Constraints and alerts

The Constraints and Alerts page summarizes the applied constraints from individual plants and periods, as well as allowing new constraints to be added to the combined model.

Tip: The grid supports sorting and grouping of items as explained in Advanced grids.

Constraints can be of several types, shown in the Type column:

• Blend Recipe: Constraints on the amount of particular components in final product blends (see: Blending).

• Calculations: Constraints defined on manually created calculations. Calculations are based on formulae shown in the Formula column.

• Global Parameters: Constraints on global parameters.

• Operating Parameters: Constraints on operating parameters for particular process units.

• Stream Routing: Constraints on split ratios from splitters and manifolds in the flowsheet.

You can deactivate a constraint by clearing the check box in the Active column. Results for inactive constraints are still returned, but the constraint is not applied during optimization.

The Model (Case) column shows the model and case from which the calculation or global parameter was imported. The Constraints and Alerts page lists the calculations imported from all the included plant cases, which can be seen in the Details page. This column is empty if a calculation or global parameter has not been imported from a plant case.

Edits to a calculation or global parameter imported from a plant case will show in the Constraints and Alerts page of the plant case and vice versa.

The Optimization column band shows the solution value for the calculation or global parameter, and the marginal value if constrained. Marginal values are only returned if selected in the Run Settings dialog.

Infeasibility breakers

Although constraints will try to be met during optimization, if your constraints result in the overall problem being infeasible, you can invoke infeasibility breakers and break one or more constraints to make the problem feasible.

For this to happen, infeasibility breakers must be enabled for the optimization (see: Infeasibility Breaker Settings), and you must select the check box in the Infeasibility Breakers column for each constraint you want to break.

If a constraint cannot be met during optimization, and infeasibility breakers are disabled for that constraint, the optimization will fail.

Show and hide rows

The Constraints & Alerts group of the Home ribbon tab allows you to hide and show rows in the table:

• Click the Show Constraints button to open a menu listing the types of constraints. A tick next to a menu entry means that the rows of that type are shown.

• Click the Show Unconstrained button to show or hide rows which do not have entered constraint or alert limits. These rows are shown when the button is highlighted.

• Click the Show Zero Solution button to show or hide rows with a Solution value of zero. These rows are shown when the button is highlighted.

  Warning: When entering new items to the table, take care with the Show Unconstrained option. If this option is not active, and you add a new row without entering constraint limits, the new row will not be visible.

Lock constraints

Solution values can be locked during subsequent runs. This means that a solution value will not change in any subsequent run, even if other input to the run is changed.

Example: The flow of naphtha from a crude could be locked, and in subsequent runs it would not be able to change. Therefore, if a fixed swing cut point was changed and the model re-run, the crude blend would have to change in order to match the fixed flow rate.

Locking a solution value acts in a similar way to fixing the value. The advantage over fixing a value is that the lock can be clearly seen in the user interface, so it is simpler to distinguish true fixed values from those which are temporarily locked.

To lock one or more solution values, select the corresponding rows in the table and do either of the following:

• Click Lock Solution in the Behaviour group of the Home ribbon tab.

• Right-click on the table and choose Locking > Lock from the context menu.

To unlock one or more solution values, select them and either click Unlock Solution in the ribbon or choose Locking > Unlock from the context menu.

Order constraints

To move one or more contiguous rows in the Constraints and Alerts grid, select the rows and click Move Up or Move Down in the Ordering group of the Home ribbon tab.

Alternatively, select the rows, right click and choose Ordering > Move Up or Ordering > Move Down from the context menu.

There are limits to the movement of rows. You cannot mix rows of different types: for example, all rows of type Calculation and Operating Parameters must be in contiguous ranges.

Show differences with the master case

Whenever a value in the row is different from the value in the parent case within the model, the unnamed column next to the Active column shows a double arrow icon () . Note that this column may not be present in all views.

The Deltas group of the Home ribbon tab lets you further manage these differences:

• Click the Show button to display a blue triangle icon in the top right corner of every cell whose value overrides the value in the parent case. This allows you to pinpoint the specific cells with overridden values, rather than whole rows.

  • A dark blue triangle means that the value is different from the parent case value.

  • A light blue triangle means that the value is the same as in the parent case, but has been entered manually, so it is still treated as an override. Future changes in the parent case value will not be reflected in the subcase value.

    

• Click Restore to change the values of the selected cells back to those of the parent case. You can also right click and choose Restore from the context menu.

  Tip: To show all the differences between cases within a model, see Compare Cases.
  To merge differences across cases, see Update Cases.

Grid

Customize the grid from the Grid group of the Home ribbon tab:

• Click Find to show or hide a text field at the top of the grid to filter the grid contents. Type in the text field to show only the rows with text matching what you have typed. The matching text is highlighted in the grid. See Filters for more details.

• Click Columns to open a small dialog windows where you can drag column headers to remove columns from the grid. You can then drag the headers back to the grid to restore the columns. See Add and Remove Columns in Manage Columns for more details.

• Choose a View from the list, or click the cogwheel icon to remove a view or save any changes. See Views for more details.

Periods

Click the Periods button in the Periods group of the Home ribbon tab to expands each row of the grid to show per-period optimization results.