Solver restrictions on buffer tanks

The solver can schedule buffer tanks and the batches connected to the buffer tank, in a limited number of cases. A buffer tank is a tank batch with a method of Flow Buffer or Batch Buffer.

These are the scenarios that Solver support:

• Tank batch with single inflow and single outflow wherein the tank batch is allocated to a group containing only one resource. Inflow and outflow can be a part of a Choice group or combination group with one or more resources.
• Tank batch with multiple inflow and/or multiple outflow wherein the tank batch is allocated to a group containing only one resource. Inflow and outflow can be a part of Choice group or combination group with one or more resources.

Solving buffer tank is more complicated when the tank batch is allocated to a resource group containing more than one resource. Resources can have different capacities, thus allowing smaller or larger fill levels. As a result, the position of inflow and outflow may be restricted differently for different resources. The solver only considers maximum capacities; minimum weight and minimum volume of a resource are considered.

These are the scenarios that the Solver supports:

• Tank batches with single and or multiple inflow and or outflow wherein the tank batch is allocated to a choice group with multiple resources in these scenarios.
  • All resources have infinite capacity (max weight set to 0 kg and max volume set to 0 liters).
  • Each resource has higher capacity than the total batch quantity.
  • Resources have the same capacity.

If a tank batch is allocated to a choice group or resources with different capacities, the Solver uses the maximum capacity of the most restrictive resource (the resource with the smallest maximum weight and maximum volume). This ensures valid positioning of inflow and outflow on a resource in the Choice group. However, it can discard a valid solution that is only available on resources with larger capacity. Alternatively, use one of the following methods:

• Change the model for all the resources to have the same capacity or infinite capacity.
• Allocate the tank batch manually before solving and lock the tank batch to the resource using status attributes.

Miscellaneous restrictions:

The Solver can handle buffer tanks with surplus or shortage as long as the quantity defects do not exceed the resources capacity.

To schedule a buffer tank with the Solver, the selection should include the buffer tank batch, all process batches flowing into the buffer tank and all process batches to which the buffer tank flows out. A sub-selection may result in unexpected behavior such as underflow, overflow, batches placed too early or too late, or not finding a solution even though one does exist.

Buffer tanks are hard to schedule. The solver may not be able to cope with buffers combined with complex resource availability, changeovers with idle times and claim. Often it is more practical to model a process step alternatively.