Overrides

The Overrides page allows you to make adjustments to your calculated results. Offsets are used to adjust the calculated property values, for example by adding a constant value to them. Overrides can be used to replace a calculated value with another value.

Tip: Avoid overrides where possible. Where property values in Plan do not match those expected, correct the underlying reason for the difference rather than using a property override. Differences between calculated and expected values are commonly caused by the incorrect setting of units of measure, mistakenly entered values, or errors in predictions by Base + Delta models. For example, do not use an override to add a stream property that is not predicted by a Base + Delta model. Edit the original model instead.
For differences between Base + Delta model predictions and expected values, it is possible to use offsets to adjust the calculated model value.

Add overrides

To add an override:

1. Right-click the Overrides table and select Add Override.

2. Select the unit where the override should be applied from the Asset Type and Asset menus.

3. Select the output stream from the unit which the override should be applied to using the Pipe menu.

4. Select the property that should be overridden using either the Modelled properties or the All properties menu.

5. Ensure the Active check box is marked to use the override.

6. Select the Variables to use in the override in the Formula tab.

7. Define the Formula that is used to calculate the override. The formula uses the symbols to refer to the input variables, and can use any standard mathematical operators in its calculation (See: Writing Formulae).

Edit overrides

The Overrides Editor is used to define the formula for the offset or override. The editor has several menus used to select the place where the offset will be applied:

• Asset Type: the type of unit in the flowsheet where the override will be applied. Choose a value to change the available entries in the Asset menu below

• Asset: the unit in the flowsheet where the override will be applied

• Pipe: the stream in the process unit to which the override is applied

• Modelled properties and All properties: the property of the stream in the flowsheet that has the override applied to.

The Formula section is used to select the variables used in the offset definition. The offset is calculated using a mathematical formula containing variables representing the properties of other streams in the flowsheet. Click the icons in the Formula tab to add, copy, edit and remove variables.

When used in the formula, the offset variables are referred to using Symbols. These are short strings that are used in the formula rather than the full property name. For example, if the formula used the total sulfur content you may refer to this by the symbol SUL in the formula, rather that writing the full property name each time. Symbols are case-sensitive and cannot contain any spaces, but may contain special characters such as !, _ or @.

Add multiple variables

Each of the variable editors described in the following section has an Add multiple variables check box. Select this check box if you want to add more than one variable without having to close and reopen the dialog window each time.

Variable types

The following variable types are available:

• Constant: A fixed numerical value. Useful if you have to use the same value in different places, or to make the meaning of the value clear to other users, since you can associate a name and a description to it.

• Flow: Equals the flow through a Pipe originating from a particular Asset, in a specific Mode or in all modes. To select the asset, choose the correct Asset Type first. You can choose among three flow types: the Total Flow of the pipe, the flow of a Feedstock or the flow of a Traded Material associated with the selected stream.

  Warning: Flow overrides are not supported in optimization. Flow overrides will be evaluated when running models in Simulate mode, but the flow is not overridden during optimization. Flow overrides should be avoided and, if used, deactivated during optimization.
  When using flow overrides you may see errors after optimization, as the override was not used during the actual optimization process, but during . Therefore the optimization solution does not include the override, but the values displayed in the user interface and reports do, and are therefore inconsistent.

• Property: Equals the value of the specified Property of a stream flowing in a given Pipe originating from an Asset, either in a particular Mode or in all modes. To select the asset, choose the correct Asset Type first.

• Operating Parameter: Equals the value of an Operating Parameter associated with a Mode of a Process Unit in a particular Model.

• Inventory Amount: Equals the Opening or Final amount of a particular Inventory.

• Inventory Property: Equals the value of a Property in either the Opening or Final stage of a particular Inventory.