Solver Restrictions on Buffer Tanks

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

The Solver supports the following cases:

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

Solving buffer tanks is more complicated when the tank batch is allocated to a resource group containing more than one resource. Resources may 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; a resources minimum weight and minimum volume are considered.

The Solver supports the following cases:

Tank batches with single and/or multiple inflow and/or outflow where the tank batch is allocated to a choice group with multiple resources:

that all have infinite capacity (max weight set to 0 kg and max volume set to 0 liters).
where each resource has higher capacity than the total batch quantity.
that have the same capacity.

If a tank batch is allocated to a choice group or resources with different capacities, the Solver will use the maximum capacity of the most restrictive resource, the resource with the smallest max weight and max volume. This ensures valid positioning of inflow and outflow on any resource in the choice group.

However, it may discard a valid solution that would only be available on resources with larger capacity. To bypass this, use one of the following:

Change the model so all resources have the same capacity or infinite capacity.
Allocate the tank batch manually before solving and lock it 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 of the above 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.