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Modernisation of a small papermill
CopYright © IFAC PRP 4 Automation, Ghent, Bdgium 1980
MODERNISATION OF A SMALL PAPERMILL K. D. Blundell Paperboard Machinery Services, PMS House, Kingston Road, Leatherhead, Surrey, U. K.
Abstract. The paper describes aspects of a typical small papermill reconstruction project going through various phases. It commences at the point of the brief, deals with laying down the principles of evaluating what is needed and then cites some design problems. It deals with equipment parameters for one subsystem and finally gives the authors impression of its effectiveness. KEYWORDS. Project Criteria. Process repeatability. effectiveness.
The object of this paper is to tell about experiences of modernising one papermachine of two complete with its own stock preparation line in a small (9 000 tons per annum) papermiB.
Most important of all was the need to keep the mill profi table, now and in the future. The main object of the modernisation was to make the product to a repeatable standard.
Particular reference will be made to the impact of instrumentation on the project.
The other main objective of the reconstruction was to improve the then range of papers to be made in terms of quality and tiqhtness of specification, particularly in terms of substance and moisture profiles and strength characteristics.
BACKGROUND. No company chairman likes to be brought to the point of finding he has to spend capi tal in order to stay competitive. But there were siqns that a major investment became due. Additionally the financial climate (in terms of govemment grants) was moving in the right direction. In this instance as a first step an independant report was looked for by top management to determine:
BEFORE RECONSTRUCTION. The types of paper made before reconstruction consisted mainly of woodfree banks and bonds, white and tinted cartridge papers, manifolds, envelopes and continuous stationary. At times deep colours were also included in the making programme.
Why the product had fallen below average market standard and was not always acceptable to the end user,
The substance ranqe was from 40 to 140 gsm and in the main the papers were made from two fumish constituents.
and
2.
Project
OBJECTS OF THE MODERNISA TION.
INTRODUCTION.
1.
Process Design.
To increase the range of products made to make the mill more versatile.
PROCESS SWINGS.
When look ing at the plant as a whole it was soon
The triqger for mill management to set the project into motion were process swinqs. These were felt by the end user who complained of the paper quality. The paper would not perform well on the converting equipment following the papermakinq process.
f~und that it was necessary to reconstruct the ~Ire
part and rethink the stock preparation plant In order to make the product more repeatable. As the survey progre ssed it bee ame clear that the key-phrase of the whole operation should be repeatability of process.
345
346
K. D. Blundell
It is not unusual to have to brinq hard evidence of such process swings to the attention of mill management before project expenditure is authorised. The followinq slides show before and after conditions of the substance profile which are not untypical for machines of the immediate postwar vintage.
THE BASIC CRITERIA FOR CAPITAL PROJECTS.
Two main tests should be applied by commerc ially motivated papermills to determine whether any specific project of a capital nature is worth while pursuing.
1.
To see whether the project would improve the commercial base of the company, i.e. make the product more versatile or better in quality or of more appeal to the market.
2.
To see whether ei ther more of the present quality of a product can be made at similar cost and/or the product can be made at a lesser unit cost if the capital project qoes forward.
With the benefit of hindsight the swinqs in this particular case were caused by a combination of all of the followinq factors:
1.
Ineffective control of from the pulper onwards.
2.
Poor consistency control followinq the machine chest.
3.
Poor control addition.
4.
Incorrect pressure screen operation in front of the machine flowbox.
5.
An open water system at the wire consisting of a number of mixinq boxes with overflows.
of
broke
consistency
consistency
NEW
Larqer companies in the Industry sometimes have a third cri teria. This is to keep up with current technoloqy, sometimes even for its own sake, in order to ensure a broader and more secure commercial base. THE BASIC BRIEF.
SORTING
OF
TrE
6.
A dated flowbox with an inadequate approachflow system.
7.
No couch pit existed thus allowing couch pit broke to fall into the wire pit, as did the much varying wire trim.
8.
Poor mechanical condition of many items of equipment.
The basic questions to be answered at the outset were:
THE BRIEF.
1.
