Engineering aspects of basis weight control

Engineering Aspects of Basis Weight Control

By:D K Singhal

Chandpur Enterprises Ltd.Chandpur-246725

Problem:

� How do you control basis weight?

� Basis weight valve� Stock flow to fan pump� Machine chest consistency control

Problem:

� What are modern techniques for basis weight control?

� Automatic machine chest consistency control

� Scanner controlling basis weight valve� Having a DCS/QCS system

Is it Really a Problem?

� Maintaining correct basis weight is the duty of machine operator.

� If he is careless, we can have good scanners, DCS/QCS systems.scanners, DCS/QCS systems.

� Why should we waste our precious time discussing such small things?

Yes!

� Yes! It IS a genuine problem.

� Please do spare a few minutes now.� Please do spare a few minutes now.

Problem:

� In fact, basis weight variation is a common problem in all the mills.

� Some mills manage to get the best results out of their system.results out of their system.

� Let us have a look at various points that are the possible sources for poor basis weight control.

Possible Sources for Poor Control

� Machine Chest� Centricleaner� Pressure Screen� Approach Flow Piping� Approach Flow Pumps� Head Box Manifold� Distributor-Rectifier Rolls� Electricity Supply System

Machine Chest

� Machine chest consistency plays its own role.

� Too high a consistency results in fiber flocculation as well as air entrapment in flocculation as well as air entrapment in pulp which creates a lot of problems in the approach flow area.

� In case consistency is too low, pulp fibers tend to settle down resulting in changing in SRBox consistency over a period of time.

Centricleaner

� In the conventional centricleaners, pulp enters from one side, it goes to centricleaner bottles, and then leaves through accept of these bottles. through accept of these bottles.

� Final accept is also at the same side. � This results in slow response of inlet

consistency change as shown in next slide.

Centricleaner Consistency Change

Pressure Screen

� Similarly, pressure screen is also a huge vessel resulting in a similar response as that by centricleaner.

� The only difference is that the outlet consistency plot will look a little smooth when compared to that with plot of centricleaner.

Approach Flow Piping

� Approach flow piping may create problems if not properly designed.

� For example, let us consider line from S R Box to fan pump. An oversized S R Box to fan pump. An oversized line results in too slow pulp speed vertically downwards.

� This results in plug flow where pulp flows in form of a plug. (Remember: pulp is not an ideal liquid)


� During the plug flow, plug like structure is developed in pipeline. These plugs are intermittent in nature as a combination separate plugs one after another.separate plugs one after another.

� When it happens, we get intermittent pulses of consistency fluctuation after the fan pump.

� This results in basis weight fluctuation.


� On the other hand, if the pulp velocity is too high (i.e. pipeline is undersized), pulp starts to show erratic behavior.

� In such a case any change in basis weight valve opening may not reflect proportional change in stock flow and hence basis weight.

Approach Flow Pumps

� All the pumps used in approach flow should be of low pulsation type.

� Pressure pulses travel at a speed much higher than that of pulp (almost equal to the speed of sound) and create fluctuation in basis weight.

Approach Flow Pumps

� Pumps must be regularly checked for gland leakage etc.

� Leaking glands are often the easiest entry point for entrained air.

� This entrained air creates unpredictable fluctuations in basis weight.

Head Box Manifold

� As discussed earlier, most of the machine direction changes, be these due to consistency change, or due to any other reason, or the desired any other reason, or the desired change in basis weight valve, enter the manifold where these are converted to cross direction changes as indicated in nest slide.

Taper Manifold

Stock Entry

Time increases ��

Effects:

� Fluctuating CD profile >>>disturbed short term MD profile

� Loss of confidence for the sample � Loss of confidence for the sample taken

� No action deadband increases� Poor basis weight control

Manifold:

� Octopus header can be used to avoid CD profile fluctuations related problems in place of taper manifold.

Distributor-Rectifier Rolls

� Distributor rectifier rolls (commonly known as holey rolls) could be a major reason for basis weight fluctuation.

