FI74061C - METHOD FOER REGLERING AV KONDENSATAVLAEGSNING FRAON EN PAPPERSMASKINS TORKPARTI. - Google Patents

Description

1 74061

A method for controlling the removal of condensate from a paper machine dryer

The method for adjusting the condensate flow from the papers-masks is torkparti

The invention relates to a method for controlling the removal of condensate in the drying section of a paper machine. In particular, the method can be applied in a drying section comprising several successive steam groups, in which case the method provides for the removal of condensate 5 per steam group. The drying cylinders are specially equipped with a rotating siphon arrangement.

An important factor for the operation of the drying section of the paper machine is to provide and maintain a sufficient heat transfer coefficient between the steam and the drying cylinder in different driving situations.

10 This depends essentially on the thickness of the condensate layer on the inner surface of the cylinder, which in turn is determined by the operation of the siphon.

In the past, and still today, the most common method is to maintain a constant pressure difference across the steam and condensate collection pipe of the group to remove condensate from the steam group (cylinder group). In other words, this so-called differential control is based on the fact that, regardless of the condensing rate and the pressure level of the group, the constant pressure difference is practically always affected across the steam group. However, this solution is disadvantageous in that the set pressure difference does not guarantee a sufficient flow rate of steam in all conditions to transport the condensate as a ring flow in the siphon tube from the cylinder. On the other hand, under some driving conditions, e.g. after a change of species or in the event of a track break, when the condensing rate and group pressure level 74061 2 change, the flow rate may increase many times over what is necessary to ensure proper condensate removal. In addition, the most significant pressure difference across the steam switch for condensate removal is not measured, so it is impossible to minimize the pressure-5 difference. In practice, a differential pressure setting independent of the driving conditions is maintained.

The practical disadvantages are thus the strong pipe erosion and the reduced production capacity of the drying section, because the pressure differences between the steam groups inevitably become large.

10 In order to reduce the adverse effects of differential pressure control, so-called flow control has been introduced, which in its original form is based on the idea that a constant pressure difference is maintained over the flow group measuring element, e.g. the measuring flange, in the operating range of the steam group. In other words, the aim is to stabilize the mass flow of the purge steam 15 by adjusting the pressure difference between the steam group and the condensate support of the steam group. In practice, this initial flow control is carried out as follows: A measuring element, e.g. a measuring flange placed in a cascade connection in the flow-through and expansion steam line between the steam groups, measures the steam mass flow of said 20 lines. However, if the dimensioning conditions of the measuring flange deviate from the measuring conditions, the measuring flange will display incorrectly. It follows that at different vapor pressures, an equal pressure difference across the measuring flange means a lower mass flow at vapor pressures below the design pressure than at high pressures according to formula (1): * TOD = * ΝΑΥΤΤ '3ossa * = “nAYTT.

ftiim - vapor density under design conditions P = actual density of the flow-through vapor in line I; 3 74061 In this case, either situations may arise in which the flow rate of steam in the piping is high and results in severe pipeline erosion, or in which the flow rate of flow in the pipeline is insufficient, causing plug flow. In this case, the condensate in the cylinders thickens and the heat transfer resistance from the steam to the web increases and the pressure level (condensing temperature) of the steam group must be raised to compensate for the deteriorated heat transfer in order to keep the drying capacity constant. Often the pressure level cannot be raised but the deteriorated heat transfer leads to a 10 decrease in production. Raising the pressure level, on the other hand, causes power generation losses at the backpressure plant.

The condensate removal control method described above is discussed in the article "FLOW CONTROL FOR DRYER DRAINAGE SYSTEMS", TAPPI, NO 11, VOL 62 (1979) pp. 63-67.

Further efforts have been made to develop flow control by connecting the aforementioned measuring element, e.g. a measuring flange, to a flow rate controller which controls the fresh steam valve of the lower pressure steam group upstream of the web as required by the high pressure group pressure level setting. In other words, the flow control has been corrected so that under all driving conditions the flow rate of steam in the siphon tube is constant and independent of the pressure of the steam group.