The competinq areas of plant requiring expenditure had to be evaluated. It soon became apparent that this particular early post second world war environment would have to overcome severe technical and social changes to survive.
The task set the Consultants by the Board of Directors was: 1.
2.
To advise and make a report on the total approach to the various problems, from marketinq strateqy to process modernisation. It was therefore necessary to deal with the problem of how to make the best use of the capital available for optimum return in a qiven and predicted commercial environment.
Which machine rebuilt first?
unit
should
be
Clearly the resources in terms of capital and manpower at the mill did not run to tackling both machine complexes at the same time.
2.
When lookinq at one of the two papermaking machine complexes, which items of plant and which of the serv ices (any, some, or all) required upgrading to achieve a calculated balanced production unit?
To implement the advice. Of particular concern were throughput, capability of fibre treatment, forminq, drying and above all, overall process repeatability.
Modernisation of a Small Papermill
The mechanical condition of many items of plant also influenced much of the thinking and eventual outcome.
3.
Which areas required reconstruction in order to improve the quality of the product?
This is a di fferent and sometimes conflicting requirement from ensuring that the machine complex is balanced as an optimum production unit.
Paper fibres are not a homogeneous material. The basic input, i.e. pulp, is grown in different areas at different times. Even a sinqle tree contains fibres of different characteristics, young and old tree growth, spring and summer growth, etc. Of course. usinq one type of pulp from one pulp mill will produce the most commercially repeatable pulp input to a papermill. Sometimes, however, for commercial and geopolitical reasons it is not possible just to use one supply source only for a product. STANDARDS.
In the event a major investment was shown to be required and sanctionted by the board. THE HOMOGENEITY OF THE INPUT. (AN ASIDE.)
In the case of this modernisation it became apparent that if the whole papermakinq process could be made more repeatable it would be possible to work to closer standards. Standards could then be moni tared.
Just a brief word for those readers who are not all that familiar with the papermaking process.
Workinq to standards requires two basic operations:
1. In a mill makinq paper from pulp there are two main production centres:
a.
The Stock preparation plant. where fibre is put into the pulper system in the shape of pulp bales. The fibres are separated from each other by a process called REFINING. More than one type of pulp can be pulped and refined for a paper production run, i.e. fibres can be blended ei ther in the pulper before refining, or be refined separately and then blended after refining. The refining after pulping is usually carried out between 4 and 6% consistency.
After this treatment the furnish goes to a chest for further fibre length control refining treatment in machine refiners. These are just in front of the papermakinq machine and before dilution, usually at about 30/0 consistency. b.
Following the machine refiners is the papermaking machine, the second production centre. The papermachine shapes the fibre into a continuous fibre mat (sheet) and then dewaters the fibre mat.
To measure the process variables and
2.
To control these variables.
The whole of the papermakinq process suffers from the difficulties of determinina standard properties continuously whilst the process is in operation and wi thout interruptinq it. This starts with the irregularity of the input. Many tests have to be carried out in the mill laboratory on a discontinuous sample basis. In the Industry such tests are described as historical tests. They are not as valuable as tests which can be conducted continuously on the production unit in time for corrective steps to be taken. The majority of control equipment to be installed in papermills can be justified on this argument alone. The pious hopes of system and control engineers are to moni tor those properties which are both capable of beinq moni tored and capable of being influenced by the mechanical papermaking plant avaible. They hope that properties which cannot be continuously monitored will remain constant by virtue of exercisinq those controls which are available. The papermakinq properties which ouqht to be measured and, as far as possible, continuously controlled durinq the making process, i.e. not
347
K. D. Blundell
348
Table 1 PROPERTY
**
UNIT
MEANS OF INFLLENCE (TREATMENT)
Freeness Refining at high consistency.
Fibrillation Fibre length
Refi ninq at high consistency. mm
Refininq at low consistency.
Table 2
PROPERTIES OF FINAL PRODUCT
MEANS OF INFLLENCE
CONTROLLER
*
Moisture (%)
Drying temperature of cylinders
Steam Valve
*
Substance (gsm).