� In most of the cases, the head box � In most of the cases, the head box manufacturer gives very good rolls, but with time, some rolls get bend, misaligned or due to journal wear out, bearing related problems, the problems start appearing.

Distributor-Rectifier Rolls

� Time series analysis (plotting machine direction basis weight at a single CD position against time) is an effective tool to predict holey roll related tool to predict holey roll related problems.

Checking for Holey Roll Related Problems:

� For the same, at least three 2 minute MD profiles are taken, one each at NDE, center and DE.

� Higher fluctuation amplitude at center � Higher fluctuation amplitude at center indicates bend holey roll.

� Higher fluctuation amplitude at any end indicates the possibility of bearing off-center at that end.

� After the problem has been identified and rectified, it is recommended to verify the results by repeating the measurements.

A typical 45 second profile:

34.50

34.70

34.90

35.10

35.30

35.50

33.50

33.70

33.90

34.10

34.30

34.50

1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 185 193

A 45 second profile:33.50

33.70

33.90

34.10

34.30

34.50

34.70

34.90

35.10

35.30

35.50

1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 185 193

� Such a profile can reveal a lot of information.

� The trend line (red color) indicates different � The trend line (red color) indicates different high basis weight pulses. These are indicated by vertical lines in the plot. These correlate to revolutions of holey roll.

� There are minor pulses also in the graph, which might be due to pulsations caused by holes or something else.

Holey Roll:

� Unfortunately, these run at much less speeds say 10-15rpm, and may result in pulsations at after 15-20m distance. The one minute MD profile is often able to reveal if a particular holey roll is bend or misaligned.

Single Position MD Profile

120.0

125.0

130.0

135.0

140.0

1

16

31

46

61

76

91

106

121

136

151

166

181

196

Stable

Stable Stable

Possibily due to testing error.

Typical Inferences:

� A bend/misaligned holey roll� To high holey roll speed� If the fluctuation is not of the same � If the fluctuation is not of the same

magnitude after each holey roll revolution, there is a possibility of loose chain/ chain sprocket driving the holey roll.

Typical Solution:

� In one instance, while checking approximately 200 samples during the machine run of one minute, the fluctuation in basis weight was found to be within + 0.5gsm.

� On the basis of the same, holey roll speed was � On the basis of the same, holey roll speed was reduced from 22 rpm to 11 rpm.

� The fluctuation immediately reduced to +0.23gsm.

� Well, in many cases, repair or replacement of holey roll becomes a must to get good basis weight control.

Electricity Supply System

� Speed of approach flow equipments vary due to voltage and frequency variations.

� Servo voltage stabilizer is able to control voltage to a maximum of ±2.5% only.

� Frequency variations, (≈2%) remain uncontrolled. � Frequency variations, (≈2%) remain uncontrolled. � Net effect: ±4.5% (i.e.fan pump speed may vary by

±75rpm if running at full speed of 1440 rpm)� One may very well understand the effect of such level

of uncontrollable speed variation of fan pump and other approach flow equipments on basis weight.

Fan Pump Speed:

� Installation of VFD will be helpful in eliminating this problem.

� So far, most of the VFD installations are aimed towards energy conservation, but in approach flow area, the improved process control is greatest advantage of VFD.

Entrained Air:� Creates disturbance! Irregularly.� Problem worsens if there are dead pockets.� Much more severe in improperly designed

pipings.� Often serious on light gsm paper, when the

speed is higher.speed is higher.� Analog (bourdon Tube) gauges do not reflect

pressure fluctuations.Digital pressure gauges with large diameter transmitters do help.

� Profile unstability Analysis• A higher value at stock entry side indicates possibility of

entrained air.• More joints from this side.

Let us now have a look at typical basis weight profile…..

