US3779863A

US3779863A - Stock flow control system for feeding a paper machine headbox

Info

Publication number: US3779863A
Application number: US00010713A
Authority: US
Inventors: D Wahren
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1969-02-18
Filing date: 1970-02-12
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18
Also published as: DE2007082A1; DE2007082B2; GB1290942A; FI51384C; SE345295B; FI51384B; DE2007082C3

Abstract

Stock feed to a paper machine headbox is controlled by a stock (fan) pump which is controlled to run at variable speeds, the input of stock to the pump originating from a thick stock containing conduit and a conduit containing mixing water. The speed of the pump motor is controlled by a signal from a control element which is responsive to measurements of the stock level or pressure within the headbox and an adjusted value of the forming wire speed. In addition, a thick stock pump located in the thick stock conduit is adjusted to run at a variable speed by a control device which is responsive to signals indicating the pressure drop across an orifice in the thick stock conduit and a signal based on the web speed. Alternatively, the thick stock pump may run at a constant speed and the control device can be used to control an orifice opening or throttling device located in the thick stock conduit between the pressure-drop determining orifice and the stock (fan) pump.

Description

United States Patent 1191 3,779,863 Wahren Dec. 18, 1973 1 STOCK FLOW: CONTROL SYSTEM FOR FEEDING A PAPER MACHINE HEADBOX lnventor:

Assignee:

Filed:

App]. No.:

Douglas Wahreln, Taby, Sweden Allmanna Svenska Ele cktriska Aktiebolaget, Vasteras, Sweden Feb. 12, 1970 Foreign Application Priority Data Feb. 18, 1969 Sweden 2196/69 US. Cl 162/253, 162/258, 162/259,

Int. Cl. n21: l /06,D21f l/08 Field of Search 162/336, 339, 259,

References Cited UNITED STATES PATENTS Taylor Primary Examiner-S. Leon Bashore Assistant Examiner-Richard H. Tushin Attorneyl(enyon and Kenyon Reilly Carr and Chapin [5 7] ABSTRACT Stock feed to a paper machine headbox is controlled by a stock (fan) pump which is controlled to run at variable speeds, the input of stock to the pump originating from a thick stock containing conduit and a conduit containing mixing water. The speed of the pump motor is controlled by a signal from a control element which is responsive to measurements of the stock level or pressure within the headbox and an adjusted value of the forming wire speed. In addition, a thick stock pump located in the thick stock conduit is adjusted to run at a variable speed by a control device which is responsive to signals indicating the pressure drop across an orifice in the thick stock conduit and a signal based on the web speed. Alternatively, the thick stock pump may run at a constant speed and the control device can be used to control an orifice opening or throttling device located in thethick stock conduit between the pressure-drop determining orifice and the stock (fan) pump.

4 Claims, 2 Drawing Figures INYENTOR. DOUGLAS WAHREN STOC K FLOW CONTROL SYSTEM FOR FEEDING A PAPER MACHINE HEADBOX BACKGROUND OF THE INVENTION The invention presented here refers to an arrangement for stock proportioning of paper machines, comprising a thick stock pump for feeding pulp to the system, which feeds a headbox system, from which stock is delivered on to a wire or wet felt. Such a system consists of a white-water chest, fan (mixture) pump, piping and, possibly, cleaning devices as well as a headbox, from which the stock suspension diluted with whitewater is uniformly distributed over the width of the machine on a wire or wet felt.

