Control loops automatic valves

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 interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

Hence, a theoretical reject pressure can be calculated from pressures measured in the field, together with a constant input from a ratio-cootrol unit. The theoretical pressure proportional to the reject flow rate is then used as the set point in the control loop. The measured variable pn, in the field can then be adjusted automatically by trimming a pressure-control valve in the reject line until the calculated pn) equals the measured pnj. 

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. 

At one of our plants, a boiler s control loops would mysteriously trip from automatic to manual, and valves... 

It does not need initialization. The position form of the algorithms requires the initial value of the controller output cs, which is not normally known in practice. For example, an operator keeps the control loop in the manual mode until a desired steady-state operation has been reached. At this point the error is zero and the position of the control valve would correspond to the cs value. Therefore, if the operator would like to transfer the control from manual to automatic, he or she should enter in the position control algorithm the value of cs, which is not normally known. This difficulty can be bypassed with the velocity form of the control algorithms, which do not need initialization [see eqs. (30.2) and (30.3)]. 

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. 

A sample of paste is automatically taken, diluted with water by a factor of ten and the pH continuously measured. The measured pH value influences the setting of the pH control valve, which is connected in parallel to the main flow control valve for sodium hydroxide. The control loop for the main flow control valve has to be set to the amount of NaOH necessary for neutralisation, and fine dosing takes place via the pH control valve in the pH control loop. 

The chemical processing industry uses a complex network of automated systems to control its processes. The smallest unit in this network is called a control loop. Control loops usually have (1) a sensing device, (2) a transmitter, (3) a controller, (4) a transducer, and (5) an automatic valve. Automatic valves (see Figure 5-7) can be controlled from remote locations, making them... 

Automatic/manual control—term describing two modes in which controllers can be operated. During plant start-up, the controller is typically placed in the manual position. In this mode, only manual control affects the position of the control valve it does not respond to process load changes. After the process is stable, the operator places the controller in automatic mode, which allows the controller to supervise the control loop function. At this point, the controller will automatically open and close the control valve to maintain the setpoint. 

Process technicians use instrumentation to controi a variety of automated processes. The key component of automatic controi is the control loop, a group of instruments that work together to controi a process (see Figure 8-1). These instruments typicaiiy inciude a transmitter coupled with a sensing device or primary eiement, a controiier, a transducer, and a control valve. 

The most common type of automated valve Is a globe valve, because of Its versatile, on-off or throttling feature. Control loops use on-off or throttling-type valves to regulate the flow of fluid In and out of a system. Automatic valves can be used to control pressure, temperature, flow, or level. 

Aspen Dynamics automatically installs some control loops on the flowsheet. This default control stmcture can be modified as required. In our flash-tank example, a pressure controller is automatically installed that controls the pressure in the vessel by manipulating the valve in the vapor line. If we had not placed a pump on the liquid line and only had a valve. Aspen Dynamics would automatically install a level controller that controls the hquid level in the tank by manipulating the valve in the hquid line. 

The most important control loop in this operation is a tray temperature controller at the middle stage of the column. The temperature is controlled at the average temperature of the minimum-boUing azeotropic temperature of the entrainer-water mixture and the normal boiling-point temperature of pure acetic acid. This will ensure that the top vapor of this colurrm will approach the binary azeotrope and the bottoms will approach pure acetic acid. The manipulated variable of this important temperature loop is the organic reflux valve. Thus the accumulation of the organic phase will be automatically adjusted throughout the batch mn. 

For control loops represented in the CCR (central control room), it is normal practice (as mentioned earlier) to furnish control stations. These may be analog pneumatic, analog electronic, or microprocessor based. In the last case, the station may be physically distinct, like an analog station, or may be represented on a CRT display as a faceplate. Each provides an indication of the process variable (flows, level, temperamre, etc.), the desired value (set point), and the valve loading signal (controller output, really). There is also a manual-automatic switch, which some vendors label hand-automatic. In the manual mode, the feedback controller is disconnected and there is a knob that enables the operator remotely to set the valve position. This may or may not be subject to restrictions imposed by feedforward compensation, overrides, and so on, depending on the design philosophy for a particular project. 

The rate of gas production is 20 000 m /hr per gasifier, hence with all 6 gasifiers running the gas production rate will be 120000 m /hr. The rate of the downstream gas compressor stage is to be controlled to match the inflow of gas so that the level of the gas holder is stabilized at typically 50%. The EUC control system comprises an automatic level control loop operating on the input control valves of the compressors supported by manual controls that allow the operator to decide if one or two compressors are to be run to balance the load. 

Once you have completed the steady-state design, use the results to size all the valves in the system. Add the control loops (two) determined from the steady-state design, with the set points which allow the control objectives to be met at all times. Inventory and pressure control will be available automatically, but you can add your own control loops if desired. Solve the column in dynamic mode. You are free to change set points or create disturbances to the system to examine how the control system performs. A good starting point would be to see how the column responds to step disturbances in the feed between the expected feed cases. You may need to tune the control loops in order to produce an adequate response. 

Pneumatic Controllers The pneumatic controller is an automatic controller that uses pneumatic pressure as a power source and generates a single pneumatic output pressure. The pneumatic controller is used in single-loop control applications and is often installed on the control valve or on an adjacent pipestand or wall in close proximity to the control valve and/or measurement transmitter. Pneumatic controllers are used in areas where it would be hazardous to use electronic equipment, in locations without power, in situations where maintenance personnel are more familiar with pneumatic controllers, or in applications where replacement with modern electronic controls has not been justified. 

If an operator saw this problem developing, he would switch the temperature loop into manual and cut back on the steam flow. The control system in Fig. 8,4fl will perform this "override control automatically. The low selector (LS) sends to the steam valve the lower of the two signals. If the steam valve is air-to-open, the valve will be pinched back by cither high temperature (through the reverse-acting temperature controller) or by low base level (through the low-base-level override controller). 

The pumping out of the batch tank at the end of the cycle lowers the level in the batch tank which signals valve AV1 to open again and a new batch of undemineralized water comes into the system. The cycle is repeated automatically for an indefinite period. The salts removed in the stack from the dilute stream are absorbed in a concentrated stream loop being pumped by concentrating pump C. Enough water is continuously added under control of valve CV and rotameter R to make up for the amount of water blown down as brine. The proportion of water which must be wasted as brine is determined by the chemical composition of the water to be treated. The blowdown is adjusted so that the least soluble salt component in the feed water will remain in solution. 

---