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TRENDS AND SOLUTIONS FOR THE PULP & PAPER INDUSTRY
viewed from a Measurement, Control and Automation point of view

Author

Jan-Erik Bergmark

Company

Endress+Hauser

Keywords

flow measurement, accuracy, repeatability, reliability, optimisation, control, selection

 

 

 

 

1. ABSTRACT

Accurate, reliable and repeatable Flow measurement is critical to the P&P process. The two main technologies for flow measurement are Magnetic and Coriolis. This paper investigates for which applications the appropriate technology should be used and more importantly what other factors need to be taken into account when installing such meters. Minor installation adjustments can lead to major improvements in the reliability of the measurement. We also take a brief look at how state of the art signal processing can introduce new measurement parameters such as Viscosity, Air content and others.

The paper furthermore takes a look at Process Optimization with a unique view that the various (and often competing) suppliers could be brought together to find optimization solutions for end users rather than looking at each component in isolation.

2. Technologies

Flow measurement is one of the most critical parameters for controlling of the processes; it is also one of the most difficult measurements to verify in the field. The selection of measurement technology to be used for a specific process area is dependent on a certain number of influencing factors e.g. required accuracy, repeatability, installation possibilities and the requested measurement parameter e.g. volume, mass, density, viscosity.

The most widely used flow meter is the Electro Magnetic type.

2.1 Electro Magnetic Flowmeter (EMF)

Faraday's law of induction states that a voltage is induced in a conductor moving in a magnetic field. In electromagnetic measuring, the flowing medium corresponds to the moving conductor. The induced voltage is proportional to the flow velocity and is detected by two measuring electrodes and transmitted to the amplifier. Flow volume is computed on the basis of the pipe's diameter. The constant magnetic field is generated by a switched direct current of alternating polarity.

Figure 1-1

2.1.1 Installation and selection considerations

The Electro Magnetic Flowmeter (EMF) is widely used in all different areas of a Pulp and Paper plant with good results, if properly installed.

To avoid problems with the flowmeter there are some factors that have to be considered

  • Sizing
    • The EMF is most accurate in the flow velocity range 2-6 m/s
    • Avoid using it with a flow velocity lower than 1 m/s, especially when
      high consistency pulp is measured.
    • For Green Liquor the flow velocity should be as low as possible to
      avoid scaling of the electrodes.
  • Electrodes and liner material
    • When the meter is installed in the digester area care should be taken to
      select a vacuum resistant high temperature liner material. To avoid bubbles in the liner the meter must be insulated if Teflon is used as liner material.
    • Make sure that the correct electrode material is used
      Avoid Hastelloy-C when measuring on Black or White Liquors.
    • Use anti-scaling shaped electrodes if the media has a tendency to create a scaling.
  • Installation
    • Lack of a good earth for the flow device is one of the most common
      problems for a EMF. When loose counter flanges are used make sure to
      connect the grounding strap from the EMF directly on the pipe and not to the
      loose flanges. Also make sure that the earth straps are connected on both
      sides of the flowmeter.
    • Flow dynamics is the most critical issue for a correct installation.
      Also the position of the electrodes can create or limit a measurement error.

Make sure to install the sensor well clear of fittings such as valves, T-pieces, elbows, etc. Inlet and outlet runs is necessary in order to ensure measuring accuracy. The normal recommendations from different suppliers are ≥ 5 x DN inlet run and ≥ 2-3 x DN Outlet run.

If these conditions are not fulfilled we can easily get large deviations on the accuracy. Below are some examples of measurement errors when the inlet or outlet runs is too short, the media in these cases is water. When e.g. pulp with higher consistency is measured, the error can be much larger.

In the first examples the meter is installed with 0 x DN inlet runs. The media is water with a flow velocity of 2,5 m/s.

Figure 1-2

In the second examples the meter is installed with 0 x DN runs on both inlet and outlet. The media is water with a flow velocity of 2,5 m/s.

Figure 1-3

In the third examples the meter is installed with 0 x DN runs on the outlet. The media is water with a flow velocity of 2,5 m/s.

