Solver Set up for Dynamic Projects 9.3
|Command Path||Project - Solver Setup|
This page is for SysCAD 9.3 Build 136 or earlier. For the latest version, please see Solver Set up for Dynamic Projects
- This command will display the following for a Dynamic project.
- SysCAD must be stopped for this command to be accessed.
This command will allow the user to change the dynamic simulation options, such as solving method, step sizes and so on prior to running the SysCAD dynamic simulation. These are done through the Dynamic dialog box. When the settings have been changed, press the OK button to save the new settings and close the dialog box.
A number of options are available for controlling how long a dynamic simulation must run or solve for:
- 'forever' where the solver never stops;
- 'to steady state' where solver runs until it detects there are no more changes in the process variables;
- 'for' a specified time span;
- 'until' a specified end time;
- 'for x steps' runs for a specified number of steps.
The current simulation date and time can be specified, unless "synchronise with clock" is selected, in which case the simulation date and time is the current real date and time.
There are three ways a dynamic model can run in terms of time management:
- As fast as possible: "Synchronise with clock" and "RealTime" must not be selected. SysCAD will start each time step immediately after the current time step is complete. The speed of the simulation will depend on computer processing power as well as size and complexity of the model.
- Real Time: "Synchronise with clock" and "RealTime" are both selected. "RealTime Multiplier" is 1. The purpose of this mode is to run at real time synchronised with your PC date and time. After a time step is completed, SysCAD solver will idle until the progress of real time is the same as the step size, at which time the next time step will start. This option is typically used for control system testing and operator training. It is important that the computers actual solver time for a step is less than the selected time step.
- Accelerated Time: "Synchronise with clock" is NOT selected and "RealTime" is selected. The user can specify a "RealTime Multiplier", for example if this is 2 SysCAD will run twice as fast as real time. This selection gives a consistent progress of time steps and as described for the above Real Time option, SysCAD will idle at the end of a time step to maintain the required multiplier. Again, care should be taken in selecting a suitable time step so that the desired real time multiplier can be achieved for a given project and PC.
SysCAD solves the dynamics of a flowsheet in incremental time steps called Step Size. The step size determines how long a time period is being simulated for when the flowsheet is being solved. A single step can have multiple iterations to solve the flowsheet and associated controls, similar to solving a steady state project. The maximum number of iterations per step is set on the Tears tab . Updates of number displays in trend windows and so on can be refreshed (or stored in historian) at the end of a step. The Steps per Update determines how many steps will be solved before the number displays are updated. Generally the number of Steps per Update is set to 1 so that the number displays are updated at the end of each step.
The method used for the dynamic simulation can be changed here. The methods available are Euler, Runge-Kutta2 and Runge-Kutta4.
The step size can be fixed or variable, depending on the integration method used. Only the Runge-Kutta4 allows variable step sizes.
- Fixed step size
- The step size is fixed. The higher value results in faster relative speed, but generally a less accurate solution. The maximum meaningful fixed step size that can be used is dependent on the dynamics (relative flow rates, volumes of units, timing of events, etc) of the process.
- Variable step size
- Only available for Runge-Kutta4 method. With a variable step size the user defines the minimum and maximum step size. SysCAD uses a step size between these limits and aims to use the largest possible. The step size used is dependent on the dynamics response of the process. If changes are occurring rapidly or a significant change occurs (valve opened, tanks overflows) when SysCAD is solving the model, the step size is reduced until conditions have stabilised.
- Once conditions have stabilised or the discontinuity has been passed, the step size is increased again.
- The Relative and Absolute Tolerances can be set if the Runge-Kutta4 method is used. The smaller the tolerance the more accurate the result, however, this will also slow down SysCAD considerably. Refer to Convergence in ProBal Setup section above.
This tab page contains the solution network settings. These can be used to improve the SysCAD speed in obtaining a solution.
- Rel Tolerance - The relative tolerance required for the solution. If this value is increased, then the model will solve faster, but will be less accurate.
- Abs Tolerance - The absolute tolerance required for the solution. If this value is increased, then the model will solve faster, but will be less accurate.
The DeRating parameter is used for convergence of the network.
For a dynamic simulation, if a tank overflows, it needs to go to an area. By default it will go to the Global Area, but if the model has other areas defined then the user may specify a different area where spillages will report.
- A tear is required to solve models that contain one or more recycle streams.
- SysCAD determines the optimum position for the tears using a modified version of the algorithm described in the following paper:
- Ollero P. and Amselem C., "Decomposition Algorithm for Chemical Process Simulation", Chem. Eng. Res. Des., 61, 303, (Sept. 1983)
- SysCAD determines the position of the tears as part of the startup sequence each time the model is solved. It is possible that tears become temporarily "inactive" during the running of a dynamic model because of, by example, the closing of valves.
- Ideally a tear (recycle) needs to be converged each time step. If tear is not converged within specified maximum iterations than the error is remembered and used as a correction in following time steps.
- All these settings can also be changed on the GlobalTear Access window page for $Solver. These global settings can be overwritten for individual tears.
- Max Iterations - The maximum number of iterations that SysCAD will perform in each time step in order to iteratively solve recycles (ie converge tears).
- Rel Tolerance - The relative tolerance used in recycle tears.
- Abs Tolerance - The absolute tolerance used in recycle tears.
The user may choose from different methods for solving Tears. See Convergence Methods for a description of these different methods, Damp as Group and the Damping Factor.
You can Activate or Deactivate flowsheets (Graphics Windows) in the SysCAD Project by selecting the flowsheet and pressing the appropriate buttons. When a Graphics Window is deactivated, all models on that graphics page will become inactive, thus not included in the solution. Any connections with other graphics pages (via the Feeder-Cross Page Connector model) will be disconnected. The user should check all feed and product streams when running a simulation with deactivated graphics to ensure correct information is used.
This is not available if the drawing is not loaded in the Graphics Window.
Date and Time:
The user may enable Set Time at Start and then set the starting time for the simulation.
Startup Reset Actions:
|Process||This will empty all pipes and flow values when the run begins. Default is off.|
|Control||This will reset all controllers back to 'Base' state, i.e. the same state as when the controller is first inserted into the project. This is mainly relevant for PID controllers. Default is off.|
|Statistics||Selecting this will Reset the Statistical blocks when scenario is started. Default is on.|
|Events/Profiles||This will reset all Event and Profiles back to start time. Default is on.|
|Empty||This Empties all containers with surge, e.g. Tanks. Default is off.|
|Preset||This resets all tanks that have the 'Preset.On' button ticked and have Preset data. Default is off.|
- Restart Historian at Start: This option clears any old data in the historian at the start of the simulation.