IQWetendMD - A Metso Automation control strategy framework for any combination of Weight, Moisture, Ash and White Water Consistency

Building on Metso’s extensive papermaking process knowhow, the IQWetendMD model predictive control (MPC) integrates fast responding wet end process controls and absolute dry end feedback controls to achieve a higher level of paper quality control performance. Paper process variability is controlled before the wet sheet is formed, attenuating any cycling or upsets that are difficult or impossible to control using only traditional feedback controls from the dry end. The result: better runnability and quality through more stable short circulation system and uniformity of the wet sheet.

Description

IQWetendMD stabilizes the wet end operation of any paper, tissue or board making process. Its scope and capability can be expanded. The control can be configured step-by-step for simple basis weight, feedforward weight and moisture control applications to more complex ash and retention control applications. The control models predict how changing one variable will affect the others and the final product quality. A full scope IQWetendMD can manage simultaneously the thick stock flow, filler flows, retention chemical flows and steam pressures to achieve smoother process operation during steady state, speed changes, and process upsets. Feedforward weight control adjusts stock flow to control the estimated future weight of the sheet as it is formed. This allows immediate compensation for changes in stock consistency and wire speed, thus preventing the occurrence of basis weight and moisture upsets.

IQWetendMD stabilizes the short circulation system operation, the forming process and the paper drying process by integrating wet end analyzers and dry end scanning sensors into a multivariable model predictive controller. The extra value of multivariable control over traditional single input, single output MD controls is realized especially when the machine is experiencing upsets, during changes, after startups or after long web breaks. IQWetendMD is a more reliable and robust control algorithm compared to traditional MD control algorithms. This is because of the unique robustness feature in predictive control algorithm and the adaptation feature built into the control. IQWetendMD continues to regulate wet sheet properties during a break by using measurements of white water consistency and headbox ash by regulating retention chemical flows, and filler flows. The sheet is re-threaded more easily and is very close to its target when it reaches the dry end.

IQWetendMD includes not only the basic feedback control applications but also it can include additional features such as coordinated machine speed and white water target calculation. Coordinated speed change utilizes the properties of the model predictive control in order to have fast speed changes without any affect on the quality of the product. White water target calculation uses total retention target and process flows to calculate the target value for the white water consistency control, thus guaranteeing that the white water consistency target is optimum for the required retention target.

It is possible to add additional feedforward features such as external disturbance variables and thick stock ash control (TSAshMD). Connecting external disturbance variables to IQWetendMD, for example the broke flow, minimizes the effect of the disturbance on paper quality. TSAshMD control maintains the filler flow into the short circulation constant by measuring the filler amount in thick stock flow and adjusting the fresh filler feed into the short circulation.

Benefits to the Papermaker

Wet end stability.
Reduced variability in paper oven dry weight, moisture and ash.
Increased machine efficiency. A significant decrease in wet end breaks and faster recovery from breaks and other upsets is achieved.
Sheet quality is on target after a break, improving sheet threading and avoiding long quality recovery times. Saleable production is achieved soon after a break.
Faster machine starups and quicker grade changes.
Savings in retention aid chemical consumption.
Stable sheet quality when broke content is changed.
CD basis weight dilution controls are better since the consistency of the dilution water is very stable.

Control configuration

IQWetendMD can be configured differently for any machine depending
on the needs of the process and the measurements available. Controls
can start from a simple weight and moisture combination and be
upgraded to a complete package of weight, moisture, ash and white
water consistency with other add-on features.

Product name	Description & Requirements
IQWetendMD, Base Application	Includes control core for the optimization and base displays. Requires metsoDNA or Damatix XDi (v. 6.1 or higher)
Oven-Dry Weight Control	Includes feedback and feedforward weight controls for one headbox. Requires basis weight and moisture measurement from the scanner and the total consistency measurement of the thick stock. Manipulates thick stock flow.
Paper Moisture Control	Includes moisture control for one drying section. One application is needed per moisture measurement. Requires moisture measurement from the scanner. Adjusts one manipulated variable, normally steam pressure but instead of steam can manipulate other variable such as air temperature of air dryer.
Paper Ash Control	Requires paper ash measurement from the scanner. Manipulates fresh filler flow to the short circulation.
White Water Consistency Control	Requires reliable white water consistency measurement. Manipulates retention aid flow to the short circulation.
White Water Consistency Target Calculation	White water consistency target calculation based on the total retention target and process flows. Requires headbox and white water consistency measurements, headbox jet flow and white water consistency control (S9746).
Headbox Ash Control During Sheet Break	Requires reliable headbox ash measurement. Manipulates fresh filler flow to the short circulation.
Coordinated Machine Speed Change	Coordinates automatically thick stock flow, filler flow, retention aid flow, steam pressure and headbox pressure in order to keep the measurements in spec throughout a production speed change. Requires interface to the machine drive system (data link or I/O).
External Disturbance Variable	External disturbance variable, such as broke flow, can be connected to the control core. The variable connected to the controller must have clear and measurable effect on the controlled variables. Up to three external disturbance variables can be connected.
TSAshMD,Thick Stock Ash Control	Requires reliable thick stock ash measurement. Manipulates fresh filler flow to the short circulation.
Tools For Process Modeling	Data collection application and tools for collecting and analyzing the process data for control tuning.

Related Controls(sold separately)

Technical Features

Utilises multivariable model predictive control (MPC) algorithm.
Uses past history and the model to calculate optimal control actions many steps forward into the future. Finds a smooth path to meet a desired setpoint trajectory.
Rate and level constraints can be included in the optimization.
Constant setpoint or desired setpoint trajectory (when and how fast to move the process) can be specified.
Models and tuning parameters can be programmed based on process events or conditions e.g. breaks, grade changes.
On-line model manager: loads, modifies, saves to library and generate deadtime plus 1st or 2nd order models.
On-line model adaptation for weight, ash and white water consistency for high ash level containing paper grades. The adaptation is based on the ash retention level changes.
Operator interface is as simple as normal PID loops. Automatic switching between paper ash (normal running) and headbox ash (during and following breaks)
IQWetendMD overview display, model, tuning and help displays are included.

Base controls: weight, feedforward weight and moisture only

Base controls and retention control (for machines without the fresh filler feed).

Base controls and ash control (for machine with fresh filler feed and without wet end analysers)

Full scope of IQWetendMD

IQWetendMD covers weight, moisture, ash, white water consistency and disturbance handling. The system can be built to
any size depending on papermaking process requirements. The block diagrams shown above are examples of building the
appropriate control solution.