RollNo. #1 #2 #3 #4 #5 #6 #7 #8 #9 Var Avg.1 115.0 115.0 119.0 119.0 120.0 119.0 120.0 120.0 119.0 5.0 118.42 112.0 112.0 116.0 116.0 119.0 118.0 115.0 116.0 116.0 7.0 115.63 120.0 122.0 121.0 120.0 122.0 120.0 119.0 120.0 123.0 4.0 120.84 117.0 119.0 114.0 117.0 115.0 116.0 118.0 118.0 119.0 5.0 117.05 112.0 116.0 114.0 118.0 112.0 114.0 114.0 115.0 116.0 6.0 114.66 116.0 118.0 114.0 115.0 112.0 116.0 118.0 120.0 118.0 8.0 116.37 118.0 118.0 114.0 114.0 115.0 120.0 120.0 120.0 117.0 6.0 117.38 120.0 122.0 118.0 119.0 120.0 120.0 119.0 119.0 120.0 4.0 119.79 122.0 123.0 123.0 121.0 120.0 122.0 121.0 122.0 122.0 3.0 121.8

10 122.0 120.0 119.0 122.0 119.0 123.0 121.0 121.0 122.0 4.0 121.011 118.0 120.0 116.0 118.0 124.0 123.0 122.0 121.0 119.0 8.0 120.112 120.0 118.0 116.0 116.0 115.0 114.0 113.0 115.0 116.0 7.0 115.913 115.0 115.0 114.0 118.0 116.0 117.0 115.0 120.0 122.0 8.0 116.914 117.0 115.0 114.0 118.0 117.0 119.0 117.0 122.0 123.0 9.0 118.015 110.0 111.0 111.0 117.0 115.0 117.0 117.0 115.0 113.0 7.0 114.016 121.0 124.0 131.0 131.0 131.0 132.0 130.0 126.0 128.0 11.0 128.217 126.0 126.0 120.0 125.0 124.0 124.0 121.0 122.0 125.0 6.0 123.717 126.0 126.0 120.0 125.0 124.0 124.0 121.0 122.0 125.0 6.0 123.718 122.0 119.0 117.0 118.0 117.0 120.0 121.0 118.0 120.0 5.0 119.119 126.0 126.0 125.0 120.0 123.0 125.0 126.0 124.0 130.0 10.0 125.020 120.0 122.0 123.0 121.0 118.0 118.0 122.0 124.0 125.0 7.0 121.421 115.0 117.0 116.0 115.0 116.0 118.0 117.0 117.0 115.0 3.0 116.222 115.0 115.0 118.0 120.0 120.0 119.0 118.0 119.0 121.0 6.0 118.323 125.0 120.0 120.0 120.0 121.0 123.0 121.0 123.0 123.0 5.0 121.824 118.0 119.0 120.0 122.0 123.0 122.0 118.0 117.0 118.0 6.0 119.725 118.0 120.0 122.0 121.0 120.0 119.0 117.0 119.0 120.0 5.0 119.626 122.0 115.0 116.0 118.0 117.0 118.0 118.0 116.0 117.0 7.0 117.427 122.0 124.0 128.0 126.0 124.0 125.0 122.0 124.0 125.0 6.0 124.428 113.0 114.0 118.0 115.0 116.0 117.0 114.0 112.0 112.0 6.0 114.629 118.0 118.0 115.0 121.0 118.0 116.0 115.0 116.0 116.0 6.0 117.0

Min 110.0 111.0 111.0 114.0 112.0 114.0 113.0 112.0 112.0 3.0 114.0Max 126.0 126.0 131.0 131.0 131.0 132.0 130.0 126.0 130.0 11.0 128.2Avg. 118.45 118.72 118.34 119.34 118.93 119.79 118.93 119.34 120.00 6.21 119.10Range 16 15 20 17 19 18 17 14 18 8 14.222

Mill Issues:

� 110-132gsm individual samples� 3-11 gsm profile variation� GSM control in 114-128� Averaged profile- 118.34-120� Average profile variation- 6.21� Poor machine runnability� Lack of operator confidence over gsm

checked

Best & Worst Profiles

125.0

130.0

135.0

105.0

110.0

115.0

120.0

1 2 3 4 5 6 7 8 9

Best Worst

Who is to blame?