A usual arrangement for such stock proportioning is shown in FIG. 1, and comprises a level box 1 provided with an overflow (by the arrow 2) or some other form of lever control, arranged at a certain height over a proportioning point 3, where the driving pressure for the thick stock flow is determined by the difference between levels in the levelbox-and white-water chest. Water, for example, previously used water (white-water, arrow 4) is fed to the proportioning point 3, and thick stock is pumped to a levelbox (at 5). With the object of obtaining a constant stock flow (constant basisweight) to the headbox and from there to the paper machine, the incoming thick stock flow is controlled by means of a manual or automatic, for example, computer-controlled, valve 6. The accuracy requirement means that the levelbox must normally be set up at a height of 4-8m over the white-water level so as to obtain the necessary pressure drop across the basis-weight valve. This system will consequently take up considerable space and will be expensive in view of the piping involved etc. This may lead to disturbances, varying degree of blocking up of the valve owing to the low pressure drop available. Setting up of the levelbox at a considerable height over the proportioning position will require a higher building (insulation, etc.) which can seldom be accomplished. In such a system the influence of friction in pipes may be aggravated and, furthermore, changes in the properties of the stock, for exam ple, beating degree, temperature or degree of cooking, cannot be simply measured or calculated. This means that difficulties may arise with automatic control of the basis-weight valve. It is also difiicult to achieve a constant stockflow owing to the presence of spontaneous flow variations caused by, for example, air admixture, wave phenomena due to the free liquid surface in the levelbox, and fluctuations in the content of additives. The influence of the air is important. A level variation of 3 cm at a head of, for example, 5 in will lead to a change in the basis-weight of only 0.2 percent. Variation of the air content leads to an altered resistance in the piping, pressure loss due to friction and thus a considerably altered proportioning. Discontinuity in the piping and accumulation of air are normal phenomena occurring in certain designs of the levelbox. In conjunction with stock proportioning it has been considered desirable to relate this to the wire speed so that a constant basisweight of the paper is maintained for changes in the machine speed or slice outflow from the headbox. Hitherto it has been difficult to satisfy these requirements, owing to the difficulty of moving the levelbox, which necessitates controlling of the basisweight valve, whose characteristics must then be stored in a computer, and to the insufficient developments in measuring techniques in this respect.

In the type of application treated here the measuring accuracy requirements are very severe, since measuring errors lead to basis-weight variations, which in their turn result in magnified moisture content variations in the finished paper. A relatively small variation in the basis-weight results in a relatively large variation in the moisture content. As an example of the order of magnitude involved it can be mentioned that a transient change in the stock' flow of 1 percent results in a transient change in the moisture content of 2 percent in absolute dimensions, i.e., 25 percent in relative dimensions.

The demand imposed on flowmeters can be expressed in such a way that their measuring accuracy must be considerably better than the spontaneous flow fluctuations, which the flow control is intended to reduce on the basis of signals from these instruments. An acceptable value of the variations between the maximum and the minimum values of the moisture content may be i 0.5 percent, which means that corresponding variations for the stock flow must be less than :t 0.25 percent, i.e., a value that is difficult to achieve with conventional instruments of the magnetic flow-meter type.

It has been experimentally found that this necessary accuracy can be achieved by the measurement of either the difi'erential pressure between maximum and minimum pressure within a comparatively short Venturi tube or the pressure drop across a suitably arranged oriiice, in which the relationship of the flow rate with the network strength of the stock is sufficiently high, without however, cavitation phenomena occurring. If the necessary flow rate is to be achieved, a larger pressure drop will be required than that usually obtained with the levelbox arrangement shown in principle in FIG. 1. When the arrangements with very long pipes outlined above are used, instability easily occurs owing to the hydrodynamic properties of the stock and/or air pockets.

. The invention presented here refers to an arrangement for solving the above-mentioned and other problems and is of the type mentioned in the first paragraph of this application. The main feature of this is that an orifice is arranged in the stock line to the headbox. The pressure drop across this orifice is measured and the stock flow is controlled on a desired value for this pressure drop, which is usually proportional to the square of the web (wire) speed.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 shows the basic principle of this invention. A headbox 12, to which pulp and water (stock) is pumped via a pump 13 (see arrow 14) located by the inlet side, is arranged in the usual manner at the point to the headbox, in this case to the inlet of the mixture pump 13. The pressure drop across the orifice is measured in the normal way for such devices, and the signal for this pressure drop is fed to a control device 19 for the prime mover 21 driving the thick stock pump 17. The prime mover 21 may be a dc. motor controlled by an S.C.R. converter or by some other similar means, but it may also be an a.c. commutator motor or a hydraulic drive motor, where the supply of the drive medium can be controlled, and thus also the motor speed. The signal (at 20) gives the actual value of the pressure drop across the orifice 18 and the speed of the motor (21) is controlled on a desired value, which is determined by the machine speed and possibly also the basis-weight as described below.

The speed control of motor 21 can be replaced by a motor intended to run at constant speed and supplemented by an extra orifice 22 for the thick stock flow. This orifice is adjustable and, when its setting is changed, it alters the the pressure drop across the orifice 18.

In addition to the solution of the above-mentioned problems, this system has the advantage in both these cases that, besides there being hardly any air admixture in the stock (unlike the case where free liquid surfaces occur or an overflow is used, see the FIG. 1), the lines 23 can be arranged vertically, which prevents the accumulation of air normally occurring with descending pulp flows.