Add/Reduce fibre
Stuff Gate
*
Ash Content
Add/Reduce additive
Control Valve
*
Colour
Add/Reduce additive
Control Valve
Refining
Control Power Input Con trol Valves
Tear
Change Fumish Burst
Air Permeabili ty
Refining
Control Power Input Con trol refi nino consistency.
Change Furnish.
Control Valves
Refining
Control Power Input Control refinino consistency.
Change Fumish
Control Valves
Size Press
Control flow.
Add/reduce Alum
Control Valve
Absorbency
Change sizing Change pH
Control Valve Control Valve
*
Pressinq
Con trol Press Nip Pressure
Calenderinq
Con trol Calender Nip Pressure.
Calendering
Control Calender Nip Pressure
*
pH
Bulk
Surface Smoothness
Modernisation of a Small Papermill
349
Table 3
PROCESS VARIABLES
MEANS OF INFLLENCE
CONTROLLER
*
Consistency
Add/Reduce water
Control Valve
*
Drying cylinder Temperature
Vary Steam (Pressure)
Control Valve
Table 4.
CONTROL LOOPS
CONTROLS
Dryer Limited
Speed Stuffgate Approach Flow system
Efflux Ratio
Total head (where applicable) Level Speed (where appli cable) Slice adjustment (where applicable)
Speed Limited
Steam Stuffgate Efflux ratio (where applicable)
Grade Change
Stuffgate Efflux Ratio Speed Steam Fumish Additives Refi ning (Where applicable)
NOTE: Properties marked Properties marked
* are normally controlled continuously as part of process control ** can normally be controlled on line as part of process control.
K. D. Blundell
350
the
Novel to this otherwise conventional system is the method of Broke addition because it is achieved with a minimum of instrumentation.
The other paper properties referred to cannot normally easily be continuously controlled, which means they have to be historically controlled.
The use of consistency regulators and expensive proportioning equipment has been minimised by unconventional system design.
FUNDAMENTAL PROJECT CONSIDERATIONS.
The author is pleased to report that the results (including the continuous dye addition) were satisfactory.
historicall y controlled. are shown in accompanying table marked thus: * or * *.
At this point it may be interestinq to list a few of the relevant fundamentals which had to be considered for this project:
Basically there are two methods of qetting fine adjustments on a stuffgate.
1.
Priorities of Investment.
2.
Process and Quality Considerations.
3.
Investment in Mechanical Plant versus Instrumen tati on.
4.
To what extent can instrumentation displace investment in mechancial plant by increasing effectiveness of exisiting plant.
5.
Difficulties to be faced by Mill not used to instrumentation. Training programme.
6.
Tackling specific process difficulties.
7.
Reduction of Process swinqs on the Papermachine particularly in terms of substance and moisture.
8.
1.
The first method consists of varying the height of the stock behind the stuffgate and changinq the stuffgate whenever a step change is required. This method of control is effective with and without computer control. The major difficulty when applyinQ this type of system is to design the headbox behind the stuffgate. However, the technology for this is available now. In practice it has been shown that surprisingl ysmall changes in substance can be effected with simple and cheap equipment and instrumentation.
Dealinq with colour changes. Presentinq al ternative system designs available and choosinq one desiqn.
To avoid qiving too long a paper the author felt it would be most interest inq in this instance only to describe say two areas of detail design which should bring out the method in which the whole project was tackled: 1.
STUFFGATE CONTROL.
Because the frequency of movement of the stuffqate is minimised maintainance also is minimised.
2.
The second method to be described here is to use the stuffga te in the conventional manner.
The Wet End System Desiqn
Description of the Flow Diagram.
Changes in the setting of the stuffgate. of course. can only be done in discreet steps. With modern valves, however, the steps are suffciently small for most paper machine applications. 1 500 steps from fully open to fully closed is a fiqure not infrequently quotedbymanufacturers. This compares with a fiqure of say 250 or so for a handoperated conventional stuffgate.
From the machine chest the stock goes via a magnetic flowmeter (to monitor the new furnish entering the system) to the machine refiners and then to the machine dilution system.
When instrumentinq small tonnage papermachines the stuffqate opening may be small with a resultant insta-
and 2.