•Is it a problem of poor designed approach flow?

•Is it a problem of Head Box?•Is it a problem of Head Box?

•Is it due to carelessness of the operators?

Average GSM

112.0114.0116.0118.0120.0122.0124.0126.0128.0130.0

� In the graph, we can see that there is a significant fluctuation in average profile variation (114-128gsm).

110.0112.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Now, may we ask?

� Were the operators careless?

� Was there so high a stock consistency � Was there so high a stock consistency variation?

� Or, there was something else responsible for the problem?

Let us try something…

� Out of the available 24 consequent profiles, we take average at different positions. Similarly, position wise minimum and maximum values can minimum and maximum values can be found out.

� The values are plotted as in next figure.

Profile Summary:

115.0

120.0

125.0

130.0

135.0

� Here, we may see that the out of 24 rolls studied, the section wise average does not vary much (118.3-120gsm)

105.0

110.0

1 2 3 4 5 6 7 8 9

Min Max Avg.

� In fact, such a stable averaged profile (Green) suggests the absence of any CD problem in machine. Typically, disturbed average profile

105.0

110.0

115.0

120.0

125.0

130.0

135.0

1 2 3 4 5 6 7 8 9

Min Max Avg.

machine. Typically, disturbed average profile can also be due to various reasons such as localized wire and felt choking, head box slice unevenness, drying constraints etc.

� All of such factors force machine operators to disturb CD profile so that they are able to maintain CD moisture profile in acceptable range.

Another Profile Report

� Let us study the another profile study, consisting of 45 profiles.

� All of these profiles were drawn from a single roll.single roll.

� These profiles are taken within 1 minute of production.

� For simplicity, instead of giving a lot of data, preliminary profile analysis is presented in form of a graph.

Profile Variation:* Best: 2.0gsm* Worst: 4.0gsm

Best & Worst CD Profiles

3839

32333435363738

1 2 3 4 5 6 7 8 9 10 11 12

Profile Variation:Best & Worst CD Profiles

3233343536373839

1 2 3 4 5 6 7 8 9 10 11 12

� Here, we may see an interesting pattern.� Out of the 45 profiles taken from a single

roll, we may get CD variations of 2.0 and roll, we may get CD variations of 2.0 and 4.0.

� So, how much CD variation we are getting is depending on our luck. Within a span of one minute, we may get a profile variation of 2.0 or 4.0 gsm.

Range of GSM Obtained at Different Positions:

38

40

32

34

36

1 2 3 4 5 6 7 8 9 10 11 12

Range of GSM Obtained at Different Positions:

32

34

36

38

40

1 2 3 4 5 6 7 8 9 10 11 12

� Here again the situation is same.As clear, there is a variation of 2 gsm at � As clear, there is a variation of 2 gsm at any individual position.

� Obviously, if the operators are drawing sample at any instance, there is always a possibility of 2gsm inaccuracy in measurement, no matter however accurate is the measuring system.

Average GSM as a Function of Time

3636.5

37

3434.5

3535.5

36

1 5 9 13 17 21 25 29 33 37 41 45

Average GSM as a Function of Time (after modification)

45.2

45.3

45.4

44.6

44.7

44.8

44.9

45

45.1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24


3434.5

3535.5

3636.5

37

1 5 9 13 17 21 25 29 33 37 41 45

� Here we can see that the average gsm is fluctuating significantly.fluctuating significantly.

� Suppose the desired basis weight is 36, and operator is getting anything as above, the problem appears should he alter basis weight valve opening or ignore the difference?

� You decide!

What is so special in these profiles?

Range of GSM Obtained at Different Positions

32

34

36

38

40

Best & Worst CD Profiles

3233343536373839

1 2 3 4 5 6 7 8 9 10 11 121 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12


3434.5

3535.5

3636.5

37

1 5 9 13 17 21 25 29 33 37 41 45

Time is Short….