The speed at the end of the web, shown here schematically by roll 37, is measured with a tacho-generator 38, whose output signal is squared in a squaring element 39. The output signal of the latter in its turn will be proportional to the square of the web speed. A suitable pulp proportioning, for example, suitable basisweight is set with a proportionalizing element such as a potentiometer 29, and this proportioning (basisweight) signal 30 is fed to the control element 19 an will constitute the desired value for the pressure drop across the orifice 18. This value is thus adjusted for altered motor speed (at 21) or altered throttling (at 22) and therefore altered proportioning (of stock to the paper machine). The desired-value signal 30 will therefore be proportional to the square of the web speed (v and is thus altered as the machine speed changes. The speed of the wire 11 (not exactly the same as v) is measured in a suitable manner by means of a tachogenerator 24 or by some other conventional means at the drive 25 of the wire. This measuring element gives 26 a signal proportional to the wire speed (v,) at 11. This signal 26 is fed to a squaring element 27, whose out-put signal is therefore proportional to the square of the speed (v,).

The v, signal can also be taken out across another proportionalizing element 31, for example, a potentiometer, which is set to a desired value of the square of the slice outflow quotient 4: i.e., the desired ratio between the slice outflow velocity and the wire speed. An actual value for these quantities can be obtained from the pressure or level signals in the headbox, which are measured at 32 and 33 and added in the control element 34 with the desired value (from 31) to form an error signal, which is used for controlling the drive 35 of a stock pump 13 for controlling the desired slice outflow pressure (arrow 36) or outflow quotient. An altered web speed thus also results here in an altered desired value and thus changing of the pump speed.

where y is the flow velocity in m/s, 1 the network strength of the stock in Newton/m and 4) the density of the suspending medium (water), approx. l0 kg/m. At the same time Reynolds number (Re) must be Re v D dB/u 5,000,

where D is the diameter of the measuring element in m and p. the dynamic viscosity (kg/m/s).

Cavitation in the measuring element should be avoided (this imposes'a practical upper limit). The pressure value across the orifice 18 can also be controlled on a value proportional to the total-head in the headbox, comprising fluid (stock) pressure plus pressure of any air cushion, plus possibly a negative pressure addition for vacuum running at low speeds.

The flow measurement can also be accomplished indirectly by measurement of the head of pump 17 and the pump speed (at 21) and combining of these values. Examples according to the above can be varied in different ways within the framework of the following patent claims.

What is claimed is:

1. Device for proportioning the pulp suspension for paper machines, comprising feed conduits for pumping pulp and water to a mixture pump for pumping pulp mixed with water (stock) to a headbox, from which (headbox) the stock is delivered to a wire for further transport of stock, the driving means of the mixture pump being controlled from a control element responsive to signals from a measuring means within the headbox measuring the level or pressure in the same and to signals from means responsive to the wire speed, one of said conduits being a pulp conduit provided with a thick stock pump for pumping pulp to said mixture pump to be mixed with water from another of said conduits before said mixture pump, in said pulp conduit being arranged an orifice for obtaining a pressure drop and means for measuring said pressure drop and control means for controlling the driving means for said thick stock pump, a reference value for desired basis weight for the suspension being obtained in reference means responsive to the produced web speed, the output of which means is fed to the input of the control means of the thick stock pump together with the output from the pressure drop measuring means, the difference between pressure drop and reference values controlling the feeding rate of said thick stock pump.

2. Device according to claim 1 characterized in that said orifice for obtaining a pressure drop comprises a Venturi tube.

3. Device according to claim 1, characterized in that the control means are connected to another orifice in the pulp line between the pumps which orifice is provided with means for altering the through-flow.

4. Device according to claim 1, including a reference device for stock pressure in the headbox, the measuring cushions and possibly plus a negative pressure addition for vacuum running at low speeds, the output signals of said measuring means and reference means together forming an error signal which is fed to the control means in the headbox measuring the stock pressure in 5 means of the driving means for the mixture pump.

eluding fluid stock pressure plus pressure of any air

Claims

2. Device according to claim 1 characterized in that said orifice for obtaining a pressure drop comprises a Venturi tube.
3. Device according to claim 1, characterized in that the control means are connected to another orifice in the pulp line between the pumps which orifice is provided with means for altering the through-flow.
4. Device according to claim 1, including a reference device for stock pressure in the headbox, the measuring means in the headbox measuring the stock pressure including fluid stock pressure plus pressure of any air cushions and possibly plus a negative pressure addition for vacuum running at low speeds, the output signals of said measuring means and reference means together forming an error signal which is fed to the control means of the driving means for the mixture pump.