Figure 1.The addition of the functions in red will limit the rate at which steam can be changed during heating.
One software function that I added is the rate limiter (RL), which has an adjustable setpoint (SP). This way, the limit on the rate at which steam demand can change can be automatically changed. So, when the slow heat-up phase is over and the reactor is switched to the reaction phase, the response of the temperature control loop is slow if the changes required in steam rate is larger, and fast if it is smaller.
The output of RL throttles the steam valve and likewise provides the external reset (ER) to the cascade master temperature controller (TRC with PID control modes) only during heating. The purpose of the switching function (SS) is to switch the ER to be received from the measurement of the slave temperature controller (TRC) when the exothermic reaction is started (the mostly cooling phase). In this way, during heat-up, the ER signal is received from the RL during cooling from jacket temperature (TT), and the setpoint of the RL can be automatically changed to vary the speed of response as a function of the sizes of upsets.
Naturally, before doing all that, first I would visit the utility building and would try to speed up the boiler(s).
BĂŠla LiptĂĄk
[email protected]
We always suggest using proper design and appropriate control strategies; we try to use ramps, limiters and other handcuffs for security.
Michel Ruel
[email protected]
Q: Is there a difference between the definitions of smart actuators and smart positioners? If there is, what are the main features of each considering their capability, limitation, advantage and future trends?
Abdullah Ghamdi
[email protected]
A: An actuator is any device capable of changing the opening of a valve or modifying the position, speed or any other operating condition of pumps, compressors, etc. They can be as simple as a coil operating an on-off solenoid or more sophisticated pneumatic or electric throttling devices, including variable-speed drives (VSD). VSDs are superior to valves because of their speed and energy savings, and they have no hysteresis.
The actuator by itself is not necessarily provided with feedback; it does not "know" if the desired position was in fact achieved. The term "smart"does not mean much. It usually means fieldbus connectivity, but it can still be just a sales gimmick, depending on the information provided. It can have real value in the areas of self-diagnostics, historical data collection, visual displays, maintenance scheduling, process property measurements, etc.
The positioner is a position control loop consisting of a position sensor and a controller. Its job is to eliminate the difference between the measurement and setpoint of this position controller. In addition, you can think of a positioner as a cascade slave controller in which the cascade master is a temperature, pressure, level, flow or any other variable.Â
The positioner can also be used to change the characteristics of the control valve artificially if the wrong valve was installed, or to change the dynamics of the loop if needed. Finally, the positioner can be part of and supplied with the actuator. As to the term "smart positioner," it can either imply just a sales gimmick, or provide valuable extra features, similar to those in "smart actuators." To determine what the term really means requires careful analysis of the bids summitted.
BĂŠla LiptĂĄk
[email protected]