Introduction
Calc levels are used to determine the order in which streams and unit operations are solved in HYSYS. Normally, it is not necessary for the average user to work with these numbers as HYSYS automatically sets calc levels that are appropriate for most applications.
However, there are certain situations where the calculation time can be reduced by specifying the order in which you want HYSYS to solve the flowsheet.
Before anyone can understand how calc levels function in HYSYS, they must understand how HYSYS solves a flowsheet. For this reason, this module will deal with both calc levels and the HYSYS solver; in reality, these two topics are inseparable.
Learning Objectives
In this module, you will learn how to adjust the calc levels in a simulation, and you will learn how HYSYS solves a flowsheet. You will also discover when it makes sense to adjust these values and when it does not.
Prerequisites
Before beginning this module, you should have a solid understanding of the HYSYS program, and its interface.
The HYSYS Solver
The HYSYS solver is responsible for all of the calculations that are performed in a Steady State simulation. The most important thing to know about the HYSYS solver is that it is a non-sequential solver. HYSYS is one of the few simulation packages available with this type of solver.
This means that calculations do not have to be performed in flowsheet order. As most users are aware, it is possible to have a solved stream as a product of an unsolved operation. This is only possible with a
non-sequential solver.
Using a non-sequential solver means that information flows both ways through the flowsheet; upstream operations can be used in the solving of preceding streams. Because HYSYS is able to move information forwards and backwards through a simulation less information has to be specified before the simulation is able to solve.
HYSYS uses a degrees of freedom analysis to trigger the solving of a unit operation or stream. Users familiar with the column operations in HYSYS will be able to relate to the degrees of freedom analysis used by the steady state solver.
When a column is entered into the flowsheet, the degrees of freedom are shown, only when the degrees of freedom = 0 is the column able to solve. While the column operations are the only operations in HYSYS that show the degrees of freedom, HYSYS is automatically keeping track of the DOF for each operation and stream in the simulation. Once the DOF = 0 for any operation or stream the solver is activated.
As operations and streams are added to the simulation, HYSYS tracks all specified values and all calculated values. All values are "filed" according to their source; every number in HYSYS knows where it came from. What the user sees as simply a value, HYSYS sees as a value and a source. The value’s source can be either a stream, an operation, or user specified.
If a change is made to a specified value, HYSYS will examine its list of calculated values and will "forget" any calculated values that can be
The steady state Solver and the dynamic Integrator are completely separate and distinct parts of the HYSYS program.
The results of the "forget" pass can be seen by putting the solver in Hold mode. The
related to the value that was changed. It is often difficult for users to see this relationship.
Once the solver is activated, there are several phases to its operation. First, it ranks all streams and operations according to their respective calc levels. The streams/operations with the lowest calc levels are ranked first. Secondly, it checks the DOF for the first item on the list. If a solution is possible, it will be solved and the solver will move unto the next item. If it can not be solved, the solver will remove that item from the current list and move onto the next item.
The solver will continue through the list until all items are solved or removed from the list.
If, after an item has been removed from the list, a change has occurred in that item, it is returned to the list and the solver will again attempt to solve the operation or stream.
A flowsheet that illustrates the process that occurs after the "forget" pass and the ranking of the individual items is given here.
Calc Levels and Subflowsheets
In older versions of HYSYS (2.0 and older) the function of calc levels in subflowsheets was quite different from their role in the newer versions. In older versions, calc levels were treated as "global." That means that it did not matter if a unit operation was in a subflowsheet or in the main PFD, it would be solved with the main solver.
In newer versions of HYSYS (2.1 and up) the calc levels are considered "local." Therefore, each subflowsheet has its own solver that is responsible for solving all streams and operations on that sheet. The default calc level for subflowsheets is 2500, that means that they will solve after all solvable streams and ordinary unit operations have been solved in the flowsheet. Note that simultaneous adjusts and recycles are still moved to the simultaneous solver and solved only after all other operations (in all flowsheets) have been solved.
Calc levels can now be used in subflowsheets in exactly the same manner as they are used in the main flowsheet.
The Simultaneous Solver
The simultaneous solver is another, separate part of the main HYSYS solver. It is responsible for calculation of all Recycle and Adjust operations that are designated as simultaneous. This solver will only run after the main solver has successfully calculated all flowsheets. When a Recycle or Adjust operation is set to calculate simultaneously, it is removed from the calculation list that the main solver uses, and is placed on the list for the simultaneous solver. Once a Recycle or Adjust operation is placed on the simultaneous solver’s list, its calc level becomes insignificant. It will always be calculated after the main flowsheet has been solved, and at the same time as the other Recycles or Adjusts.
