US3575798A - Process for maintaining steam dryer pressure below the maximum available - Google Patents

Abstract

A METHOD OF CONTROLLING A PHYSICAL PROCESS, LIKE A PAPER MAKING MACHINE, IS DISCLOSED. THE PROCESS HAS AT LEAST TWO VARIABLES. THE FIRST VARIABLE IN THE PROCESS IS FREQUENTLY ADJUSTED. THE SECOND VARIABLE IS LESS FREQUENTLY ADJUSTED. THE SECOND VARIABLE IS RELATIVELY STABLE. SETTING THE RELATIVELY STABLE VARIABLE ESTABLISHES AN OPERATING RANGE FOR THE FIRST VARIABLE. THE MEASURABLE PRODUCTION FROM THE PHYSICAL PROCESS IS THEREBY IMPROVED AND/OR INCREASED. IN A PAPER MAKING MACHINE, MACHINE SPEED IS ADJUSTED INFREQUENTLY IN RESPONSE TO THE MOISTURE CONTENT OF PAPER AFTER A DRYER SECTION, TO ESTABLISH AN OPERATING RANGE FOR DRYER STEAM PRESSURE WHICH IS BELOW THE MAXIMUM AVAILABLE STEAM PRESSURE, THE DRYER STEAM PRESSURE

THEN BEING ADJUSTED WITHIN THE RANGE TO CORRECT FOR MOISTURE VARIATIONS.

Description

Apri! 20, 1971 PROCESS FOR MAINTAI E B. DAHLIN NING, STEAM DRYER PRESSURE BELOW THE MAXIMUM AVAILABLE Filed July 3, 1968 INPUT 0F 42 MATERiAL N Fae-i 2 SENSOR FOR VARIABLEi i so 2 PROCESS 322 CONTROLLER 26 2a A 2 SENSOR FOR 2 VARIABLE2 24 25 V s SENSOR FOR 2 M v oUTPU F r mHED VAR'ABLE 3 SENSOR SENSOR 26 CONTROLLER I [In 12202. ERIK B. DAHLIN F l6. 2 ,2

United States Patent 3,575,798 PROCESS FOR MAINTAINING STEAM DRYER PRESSURE BELOW THE MAXIMUM AVAILARLE Erik B. Dahiin, Saratoga, Calif, assignor to International Business Machines Corporation, Armonk, N.Y. Filed July 3, 1968, Ser. No. 742,318 Int. Cl. 1321f 5/02 U.S. Cl. 162-198 3 Claims ABSTRACT OF THE DISCLOSURE A method of controlling a physical process, like a paper making machine, is disclosed. The process has at least two variables. The first variable in the process is frequently adjusted. The second variable is less frequently adjusted. The second variable is relatively stable. Sett ng the relatively stable variable establishes an operating range for the first variable. The measurable production from the physical process is thereby improved and/or increased. In a paper making machine, machine speed is adjusted infrequently in response to the moisture content of paper after a dryer section, to establish an operating range for dryer steam pressure which is below the maximum available steam pressure, the dryer steam pressure then being adjusted within the range to correct for moisture variations.

CROSS REFERENCES TO RELATED APPLICATIONS An invention described in US. patent application, Ser. No. 599,878, filed Dec. 7, 1966, now Pat. No. 3,534,400, by the same inventor and entitled Parameter Identification Method For Process Control Systems can be used in conjunction with the invention of this application. The same is true of another invention described in earlier filed US. application, Ser. No. 599,879, filed Dec. 7, 1966 by the same inventor and entitled Coeflicient Tuning Methods For Process Control Systems. Both the noted patent applications are assigned to the same assignee as this patent application.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates to a method of controlling a physical process. More particularly, it relates to increasing the production rate, and the quality, of a paper making machine.

(2) Description of the prior art This invention relates to the art involving paper making machines, particularly those that produce heavy paper, such as board or bag paper. Such machines have been characterized in the past by dryer-limited operation. This means that the production rate of the machine is limited by the machines ability to dry the paper stock. The speed of the machine must be kept down so as to allow the paper stock to be dried to the proper degree before exiting from the dryer section of the machine.

Steam filled rollers are used as dryers. The steam is frequently supplied to such rollers in an unregulated manner; that is, the rollers receive as much steam as the central boiler can produce.