The method is called constant flow rate control. It follows from this correction that the flow rate of steam through the siphon tube must be selected to be excessive to ensure condensate removal over the entire operating range of the machine, because the flow rate of steam through the siphon pipe required to remove condensate varies with group pressure. The above solution is presented in the presentation "NOUVELLE 30 METHOD OF REGULATING THE MODERN SECURITY". The presentation was given at the IP 83 conference in Grenoble, France, November 22-23, 1983.

4,74061

The two systems can be summarized as follows:

In a system based on a constant pressure difference across the measuring flange, the mass flow must be selected so as to ensure a sufficient flow rate of vapor through the siphon tube when the group pressure is high.

At low steam pressures, on the other hand, the flow rate of steam through the eifon pipe and piping therefore increases unnecessarily, causing the above-mentioned disadvantages.

In a system based on a constant flow rate, the rate 10 must be selected so that the condensate is removed when the group pressure is low. At high pressures, on the other hand, this speed is too high, causing the above-mentioned disadvantages.

This is further illustrated in Figure 1. Curve No. 1 of the figure shows the rate of purge and expansion steam required to remove condensate at different pressure levels, i.e. the so-called The optimum curve. Curve No. 2 depicts the flow rate in a system based on the constant pressure difference across the measuring flange and curve No. 3 describes the flow rate in a system based on a constant flow rate. The hatched area in the figure indicates the range of too high a flow rate v 20.

\

The disadvantages presented above and the shape and existence of the optimum curve shown in Figure 1 have been found in practice in measurements performed on a production machine.

It is an object of the present invention to provide a control method for a paper machine with a multi-cycle blade dryer to remove condensate at a sufficiently high but still optimized for driving conditions siphon tube flow rate, i.e. to adjust the flow rate so that the flow rate follows the optimum. The method can eliminate the disadvantages of the prior art solutions described above and thus increase the state of the art in the art.

5 74061 In order to achieve this purpose, the method is mainly characterized in that in order to remove the condensate, the velocity of the flow-through steam in the siphon tube is controlled by the as a function of the pressure group of the steam group according to a curve which implements constant flow rate control of the steam group 5 at low pressure level and flow rate control of the steam group at high pressure level by means of paper. The shape of the control curve can be set in the controller either in advance or determined by machine measurements.

The invention is illustrated in the following description by an application example while referring to the accompanying drawing. The drawing schematically shows an apparatus arrangement for carrying out the method 15.

In Fig. 2, reference numerals 1, 2 and 3 show groups of steam cascaded against the direction of travel of the paper web. The direction of travel of the web is indicated by arrow 4. Fresh steam is introduced to the cylinder groups along the main steam pipe 5. Steam groups 1, 2 and 3 20 are connected by control valves 6, 7 and 8 to the distribution group 9, 10 and 11 of the respective steam group. , 13 and 14. From the condensate tank 12, the flow-through and expansion steam is led from the upper part of the condensate tank belonging to the steam group 1 to the manifold 10 of the next steam group 2. Correspondingly, the procedure with the steam group 2 is also followed. The steam-condensate mixture of the cylinders of the third steam group is led to the condensate tank 30 14. From the condensate tank, the flow-through and expansion steam is led to the surface condenser 15 to provide the necessary vacuum 6 74061 to the condensate tank 14. In order to achieve a sufficiently low pressure in the third steam group to prevent "sticking of the web", the group is further provided with a separate pressure control valve 17.