When Recycles are set as nested, they are calculated by the main solver as was discussed previously. Setting the Recycles to calculate
simultaneously helps to speed up convergence if there are interactions between the multiple recycle operations.
The Default Calc Levels
As mentioned previously, HYSYS automatically sets the calc levels for operations and streams based on preset defaults. These default values will be given here.
As you can see, the default calc levels for logical operations will mean that these will calculate last.
Item Default Calc Level
Material Streams 500
Energy Streams 500
Ordinary Unit Operations
-Pumps, Heaters, Coolers, Valves, Heat Exchangers, Separators, LNG Exchangers, Reactors, Spreadsheets, Mixers, Tees, etc.
500
Column Operations and Subflowsheets -Distillation Columns, Liquid-Liquid Extractors, Refluxed Absorbers, Reboiled Absorbers, etc.
2500
Logical Operations -Adjusts, Recycles, Sets
3500
For calc level purposes, the Component Splitter and the Shortcut Distillation column are considered ordinary unit operations and have a calc level of 500.
Information Summary
The information presented here is not new; rather this section will serve as a brief summary of the information presented in this module.
The HYSYS Solver:
• Is responsible for all steady state calculations in the HYSYS program.
• Is a non-sequential solver, information can flow forward and backward through the flowsheet.
• Uses a Degrees of Freedom analysis to trigger solving of unit operations and streams.
• Tracks all numerical values in HYSYS according to their source.
• Runs after the "forget" pass that erases any numerical value that can be traced back to the changed value.
• Ranks all streams and unit operations according to their calc level. The items with the lowest calc level will be solved first, if possible.
Calc Levels and Subflowsheets
• The operation of calc levels in subflowsheets has changed in the newest versions of HYSYS (2.1 and up). They are now considered local and operate in a manner identical to their operation in the main flowsheet.
• The default calc level for subflowsheets is 2500.
The Simultaneous Solver
• The Simultaneous Solver is a separate and distinct part of the main HYSYS solver. It is responsible for solving all Recycles and Adjusts that are specified as simultaneous.
• The Simultaneous Solver will only solve after all other streams, operations, and subflowsheets have been solved.
• The calc levels of simultaneous recycles and adjusts are not considered by the simultaneous solver. They are all solved simultaneously.
Exercise
In the flowsheet shown above, all of the user specified information is shown in the tables. The Adjust is used to set the mass flow rate of stream 2, so that the mass flow rate of stream 6 is 1 kg/hr (2 lb/hr). Answer the following questions based on the material presented in this module.
Which item will be ranked last in the solver’s list; i.e. which item has the highest default calc level? ___________ If this entire simulation is unsolved, which item will be solved first? Hints: Only one item has DOF = 0. This item must be solved before any other stream or operation can solve. ______________
If the solver is in Hold mode, and the pressure drop across the valve is changed, what six items will be affected? _______________, _______________, __________________, _______________, _______________, and ______________. What will happen if the Adjust operation is assigned a calc level of 400? ______________________________________ ________________________________________________. Would there be any advantage to setting the calc level for stream 7 at a level greater than 3500, yes or no ? __________ If yes, what is the advantage? ___________
Additional Exercise
Subflowsheet #1
Subflowsheet #2
Examine the flowsheets on the previous two pages and answer the following questions.
If the recycles are set to simultaneous, how many separate and distinct solvers will this simulation have? ___________
Assuming that the flowsheets will solve from left to right, will the ADJ-1 in Subflowsheet #3 (calc level = 3500, non-simultaneous) solve before stream 71 in Subflowsheet #2 (calc level = 500), yes or no? ___________ Why? ______________________________________________ What if the ADJ-1 was set to simultaneous? ____________ Why? ______________________________________________
The two recycles in the main flowsheet are set to calculate simultaneously. Will the column (T-100) be solved before the recycles will start their calculation? ___________
If the two recycles were set to Nested rather than
simultaneous, would the ADJ-1 in the third subflowsheet be solved before the recycles began calculation? __________
Is there any way that the calc levels can be manipulated in order to speed up the solving of the simulation, yes or no? _________ How? ____________________________________
Can you foresee any difficulties in solving if a non-simultaneous adjust operation was added to manipulate the feed flowrate in order to produce a certain liquid volume flowrate in stream 20, yes or no? ____________ What will happen? __________________________________
Optional Exercise
Open the HYSYS case ADV Flowsheet.hsc found on the course disk. Try to decrease convergence time by manipulating the calc levels for various streams and operations.
