The characteristics of a flow control loop are influenced by several factors. These include what the flow stream is (liquid, gas, or two-phase liquid and vapor), how the flow is measured, how the flow is manipulated (i.e., the final control element), the relationship between the final con- trol element and its piping environment, and the form of the controller itself. As a quick sum- mary, a flow loop can be characterized as relatively fast, nonlinear, and often noisy. Let us explore each of these attributes.
Figure 3-21. Multiple-Input, Multiple-Output Processes (Interacting Control Loops) 352&(66 P P P1 [ [ 9$/9(6 25)/2: &21752//(5 6(732,176
The dynamic character of a flow loop is most often dominated by the dynamics of the final control element. If this element is a traditional control valve, then the speed of the actuator is the dominating dynamic element. A spring-opposed air-operated actuator will act as a first- order lag for small signal changes. For large signal changes, the changing volume within the actuator acts as a velocity limit on changes in stem position. In addition, the friction of the stem packing introduces a hysteresis effect between the signal-to-valve and the actual stem position. A valve positioner will decrease the effect of hysteresis, thereby improving the response of the stem positioning. While valve positioners are highly recommended for most control loops, in a flow loop the valve positioner and the flow controller may respond on approximately the same time scale, therefore causing the two to interact. (See chapter 9 for further discussion of the interaction of cascaded controllers.) Hence, it is often recommended that valve positioners not be used on flow loops. Many practitioners disagree with this recom- mendation, however, preferring to use a positioner on all control valves, even flow. Any ten- dency toward interaction between the positioner and the flow controller is then compensated for by reduced controller tuning. In general, it can be said that most flow loops, if they oscil- late, will have a period of around one to three seconds.
We are interested in linearity because our preference would be that the loop have the same response with the set point at, say, 70 percent as it does when the set point is at, say, 30 per- cent. Flow loops are frequently nonlinear and have the maddening aspect that the type of non- linearity found in one loop may not be the same as that found in another loop.
The nonlinearity is determined by the characteristics of the final control element, the type of flow measurement used, and the effect of other restrictions in the flow line. We discussed installed valve characteristics earlier in this chapter.
Another factor to consider when designing flow control loops is the flow sensor and its signal to the controller. If flow is measured by measuring differential pressure across an orifice plate, then the signal is proportional to the square of the flow, not to the flow itself. In older installa- tions, the transmitted and displayed signal was proportional to the square of the flow. The dis- play was on a nonlinear scale as shown in Figure 3-22. If this signal were also used as the process variable for a controller, then the squared relationship introduced an additional nonlin- earity into the control loop. An equal-percentage valve would produce a great variation in pro- cess gain over the full range of the valve. A better choice of valve would be a quick-opening valve if there is no change in pressure drop across the valve, or a linear valve if there is a sig- nificant decrease in pressure drop across the valve as the flow increases.
With current technology, square root extraction is probably performed someplace within the loop, either in the sensor transmitter or in software in a digital-based controller. This removes one of the nonlinearities. As a result, an equal-percentage valve should be used if there is a sig- nificant decrease in pressure drop across the valve with increasing flow; a linear valve should be used if the pressure drop is relatively constant.
In addition to nonlinearities, another consideration in flow control loops is the measurement noise caused by turbulence in the line. Some types of sensors will produce more noise than
others. Head (differential pressure)-producing devices and vortex meters will be the noisiest. Magnetic flow meters and Coriolis meters will be less noisy, and turbine meters will be the least noisy. To some extent, the noise can be filtered out, either in software or hardware, but doing so will retard the response of the control loop. There are two primary reasons for filter- ing out the measurement noise—first, to improve the appearance of the chart record or indica- tion and second to preprocess the signal before it is sampled for logging and/or archiving purposes. In most analog controllers, the same signal is used for indication as for control. Hence, any filtering performed will affect both. In many digital-based systems, the control sig- nal can be separated from the indication signal. Here, filtering can be applied to the indication signal without filtering and retarding the control signal.
Flow loops are usually tuned with a low gain (wide proportional band) and a relatively short integral time. (See chapter 4 for a description of tuning parameters and terminology.) This is especially true with digital flow controllers, in which the dead time caused by the scan time is significant compared to the rather short lag in the process. Analog controllers were often tuned with a high gain and long integral time. Because of the measurement noise present, derivative is never used in a flow loop.
Figure 3-22. Nonlinear Display Scale When Flow Is Measured by a Differential Pressure Sensor without Square Root Extraction