NAMUR
5.6 Allocation and Arbitration
Batch information must be recorded with appropriate key fields, such as the source of the data and the associated equipment identification, time stamp, and value. The information will be sorted into various views in order to make it more useful. The start and end times of a batch along with the equipment ID of the first unit make it possible to sort out everything for one batch, including batch and common data. Sorting is even easier if batch ID is included, although that is not possible for common data.
5.5.4 Batch Reports
Each report represents a different view of batch history. Once the history is captured, reports can be requested whenever recipients need them. Report formats or views may be altered to show different data without modifying the batch history records, unless the data is not in the history. Graphic data may be required. The following is a partial list of possible reports and recipients:
• Production management—Summaries of manufacturing costs, actual and prom- ised manufacturing dates, cost of inventory, equipment utilization, human resource levels, and so on.
• Product development—Detailed reports of equipment activities and analysis results, statistics for many batches, comparisons of similar data among equipment, and so forth.
• Plant operations—Reports of current operations in the plant, summaries of past production.
• Quality management—Reports of data associated with quality for specific batches or equipment, statistical charts summarizing recent and long-term production, and others.
• Authorities—Carefully compiled reports of compliance with regulatory require- ments, with nothing extraneous that might be misunderstood.
• Customers—Whatever they want that they can’t be talked out of, usually related to product quality and process uniformity. Again, nothing extraneous should be included.
If a batch schedule does not name the equipment to be used, then the cell must allo- cate a starting unit when a scheduled batch is due to start. Allocation is done by coordination control. In the event that two or more resources are available, some algorithm is necessary to choose one of them. Hotels use “room nearest the elevator”
and restaurants use “table nearest the kitchen.” A process cell might choose “least- used equipment” in order to distribute wear on the equipment.
If things are not so simple then the cell or the unit or the common resource must have a way to resolve any conflicts. Conflicts are reduced, but not eliminated, by making reservations. Since the decision is made by the resource and the equipment entity that is competing for the resource has no recourse to a more favorable decision, the proce- dure for resolving the conflict is called arbitration. If there is no priority structure then an algorithm like “please hold, your call will be answered in the order in which it was received” or “serve the smallest request first” may be sufficient. If there is a priority structure then the request queue must be sorted by priority each time a new request comes in. Things are more complex if a priority request can stop a service in progress.
Part of the contents of a supply tank may be reserved for a specific batch, which may result in arbitration. This also applies to space in a storage tank.
The following discussion is about equipment, but it works as well for operators and other resources.
5.6.1 Allocation
Batch processing is performed in process cells that contain units and common resources. If you have only one unit then you probably won’t find the rest of this very interesting, though it might be useful for planning future growth.
Process cells generally don’t do anything that isn’t requested by a schedule. When a batch is scheduled to run, the cell must allocate at least one unit for that batch. The choice may depend on the connectivity to following units in order to assure completion of the batch. After the batch is started, the unit does its processing relatively independ- ently of the rest of the process cell. That is, the unit is independent until it needs the services of a common resource or another unit, perhaps for transfer. If the cell has suc- cessfully allocated the resource then there is no problem. If the unit requests service from the resource without warning, then the resource may not be available.
Some batch processes may require a resource for a fraction of the batch processing time, leaving the resource free to serve other similar processes. No one wants an idle resource in their plant, unless it is the fire pump or something like that, so processes are designed to time-share the resource, now called a common resource. A common resource may be a unit, if the resource must actually process the batch. More likely, the resource may provide some service to the unit containing the batch, like delivering a material, filtering the product, or providing additional heat transfer capability. In that case, the resource can not be a unit but may be an equipment module. The equipment module may have procedural control at the phase level.
Common resources have also been called shared resources, but this is imprecise.
Common resources can be exclusive-use resources when they are dedicated to a requesting unit or shared-use resources if more than one but less than all units may use the resource at the same time. If all of the units can use it at the same time then it does not require allocation.
Exclusive-use resources may become bottlenecks in cell processing. A bottleneck is a narrow passage that slows down everything behind it when it is overloaded. That’s why some beer bottles have wide mouths and no neck, so that the beer is not delayed on its journey to becoming fat and waste. An exclusive-use resource must have some idle capacity. If not, then it is limiting the production rate, possibly leaving much more expensive units idle while they wait for the resource. In any event, exclusive-use common resources require allocation and possibly arbitration.
Shared-use common resources reduce the bottleneck effect but may not eliminate it.
A shared-use resource may be a pump and two flow totalizers, with paths for any totalizer to any of four units. The pump can handle two charges at once but no more.
The result is that only two batches may acquire this equipment module at the same time. The third request will have to be allocated to a queue or arbitrated.
Allocation or acquisition is required when two or more units may compete for a common transfer line. Consider the case of a multipath transfer header that is too complex for reasonable coordination control among units, but the header can only serve one source and destination. The header and its control comprise an exclusive- use common resource that is common to many sources and destinations. The use of the header may be decided by anything from simple allocation to preemption.
5.6.2 Arbitration
Arbitration algorithms are used to resolve allocations of a single resource to multiple requests. A simple solution is a queue of the first-in, first-out variety. Each request in the queue represents a delayed allocation.
A reservation system helps to reduce arbitration by allocating requests to time slots. A hotel reservation system allocates people to rooms on certain dates. This usually works very well, as long as the schedule stays on track. A priority scheme is required when there are more requests than there are open reservation slots. Perhaps the system already has a priority scheme. Either way, when priority is exercised the cur- rent occupant of the resource or a reservation slot must be preempted. Then something must be done with the preemptee. A falling-domino effect may ensue.
Preemption means that a higher priority request for a resource is allowed to stop the service to the current user and start serving the preemptor. A simple example is an exclusive-use common resource that charges solvent to reactors. The charges take a long time. A unit that is in the final stages of the reaction may discover that it needs a comparatively small amount of solvent to complete the reaction. This unit is allowed
to preempt the solvent charge in progress, obtain what it needs, and return control to the charge in progress. Other examples require that the batch in progress be pumped to storage so that the preempting batch may use that unit.