In order to control the drying rate in such equipment, the speed at which the paper passes over the dryer rolls (i.e. machine speed) is varied. Due to the mechanical nature of the drive mechanism, this speed control operation has generally been implemented at no less than fifteen minute intervals. Thus, should a measurement of the papers moisture content indicate that the drying rate is improper, it can be seen that paper of sub-standard 3,575,798 Patented Apr. 20, 1971 ice quality can be produced for as much as fifteen minutes before suitable control action is taken. Typical paper machine production rates are in the vicinity of tons of paper per hour. This means that a significant amount of below grade, or low quality, paper can be coming out of the machine during a fifteen minute interval. Such paper must either be scrapped, or sold at a reduced price, thereby adversely affecting the entire operation of the particular paper mill in question.

Accordingly, it is a general object of this invention to improve the art of controlling physical processes and particularly paper making processes.

A more particular object of this invention is to exercise control action in a manner most efiicient from the viewpoint of utilizing time; that is, adjust one manipulated variable infrequently so as to establish an operating range for another manipulated variable and then adjusting the last-mentioned variable frequently.

A still more particular object of this invention is to improve the production rate of a paper making machine by controlling and adjusting machine speed less frequently than dryer pressure.

SUMMARY OF THE INVENTION In accordance with one aspect of my invention, a method for controlling physical processes is disclosed. That method comprises the steps of adjusting one manipulated, and relatively stable, variable infrequently so as to establish an operating range for a frequently changed variable. The value of that variable is thereby optimized. The frequently changed variable performs a fine regulation role. The entire associated physical process, involving the manipulating of physical quantities and operating thereon in accordance with rules of thermodynamics, etc. is improved.

In accordance with another aspect of my invention related specifically to paper making machines, the machine speed is repetitively measured. In addition, the moisture content of the paper is also repetitively measured along the length of the paper being formed. Further, the maximum available steam pressure for the dryers is measured. The machine speed is then adjusted so that it is at a suitable value in reference to a steam pressure of some increment less than the maximum pressure available. This setting of the dryer speed takes place every fifteen minutes or so. Then, adjustments are made to the steam pressure within the increment of steam pressure available. These adjustments refine the degree of drying effected, or implemented, by the paper machine.

To contrast this invention with the prior art requires an understanding of the problem and earlier solutions. The problem of increasing the production rate of a paper machine, or any physical process for that matter, has been known to the prior art. Various control schemes using one or another parameter, or even a plurality of parameters, adjustments have also been known to the prior art.

The advantage of this invention lies in that it teaches a specific combination of adjustments to process variables that takes advantage of the characteristics of each variable. A variable, which is only adjusted infrequently, is set for most economical operation. An unstable variable, which by contrast is varied, or adjusted, frequently, keeps the stable variable within the most economical operating range.

Exploring this more fully and looking specifically at paper making machines, the most typical approach to solving the dryer limited operating problem is simply to operate the paper making machine at maximum dryer steam pressure. That is, all the steam supplied by the boiler is forwarded to the dryers in an unregulated manner. The machine speed is then adjusted every fifteen minutes or so in accordance with the steam pressure then available to the dryers. As noted earlier, this means that significant amounts of paper of a lower than desirable quality may well be produced.

Alternatively, the prior art has adjusted the steam supplied to the dryers. That is, some dryers have not operated at wide open steam pressures, but rather the steam supplied to them has been regulated. Nowhere in the prior art has there been a teaching of the combined steps involving adjusting the machine speed infrequently and the steam pressure frequently as is taught in this invention.

This invention is particularly applicable to implementation on an automated paper making machine. That paper making machine could be under the control of an IBM 1800 Process Control Computer, for example. The already improved operation resulting from such a control mode is enhanced by the method of this invention. However, the method of this invention could be practiced by hand if necessary or by combinations of analog and digital equipment if necessary.

The primary economic advantage from this invention is that the production rate of a paper machine is, in view of the dryer limited problem, increased to a practical upper limit. Below quality paper, as measured in tons, is significantly reduced over given time periods. The quality of production being maintained relatively constant over a long time period results in increased profits for the owner and operator of the paper making machine so controlled.

Accordingly, the foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a generalized physical process being controlled according to the method of my invention.

