Valves, control installed flow characteristic

FIG. 8-82 Installed flow characteristic as a function of percent of total system head allocated to the control valve (assuming constant head pump, no elevation head loss, and an R equal 30 equal-percentage inherent characteristic). 

If the manipulated valve has a linear installed flow characteristic (preferred), and if there is no level self-regulation (if Ap, does not change appreciably with change in level), then the dynamic response of the proportional-onlyt level control system may be defined by a first-order time constant ... 

Since we will be looking increasingly at column control systems where column pressure is allowed to float, care must be taken to check valve maximum capacity at minimum APp and valve turndown at minimum flow at maximum APp. These flows, in many cases, will be manipulated to control liquid level, which means that a linear installed flow characteristic is desired. Cascading from level control to flow control is recommended. 

If the PI level control system (or proportional-only) is not a cascade level-flow system, then it is desirable to have a control valve with a linear installed flow characteristic, that is, dQ lddc = constant. Then control-loop dynamics would be independent of flow rate and... 

For a cascade level-flow system, the control-valve installed-flow characteristic should also be linear. ... 

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

Boger, H.W., Flow Characteristics for Control Valve Installations, ISA Jour., 13, 11,50, 1966. 

Both the temperature T and the liquid height H in a tank are controlled simultaneously by manipulating the flow rates of hot water Fh and cold water Fc into the vessel. Liquid leaves the vessel through a hand valve, so the flow rate out of the vessel depends on the square root of the height of liquid in the tank F = K Jh. The inlet temperature of the cold water is 70°F and for the hot water it is 160°F. The diameter of the tank is 12 inches. The temperature transmitter range is 50-200°F. The level transmitter range is 0-16 inches of water. The hot and cold water control valves have linear installed characteristics and can pass 3 gpm and 4 gpm, respectively, vdien wide open. 

An in-line pH control system is installed when the brine is to be acidified. HCl is added upstream of an inline mixer, with measurement downstream of the mixer. This is a simple case of pH control, the end point being fairly low. The process gain is also low in this case, and control is easier than in the first stage of the pH adjustment, described in the previous section. The acid valve should be a fail-closed globe valve of equal percentage characteristic with full PTFE lining and FIFE bellows. An interlock should close the acid feed valve whenever there is no brine flow or no current on the cells. 

Another factor that affects the linearity of the response is the characteristic of the valve selected. Three of the most common valve characteristics include quick opening, linear, and equal percentage. If the majority of the system pressure drop is taken across the valve, then a linear valve should be used since its installed characteristic will also be linear, giving the linear response desired. However, if the pressure drop across the valve is a small part of the total line drop and is not constant, then an equal percentage valve can be used since its installed characteristic will be close to linear. Quick-opening valves are most commonly used with on-off controllers where a large flow is needed as soon as the valve begins to open. More information on these valve flow characteristics can be found in Chapter 2. 

Another approach to this situation would be to put a component in the loop that would have a complementary gain to the process gain. An example of this is using a square-root extractor with a head flow meter in the flow control loop. If the pressure drop across the valve remained fairly constant, then the valve and installed characteristic would be nearly the same. An equal percentage valve could be used to complement the process, and the product of the valve and process gain KyKf) would almost be constant, as illustrated in Figure 7.13. 

The liquid flow rate from a vertical cylindrical tank, 10 feet in diameter, is flow-controlled. The liquid flow into the tank is manipulated to control liquid level in the tank. The control valve on the inQow stream has linear installed characteristics and can pass 1000 gpm when wide open. The level transmitter has a span of 6 feet of liquid. A proportional controller is uaed with a gain of 2. Liquid density is constant. 

A flow control valve will be installed in a process system in which the flow may vary from 100 to 20 percent while the pressure drop in the system rises from 5 to 80 percent. What is the required rangeability of the control valve What type of control-valve characteristic should be used Show how the effective characteristic is related to the pressure drop the valve should handle. 

At low flow rates, most of the pressure drop is taken over the control valve since the pressure drop over the rest of the process equipment is low. At high flow rates, the pressure drop over the control valve is low. In this situation the equal-percentage trim tends to give a more linear relationship between flow and control valve position than does linear trim Figure 3.8 shows the installed characteristics of linear and equal-... 

The inherent characteristics are those that relate flow to valve position in the situation where the pressure drop over the control valve is constant. These are the (AP///A/, .) = 0 curves in Fig. 3.8. Installed characteristics are those that result from the variation in the pressure drop over the valve. 

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