� All these profiles have been taken from a SINGLE roll, one after another.

� For a machine speed of 200mpm, and roll diameter of nearly 1M, this can be said that these profiles are one second apart from these profiles are one second apart from each other.

� Thus, 45 profile means nearly 45 seconds of production.

� If, basis weight can fluctuate between 34.0 to 36.5 within a short span of 45 seconds, how can we expect to control the same?

Scanner Malfunctioning…

� The most important aspect about such a case is that the basis weight scanners installed for DCS & QCS systems seem to malfunction. You may take a sample which could be 34.0gsm or 36.5gsm, and most could be 34.0gsm or 36.5gsm, and most probably the DCS system would be displaying some other basis weight on its console.

� In such a case, most papermakers will say that the scanner is giving wrong results.

� So, if you are not satisfied with your scanner, please read ahead.


� Look at the following 1 minute gsm plot.

51.50

48.00

48.50

49.00

49.50

50.00

50.50

51.00

51.50

1 21 41 61 81 101 121 141 161 181 201 221 241 261 281


� In this plot if canner is indicating 50

48.00

48.50

49.00

49.50

50.00

50.50

51.00

51.50

1 21 41 61 81 101 121 141 161 181 201 221 241 261 281

� In this plot if canner is indicating 50 gsm and your sample is 49.5 gsm you may say that scanner is indicating basis weight higher by 0.5 gsm.

� But, if your sample is 50.5 gsm?


� Scanners work on a scientific principle of radioactive rays penetration properties of cellulose fibers.

� A calibration problem may result in over or � A calibration problem may result in over or under reporting of basis weight, and this can be corrected easily.

� But, in case there is a fluctuation in basis weight as seen earlier, one may doubt that the scanner is not functioning well.

Advantages of Such Analysis:

� It is possible to analyze different reasons and their contribution to existing basis weight variation problem.problem.

� Common inferences that can be drawn by such study are..

Typical Inferences:

� A bend/misaligned holey roll� Entrained air in approach flow

systems� Inadequate inlet pressure at pressure

screen inlet� Centricleaner pit level and

consistency fluctuations

In House Troubleshooting:

� Single position MD profiling has been found a very good technique for such problems.

� The following slides indicate how you may use this technique to solve basis weight related problems in your plant.

Single Position MD Profiling:

� Single position MD profiling is a technique, in which data are obtained for basis weight profile in machine direction, and plotted against time.

� Any specific pattern indicates the possible source of the problem, which can be considered for rectification.

� Since, data are taken only for a small time of paper manufactured, sometimes, it may be required to repeat the same.

Single Position MD Profiling

Cutter

Unwind

�

12 11 10 9 8 7 5 6 5 4 3 ….

Unwind

# A 2 minute profile can be used to reveal much information.

# For a longer duration, 10th or 20th alternate samples can also be used.

Another Typical Single Position MD Profile:

135.0

140.0

Stable


120.0

125.0

130.0

1 18 35 52 69 86 103 120 137 154 171 188

StableStable

Issues:

� Different zones of relatively stable gsm• These zones exist for a small time duration of

say 30-50 seconds.• Had the operator taken sample 30-50

seconds earlier or later, what gsm value he would have observed?


130.0

135.0

140.0

Stable

Stable Stable


would have observed?• For a two minute MD plot (gsm range 122-

132 in the plot), to achieve 120 gsm, should the operator reduce gsm by 2 or by 12?

� Fluctuations within stable zones• 127 to 132 gsm in first stable zone.• What should the operator do when he checks

that the sample is of 127 gsm or 132 gsm?

� Why does it happen?

120.0

125.0

11

63

14

66

17

69

11

06

12

11

36

15

11

66

18

11

96

Stable

A Typical Approach Flow:

� Often, the conventional approach flow system looks perfect, as below…

Seems a perfect system!