Figure 1-4

In the last examples the meter is installed with 5 x DN runs on the inlet and 2 x DN on the outlet. The media is water with a flow velocity of 2,5 m/s.

Figure 1-5

As we can see the installation is a major factor to achieve high accuracy and this must be considered when selecting the type of meter to be used. If the actual application doesn't allow us to install the meter in a correct way and we still need the high accuracy the Coriolis-massflow meters are an alternative.

2.2 Coriolis-massflowmeter

Coriolis meters offer the best accuracy in mass flow measurement. Due to the very good zero point stability of today's meters, this accuracy is obtained over a wide flow range. The physical principle of operation is very elegant, with no moving parts in the process subject to wear.

The tubes are oscillated in resonance. The resonance frequency gives information about density, which is also an important parameter for quality and process control. Coriolis mass flowmeters have proven to record density with excellent accuracy e.g. 0,1 % for massflow and 0,001 % g/cc for density.

With an array of Coriolis sensors on the market mass flowmeters can be applied to almost any application. There are two different types of measuring principles available today, one with a single straight tube and another with two parallel tubes see figures 1 and 2

Figure 1

Figure 1 Two parallel tube principle

Figure 2

Figure 2 Single tube principle

2.2.1 Installation and selection considerations

For those applications, where accuracy, low pressure drop and good cleanability is needed, the single straight tube is the best alternative. This type of meter was introduced several years ago, and now serves as a platform for new application possibilities such as viscosity measurement.

Even that the Coriolis are able to work properly even without any straight pipe runs there are some factors that have to be considered

  • Sizing
    • Same basic sizing rule as for the EMF applies to the Coriolis meter.
  • Installation
    • A back pressure of minimum 0,2 bar is preferred to guarantee that the air bubbles are
      compressed inside of the measuring tube and that no cavitation take place downstream of the meter.

Figure 2-1

  • If a meter with a single tube is used the installation be done in all directions.

Figure 2-2

  • If two parallel tubes are used the location of the meter must be as in the picture below

Figure 2-3

2.2.2 Measurement parameters and applications

The Mass Flow measuring principle, in combination with new technology used in signal processing today, have created a possibility to measure and to combine different measurement parameters.

By doing this we can use the meter to measure e.g.

  • Viscosity
  • Concentration e.g BAUME or % Black Liquor
  • Density
  • Air content e.g. air content in white water to verify the efficiency of used defoaming agents.
  • Temperature

and the most common plant areas where it is getting installed are e.g.

  • Digesters, black liquor
  • Evaporation / Recovery Boiler, black liquor
  • Chemical Preparation
  • Bleaching Chemicals
  • Stock Preparation
  • Coating Kitchen
  • Diesel Oil, Fuel Oil, Tall Oil,  (media properties)

To give a practical example we will explain how the measurement of viscosity could be implemented as a new parameter for the single tube Coriolis meter.

The key of this instrument is the patented TMBTM–system of the meter (see Flomeko´98 proceedings for further details). A  pendulum is attached to the middle of the oscillating measuring tube. This pendulum oscillates counterphase to the tube, thus compensating for the momentum of the measuring tube.

Figure 3-1

The torsional movement of the measuring tube (see figure 2) can be utilised  to measure the viscosity of the fluid. Figure 3 is a cross sectional view of the measuring tube. Due to the rotational motion v of the tube, the fluid is forced to a rotational motion. Depending on the viscosity, we find different velocity profiles v(r) of the fluid.

Figure 3

Figure 3   cross sectional view of the measuring tube and velocity profile v(r) of the fluid

Due to the velocity gradient of the velocity profile there are shear forces in the fluid. These shear forces dampen the oscillation of the measuring tube and can be measured via excitation current, which is necessary to maintain the tube oscillation. Thus, measuring the excitation current, the viscosity of the fluid can be determined: = f ( Iexc , ...)

This new viscosity measurement feature, increases the multivariable functionality of the coriolis meter and thus offers customers the possibility to control their core process variables flow, density, temperature and viscosity with one single meter.

3. Process Optimization

Pulp and paper making is a dynamic process, and the process condition change every day.

