Manual mode

The safe operation of a chemical process requires continuous monitoring of the operation to stabilize the system, prevent deviations, and optimize system performance. This can be accomplished through the use of instrumentation/control systems, and through human intervention. The human element is discussed in Chapter 6. Proper operation requires a close interaction between the operators and the instrumentation/control system. To a large extent, batch operations have simple control systems and are frequently operated in the manual mode. The instrumentation system is the main source of information about the state of the process. Some of the typical functions of the instrumentation/control system are... 

Manual mode control operation is very common leading to increased potential for human error. 

Minimize use of manual mode control through well designed automatic mode of operation... 

Ensure that SIS is not disabled when operating in manual mode... 

A switch SW2 is provided for the selection of the engine to be started in auto or manual modes ... 

In manual mode, the engine can be started and stopped locally, through a pair of start and stop push buttons Nos 17 and 18. provided on the AMF panel (Figure 16.11). 

To prevent surges, a well-trained operator would put the controller in manual mode and freeze the valve in an open position. This stops the control loop oscillations and decreases the compressor discharge resistance, thus breaking the surge cycle. Unfortunately, the operator has no way of knowing how much to open the valve and, subsequently, how much to close it. 

The flow controllers are often used to set desired flows for the fresh feed, stripping steam, and dispersion steam. Each flow controller usually has three modes of control manual, auto, and cascade. In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set-point. In cascade mode, the controller set-point is an input from another controller. 

To make use of empirical tuning relations, one approach is to obtain the so-called process reaction curve. We disable the controller and introduce a step change to the actuator. We then measure the open-loop step response. This practice can simply be called an open-loop step test. Although we disconnect the controller in the schematic diagram (Fig. 6.1), we usually only need to turn the controller to the manual mode in reality. As shown in the block diagram, what we measure is a lumped response, representing the dynamics of the blocks Ga,... 

After proper pairing of manipulated and controlled variables, we still have to design and tune the controllers. The simplest approach is to tune each loop individually and conservatively while the other loop is in manual mode. At a more sophisticated level, we may try to decouple the loops mathematically into two non-interacting SISO systems with which we can apply single loop tuning procedures. Several examples applicable to a 2 x 2 system are offered here. 

The operator can with one keystroke change the console display, go into manual mode, or abort the batch. In manual mode, the reactor... 

A manual output adjust knob, in the center of the M-A transfer switch, varies the controller output signal in manual mode of operation. The knob is rotated clockwise to increase the signal and counterclockwise to decrease the signal. 

MAN. This position places the controller in the manual mode of operation. Controller output is now varied by adjusting the manual output adjust knob. This adjustment is indicated on the output meter. The deviation meter indicates any deviation between controlled variable and setpoint. 

MAN side). The controller is still in the manual mode of operation, and the deviation meter indicates controlled variable value (0% to 100%) as it did in the 0-100 (AUTO side) position. 

BAL. In many cases, controller output signals of the automatic mode and manual mode may not be the same. If the controller were directly transferred from automatic to manual or manual to automatic, the controller output signal could suddenly change from one value to another. As a result, the final control element would experience a sudden change in position or "bump." This can cause large swings in the value of the process variable and possible damage to the final control element. 

Bumpless transfer is the smooth transfer of a controller from one operating mode to another. The balance (BAL) position provides this smooth transfer when transferring the controller from the automatic to manual mode. In the BAL position, the controller is still in the automatic mode of operation, but the deviation meter now indicates the difference between outputs of manual and automatic modes of control. The manual output is adjusted until the deviation meter shows no deflection. Now, the controller can be transferred smoothly from automatic to manual. 

Mode indicating lights. Located directly below the manual push buttons, these lights indicate the operating mode of the controller. When in manual mode, the left light, labeled "M", will be lit when in the automatic mode, the right light, labeled "A", will be lit. 

Mode selection buttons. Located directly under each mode indicating light, each button will select its respective mode of control. If the button below the "M" mode light is depressed, the controller will be in the manual mode of operation if the button below the "A" mode light is depressed, the controller will be in the automatic mode of operation. 

The transfer of materials to and from the fluid-bed processor is an important consideration. The loading and unloading of the processing bowl can be accomplished by manual mode or by automated methods. 

Add the rinsings to the mixer. Set the mixer temperature, 25°C mixer speed, 18 rpm manual mode vacuum, 0.5 bar. 

Sprinkle item 1 slowly while mixing at 20 rpm in manual mode. Check that item 1 is dispersed completely without forming lumps. 

Installation was timed to match startup of the automated plating lines. The waste treatment system was started up in manual mode under TRSI supervision. During this period a number of small glitches were encountered, as is common with projects of this size. These were quickly resolved. The system progressed to its fully automated capabilities with no major interruptions to production. 

Early controller designs required balancing of the controller output prior to switching to or from automatic and manual modes. This procedure minimized inadvertent disturbance to the process caused by potentially large differences between the automatic and manual output levels. Later designs featured bumpless or procedureless automatic-to-manual transfer. 

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