What's New in VMGSim 10.0 Dynamics
This article summarizes the major new features in the dynamics engine. Some of these features have been trickling out in VMGSim 9.5 patches, while others are brand new to this release.
This is a major feature in VMGSim 10.0. A separate newsletter article with more detail on this feature will appear in a future issue.
VMGSim Dynamics is the first chemical process simulator to utilize multirate techniques, and this feature reinforces our commitment to being an industry innovator, and to delivering best-in-class functionality.
Multirate integration is the ability to integrate different parts of a model with different step sizes, and to adjust those step sizes based on convergence and error magnitude. This has two big advantages: it allows focusing computational effort where it is needed, and it allows compute-intensive convergence strategies to be used selectively rather than on the entire model.
Multirate integration works with both fixed step size, and with variable step (error control).
For users who would like more background on multirate integration in general, the home page of Dr. Valeriu Savcenco is a good starting point.
2nd Order BDF Integration
Implicit Euler has been the method of choice for integrating the set of differential and algebraic equations in the simulator. The advantages of the method are simplicity and robustness. It is however, a first order (linear) method, and for cases where high accuracy is required, it may results in small time steps being needed. Implicit Euler is part of a family of methods called Backward Difference Formulas (BDF). The next member of the family is BDF2 which uses a quadratic function instead of a linear function. For cases where the trajectories are smooth, BDF2 will in general be able to use larger step sizes to achieve comparable accuracy. We now offer the option to use BDF2 to integrate the equations in the pressure-flow solver.
Accelerate to Steady State / Stop When Steady State Reached
"Run To Steady State" offers the ability to run a model and have the integrator stop when the model has lined out (steady state has been achieved).
Reaching a lined out condition can take a long time - particularly because the closer you get, the slower the rate of change becomes. "Approach Steady State" accelerates reaching a lined out condition by automatically increasing the integrator step size for holdups where this can be achieved without introducing oscillation or instability.
Thermodynamic property calls are often the largest percentage of total CPU time, particularly when there is a large component slate. We have developed a set of sophisticated shortcut methods that reduce compute time significantly when calculating primary properties such as density, enthalpy, and fugacity, and their derivatives with respect to temperature and pressure. Transport properties like viscosities and thermal conductivities can become a major bottleneck in pipe calculations and heat loss calculations, so we have also developed shortcut models for prediction of viscosities and thermal conductivities.
Clipped Transient dP in Pipes
Calculating the dP associated with fluid acceleration makes pipe modeling more accurate, and can also stabilize the calculation of flowrates when frictional resistance is very low. However, there are two constraints to applying these calculations successfully:
- Getting an accurate estimate of the acceleration term with multiphase flow during a transient
- Shock waves caused by bubble collapse (similar to pump cavitation)
We have improved our calculation techniques sufficiently to where the first item is rarely a problem. The second item will always be a problem, since it is caused by a physical phenomenon. It can be addressed by sufficient discretization in space and time, but, unless there is a specific need to track such phenomena, a better approach is to filter out these disturbances, since they do not impact the macro behavior of the system. Hence, we now offer an option to calculate the acceleration dP, but with a filter to mitigate the pressure spikes associated with bubble collapse or any other fast transients such as a valve slamming shut.
Pipe Rupture / Blowdown
We have made a number of improvements to the pipe unit operation to facilitate accurate simulation of pipe rupture / blowdown events. More information on this topic can be found here.
Flow in Pipe Annulus / Concentric Flow
We now have the ability to associate two pipe segments with each other, specifying that one is contained inside the other. The solution methodology accounts for this, allowing for countercurrent flow, and possibly computing mechanistic model flow regimes for flow in an annulus. The heat transfer calculation for the inner pipe recognizes that the outer surface is another fluid and correctly calculates to heat flux between fluids.
Import GIS (latitude/longitude data) for long pipelines
VMGSim 9.5 allows the user to import pipeline survey data, and then to create a simpler, but approximately equivalent model in the simulator. The input format required information as to segment length and elevation.
VMGSim 10.0 allows to user to specify latitude/longitude co-ordinates instead of segment length, and computes the total bend angle from both direction change and elevation change.
Uneven Phase Split in Tee Junctions
When a fluid splits between the through-run and the branch in a tee, the phase distribution is not always symmetric - it depends on the tee angle and orientation, and the overall velocity in the branches. VMG Dynamics now offers the option to model this asymmetrical behavior. More information on this topic can be found here.
ISO 4126 Part 10 (Diener Schmidt) for Non Equilibrium Flow in Valves and Relief Valves
Dynamics now supports ISO 4126 part 10 (the Diener Schmidt method) for calculating flashing flow in relief valves in the dynamics engine (available in build 9.5.60 and later). The calculation method for relief valves now has three options:
Diener Schmidt can be used for all venting conditions, and will produce results that match HEM when the vapor fraction change across the valve is small. It will be a closer match to reality under flashing conditions. Below is a plot showing measured data for steam/water at 6.8 bar and various qualities, as well as calculated curves using HDI/HEM and HDNI (Diener Schmidt). I have overlaid points extracted from the attached simulation case showing predictions from VMGSim. Magenta points are HEM, and green points are Diener Schmidt.
The Diener Schmidt method is also applicable to valves and orifice plates, and we have implemented it for those unit ops as well, using the recommendations from the Diener Schmidt paper. The associated options are “Boiling Delay Model” and “Boiling Delay Exponent”. There is currently no ISO/API/ISA standard associated with its usage for valves/orifice plates.
Column Builder Assistant
Building, sizing, initializing, and finally running a distillation column in dynamics can be very challenging. Even if all of the sizes are known, it can be tedious to assemble and configure all of the unit ops, and doing so in such a way that the column will run stably is even more difficult. To help simplify this process, there is a new Column Builder feature that will create a dynamic column model from an existing steady-state column.
The column builder scope goes beyond the basic condenser/tray/reboiler model in competitor products, and includes separators, valves, pumps, and a basic control layer. In addition, automatic sizing is performed, such that the column will run in a stable manner.
Separator Internals (Buckets and Weirs)
In previous versions of VMGSim Dynamics, all of the historian data was stored in memory. This has the advantage that it optimizes speed when rendering the strip charts, but it becomes problematic for large models, since it competes for memory with the model itself.
VMGSim 10.0 offers the option to store historian data to a SQL database, and keep only a more limited current window of data in memory.
An additional benefit to this approach is that when the simulation case has been saved, the database can be extracted and then processed or rendered by other tools.
Mark Beyleveld, B.Sc.(Eng), B.Com.
Please contact your local VMG office for more information.