Design influences Gauqing System.
regarding
the
THE WET END SYSTEM.
Modernisation of a Small Papermill
bili ty. The opening is pressure dependant and difficulties are common in this area for small machines or machines using long fibred raw materials.
Both substance and moisture could be kept to considerably closer limits. In this instance three basic considerations dominated the choice of the gauging system.
1.
The sui tabili ty of the sensinq heads to be used on this wide range of papers. These included deep tints and blacks. colours known to interfere with moisture sensinq.
2.
Price.
3.
The uni t had to be simple to use (from the training and maintenance points of view) and had to have good service facilities.
In the case described the second method of stuffgate control was adopted.
FLOW CONTROL. As an intermediate device between computer control and hand control quite a few machines now are equipped with a magnetic flowmeter in the stock line controlling a stuffgate. To succeed with. such an exercise the whole system ought to be pressurised. On several occasions experience has shown that an open system cannot be controlled effectively with such a device through a usual machine making programme. In the case history being reported the stuffgate computer controlled system was commissioned in two stages. The first stage consisted of operatinq with a constant headbox on the delivery side of the refiner feedinq the stuffgate, i.e. an open system. The second st age consisted of pressurising the whole stuffgate flow system. The controls achieved with no other alteration but pressurising the sytem once again confirmed the superiority of the pressurised system for control purposes.
The actual unit chosen was micro processor based with VDU and had sufficient room to put the followinq process variables into the system to be computer controlled diqitally:
a. b. c.
Consistency. Thick stock flow. Steam.
d.
A simple method of incorporatinq Efflux ratio calculations.
The VDU and the main control panels for the papermachine were positioned at the dry end inside a small portable air condi tioned buildinq. The VDU pages for monitoring control which were part of a standard package were:
1.
A data paqe.
2.
Current substance, bone dry weight and moisture profiles in the cross machine direction.
3.
Present, past and twice past profiles which can be displayed as substance, bone dry weight or moisture in the cross machine direction.
4.
Substance, bone dry and moisture profiles in the machine direction on the basis of cross machine direction average when applicable.
5.
A useful page for management control and fulfilling a basic need to control the important papermaking process variables was the provision of a histogram.
THE CHOICE OF A GAUGING SYSTEM. With a view to making the whole papermaking process repeatable so that standards could be kept to fairly close limits the inclusion of a gauging system was considered necessary. Such a system constituted a major change in F or the fi rst outlook for the mill opera ti ves. time in many areas of the process they could get aw.ay from historical testinq so that process adjustments could be made before it was too late.
Further, the papers could be made to closer specification because information was presented on a VDU and augmented where necessary. This Was ~ore informative and comprehensive than anything available to them before.
351
K. D. Blundell
352
IN RETROSPECT. Unfortunately it has not been possible to make a project audit. Time has been responsible for this omission. So the next best thinq that can be done is to give the author's impression of the effectiveness of the installation. The availabili ty to the manufacturinq process of the gauqinq system and the instrumentation has been high. There is no quest ion at all about any of the mechanical work which was done. This has been proved essential. Wi thout the process and mechanical changes the plant would have had to shut. Few specific snags have been found. The papers now made are made to predictable repeatable tolerances. The order book encompasses papers not previously made commercially. The mill staff are much more aware of the need to produce a higher quality product. There is still more process and mechanical work But the cash which needs doing at the mill. flow position is still difficult. There is a sober realisation, certainly with middle management, that to keep up to date is a continuous process needing planned investment, i.e. an ever pressinq demand for cash to keep up to date. The present (at the time of qoing to press) almost worldwide exceptionally high interest rates, primarilY to combat inflation, make risk investment unattractive with many investors. Thus also by implication with many major Boards of Directors unless a high return can be shown. The task of top management is becominq increasingly delicate regarding the continuing commi tment of funds for basically dated mechanical equipment. With this type of plant it is frequently a question of protectinq a previous investment. As we get older we all know that we have a li fespan. In the world of manufacturing industry we can only prolonq this li fespan by the ongoing ploughing back of capital. By investinq in a balanced mixture of process knowhow, mechanical plant and modern electronics, the li fespan of this papermakinq plant has been prolonged.
