Automatic control reset

The sulfonic acid flow passes through a second mass flow meter. If the mass flow of the sulfonic acid is different from the set point, it means that the S03/ organic ratio is incorrect (i.e., the resulting product may be under- or over-sulfonated) and an automatic controller resets the organic flow controller accordingly. 

EO 1.4 DESCRIBE the characteristics of the following types of automatic control systems d. Proportional plus reset control system... 

Tuning. Typical values are K.. = O. IK, Tj = 0.75 (0 + x), and = 0.0. The proportional band (lOO/A" ) and the reset time (1/T,) can be calculated from these. When fine tuning, observe the behavior of both the controlled and manipulated variables. Always use an implementation that includes antireset windup protection for use when the manipulated variable encounters a constraint. Use an implementation that includes initialization that starts automatic control bumplessly from the last manual condition. The digital execution period should be fast with respect to the feedback dynamics, with At < 0.05 (0 -i- x) where possible. 

Provide for manual override and reset capability for automatic control fimctions utilized within the system ... 

By approximately 11 00 pm Friday evening, the local electricity demand had subsided, and the load dispatcher gave permission for Chernobyl Unit 4 to continue with its power reduction. At this point, another step in a string of unfortunate events occurred when the automatic reactor control unit did not adjust for the new lower power level. The operators had reset the reactor monitoring system to the requested level, but had failed to reset the reactor automatic controller. The reactor s response was a dramatic drop in power, down to 30 MWt, 1 % of the normal operating level (US NRC, 1987). In effect, the automatic controller inserted control rods, which drove the overall core power dramatically lower than was intended. 

The cool-down time should be as short as possible as this is unused analysis time. The controller as well as the oven should be capable of automatically resetting to the initial temperature. An indication that the column temperature has re-equilibrated after cool-down is very desirable. 

The PTGC technique involves increasing the column temperature at a preset rate during the elution process. This rate may be constant throughout the run, or periods of isothermal operation may be automatically programmed at set times between temperature increases. Generally, the electronically controlled ovens are designed to increase temperature at rates from 0.5-30°C per minute. The initial temperature should be chosen to minimize the retention time for the least retained solute, while the final temperature must be sufficient to elute the least volatile compound in a reasonable time. The instrument then automatically resets the temperature to the initial value in preparation for the next sample. 

These tertiary crushers employ smooth or toothed heavy-duty impact and abrasion-resistant steel-rimmed rolls. The rolls are mounted inline in a horizontal manner and turn toward each other at equal speeds to create a nip into which a friable feed material is introduced (Fig. 4). Heavy-duty compression springs with automatic reset are used to dampen crushing shock and to protect the crusher from tramp iron and oversize material. An adjustable screw that adjusts spring tension changes the crusher opening. A flywheel is used to even out pulses and economize on power consumption. These crushers have a theoretical maximum reduction ratio of 4 1 and will only crush materials to about 10 mesh. Roll crushers produce a controlled product size distribution without a lot of fines. The narrow particle size distribution is achieved by controlling a combination of variables including roll speed, gap measure, differential speed, feed rate, and roll surface. 

In process control only a few types of control action (control modes) are important, namely (1) on-off or two-position control (2) proportional control (3) integral control or automatic reset (4) derivative or rate action. 

With the exception of derivative action any of these control modes may be used alone in certain applications. Integral and derivative actions are most usually combined with proportional control to give proportional plus integral control (proportional control with automatic reset) proportional plus derivative control or three-mode control, which is proportional plus integral plus derivative. 

AUTOMATIC RESET (INTEGRATION) - Is a type of control, in which the controller output, changes at a rate proportional to the deviation or error. The output will continue to change as long as any deviation or error exists. 

RESET - A process of automatically adjusting the control point of a given controller to compensate for changes in outdoor temperature. The hot deck control point is normally reset upward as the outdoor... 

Fig. 1-8. Example of temperature variation with proportional control of an extruder barrel (no automatic reset).

Fig. 1-9. Variation of temperature in an extruder barrel with time, using proportional plus automatic reset control.

There is one major disadvantage to using an automatic reset barrel temperature controller the coefficients of the proportional, the reset, and the rate terms all have to be adjusted properly to obtain desired performance. It is not difficult to do this, but it can be time-consuming. One must follow the manufacturer s instructions. 

Many processes require speed controls. Performance and reliabilities of these controls are very similar to those of the temperature controls—you get what you purchase. Early speed controllers, like the temperature controllers, were mechanical. Speeds were held within 5 percent, resulting in poor plastic melt control. Where better speed control is desired, the solution is the same as in temperature control only the equipment names are changed. A device is added to the motor, and an integral characteristic is provided, corresponding to the automatic reset with heat. It brings the speed closer to the set point. A derivative characteristic, corresponding to the rate in heat, ensures a prompt response to any upsets. 

For cell room use, the dilution water is demineralized, and the mixing device is of nickel or a high-nickel alloy. The cell inlet concentration is controlled, while the variable of more interest is the concentration in the cells. We have the same situation that applies with temperature control, discussed above. Given a measurement of the outlet caustic concentration, again we can reset the control point at the irdet This might be automatic or by occasional operator intervention. 

In accordance with the requirement of NUREG-0737, the design of instrumentation and control systems for the automatic containment isolation valves is such that resetting the isolation signal does not result in the automatic reopening of the valves. Reopening of containment isolation valves requires deliberate operator action to open valves on an individual containment penetration basis (see CESSAR-DC Section 6.2.4.5). 

The nuclear system protection system initiates the rapid insertion of the control rods to shut down the reactor. The system is of the fail-safe design where it will trip on loss of electrical power but will not trip and cause a scram on the loss of a single power source. The four trip channels are physically separated from each other and from other equipment precluding the possibility of interactions that could cause possible false scrams or failure to scram. The logic requires a manual reset by the operator, which is automatically inhibited for 10 s. One reset switch is used for each trip channel. Failure of a single trip channel, division logic, or a system component will not prevent the normal protective action of the nuclear system protection system. 

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