using calc levels

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Using Calc Levels 1

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Using Calc Levels

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ADV 10.pdf


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IntroductionCalc 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 ObjectivesIn 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 itdoes not.

PrerequisitesBefore beginning this module, you should have a solid understanding

of the HYSYS program, and its interface.


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The HYSYS SolverThe 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-sequentialsolver.

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 values 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

traced back to the specified value that has changed. This is known as

the "forget" pass. If the solver is not in Hold mode, the solver will begin

calculations immediately after the "forget" pass is complete, but not

before. Note that HYSYS will only forget those value that are somehow

The steady state Solver and thedynamic Integrator arecompletely separate anddistinct parts of the HYSYSprogram.

The results of the "forget" passcan be seen by putting thesolver in Hold mode. The"forget" pass will stillfunction, but the solver willnot solve.


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related to the value that was changed. It is often difficult for users to seethis 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.


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Calc Levels and SubflowsheetsIn 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 ownsolver that is

responsible for solving allstreams 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 havebeen 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 onlyrun 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 solvers 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 interactionsbetween the multiple recycle operations.


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The Default Calc LevelsAs 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, theComponent Splitter and theShortcut Distillation columnare considered ordinary unitoperations and have a calclevel of 500.


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Information SummaryThe 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 HYSYSprogram.

Is a non-sequential solver, information can flow forward andbackward through the flowsheet.

Uses a Degrees of Freedom analysis to trigger solving of unitoperations and streams.

Tracks all numerical values in HYSYS according to theirsource.

Runs after the "forget" pass that erases any numerical valuethat can be traced back to the changed value.

Ranks all streams and unit operations according to their calclevel. The items with the lowest calc level will be solved first, ifpossible.

Calc Levels and Subflowsheets

The operation of calc levels in subflowsheets has changed inthe newest versions of HYSYS (2.1 and up). They are now

considered local and operate in a manner identical to theiroperation 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 themain HYSYS solver. It is responsible for solving all Recyclesand Adjusts that are specified as simultaneous.

The Simultaneous Solver will only solve after allother streams,operations, and subflowsheets have been solved.

The calc levels of simultaneous recycles and adjusts are notconsidered by the simultaneous solver. They are all solved

simultaneously.


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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 solvers list; i.e. whichitem has the highest default calc level? ___________

If this entire simulation is unsolved, which item will besolved first? Hints: Only one item has DOF = 0. This itemmust be solved before any other stream or operation cansolve. ______________

If the solver is in Hold mode, and the pressure drop acrossthe valve is changed, what six items will be affected?_______________, _______________, __________________,_______________, _______________, and ______________.

What will happen if the Adjust operation is assigned a calclevel of 400? ______________________________________

________________________________________________.Would there be any advantage to setting the calc level forstream 7 at a level greater than 3500, yes or no ? __________

If yes, what is the advantage? ___________


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Additional Exercise

Main PFD


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Subflowsheet #1

Subflowsheet #2

Subflowsheet #3


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Examine the flowsheets on the previous two pages and answer thefollowing questions.

If the recycles are set to simultaneous, how many separateand 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 calculatesimultaneously. Will the column (T-100) be solved beforethe recycles will start their calculation? ___________

If the two recycles were set to Nested rather thansimultaneous, would the ADJ-1 in the third subflowsheetbe solved before the recycles began calculation? __________

Is there any way that the calc levels can be manipulated inorder 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 manipulatethe feed flowrate in order to produce a certain liquidvolume flowrate in stream 20, yes or no? ____________

What will happen? __________________________________


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Optional Exercise

Open the HYSYS caseADV Flowsheet.hscfound 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

Feedstream to result in a Liq. Vol. Flow of 150m3/h(22,500 bbl/day) in

the columns liquid product stream( #20). Attempt to solve theflowsheet.

Can you decrease convergence time by manipulating thecalc levels? ______________

What happens when this flowsheet tries to solve? _________

What can be done to prevent and correct this behaviour?________________


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Answer Key

First Exercise, Page 8

Which item will be ranked last in the solvers 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 solvers 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 anyother stream or operation can solve. ______________

The only item with enough user specified information to solve isthe Stream #1. All other streams or operations requireinformation 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, Stream6, 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 thesolvers 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 itwill solve after all other operations and streams. This meansthat the adjust will have completed its calculations. If thisstream were connected to a large column, then the columnwould only have to solve once rather than several times. If thiswere the case, the advantage would be much greater.


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Additional Exercise, page 11If 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) (donot forget that the column is a separate subflowsheet), and thesimultaneous 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 besolved 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 abovewill change. All Simultaneous adjusts and recycles must besolved after all operations and streams in all other solvers havebeen 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 andstreams in all other solvers must be solved before thesimultaneous 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? __________

Lets look at the calc levels for these operations, the recyclescalc level is 3500 and the sub flowsheets calc level is 2500.Therefore, the solver for the subflowsheet will be activatedbefore the recycles will start calculating. In fact, if possible, all

streams and operations in subflowsheets will be solved beforethe solver will move onto the next level. Therefore, yes; theADJ-1 operation in the third subflowsheet will be solved beforethe recycles will start calculating.


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Is there any way that the calc levels can be manipulated in order tospeed up the solving of the simulation, yes or no? _________

How? ____________________________________

No; changing the calc levels will have little, or no, effect on thespeed 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 flowsheetwill cause a severe error in the simulation. The adjust will

change the flowrate of the feed stream, but since the Recycleoperation is not solving yet, the flowrate of stream Rec 2 Outwill not change. If this stream does not change, it is impossiblefor the flowrate of stream 20 to change; so the adjust operationwill again change the flowrate of feed, but again the flowrate ofstream 20 will not change. This problem can be correctedeither by setting the adjust to simultaneous, or removing thesimultaneous setting from the recycle operation.

using calc levels

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