VFD is not appropriate with this area control system, but can be used effectively with either pressure control or flow control, discussed next. 

When a centrifugal pump is installed in a pumping system and operated at a particular rotational rate, N, (usually 1,750 to 3,450 rpm), the flow rate can be varied by changing the opening on a valve located in the pump discharge line. The variation of H with Q defines a unique characteristic curve for the particular pump operating at N with a fluid of a particular viscosity. Each make and model of a centrifugal pump is supplied by the manufacturer with a... 

The functional relationship between the mass flow rate of coolant and the controller output is usually given by the controller manufacturer, or can be determined experimentally. Figure 4.7 gives the control valve characteristic curve determined for this system. The fraction controller output is related to the proportional gain through... 

For pumped systems rapidly increasing energy costs have aroused considerable interest in designing for very low valve-pressure drops. This pushes us toward line-sized valves. If piping and equipment pressure drops are large in comparison with valve-pressure drops, valve turndown sui rs (becomes smaller). The installed flow characteristic also becomes very different ftom the inherent flow characteristic. A linear valve tends toward an installed square-root flow characteristic, while an equal-perccntJ e valve tends toward an installed linear characteristic. At the hi -flow, high-lift end of the plot, however, all curves, r ardless of y alve inherent characteristic, level off at a maximum flow. This is so because the valve has run out of pressure drop. 

If pvunp and valve curves are available, and if die hydraulic resistance and static heads of the process equipment are known, we can usually calculate the installed flow characteristic. Alternatively, with permission from production supervision, we can experimentally make a plot of valve loading signal versus flow. The slope of this curve at average rate is tlQj,ldQc-... 

The inherent characteristic of a valve is a plot of Cv versus valve opening. This curve is usually plotted as Cy as a percentage of maximum flow (or Cv). Inherent characteristics are usually plotted in this way rather than actual Cv versus actual lift so that the same curve will apply to a set of geometrically similar valves, irrespective of size. If the characteristic curve and the maximum Cy are known, then the Cy at any intermediate lift or opening can be determined. 

If the pressure drop did not change across the valve with valve opening, then the flow would vary proportionally with Cy, and thus the operating characteristic would be the same as the inherent characteristic curve if both were plotted as a percentage of maximum. However, as the valve closes, the pressure drop across it increases. This... 

System Curves In addition to the pump design, the operational performance of a pump depends upon factors such as the downstream load characteristics, pipe friction, and valve performance. Typically, head and flow follow the following relationship ... 

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