� Ideally, if inlet consistency is fixed, basis weight should remain fixed.

� But, if the consistency is changing at different points, basis weight fluctuation cannot be controlled.

� But, the inlet consistency to head box keeps on changing.

Inlet Consistency Changes! How?

� Yes, it keeps on changing due to various reasons e.g. � Pump speed variations due to voltage,

frequency etc.frequency etc.� Presence of entrained air through

pump glands etc.� Centricleaner pit level and consistency

variations� As a result, profile gets disturbed.

Effects:

In most of such cases DCS & QCS systems malfunction as scanners do not seem to sense correct basis weight.


120.0

125.0

130.0

135.0

140.0

11

63

14

66

17

69

11

06

12

11

36

15

11

66

18

11

96

Stable

Stable Stable


Reasons:

� Fan Pump Pulsations:� RPM: 1500, vanes: 6, Speed: 200mpm� Pulse to pulse difference: 200/(1500*6)� i.e. 0.02 m

So, for such a case, on lower speeds, fan pump � So, for such a case, on lower speeds, fan pump pulsation can’t harm you; but if the speed itself is fluctuating….?

� Similarly, effect of other equipments e.g pressure screen, holey rolls etc. can be considered.

� At higher machine speeds fan pump pulsations create significant problems.

Typical Inferences:

� A bend/misaligned holey roll� Entrained air in approach flow

systems� Inadequate inlet pressure at pressure

screen inlet� Centricleaner pit level and

consistency fluctuations

Typical Actions Taken:

� Installation of a new closed centricleaner

� Installation of VFDs for all approach flow pumps

� Provision of air venting lines at different piping locations

� Correct alignment of holey roll.

Typical Results:

For a yankee machine(No consistency regulator, no DCS)-� Average basis weight for 24 hours varied

between 48.2-49.4gsm; Individual samples between 47.4-50.2gsm

� Average basis weight for 72 hours varied � Average basis weight for 72 hours varied between 54.7-57.0gsm; Individual samples between 54.0-57.8gsm

� Reduction in CD profile variation by more than 50% after such studies were made and suitable actions were taken.

Extra Benefits:

For a yankee machine(No consistency regulator, no DCS)-� Machine joints reduced to just 25-30% of

earlier� Increase in machine speed by 3-5%

How to begin?

� If you are not satisfied with the basis weight control you are having, just follow the following steps-


� Collect data on single position in machine direction at drive end, non-drive end and at centre.

� Ensure that the data are being taken from � Ensure that the data are being taken from such a roll of paper that no process change had been done prior to at least 10 minutes before producing that roll. Also, the basis weight control should be running in manual control mode.

Plot a Graph

� Make a graph plotting time at ‘X’ axis, and basis weight at ‘Y’ axis. Such study is known as Time Series Analysis in mathematics.Analysis in mathematics.

� Identify any repeated pattern. If hills and valleys are repeated, compute their frequency.

Analyze

� Now, look at the equipment in the machine the frequency of which matches with the plot. This is going to be your first enemy. Do the needful to be your first enemy. Do the needful to minimize the problem it is creating.

Correct

� Take corrective action to solve the problem identified. Once the problem is solved the variations from such section are reduced.section are reduced.

Repeat:

� Having rectified the problem of earlier identified culprit, repeat single position MD profiling, and quite possibly you may find another culprit.may find another culprit.

� Very soon, you would be observing a better basis weight control.

Finally….

� You will be getting better basis weight control.

� Please do not forget to share your experiences with others. I’d really like to have your email containing your achievements in my inbox.

Average GSM as a Function of Time (after modification)

3434.5

3535.5

3636.5

37

44.6

44.7

44.8

44.9

45

45.1

45.2

45.3

45.4

1 2 3 4 5 6 7 8 9 101112131415161718192021222324

1 5 9 13 17 21 25 29 33 37 41 45

Thank you.

D K [email protected]

Technology

Engineering aspects of basis weight control