FIG. 2 is a block diagram of a paper making machine being controlled according to the method of my invention.

DESCRIPTION OF PREFERRED EMBODIMENT Both FIGS. 1 and 2 will be used to describe a preferred embodiment of this invention. Before entering into that description, it should be noted that my invention includes a new method of interrelating measured and controlled variables in a process. It involves taking a series of physical measurements, performing certain mathematical operations, on the values obtained by these measurements, and then adjusting certain variables in accordance With the results obtained from the computation. Should a computer be utilized to perform the computation, the computer program is not my invention. My invention is a control technique, or more explicitly, my invention is a method of adjusting a setting of process variables in accordance with signals obtained from the process being controlled.

With that as an introduction, FIG. 1 will be described. Process in FIG. 1 receives an input of material 12 and generates an output of modified material 14. The material provided to process 10 from input 12 is operated upon by the process in accordance with certain rules of thermodynamics and kinetics, etc. For example, should it be a paper making process, an input of water and pulp is cooked and dried until finished paper is formed. In that example, the finished paper is the output of modified material 14.

A controller 16 can control identifiable variables present in process 10. In order to do this, it is necessary to know the state of the variables. Sensor 18 receives as a signal from process 10 on line 20' the value of a first variable. That value is represented by an electrical signal and forwarded on line 22 to controller 16. Simimarly, sensor 24 receives as an input from process 10 on line 26 a value, or measurement, of a second variable. That value is represented by an electrical signal on line 28 and forwarded to controller 16. Sensor 25 measures a property of the output of modified material 14 (a third variable. The value of that property is represented by an electrical signal on line 27 and forwarded to controller 16. Controller 16 evaluates the signals received on lines 22, 27, and 28 and decides whether or not the values of variables 1 and 2 are to be modified. If so, signals are forwarded on lines 30, 32 to process 10, and the values of variable 1 and variable 2 are modified in accordance with the signals present on lines 30, 32.

In a preferred embodiment of this invention, variable 1 could be a relatively stable variable and one that is infrequently adjusted. Variable 2, by contrast, could be adjusted relatively frequently; that is, its value could change rapidly as a function of time. This being so, variable 1 would be adjusted in response to a signal on line 30 relatively infrequently when compared to the adjustment of variable 2 in response to signals on line 32. This type of adjustment results in a smoothing operation on the output of modified material 14.

The method of my invention will become clearer by reference to FIG. 2. There, a preferred embodiment referencing a paper making process is shown and described.

A knowledge of paper making machines is assumed on the part of one skilled in this art. A description of such a machine can be found in McGraw-Hill Encyclopedia of Science and Technology, vol. 4, p. 289-1960 edition and also in vol. 9 at p. 540 of the same encyclopedia. Thus, the apparatus shown in FIG. 2 is depicted schematically and only elements pertinent to this invention are set forth.

With reference then to FIG. 2, a mixture 200 of liquid and pulp is contained in head box 202. It is ejected through portal 204 as a spray and deposited upon endless belt 206. Belt 206 is driven by drive means 208, shown here as a motor driven roller which must necessarily co operate with an associated idler roller, or even another positive drive roller, 210. The mixture 200 of pulp and fluid thus deposited on endless belt 206 is formed into finished paper and Wound on takeup reel 212. To convert the mixture 200 of pulp and fluid into paper, it is necessary to remove some of the fiuid from that mixture. This is accomplished by dryer means 214, shown here as a conventional steam heated cylinder receiving steam from a boiler 216. (In actual practice, more than one cylinder would be present.) To convert the mixture 200 of pul and fluid into finished paper of certain characteristics requires the measurement and manipulation of many variables, only two of which are considered in the teaching of this invention; namely, machine speed and steam pressure. Machine speed can be measured in feet per unit of time and the measurement is taken in relation to endless belt 206, while steam pressure can be expressed in the conventional units of pounds per square inch gauge or absolute. Controller 16 is also shown, as are sensors 18 and 24. Completing the description of the exemplary equipment set forth in FIG. 2, note the provision of a valve 218 in the steam supply line 219.

