Transmitters, control loops

A more sophisticated implementation is full metering control (Fig. 10.6). In this case, we send the signals from the fuel gas controller (FC in the fuel gas loop) and the air flow transmitter (FT) to the ratio controller (RC), which takes the desired flow ratio (R) as the set point. This controller calculates the proper air flow rate, which in turn becomes the set point to the air flow controller (FC in the air flow loop). If we take away the secondary flow control loops on both the fuel gas and air flow rates, what we have is called parallel positioning control. In this simpler case, of course, the performance of the furnace is subject to fluctuations in fuel and air supply lines. 

The temperature control loop consists of a temperature transmitter, a temperature controller, and a temperature control valve. The diagonally crossed lines indicate that the control signals are air (pneumatic). 

B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The temperature transmitter is the feedback element. It senses the controlled output and lube oil temperature and produces the feedback signal. 

We will consider all the components of this temperature control loop in more detail later in this book. For now we need only appreciate the fact that the automatic control of some variable in a process requires the installation of a sensor, a transmitter, a controller, and a final control element (usually a control valve). Most of this book is aimed at learning how to decide what type of controller should be used and how it should be tuned, i.e., how should the adjustable tuning parameters in the controller be set so that we do a good job of controlling temperature. 

The part of the control loop that we will spend most of our time with in this book is the controller. The job of the controller is to compare the process signal from the transmitter with the setpoint signal and to send out an appropriate signal to the control valve. We will go into more detail about the performance of the controller in Sec. 7.2. In this section we will describe what kind of action standard commercial controllers take when they sec an error. 

Let s consider a typical basic process control loop (Figure 2.1). A pressure-indicating transmitter (PIT) sends the pressure signal to a proportional integral derivative controller (PID), which sends a signal to the control valve to... 

In addition to device-level diagnostics, networked final control elements, process controllers, and transmitters can provide loop level diagnostics that can detect loops that are operating below expectations. Process variability, time in a limit (saturated) condition, and time in the wrong control mode are metrics used to detect problems in process loop operation. 

Two 1-min temperature measurement lags are included in the temperature control loop. A 50°C temperature transmitter span is used. Controller gain is 10, and integral time is 10 min. 

Control with Only Bypass The important control loop in this process is the temperature controller that manipulates the bypass flow to control the temperature of the mixed hot and cold streams. The controller is direct acting (an increase in temperature opens the bypass valve). A 1-min deadtime is inserted in the loop, and a relay-feedback test is run that gives Tyreus-Luyben settings Kc = 0.48 and T/ = 4.0 min. The temperature transmitter span is 350-450 K. 

The control system consists of five measurements, two manipulated variables and five control loops. In the first control loop, loop 1, a temperature measurement TTi is made of the reactor temperature T (where the first T refers to Temperature and the second T to Transmitter) which is sent to a feedback controller TCi (where the T refers to Temperature and the C to Controller) which is normally a Pi-controller in order to avoid offset. 

A DC motor is feedback-controlled by a current sub-control loop and a primary speed control loop. In order to close the control loop, the actual current value is fed back to the current control loop and a speed signal to the speed control loop. While current is measured in the power converter, a shaft encoder on the motor is required for speed signal feedback. Either a tacho-generator or a digital encoder is used as a speed transmitter. If speed measurement accuracy is not very important, the speed feedback can be measured via armature voltage. In this case, this measurement can also be done within the power converter. Static control accuracy reaches... 

Automatic valves are part of a control loop, which is shown in Figure 8.6. The loop contains a primary element, which measures the controlled variable, such as temperature, pressure, flow rate, and liquid level. The operation of a control loop is the same regardless of what variable is controlled. In the case of flow-rate control, the controller obtains the flow rate from transmitter a flow meter and compares the measured flow rate with a value that has been preset in the controller. If the flow rate is greater than the preset value, the controller increases air pressure on top or bottom of a diaphragm in the valve. Then, the valve partially closes to reduce the flow rate. On the other hand, if the flow rate is below the preset value, the controller will act to reduce the air pressure on the diaphragm, and hence the valve opens wider. Electric motors can also operate automatic control valves. 

The P and I diagram shows all the components that make up a control loop. For example. Figure 5.8 shows a field-located pressure transmitter connected to a shared display pressure indicator-controller with operator access to adjustments and high and low alarms. The pressure controller sends an electric signal to a fail-closed diaphragm-actuated pressure control valve. 

