Sequence Logic

Sequence Logic Sequence logic must not be confused with discrete logic. Discrete logic is especially suitable for interlocks or per-... 

Sequence logic is often, but not necessarily, coupled with the concept of a process state. Basically, the sequence logic determines when the process should proceed from the current state to the next, and sometimes what the next state should be. 

Sequence logic must encompass both normal and abnormal process operations. Thus, sequence logic is often viewed as consisting of two distinct but related parts ... 

Normal logic. This sequence logic provides for the normal or expec ted progression from one process state to another. 

No single approach has evolved as the preferred way to implement sequence logic. The approaches utihzed include the following ... 

Discrete logic. Sequence logic can be implemented via ladder logic, and this approach is common when sequence logic is implemented in programmable logic controllers (PLCs). 

Programming language.s. Traditional procedural languages do not provide the necessary constructs for implementing sequence logic. This necessitates one of the following ... 

As none of the above have been able to dominate the industry, it is quite possible that future developments will provide a superior approach for implementing sequence logic. 

Link Trees - generates sequence logic by using the event tree logic and applying the linkage rules. 

Sequence Logic - edits sequence names and end states and assigns frequencies after an event tree has been created. 

Edit Accident Sequence Logic 2 Edit Base Case System Failure Probabilities... 

The interface between continuous controls and sequence logic (discussed shortly) is also important. For example, a feed might be metered into a reactor at a variable rate, depending on another feed or possibly on reactor temperature. However, the product recipe calls for a specified quantity of this feed. The flow must be totalized (i.e., integrated), and when the flow total attains a specified value, the feed must be terminated. The sequence logic must have access to operational parameters such as controller modes. That is, the sequence logic must be able to switch a controller to manual, automatic, or cascade. Furthermore, the sequence logic must be able to force the controller output to a specified value. 

An industrial example requiring simple sequence logic is the effluent tank with two sump pumps illustrated in Fig. 8-59. There are two sump pumps, A and B. The tank is equipped with three level switches, one for low level (LL), one for high level (LH), and one for high-high level (LHH). All level switches actuate on rising level. The logic is to be as follows ... 

FIG. 8-60 (a) Ladder logic, (b) Sequence logic for effluent tank sump pumps. 

Some reactors are equipped with multiple vacuum pumps, with different operating modes for moderate vacuum versus high vacuum. Sequence logic is usually required to start vacuum pumps and establish vacuum. 

Such challenging applications for recipe management and sequence logic require a detailed analysis of the production equipment and the operations conducted within that production equipment. While applications such as the sump pumps in the effluent tank can be pursued without it, a structured approach is essential in flexible batch facilities. 

Interlock 1 is also provided with rotating sequencer logic, which serves to equalize run times between machines and protects the same machine from being started and stopped too frequently. A simple approximation of these goals is achieved if the machine that operated the longest is stopped and the one that was idle the longest is started. 

Thoughtful Transition/Sequencing-Logical/Effective Pacing Controlled/Flows Smoothly... 

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