DD204964A5 - METHOD FOR PREVENTING THE CONTAMINATION OF A SIEVE DEVICE - Google Patents

Abstract

The aim and object of the invention are to provide a method for preventing the contamination of a screening device, with which it is possible that holes or slots in a screening device are not polluted and blocking in the valve within the waste line d. Siebes is excluded. The object is achieved in that the valve is opened from a first to a second position when the pressure drop exceeds a first predetermined value and the valve is reset after a time to its first position. DD # AP

Description

Berlin, don 22,2,1983 61 538/13

Method for preventing the contamination of a screening device

Field of application of the invention

The present invention relates to a method for preventing the contamination of a screening device within a screen and / or the cutting off of a Abproduktventils disposed in a Abproduktleitung for screening a fiber suspension, further comprising a flow of accepts, which is excreted by an accepts line, and Pressure drop at the screening device, which is detected for the control of the diversion of a waste product by means of the valve in the waste line.

characteristics the known technical solutions

Sieves of different designs are used in pulp and paper shredding for cleaning cellulose fiber suspensions. In the printing screens most commonly used today, the screen surfaces are either fixed or rotating. In order to prevent clogging or soiling of the screening device, the screens are provided with blades which have an ^i: relative speed with respect to the screen surface and which, during sieving, have a destructive effect on the charge scraping on the fiber mesh formed on the screen surface and thereby prevent complete fouling of the holes or slots in the screening device. The tendency to clogging can often, but not always, by increasing the

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Relative rotational speed between the surface of the screening device and the film can be prevented * A significant increase in this rotational speed can lead to the desired cleaning _s but at the cost of increased costs for the necessary energy consumption »

However, clogging or soiling may also occur in the valve used to control the waste diversion. * This fouling usually occurs "when the amount of waste is small."

The SE-PS 304 677 discloses a sieve device, which contains a valve in the waste product line of the sieve »The said conduit has a regulating device, which is characterized in that it is arranged in a predetermined best possible regulation position, further comprising an actuator for opening the Valve in a position that opens much further than the said best regulation position, without disturbing the setting of said regulator * Also included is a sensor which responds to a predetermined minimum flow rate in said waste line to the valve by said actuator The sensor is characterized in that the valve does not open in the best regulation position in response to changes in the flow rate through the waste line; These changes are not sufficient to reduce the flow to the minimum value. The sensor will operate to move the valve to the best position

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reset when the flow rate has increased to a value substantially above the minimum value,

Object of the invention

An object of the present invention is that holes or slots in a screening device are not polluted »

the essence of the invention

The invention has for its object to provide a method for preventing the contamination of a screening device, which makes it possible to -sine blocking in the valve within Öer Abproduktleitung the screen to prevent

According to the invention, this is accomplished by detecting the pressure drop across the screen and controlling the product depletion: by means of a bleed valve , opening this valve from a first to a second position when the sensed pressure drop exceeds a first predetermined value, and jerking the valve in the same first position after a certain time ', d, h "in its position before the opening,., The waste product valve is preferably fully open, d. h, _t the second position is the one in which the valve is fully open. The valve in the waste line is regulated back to the first position , i. h, in the one who took the valve before the change, either after a gege »

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a predetermined time, or when the detected pressure drop across the screen falls below a second given value.

The diversion of the product is carried out continuously; the amount of waste diversion is constant »The constriction of the accepts stream and the opening of the waste product valve are carried out simultaneously» The sieve has two or more stationary sieve devices and has rotating foils,

In order to further enhance the effect achieved according to the invention, the flow through the accepts line can be narrowed by means of a valve located therein. This constriction may occur when the valve for the waste products is in the second position, preferably simultaneously with the opening of this valve. It is particularly preferred when the opening of the valve for the waste products takes place rapidly , ie, without delay,

Working Example

The invention will now be explained with the aid of drawing, in which represents "In the drawings;

Fig _e 1: the schematic representation of a sieve, which is controlled according to the invention, and

Fig. 2: shows schematically the degree of separation for two different impurities as a function of the amount of waste product.

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The sieve S in FIG. 1 can be a sieve with a fixed sieve and rotating foils, eg. 3, as it is described in SE-PS 343,621.

Dia fiber suspension j which is to be screened is through

a line 1 to a sieve 'S passed. A portion of this suspension, d * h, the majority in almost all applications, flows through the holes or slots in a sieve 4 and leaves the sieve through a conduit 2 as accept. The remaining portion of the artommenden suspension, which is not through the openings of the screening device

4, is diverted from a conduit 3 as a waste «The flow rate through the conduits 2 and 3 is regulated by the valves 5 or 6« The pressure drop across the sieve

5 is detected by a pressure detecting device PG »A signal proportional to the pressure drop is fed to a control device MP. The control device MP controls the setting of the valves 5 and 6 in response to the signal of the pressure sensing device PG and the programming eines.Mikroprozessors in the control device MP by means of two valve actuating means 7 or "S _e

In. the waste product line 3 can also, as shown in the preferred embodiment in Fig, I,. a flow meter to be arranged. "A signal proportional to the detected flow rate of the waste products is fed to a control device FC which has the task of keeping the flow rate of the waste products constant. When the detected flow rate of the waste products deviates from a predetermined fourth , a signal proportional to the deviation becomes 3e ~

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actuation means 8 to change the valve opening with respect to the predetermined 'Vertes the flow rate' This flow control device FC is only

capable of opening the valve 5 to a predetermined maximum distance, far from the full opening. The control device FC can only change the valve opening within a narrow range.