With both recycle operations set to simultaneous, add a
non-simultaneous adjust operation to manipulate the molar flowrate of the
Feed stream to result in a Liq. Vol. Flow of 150 m3/h (22,500 bbl/day) in
the column’s liquid product stream( #20). Attempt to solve the flowsheet.
Can you decrease convergence time by manipulating the calc levels? ______________
What happens when this flowsheet tries to solve? _________ What can be done to prevent and correct this behaviour? ________________
Answer Key
First Exercise, Page 8
Which item will be ranked last in the solver’s list; i.e. which item has the highest default calc level? ___________
• The Adjust operation (ADJ-1) has the highest default calc level (3500), and will be ranked last in the solver’s list.
If this entire simulation is unsolved, which item will be solved first? Hints: Only one item has DOF = 0. This item must be solved before any other stream or operation can solve. ______________
• The only item with enough user specified information to solve is the Stream #1. All other streams or operations require
information from another source before they are able to solve.
If the solver is in Hold mode, and the pressure drop across the valve is changed, what six items will be affected? _______________,
_______________, __________________, _______________, _______________, and ______________.
• The six affected items are: VLV-100, Stream 5, V-100, Stream 6, Stream 7, and the Adjust (ADJ-1).
What will happen if the Adjust operation is assigned a calc level of 400? ____________________________________________________.
• Essentially nothing, it will be ranked first, instead of last, in the solver’s list, but it cannot solve until stream 6 has been solved. Therefore, nothing will happen to the solving order.
Would there be any advantage to setting the calc level for stream 7 at a level greater than 3500, yes or no ? __________
If yes, what is the advantage? ___________
• Yes, though in this case the advantage is so small it can not be seen. Setting the calc level for stream 7 at 4000 means that it will solve after all other operations and streams. This means that the adjust will have completed its calculations. If this stream were connected to a large column, then the column would only have to solve once rather than several times. If this were the case, the advantage would be much greater.
Additional Exercise, page 11
If the recycles are set to simultaneous, how many separate and distinct solvers will this simulation have? ___________
• This simulation will use six (6) separate and distinct solvers, one for the main flowsheet, one for each subflowsheet (4) (do not forget that the column is a separate subflowsheet), and the simultaneous solver.
Assuming that the flowsheets will solve from left to right, will the ADJ-1 in Subflowsheet #3 (calc level = 3500, non-simultaneous) solve before stream 71 in Subflowsheet #2 (calc level = 500), yes or no? ___________ Why? ______________________________________________
• Yes; once the solver has entered a subflowsheet, it must be solved to the highest possible level before exiting.
What if the ADJ-1 was set to simultaneous? ____________ Why? ______________________________________________
• If the Adjust were set to simultaneous, then the answer above will change. All Simultaneous adjusts and recycles must be solved after all operations and streams in all other solvers have been solved.
The two recycles in the main flowsheet are set to calculate
simultaneously. Will the column (T-100) be solved before the recycles will start their calculation? ___________
• Yes; according to the answer above, all operations and streams in all other solvers must be solved before the simultaneous solver will begin its calculations.
If the two recycles were set to Nested rather than simultaneous, would the ADJ-1 in the third subflowsheet be solved before the recycles began calculation? __________
• Let’s look at the calc levels for these operations, the recycles’ calc level is 3500 and the sub flowsheet’s calc level is 2500. Therefore, the solver for the subflowsheet will be activated before the recycles will start calculating. In fact, if possible, all streams and operations in subflowsheets will be solved before the solver will move onto the next level. Therefore, yes; the ADJ-1 operation in the third subflowsheet will be solved before the recycles will start calculating.
Is there any way that the calc levels can be manipulated in order to speed up the solving of the simulation, yes or no? _________ How? ____________________________________
• No; changing the calc levels will have little, or no, effect on the speed of the solver.
Can you foresee any difficulties in solving if a non-simultaneous adjust operation was added to manipulate the feed flowrate in order to produce a certain liquid volume flowrate in stream 20, yes or no? __________What will happen? __________________________________
• Adding a non-simultaneous adjust operation to the flowsheet will cause a severe error in the simulation. The adjust will change the flowrate of the feed stream, but since the Recycle operation is not solving yet, the flowrate of stream Rec 2 Out will not change. If this stream does not change, it is impossible for the flowrate of stream 20 to change; so the adjust operation will again change the flowrate of feed, but again the flowrate of stream 20 will not change. This problem can be corrected either by setting the adjust to simultaneous, or removing the simultaneous setting from the recycle operation.