A flow measuring member, e.g. a measuring flange 19, is connected to the flow and expansion steam line 18 from the first condensate tank belonging to the steam group. When the mass flow of the flow-through and expansion steam is above the set value, the controller FIC / 2 controls the fresh steam valve 7 open, thus causing an increase in the back pressure and a consequent decrease in the mass flow. When the mass flow of the flow-through and expansion steam is below the set value, the fresh steam valve 7 closes and the blow-out valve 20 opens, respectively. The controller FIC / 2 obtains its external setpoint from the pressure control circuit PIC / 1 of the steam group 1, which controls the pressure of the steam group 1 by means of a valve 6. The pressure control circuit, on the other hand, obtains its external setting value either manually or, for example, from the difference between the set value of the winder web moisture and the measured value, which is modified in the computer system 21 to correspond to a certain pressure set value. The actual pressure value from the pressure control circuit PIC / 1 in the calculation unit FDE / 2 is modified so that the output of the calculation unit 25, which is transferred to the setpoint for the control circuit FIC / 2, corresponds to a steam mass flow which implements the optimum steam velocity curve It is further desired from Figure 2 that the pressure control circuit PIC / 1 also provides an external setpoint for the flow control circuit FIC / 3, which operates like FIC / 2 when applied to the operation of the steam group 2. It should be noted, however, that the position of the control curve with respect to the steam flow velocity axis (Fig. 1) has changed because the condensing rate in the cylinder has changed.

I: 74061

The pressure of the third steam group is regulated by the pressure control circuit PIC / 3, which derives its external value from, for example, the machine control panel list. The actual pressure value obtained from the pressure sBBcircuitstB is used as a setting-5 value for the flow control circuit FIC / 4 after modification as above. The flow measuring element 22 is connected to a controller FIC / 4, which tends to keep the flow rate * and the expansion steam mass flow at its set point by adjusting both the "backup valve" 23 of the vacuum pump and the set value of the surface condenser cooling water temperature control circuit. When the flow-through and expansion steam mass flow rate 10 exceeds the aeethue value, the controller FIC / 4 first controls the "backup valve" and then raises the setpoint of the control circuit TIC / 4, thus causing a back pressure rise in the condenser tank 14 and a consequent decrease in flow. the "reserve valve" 15 opens and the setpoint of the control circuit TIC / 4 decreases, as a result of which the pressure level in the condensate tank 14 decreases.

In practice, it has been found that with the optimum control curve according to the invention the siphon tube flow rate range will be about 15 m / s, when the corresponding value in the initial flow rate control would be about 30 m / s and in constant flow rate control according to the control strategy. Due to the criteria for determining the constant velocity line, the velocity of the steam at high pressures will be about 15 m / s higher than the flow velocity according to the invention. The curve v = f (p) is adjusted to continuously decrease in shape in the coordinate system, where v * is the radial velocity of the purge vapor in the siphon tube and p * is the pressure of the purge vapor.

As a result, it is precisely in a situation with limited dryness that both production is lost due to pressure differences between oversized groups and severe pipeline erosion is caused. While using the control strategy according to the invention, both maximum production is achieved when the pressure differences between the steam groups are minimized and the minimum pipe erosion is minimized.

74061 8

Because the cylinders belonging to the word steam group condense in different ways, e.g. 1, e.g. stepwise. When setting the alarm limit, the bets between the different operating groups must be taken into account, which are usually very standard.

I,

Claims (3)

9,74061 Claims; A method for controlling condensate removal in a drying section of a paper machine, the drying section comprising two or more successive groups of steam, the condensate removal of which is controlled on a steam group-by-steam group basis, the drying cylinders being provided with a rotating siphon arrangement. as a function of the pressure level (p) of the vapor group according to a curve (1) which implements the constant flow rate control (3) at the low pressure level of the vapor group and the radial velocities of or by measurements on a paper machine. Method according to Claim 1, characterized in that the curve (1), v = f (p), is adjusted to continuously ... decrease in shape in the pv coordinate system, where v = blow-off steam. radial velocity in the siphon tube and p = purge vapor pressure. Method according to Claim 2, characterized in that the position of the curve (1) in the coordinate system is monitored on the basis of the torque measurements of the operating groups, wherein the increase in torque raises the level of the curve (1), e.g.