MODERNISATION OF A SMALL PAPERMILL K. D. Blundell Paperboard Machinery Services, PMS House, Kingston Road, Leatherhead, Surrey, U. K.
Abstract. The paper describes aspects of a typical small papermill reconstruction project going through various phases. It commences at the point of the brief, deals with laying down the principles of evaluating what is needed and then cites some design problems. It deals with equipment parameters for one subsystem and finally gives the authors impression of its effectiveness. KEYWORDS. Project Criteria. Process repeatability. effectiveness.
The object of this paper is to tell about experiences of modernising one papermachine of two complete with its own stock preparation line in a small (9 000 tons per annum) papermiB.
Most important of all was the need to keep the mill profi table, now and in the future. The main object of the modernisation was to make the product to a repeatable standard.
Particular reference will be made to the impact of instrumentation on the project.
The other main objective of the reconstruction was to improve the then range of papers to be made in terms of quality and tiqhtness of specification, particularly in terms of substance and moisture profiles and strength characteristics.
BACKGROUND. No company chairman likes to be brought to the point of finding he has to spend capi tal in order to stay competitive. But there were siqns that a major investment became due. Additionally the financial climate (in terms of govemment grants) was moving in the right direction. In this instance as a first step an independant report was looked for by top management to determine:
BEFORE RECONSTRUCTION. The types of paper made before reconstruction consisted mainly of woodfree banks and bonds, white and tinted cartridge papers, manifolds, envelopes and continuous stationary. At times deep colours were also included in the making programme.
Why the product had fallen below average market standard and was not always acceptable to the end user,
The substance ranqe was from 40 to 140 gsm and in the main the papers were made from two fumish constituents.
and
2.
Project
OBJECTS OF THE MODERNISA TION.
INTRODUCTION.
1.
Process Design.
To increase the range of products made to make the mill more versatile.
PROCESS SWINGS.
When look ing at the plant as a whole it was soon
The triqger for mill management to set the project into motion were process swinqs. These were felt by the end user who complained of the paper quality. The paper would not perform well on the converting equipment following the papermakinq process.
f~und that it was necessary to reconstruct the ~Ire
part and rethink the stock preparation plant In order to make the product more repeatable. As the survey progre ssed it bee ame clear that the key-phrase of the whole operation should be repeatability of process.
345
346
K. D. Blundell
It is not unusual to have to brinq hard evidence of such process swings to the attention of mill management before project expenditure is authorised. The followinq slides show before and after conditions of the substance profile which are not untypical for machines of the immediate postwar vintage.
THE BASIC CRITERIA FOR CAPITAL PROJECTS.
Two main tests should be applied by commerc ially motivated papermills to determine whether any specific project of a capital nature is worth while pursuing.
1.
To see whether the project would improve the commercial base of the company, i.e. make the product more versatile or better in quality or of more appeal to the market.
2.
To see whether ei ther more of the present quality of a product can be made at similar cost and/or the product can be made at a lesser unit cost if the capital project qoes forward.
With the benefit of hindsight the swinqs in this particular case were caused by a combination of all of the followinq factors:
1.
Ineffective control of from the pulper onwards.
2.
Poor consistency control followinq the machine chest.
3.
Poor control addition.
4.
Incorrect pressure screen operation in front of the machine flowbox.
5.
An open water system at the wire consisting of a number of mixinq boxes with overflows.
of
broke
consistency
consistency
NEW
Larqer companies in the Industry sometimes have a third cri teria. This is to keep up with current technoloqy, sometimes even for its own sake, in order to ensure a broader and more secure commercial base. THE BASIC BRIEF.
SORTING
OF
TrE
6.
A dated flowbox with an inadequate approachflow system.
7.
No couch pit existed thus allowing couch pit broke to fall into the wire pit, as did the much varying wire trim.
8.
Poor mechanical condition of many items of equipment.
The basic questions to be answered at the outset were:
THE BRIEF.
1.
The competinq areas of plant requiring expenditure had to be evaluated. It soon became apparent that this particular early post second world war environment would have to overcome severe technical and social changes to survive.