It is desired to carry out a cleaning of the fiber suspension by sieving. Consequently, the impurities in the smaller of the two streams leaving the screen are concentrated in the stream of waste products, while the other, larger streams have little impurities. The effluent from a greater level of impurities contains the effluent It is natural to strive to minimize the amount of fibers in the waste product. This purification or separation is a function of Abprodu-ktmenge, d _# h * the ratio between the flow amount of the Abprodukts and the flow rate of feed, apart from parameters depending on the screen and the sieve used, d * h _e of hole diameters and the entire "Cf.

In Fig. 2, the relative separation of the impurities of two different types is shown as a function of the amount of waste. As can be seen from Fig. 2, the amount of waste product must exceed a given minimum value in order to achieve an acceptable cleaning of the suspension, without the fiber loss being unreasonably large. The amount of waste product is usually 10 to 25 %, preferential

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as .10 to 15%, the volume flow "If a specific thickening of the Abproduktes is given during the screening, which may be of the order of magnitude of the value of 2, the fiber content is approximately twice as large at constitute feed or inlet" That means in that with a quantity of waste material of approximately 10 to 15 % of the volume flow, a fiber loss of approximately 20 to 30% results _a

The branching of the waste product is usually carried out continuously and is then preferably in a constant ratio to the feed. In certain applications, a periodic diversion of the waste product is possible. Usually the waste product is continuously made »

For smaller quantities of waste, the valve 6 controlling the amount of waste is minimally adjusted, ie, the flow in the valve has a small cross-sectional area. Valves are often characterized by having a large ratio between the cross-sectional perimeter and the flax of the flow area of the valve at a small degree of opening. This results from the fact that the risk of contamination increases with strong constriction of the valve. This in turn means that some fouling will occur if the flow area of the waste product valve is kept constant. "This is sensed by a pressure detector as soon as the pressure above the screen is increased. If this pollution is not eliminated , this eventually leads to a complete stoppage *

During sifting also occurs a slow pollution

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said screening means on the prevention of this contamination is usually sought by increasing the d Un re hu η gsgesch wi η di gkeitd β r -FoI1en with de m e _r result. that the energy consumption increases !. In certain cases this is not sufficient, and the consequence of this is that the sieve is completely blocked,

According to the invention, the contamination of the strainer openings and / or the blocking of the screen waste product valve is monitored by measuring the pressure drop across the screen. A signal which changes as a result of this pressure drop acts on the control device MP containing a microprocessor this signal exceeds a predetermined value, the control device actuates the valve 5 as one of its responsible functions via an actuating device 8; the valve is then placed in a second, preferably fully open position. The pressure drop across the screen 4 thus decreases, the effluent becomes larger and is accompanied by a possible accumulation of fibers in the valve opening. The load on the screening device eventually becomes smaller. Moving films can now more easily prevent possible contamination. When the drudgod drop has dropped below a second, predetermined value, the valve 5 is actuated to return to its previous value.

The cleaning effect can in this question / if necessary, reinforced by operating the valve 5 v / ground when the valve 5 is brought into its open position or itself

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This operation has a suitable constriction of the accepts flow result. The valve 5 bav / at the same time preferably affects a return to the earlier position as soon as the valve 5 assumes its first position »

In dom illustrated example, the return of the valve S and possibly of the valve 5 to its first positions is controlled by the pressure drop across the screen. This reset, d, h, the duration of the cleaning process, may also be controlled to occur after a given predetermined time. In this case, the microprocessor of the controller 10 may be programmed so that, regardless of the detected pressure drop, a signal after a given number of seconds is discharged, z "B, 5 seconds, and said signal actuates or 5 and 6 the actuator means 7, the valves 8 _S so that the latter occupy their first position.

one possibility is also to program the microprocessor so that the specified cleaning is carried out periodically and independently of the pressure drop across the wire 5.

The valves 5 and 6 should be designed so that the ratio between the cross-sectional area of the flow and its periphery is as large as possible * Example of preferred valves are ball valves and ball segment valves «

The valves 5 and 6, in particular the valve 5, should have such a structure that rapid regulation can be made; this increases the cleaning effect ♦

Claims (5)

22,2,1983 61 638 13 1 I Π J &# Invention claim . A method of screening a fiber suspension to prevent fouling of a screening device within a screen and / or blocking a waste product valve provided in a waste line, wherein a flow of accept material is discharged through an accept line and the pressure drop across the screen means controls the screen Abzweigung a waste product is detected by means of the valve in the waste line, characterized in that the valve is opened from a first to a second position, when the pressure drop exceeds a first predetermined value, and that the valve is reset after a time in its first position , 2. The method according to item 1, characterized in that the valve is fully open in the second position, 3. The method according to the items 1 or 2, characterized in that the valve is returned to its previous or first position when the pressure drop drops below a second, predetermined value. 4. The method according to the items 1 or 2, characterized in that the valve is returned to its previous or first position after a predetermined period of time. A method according to any one of items 1 to 4, characterized in that the diversion of the waste product is carried out continuously. _., 22.2.1983 4 5 1 6 b * 4 - ii - ei 533/13 5 »Method according to item 5, characterized in that the quantity of the Abprcduktabzv / sigung is constant, 7, method according to any one of items 1 to 5, characterized in that in addition the accept flow nittoJU a second valve in the accept line is narrowed, when the Abproduktsvilant is in its z'.voiton position » 8 * ₃ The method of item I wherein _s DSSS st narrowing dec accepted stock: romes and the opening of the-Abprcduktventils be performed simultaneously * 9 _t The method according to any one of items 1 to S, characterized in that the screen has one or more stationary screening devices " 10. The method according to item 9 _f, characterized in that the screen has rotating wings. For this 1 page drawings