Abstract:
The invention relates to a process for measuring and controlling the nip pressure in the press of a paper machine, particularly in a shoe press, across and/or along the web running direction. It is largely characterised by the hydraulic, static pressure being measured at reference points through measurement boreholes in the press nip and adjusted continuously. In addition, the invention relates to a device for carrying out the process.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a process for measuring and controlling the nip pressure in the press of a paper machine, particularly in a shoe press, across and/or along the web running direction. In addition, the invention relates to a device for carrying out the process.  
           [0002]    In order to control dewatering by presses used in the paper-making sector, the appropriate nip pressure must be set. That is why there are a number of theoretical calculation approaches. These can be applied as long as conditions are ideal. Particularly in the edge zones of a paper web however, for example at the creping cylinder of a tissue machine, non-uniform conditions occur that can no longer be expressed precisely in a calculation. On the other hand, there are also operating parameters that can be calculated, but production realities prevent them from being entered as actual settings.  
         SUMMARY OF THE INVENTION  
         [0003]    The aim of the invention is thus to create a process and a device to measure and control the nip pressure across and/or along a press in order to obtain an even moisture profile or to set paper quality parameters.  
           [0004]    The invention is characterised by the hydraulic, static pressure in the press gap (press nip) being measured at reference points through measurement boreholes exposed to the nip. By measuring at reference points, it is very easy to determine the nominal pressure value for controlling the nip pressure in the cross direction and the actual pressure curve in running direction.  
           [0005]    An advantageous further development of the invention is characterised by the nip pressure in the press being controlled at this nominal value in cross direction and thus, also being adjusted continuously. With this control method it is possible to obtain uniform press duty across the machine running direction at any production status.  
           [0006]    A favourable configuration of the invention is characterised by differential set values for the pressure across the web width. As a result, the moisture profile of the paper web can be influenced selectively.  
           [0007]    The invention is also directed to a device for measuring and controlling the nip pressure in the press of a paper machine, particularly in a shoe press, across and/or along the web running direction. It is characterised by measurement boreholes being provided at several points in the press across and/or in web running direction, and which extend into the press nip. By measuring the pressure at the reference points, it is possible to determine the entire pressure progression exactly and control it accordingly.  
           [0008]    If measurement boreholes are provided in the edge zone, the areas that cannot be calculated can be controlled particularly well.  
           [0009]    If several pressing elements, for example pressing pistons, are provided across the web width and which are of a design that allows them to be controlled partly or entirely, the nip pressure profile or also the moisture profile across the web running direction can be controlled particularly well.  
           [0010]    Due to the adjustable shoe limit stop, the position of the press shoe can be moved in longitudinal direction and the longitudinal profile of the nip pressure in the nip thus set and/or controlled.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention will be described using the examples in the drawings, where;  
         [0012]    [0012]FIG. 1 shows a diagram of the pressure progression in a press;  
         [0013]    [0013]FIG. 2 shows a cross-section through a press roll;  
         [0014]    [0014]FIG. 3 contains a horizontal projection and cross-section of the press shoe in a shoe press according to the invention;  
         [0015]    [0015]FIG. 4 shows a sectional view along the line marked A-B in FIG. 2; and  
         [0016]    [0016]FIG. 5 contains a control diagram for a plant according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    [0017]FIG. 1 shows a progression or profile of the pressing force of a shoe press in the web running direction x. The various curves shown,  1   a ,  1   b , and  1   c , refer to different products. In addition, potential locations,  2   a ,  2   b , and  2   c , are marked for pressure measuring points in the web running direction. If the pressure measurements at these points are known, the influence this has on the paper grade can be determined and corrected or controlled. An analogous profile can be obtained in the cross-web direction.  
         [0018]    [0018]FIG. 2 shows a shoe press roll  4  with a mating roll  5 . This mating roll  5  can also be the drying cylinder of a tissue machine. The roll consists of an outer press shell  6  and a supporting ring  7  on which the press shoe  8  rests. The press shoe  8  is loaded against the press shell  6  by pressing elements  9 . A return spring  10  is provided as a counterforce and to retrieve the press shoe when pressure is relieved. Due to the pressure generated on the press shell  6  by the press shoe  8 , the paper web  12  transported on the felt  11  is pressed against the mating roll  5  and dewatered. The paper web  12  can then be carried onwards together with the mating roll. If this mating roll  5  is the drying cylinder of a tissue machine, this is where the thermal drying process of the paper begins. The more water removed from the web by the press, the higher the drying capacity of the drying cylinder.  
         [0019]    In order to control the moisture profile across the web running direction and also compensate edge zone influence, several pressing elements  9  are provided across the web running direction which can be adjusted either individually or partly separated.  
         [0020]    The shoe limit stop  15  can be moved in the web running direction in order to exert a targeted influence on the longitudinal profile of the nip pressure and/or on the paper quality.  
         [0021]    [0021]FIG. 3 contains a horizontal projection of a pressure shoe  8 . This figure shows measurement boreholes  13 ,  13 ′,  13 ″ and  14 ,  14 ′,  14 ″,  14 ′″. The measurement boreholes  14 ,  14 ′,  14 ″,  14 ′″ are located across the web running direction, while measurement boreholes  13 ,  13 ′, and  13 ″ are aligned with borehole  14  in web running direction. These measurement boreholes correspond to measuring points  2 ,  2   a ,  2   b  and  2   c  in FIG. 1.  
         [0022]    [0022]FIG. 4 shows a sectional view along the line marked A-B in FIG. 2.  
         [0023]    Here, the shoe press roll  4  and mating roll  5  are shown. Several pressing elements  9  are provided across the web width for loading the press shoe  8 . These can be actuated independently of one another via an oil feed pipe  16  and associated distribution lines  17  (shown in phantom). Pressure is relieved via the return springs  10 .  
         [0024]    [0024]FIG. 5 contains a control diagram for a plant according to the invention. As a basic principle, the plant is split into zones across the machine width for control purposes. A difference is made between the main zone A 1  in the centre of the machine and the edge zones on the front and rear sides (Y 1  . . . Yn, Z 1  . . . Zn), where the number of edge zones is variable. The pressure in the oil feed pipe to the main zone (pre-pressure controlled by PID controller) is controlled based on the theoretical calculation of the oil pre-pressure for the pressing elements in the main zone required to obtain a selected line load. The operator selects the desired line load in the process control system. The oil pressure in the nip resulting from the pre-pressure control is measured redundantly in the nip of the main zone itself, the values measured are averaged and then used for edge zone control after evaluation with a coefficient of correction as theoretical set value. The measurement in the main zone is taken as the basis for the set values in the edge zones. First a calculation (evaluation) is made according to theoretical formula with the influence of the pressure in the edge zones for the dry content. From this a coefficient of correction with reference to the pressure in the main zone is determined and used as the basis for specific set values for the pressure in the specific edge zones. The theoretical formula includes parameters such as the paper quality, basis weight, and level of dry content.  
         [0025]    The position of the pressure measurements for the main zone is also variable. This common edge zone set value is now adjusted again for each individual edge zone according to the operating requirements and fed to the individual PID controllers as remote set value. In this case, each edge zone has its own PID controller, where the final control element is located ahead of the roll in the oil feed pipe to the loading element of the appropriate edge zone and the pressure is measured directly in the nip in the position of the appropriate edge zone.  
         [0026]    This control concept provides a facility for the first time to maintain the pressure in the nip itself constant over the entire width and thus, to obtain an even line load in real terms.