The task set the Consultants by the Board of Directors was: 1.
2.
To advise and make a report on the total approach to the various problems, from marketinq strateqy to process modernisation. It was therefore necessary to deal with the problem of how to make the best use of the capital available for optimum return in a qiven and predicted commercial environment.
Which machine rebuilt first?
unit
should
be
Clearly the resources in terms of capital and manpower at the mill did not run to tackling both machine complexes at the same time.
2.
When lookinq at one of the two papermaking machine complexes, which items of plant and which of the serv ices (any, some, or all) required upgrading to achieve a calculated balanced production unit?
To implement the advice. Of particular concern were throughput, capability of fibre treatment, forminq, drying and above all, overall process repeatability.
Modernisation of a Small Papermill
The mechanical condition of many items of plant also influenced much of the thinking and eventual outcome.
3.
Which areas required reconstruction in order to improve the quality of the product?
This is a di fferent and sometimes conflicting requirement from ensuring that the machine complex is balanced as an optimum production unit.
Paper fibres are not a homogeneous material. The basic input, i.e. pulp, is grown in different areas at different times. Even a sinqle tree contains fibres of different characteristics, young and old tree growth, spring and summer growth, etc. Of course. usinq one type of pulp from one pulp mill will produce the most commercially repeatable pulp input to a papermill. Sometimes, however, for commercial and geopolitical reasons it is not possible just to use one supply source only for a product. STANDARDS.
In the event a major investment was shown to be required and sanctionted by the board. THE HOMOGENEITY OF THE INPUT. (AN ASIDE.)
In the case of this modernisation it became apparent that if the whole papermakinq process could be made more repeatable it would be possible to work to closer standards. Standards could then be moni tared.
Just a brief word for those readers who are not all that familiar with the papermaking process.
Workinq to standards requires two basic operations:
1. In a mill makinq paper from pulp there are two main production centres:
a.
The Stock preparation plant. where fibre is put into the pulper system in the shape of pulp bales. The fibres are separated from each other by a process called REFINING. More than one type of pulp can be pulped and refined for a paper production run, i.e. fibres can be blended ei ther in the pulper before refining, or be refined separately and then blended after refining. The refining after pulping is usually carried out between 4 and 6% consistency.
After this treatment the furnish goes to a chest for further fibre length control refining treatment in machine refiners. These are just in front of the papermakinq machine and before dilution, usually at about 30/0 consistency. b.
Following the machine refiners is the papermaking machine, the second production centre. The papermachine shapes the fibre into a continuous fibre mat (sheet) and then dewaters the fibre mat.
To measure the process variables and
2.
To control these variables.
The whole of the papermakinq process suffers from the difficulties of determinina standard properties continuously whilst the process is in operation and wi thout interruptinq it. This starts with the irregularity of the input. Many tests have to be carried out in the mill laboratory on a discontinuous sample basis. In the Industry such tests are described as historical tests. They are not as valuable as tests which can be conducted continuously on the production unit in time for corrective steps to be taken. The majority of control equipment to be installed in papermills can be justified on this argument alone. The pious hopes of system and control engineers are to moni tor those properties which are both capable of beinq moni tored and capable of being influenced by the mechanical papermaking plant avaible. They hope that properties which cannot be continuously monitored will remain constant by virtue of exercisinq those controls which are available. The papermakinq properties which ouqht to be measured and, as far as possible, continuously controlled durinq the making process, i.e. not
347
K. D. Blundell
348
Table 1 PROPERTY
**
UNIT
MEANS OF INFLLENCE (TREATMENT)
Freeness Refining at high consistency.
Fibrillation Fibre length
Refi ninq at high consistency. mm
Refininq at low consistency.
Table 2
PROPERTIES OF FINAL PRODUCT
MEANS OF INFLLENCE
CONTROLLER
*
Moisture (%)
Drying temperature of cylinders
Steam Valve
*
Substance (gsm).
Add/Reduce fibre
Stuff Gate
*
Ash Content
Add/Reduce additive
Control Valve
*
Colour
Add/Reduce additive
Control Valve
Refining
Control Power Input Con trol Valves
Tear
Change Fumish Burst
Air Permeabili ty
Refining
Control Power Input Con trol refi nino consistency.
