Contend for Shared Equipment

In most plants, there are situations where one piece of equipment is shared by many units. In the following example process, Pump 1 is used to transfer product from Unit 1 to Unit 3, Unit 1 to Unit 4, Unit 2 to Unit 3, and Unit 2 to Unit 4.

You can use several methods for managing the contention of the use of the pump. One of the methods defines each source unit and destination unit combination as a connection in the process model. With this method, only one of the transfers (one connection) can be functioning at any one time because there is only one Pump. For this method in the previous figure, there is one connection phase for each connection. Therefore, in our example, there are four phase logic blocks in the control system. Since there is only one pump to handle all four connections, the four phase logic blocks must be interlocked with one another.

Using this method, contention for use of the pump is handled as follows. When the batch in Unit 1 or Unit 2 is ready to transfer to either Unit 3 or Unit 4 and the destination unit is Ready, Batch Manager allocates the destination unit. Knowing the source unit and the destination unit, Batch Manager can determine which phase logic block to enable to run the transfer. If a transfer phase is running between Unit 1 and Unit 3, the phase blocks associated with the three other connections are inhibited because of the interlocking done in the control system. If Batch Manager needs to run a transfer between Unit 2 and Unit 4, it waits until the Unit 1 to Unit 3 transfer is complete. When the transfer is complete, the interlocks are released on the three other connection phases. The phase status changes from Interlocked to Ready, and Batch Manager can then proceed with the transfer.

Another method requires defining segments, as well as connections, in the Process Model Editor. The segments would assume one of the statuses defined in the model. The availability of each of the connections would correspond to the status of all of the segments that have been assigned to the connection. Batch Manager would only be able to use connections that were available.

Using this method, contention for use of the pump is handled as follows. When the batch in Unit 1 or Unit 2 is ready to transfer to either Unit 3 or Unit 4 and the destination unit is Ready, Batch Manager evaluates the availability of the connection from the status of each segment assigned to the connection. If an available connection is found, the destination unit and connection are allocated, and the proper connection phase is enabled. This method requires no extra control system interlocking. When the transfer is complete, the segment statuses change accordingly, and the availability of the connection are automatically updated.

If connections and segments are not defined in the process model and transfers are performed by coordinating the simultaneous processing of multiple process phases, the Batch Manager evaluation process is different. Batch Manager ensures that a suitable source unit and destination unit are allocated. However, the control system is responsible for interlocking the remaining process phases from running until the transfer of material has been completed.