Process model drivers
- Last UpdatedAug 11, 2025
- 7 minute read
When a process unit is modeled using Base + Delta models, the models are usually configured to vary in response to the change in one or more input properties, known as drivers. For example, a hydrotreater model will change its results in response to the sulfur content of the input feed, therefore sulfur is typically a driver for hydrotreater models.
The Drivers page (in the Model Structure tab of the Process Units page) shows the drivers configured in the Base + Delta model associated with the selected process unit. To modify the drivers, you need to edit the associated model.
Note: If a process model has been imported from a password-protected file, model details are unavailable.
Drivers can only be edited if the process model is editable. Adding a new driver adds a new driver column to the Base + Delta model, which must have a base and delta value entered for every property prediction of every base in the model.
When adding a new driver, you must ensure that every unit feeding this process model predicts the driver property. Otherwise the model will not be able to function and an error message will be displayed.
You can disable a driver by clearing the check box in the Active column for the corresponding row. When you disable a driver in the Drivers page, the corresponding column in the tables under Base Delta Structure is disabled as well. If the process model has multiple bases, you cannot disable the driver used as the interpolation variable.
Driver types
Process unit models support different driver types which are used to support different ways of affecting the model results. The driver types are:
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Property: a property of an input stream such as the density of the feed.
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Operating Parameter: process unit parameter controlling the unit performance such as FCC riser temperature.
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Feed Flow: the flow rate of the material being fed to the process unit.
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Cross Term: a parameter derived from by combining two or more existing drivers together through multiplication or an exponential function.
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Equation: an expression linking several other drivers together using a mathematical formula.
Property drivers
Property drivers cause changes in the Base + Delta model's output based on changes in an input stream property.
For each set of drivers you add for input streams, you can add corresponding predictions for the output streams. See the table below for details.
You can add drivers for multiple properties without closing the Property Editor. Click Add to add a set of drivers (and optionally predictions), then change the property to add another set. When you are done, click Close to leave the Property Editor.
|
Item |
Description |
|---|---|
|
Inputs |
The streams for which the property should be calculated. Select the check box next to each stream name to calculate the property for the stream. When a driver for the corresponding stream and property already exists, Exists appears in the Status column. |
|
Outputs |
The outputs for which a corresponding prediction should be created. Click the check box next to the table to enable this feature. Then click the check box next to each output name for which you want to create a prediction. If a prediction for the corresponding property and stream already exists, Exists appears in the Status column. |
|
Property |
The property which is used as a driver. |
|
UoM |
The unit of measure when used as a driver. |
|
Blend Rule |
If one or more blend rules are defined for the selected property in the Blend Rule page, you can select the option button and choose the blend rule you want to use. (See: Drivers using Blend Rules.) |
|
Property x Yield |
Enabled only if you have chosen to create matching predictions. Select the check box if you want the predicted property to be adjusted relative to the yield of the stream containing that property. (See: Property × Yield Predictions.) Select the Auto Calculation check box if you want to enter the property value in the Base Delta Structure table, instead of the value multiplied by the yield. AVEVA Unified Supply Chain will perform this calculation automatically when simulating or optimizing the case. |
Operating parameter drivers
Operating parameter drivers cause changes in the Base + Delta model's output, based on changes in an externally controlled operating parameter. These parameters may be optimized, but are also often set to particular fixed values depending on the flexibility of the process unit.
Warning: Before you can add an operating parameter driver you must ensure that the operating parameter is already associated with the process model. For process models published from Process Model Manager, the operating parameter must be set as a driver in the model and will automatically be added as an operating parameter and driver to the model. For process models created in Plan, the operating parameter must be manually added to the model first on the Operating Parameters tab.
Feed flow drivers
Feed flow rate drivers cause changes in the Base + Delta model's output based on changes in the flow rate to the process unit.
Cross term drivers
Base Delta property outputs are calculated using the following formula:
Result = Property Base + Offset + ∑ (Driver Value - Driver Base)/Driver Step * Driver Delta
Each driver is evaluated separately, and its relative step multiplied by its delta, and each cumulatively added to the base property value.
|
Driver |
Actual values |
|||
|---|---|---|---|---|
|
Density (g/cc) |
0.5 |
|||
|
Driver Base |
0 |
|||
|
Driver Step |
1 |
|||
|
Property |
Property Base |
|||
|
Yield (%) |
0 |
100 |
=0+(0.5-0)/1*100 =50 |
When a driver is a cross term, the driver is evaluated using the following formula:
Cross Term Component = (((Cross Term Driver Value - Cross Term Driver Base)*(Cross Term Driver Value - Cross Term Driver Base))/Cross Term Driver Step) * Cross Term Driver Delta
That is, for each driver component of the cross term, the individual component driver delta is calculated, and these deltas are combined using the formula defined for the cross term. This is then made relative by dividing by the cross term driver step value. This relative value is then multiplied by the cross term delta. This cross term value is then cumulatively added to any other drivers and the base property offset.
|
Cross term driver |
Actual values |
|||
|---|---|---|---|---|
|
Density (g/cc) * Density (g/cc) |
Density 0.5 |
|||
|
Cross Term Driver Base |
0 |
|||
|
Cross Term Driver Step |
1 |
|||
|
Property |
Property Base |
|||
|
Yield (%) |
0 |
100 |
=0+(((0.5-0)*(0.5-0))/1)*100 =25 |
A cross term may feature the same driver two or more times, or may feature different drivers. When a cross term driver features different driver components these are still evaluated to find their difference relative to the base value for the same base value for the cross term.
Example: If the cross term has two components, density and reactor temperature, and the base value for the cross term is 0, then the difference in density (0.5-0) is multiplied by the difference in reactor temperature (300-0) before being divided by the cross term step and then multiplied by the cross term delta.
|
Cross term driver |
Actual values |
|||
|---|---|---|---|---|
|
Density (g/cc) * Reactor Temperature (°C) |
Density 0.5 Reactor Temperature 300 |
|||
|
Cross Term Driver Base |
0 |
|||
|
Cross Term Driver Step |
1 |
|||
|
Property |
Property Base |
|||
|
Yield (%) |
0 |
0.1 |
=0+(((0.5-0)*(300-0))/1)*0.1 =15 |
In the Cross Term Driver Editor you can choose up to three drivers as components.
Equation drivers
Equation drivers cause changes in the Base + Delta model's output based on changes in other drivers. The values of these other drivers are combined using a mathematical expression, and the change in the result of this expression is used to drive the model.
To add an equation driver:
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Select Add > Equation from the Home ribbon tab.
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Enter a name for the equation in the Equation Driver Editor.
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Click the Add Variable button
to add a new variable.
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If necessary click in the Driver column to select the existing driver to add; edit the Symbol name if necessary.
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Write the mathematical expression linking the drivers. (See: Writing Formulae.)
Drivers using blend rules
Along with predictions, drivers can also be used in their index space, rather than in the original unit of measurement. This means that the driver base, step and delta are entered in an indexed manner, and not real unit of measure.
Example: If the pour point index was used as a driver, then the base would be expressed in pour point index (not temperature), and the delta would also be expressed in pour point index.
Indexed property drivers (and associated index property predictions) are useful for pass-through properties; that is, properties which do not change within the process unit.
Example: Pour point is often passed through a distillate hydrotreater. The pour point of the hydrotreated product is equal to the pour point of the feed to the unit.