Change Furnish.
Control Valves
Refining
Control Power Input Control refinino consistency.
Change Fumish
Control Valves
Size Press
Control flow.
Add/reduce Alum
Control Valve
Absorbency
Change sizing Change pH
Control Valve Control Valve
*
Pressinq
Con trol Press Nip Pressure
Calenderinq
Con trol Calender Nip Pressure.
Calendering
Control Calender Nip Pressure
*
pH
Bulk
Surface Smoothness
Modernisation of a Small Papermill
349
Table 3
PROCESS VARIABLES
MEANS OF INFLLENCE
CONTROLLER
*
Consistency
Add/Reduce water
Control Valve
*
Drying cylinder Temperature
Vary Steam (Pressure)
Control Valve
Table 4.
CONTROL LOOPS
CONTROLS
Dryer Limited
Speed Stuffgate Approach Flow system
Efflux Ratio
Total head (where applicable) Level Speed (where appli cable) Slice adjustment (where applicable)
Speed Limited
Steam Stuffgate Efflux ratio (where applicable)
Grade Change
Stuffgate Efflux Ratio Speed Steam Fumish Additives Refi ning (Where applicable)
NOTE: Properties marked Properties marked
* are normally controlled continuously as part of process control ** can normally be controlled on line as part of process control.
K. D. Blundell
350
the
Novel to this otherwise conventional system is the method of Broke addition because it is achieved with a minimum of instrumentation.
The other paper properties referred to cannot normally easily be continuously controlled, which means they have to be historically controlled.
The use of consistency regulators and expensive proportioning equipment has been minimised by unconventional system design.
FUNDAMENTAL PROJECT CONSIDERATIONS.
The author is pleased to report that the results (including the continuous dye addition) were satisfactory.
historicall y controlled. are shown in accompanying table marked thus: * or * *.
At this point it may be interestinq to list a few of the relevant fundamentals which had to be considered for this project:
Basically there are two methods of qetting fine adjustments on a stuffgate.
1.
Priorities of Investment.
2.
Process and Quality Considerations.
3.
Investment in Mechanical Plant versus Instrumen tati on.
4.
To what extent can instrumentation displace investment in mechancial plant by increasing effectiveness of exisiting plant.
5.
Difficulties to be faced by Mill not used to instrumentation. Training programme.
6.
Tackling specific process difficulties.
7.
Reduction of Process swinqs on the Papermachine particularly in terms of substance and moisture.
8.
1.
The first method consists of varying the height of the stock behind the stuffgate and changinq the stuffgate whenever a step change is required. This method of control is effective with and without computer control. The major difficulty when applyinQ this type of system is to design the headbox behind the stuffgate. However, the technology for this is available now. In practice it has been shown that surprisingl ysmall changes in substance can be effected with simple and cheap equipment and instrumentation.
Dealinq with colour changes. Presentinq al ternative system designs available and choosinq one desiqn.
To avoid qiving too long a paper the author felt it would be most interest inq in this instance only to describe say two areas of detail design which should bring out the method in which the whole project was tackled: 1.
STUFFGATE CONTROL.
Because the frequency of movement of the stuffqate is minimised maintainance also is minimised.
2.
The second method to be described here is to use the stuffga te in the conventional manner.
The Wet End System Desiqn
Description of the Flow Diagram.
Changes in the setting of the stuffgate. of course. can only be done in discreet steps. With modern valves, however, the steps are suffciently small for most paper machine applications. 1 500 steps from fully open to fully closed is a fiqure not infrequently quotedbymanufacturers. This compares with a fiqure of say 250 or so for a handoperated conventional stuffgate.
From the machine chest the stock goes via a magnetic flowmeter (to monitor the new furnish entering the system) to the machine refiners and then to the machine dilution system.
When instrumentinq small tonnage papermachines the stuffqate opening may be small with a resultant insta-
and 2.
Design influences Gauqing System.
regarding
the
THE WET END SYSTEM.