Option 3 Costs for Instrumentation of Particular Types of Equipment Table 16.15 fists typical DCS (control loops, C alarms. A and transmitters, T) and the types of field instruments (indicators, I, and relief valves, R) for different types of equipment and their FOB cost. For this list, we give an estimate of the installed instrument cost in Table 16.15. These results are reasonably consistent with the previous two methods. 

For example, consider the case where a flow control loop is the initiating cause. This initiating cause includes a flow transmitter, a controller, and a control valve. In order to allocate risk reduction to a pressure control loop in the BPCS, the pressure transmitter should be wired to an independent controller, modulating an independent final element (for example, vent valve to flare system). 

Figure 15-1 shows a set of equipment. Transmitter T2 is cormected directly to the SIS, and transmitter T1 is the sensor for the BPCS control loop. The SIS logic solver is separate and independent from the BPCS controller. The SIS final element is an independent isolation valve with a three way solenoid as an interface. The BPCS controller is connected to a modulating control valve. 

Clearly, the key control loops are the two temperature controllers. Temperature control by manipulating reboiler duty in the high-pressure column is conventional. Relay-feedback testing and lyreus-Luyben tuning give ATc = 0.157 and ti= 10.6 min for a temperature transmitter range of 250-350 °F and an output maximum of 94.8 x 10 Btu/h. The TC2 controller is reverse acting. 

Another useful element is the Scale block that converts one type of signal into another set of units. For example, if the process variable is temperature and we want to generate a percent of scale signal (or an electronic or pneumatic signal) that would come from a temperature transmitter, a Scale block provides this conversion. On the other end of the control loop, the output signal from the controller (%, ma or psig) can be converted into the appropriate manipulated variable units (GJ/h, Ib/h, etc.) by the use of aMultiply block. The use of the Scale block will be illustrated in the second example, which is more complex than the first example discussed below. 

If the chlorate reactor is a vessel, a level control loop is necessary. The transmitter should be of the flush-diaphragm type with wetted parts of tantalum. The control valve, located in the pump discharge piping, should be fully lined with PTFE. The size of the system will determine the type of valve to be used. 

Level control is critical because an unsteady level will result in varying discharge flow, which directly affects the brine neutralization control loop. The level transmitter should be a dual remote d/p cell with wetted parts of tantalum. The control valve should be a fully PTFE-lined butterfly valve in the pump discharge line. If powered by a UPS, this pump can be a source of flushing brine during a total power outage. TTie dechlorinated brine receiver may be elevated in order to provide more suction head to the discharge pump. 

The alternative to separate control of the header pressures is to control one pressure directly and the other through a differential pressure controller referred to the first. With this alternative, the more frequent choice is to control the chlorine pressure and the hydrogen/chlorine differential pressure. This takes advantage of better control dynamics. An argument for this approach is that it is the quality of the differential pressure control that may determine the life of the membranes, and there is only one control loop variance to deal with. In a poorly designed system, however, responses to the two control signals can be out of phase and introduce fluctuations in the pressures. In order to have a consistent approach in this presentation, we assume that the two header pressures are controlled separately and directly. Differential pressure is measured directly or computed from the outputs of the two pressure transmitters. Monitoring, alarm, and supervisory systems can be appropriate to individual applications. 

An example of a feedback control loop is shown in Figure 8.1 for heating the process water with live steam injection. The temperature of the water leaving the process is measured by a temperature sensor, and a transmitter sends the signal to the controller. The desired temperature setpoint is adjusted in the controller, and the difference between the water temperature out of the process and the setpoint is called the error. In this example, the process water temperature out of the process is the controlled process variable, that is, controlled to a setpoint. The manipulated variable is the steam flow rate, which is manipulated by the automatic valve that is connected to the controller output. 

Product concentration can be controlled by measuring a number of physical properties. On-stream composition analyzers are often used. Commonly used physical properties include density, boiling point rise, temperature/pressure combinations, temperature difference, conductivity, differential pressure, refractive index, buoyancy float, and viscosity. Each method has certain advantages as well as limitations. In all cases, however, a representative measurement location must be carefully selected to eliminate entrained air bubbles or excessive vibration, and the instrument must be mounted in an accessible location for cleaning and calibration. The location of the product quality transmitter with respect to the final effect should be considered also. Long piping runs between the product and the instrument increase deadtime, which in turn reduces the effectiveness of the control loop. 

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