Normally the controls are tuned during commissioning. Then after that the process condition changes and equipment wears out, which deteriorate the efficiency of the process; in many cases the unmatched control tuning makes things worse, and eventually the process will have its own behaviour which is uncontrolled. In most cases the operator learns to live with the limitation.

As we have seen the installation of the instruments can largely affect the measurement accuracy and expected functionality in the control strategies of the applications and this has also to be considered when an optimization takes place.

Normally when a mill is looking for improvement of a process they are contacting the different suppliers of equipment separately with no intention of bringing them together as a team where each and every supplier contribute with specialist knowledge of their area of supply.

As an example of how a "team" of people with different expertise can work together instead of being competitors and really make a difference I will give examples from an audit done by ABB Pulp and Paper experts in Singapore.

The audit included a complete optimization of a paper machine with focus on the improving the quality of the paper and at the same time reducing the number of sheet brakes that occurred frequently.

The basis for the team was to involve people with expertise knowledge of e.g. QCS, DCS, Instrumentation, Valves, consistency measurement and of course the mills own instrumentation and automation experts.

By collecting a large number of data momentarily with a Data Analysis Tool and then analyse it from each and every different component aspect it was possible to detect a large number of problem areas which would not been possible if we only looked at e.g. the QCS measurement variation as an isolated parameter.

The following application findings will give some practical examples of how the installation and setup of the instrument can be undetected or even hidden and cause large problems in the future when the processes has changed due to increased production or other process related improvements.

The following examples are only from the instrumentation related part, of course was there a lot of findings in all other areas such as. QCS, DCS control strategy, valves and so on  that was detected and corrected.

Here is an example of a measurement, with high filtering. The flow measurements were dampened with filters, when the filtering was removed; the problem with the control was exposed. The sticky control valve causes pulsation as shown in the measurements below. Servicing control valve positioner and providing additional filters in the instrument airline eliminated this problem.

Figure 4

Next application shows that the real noise level on the flow measurement is about 150lpm on a range of 0 to 350lpm (>40% noise). This was not possible to see until the heavy damping of the flowsignal in the flow transmitter was removed. This noise was caused by bad installation of the flow tube.

Figure 5

In the next example It was noticed that the filler flow dropped by about 100lpm while pump output was constant. This caused the Ash measurement to drop as shown below and it also caused sheet break. This was not noticed since the dip in flow last only for few seconds. This has been a result of the switching filter on the filler line for cleaning. Operating procedure was improved to avoid the discontinuity in the filler flow.

Figure 6

What was the outcome of the audit?

Sheet break reduction after 4 months

Sheet break duration reduction 29% (18.5 hours extra productive hours in a month)

Number of sheet break reduction 25% (reduction of 58 breaks)

The following are the percentage reduction on long-term variability of 32 reels (about 1500tons) comparison before and after the Audit

  • Basis weight   53%
  • Bone dry weight  51%
  • Moisture   60%
  • Ash    80%

These results showed that the control and process optimization resulted in improved machine performance and reduced variability in the paper quality. Projected increase profit on this machine was above US $2,000,000; This was achieved with few weeks of asset optimization audit and by utilizing the different expertise knowledge that is seldom combined.

The largest contribution in achieving the results was the possibility to collect the data in a controlled way and the ABB experts who could analyse the results and call for the right actions.

4. Conclusion

This paper has shown that the selection and installation of flowmeters can effect the measurement results in a very drastically way. By selecting the correct flow meter for the application and installing it in the process correctly major improvement can be achieved. Wrong installation or selection can be a major restriction for future process improvements.

We have also seen that the possibility to utilize different supplier's expertise and put them together as a team can make a large difference in the possibility to achieve process optimization results.

The team must contain of people with high level of control expertise, automation system knowledge and process knowledge perform the optimizations audit. The key factor in these audits is the analysis tools, which helps the experts to identify the issues by using statistical and time series analysis and also identification of process model for tuning.

It has also shown that the new technologies and modern signal processing can result in new utilization of the multi variables that is possible to measure with the Coriolis meters.

In the future we will see many new inventions in this area.

References

B.Eng. Hege Bjönnes, Endress+Hauser Flowtec AG

S.R. Subramani, ABB Industry, Singapore

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