Modernisation of a Small Papermill
bili ty. The opening is pressure dependant and difficulties are common in this area for small machines or machines using long fibred raw materials.
Both substance and moisture could be kept to considerably closer limits. In this instance three basic considerations dominated the choice of the gauging system.
1.
The sui tabili ty of the sensinq heads to be used on this wide range of papers. These included deep tints and blacks. colours known to interfere with moisture sensinq.
2.
Price.
3.
The uni t had to be simple to use (from the training and maintenance points of view) and had to have good service facilities.
In the case described the second method of stuffgate control was adopted.
FLOW CONTROL. As an intermediate device between computer control and hand control quite a few machines now are equipped with a magnetic flowmeter in the stock line controlling a stuffgate. To succeed with. such an exercise the whole system ought to be pressurised. On several occasions experience has shown that an open system cannot be controlled effectively with such a device through a usual machine making programme. In the case history being reported the stuffgate computer controlled system was commissioned in two stages. The first stage consisted of operatinq with a constant headbox on the delivery side of the refiner feedinq the stuffgate, i.e. an open system. The second st age consisted of pressurising the whole stuffgate flow system. The controls achieved with no other alteration but pressurising the sytem once again confirmed the superiority of the pressurised system for control purposes.
The actual unit chosen was micro processor based with VDU and had sufficient room to put the followinq process variables into the system to be computer controlled diqitally:
a. b. c.
Consistency. Thick stock flow. Steam.
d.
A simple method of incorporatinq Efflux ratio calculations.
The VDU and the main control panels for the papermachine were positioned at the dry end inside a small portable air condi tioned buildinq. The VDU pages for monitoring control which were part of a standard package were:
1.
A data paqe.
2.
Current substance, bone dry weight and moisture profiles in the cross machine direction.
3.
Present, past and twice past profiles which can be displayed as substance, bone dry weight or moisture in the cross machine direction.
4.
Substance, bone dry and moisture profiles in the machine direction on the basis of cross machine direction average when applicable.
5.
A useful page for management control and fulfilling a basic need to control the important papermaking process variables was the provision of a histogram.
THE CHOICE OF A GAUGING SYSTEM. With a view to making the whole papermaking process repeatable so that standards could be kept to fairly close limits the inclusion of a gauging system was considered necessary. Such a system constituted a major change in F or the fi rst outlook for the mill opera ti ves. time in many areas of the process they could get aw.ay from historical testinq so that process adjustments could be made before it was too late.
Further, the papers could be made to closer specification because information was presented on a VDU and augmented where necessary. This Was ~ore informative and comprehensive than anything available to them before.
351
K. D. Blundell
352
IN RETROSPECT. Unfortunately it has not been possible to make a project audit. Time has been responsible for this omission. So the next best thinq that can be done is to give the author's impression of the effectiveness of the installation. The availabili ty to the manufacturinq process of the gauqinq system and the instrumentation has been high. There is no quest ion at all about any of the mechanical work which was done. This has been proved essential. Wi thout the process and mechanical changes the plant would have had to shut. Few specific snags have been found. The papers now made are made to predictable repeatable tolerances. The order book encompasses papers not previously made commercially. The mill staff are much more aware of the need to produce a higher quality product. There is still more process and mechanical work But the cash which needs doing at the mill. flow position is still difficult. There is a sober realisation, certainly with middle management, that to keep up to date is a continuous process needing planned investment, i.e. an ever pressinq demand for cash to keep up to date. The present (at the time of qoing to press) almost worldwide exceptionally high interest rates, primarilY to combat inflation, make risk investment unattractive with many investors. Thus also by implication with many major Boards of Directors unless a high return can be shown. The task of top management is becominq increasingly delicate regarding the continuing commi tment of funds for basically dated mechanical equipment. With this type of plant it is frequently a question of protectinq a previous investment. As we get older we all know that we have a li fespan. In the world of manufacturing industry we can only prolonq this li fespan by the ongoing ploughing back of capital. By investinq in a balanced mixture of process knowhow, mechanical plant and modern electronics, the li fespan of this papermakinq plant has been prolonged.