To operate according to the principles of this invention, assume that the speed of drive roller 208, as well as the setting of valve 218, have been established at some initial value. Sensor 18, which could be a moisture gauge, such as the Moistron gauge of Industrial Nucleonics, then begins to measure the moisture content of the paper leaving the dryer section. These measurements are translated into electrical signals which are relayed on line 22 to controller 16. Similarly, the value of steam pressure supplied to drum 214 is monitored by sensor 24 and those values are transmitted to controller 16 in the form of electrical signals on line 28. Equipment for sensing steam pressure and converting it to electrical signals is commercially available from companies like Taylor Instruments or Leeds and Northrup. Controller 16 has stored within it a set point value for the maximum pressure available from boiler 216. It also has within it an algorithm expressing a relationship of average current steam pressure and machine speed for a particular paper quality. One such algorithm is set forth in Equation 1:

(l) Speed =speed |-A (maximum steam pres sure-average current steam pressure-j-A where n=a given sampling time A A are constants.

Having received these measurements on a continuing basis from sensors 18, 24, controller 16 will adjust the machine speed every fifteen minutes or so. This adjustment is made by means of signals on line 30 to a motor associated with drive means 208. The machine speed is so set that not all of the drying effect available from dryer 214 is used; that is, a slight range for steam adjust ment is left. However, by the algorithm (1), the controller sets the machine at the maximum speed possible under existing steam demand. Still, a fine moisture regulation, using small variations in the steam pressure, is available. More frequently, the controller 16 determines how the steam pressure should be varied within that range so as to maintain constant moisture contents in the sheet. Controller 16 does this by sensing the pressure available from boiler 216 and making the necessary adjustments by means of signals on line 32 to valve 218. The following algorithm expresses the relationship between moisture content and steam pressure.

The following table gives an example of typical variable values in accordance with the teachings of my invention.

Steam Moisture pressure, content, p.s.i.g. percent While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

*1. A method of maintaining the quality of paper produced by a paper making machine having a dryer section, means for controlling dryer header steam pressure, and means for controlling machine speed, comprising the steps of:

measuring moisture content of paper stock at a predetermined point after said dryer section in said machine to provide a signal indication of said moisture content thereat;

providing a signal indication of the maximum available steam pressure for said dryer;

adjusting, in response to said signal indication of maximum available steam pressure, said machine speed at predetermined infrequent intervals to a value estimated to require dryer header steam pressure a predetermined deviation less than said maximum available steam pressure, there-by establishing an operating range for adjustment of said dryer header steam pressure;

comparing said moisture content signal to a predetermined indication of desired moisture content to provide an error signal; and

adjusting, in response to said error signal, said dryer header steam pressure at predetermined frequent intervals such that a plurality of said frequent intervals are included within each of said infrequent intervals, such adjustment being in a direction to cause the reduction of said error signal to zero.

2. The method of claim 1 wherein said step of adjusting machine speed additionally comprises:

periodically measuring said dryer header steam pressure to provide signal indications thereof;

averaging said signal indications of dryer header steam pressure over a preceding time interval of fixed duration to provide a signal indication of the average current header steam pressure;

subtracting said signal indication of averaged header pressure from said signal indication of maximum available steam pressure and a constant value to generate an error term signal, said constant value representing said predetermined deviation; and

adjusting said machine speed at said predetermined infrequent intervals in response to said error term signal in a direction and of a value to cause the reduction of said error term signal to zero.

3. The method of claim 1 wherein said step of adjusting machine speed comprises:

periodically measuring said dryer header steam pressure to provide signal indications thereof;

averaging said signal indications of dryer header steam pressure over a preceding time inter-val of fixed duration to provide a signal indication of the average current steam pressure; and

generating a speed control signal to operate said speed control in response to said signal indications of maximum available steam pressure and of average current steam pressure at said predetermined infrequent intervals in accordance with the following algorithm:

speed =speed +A (maximum steam pressure average current steam pressure +A Where n=a given sampling time; A =a predetermined constant relating steam pressure to machine speed for a vine desired paper; A =a constant representing said predetermined deviation of dryer header pressure below the maximum available steam pressure.

References Cited UNITED STATES PATENTS 1/1970 Hart et al. 162252X 4/1927 Witham, Jr. 162263X 9/1933 Kruse 162198 1/ 1963 Petitjean 162263X 7/1966 Canter, Ir. 162-263X US. Cl. X.R.

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