Abstract:
The invention relates to a method and an arrangement for measuring a fluid. Electromagnetic radiation is used to measure the consistency, flow and/or gas content of a fluid flowing in a process pipe ( 102 ). At least two of the measurements are carried out substantially from the same measurement point in the fluid in a measuring tube ( 202 ) by utilizing one integrated measuring equipment ( 200 ) comprising transmitters and receivers for electromagnetic radiation in order to carry out the aforementioned at least two measurements.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method for measuring a fluid, wherein the consistency, flow and/or gas content of a flowing fluid is measured by means of electromagnetic radiation.  
           [0002]    The invention also relates to an arrangement for measuring a fluid, comprising units for measuring the consistency, flow and/or gas content in order to measure a fluid that flows in a process pipe by means of electromagnetic radiation.  
         BACKGROUND OF THE INVENTION  
         [0003]    Flow measuring equipments are generally used to measure the flow of a fluid in a pipe. There are various types of prior art measuring equipments, which are typically based on such physical phenomena as the Coriolis force, pressure difference and voltage induced by movement of a conductive material in a magnetic field. Further, ultrasound technology has been used to implement measuring equipments based on correlation and the Doppler effect. Microwave technology has also been used to provide flow measuring equipments based on the Doppler effect. There are also arrangements utilizing the correlation of microwaves by means of a metallic process pipe, disclosed for example in U.S. Pat. Nos. 4,423,623, 4,888,547 and WO 94/17373. In the arrangements disclosed in U.S. Pat. Nos. 4,423,623 and 4,888,547, the process pipe is used as a waveguide, and variations in the cut-off frequency of the waveguide serve as correlating signals. The arrangement of WO 94/17373 employs the correlation of signals on the same frequency or at least the same frequency band after the signals have propagated through the flowing material.  
           [0004]    The manufacture of paper of good quality requires accurate measurement and adjustment of the water or solids content of the papermaking pulp, in other words the pulp consistency. If the consistency of the pulp is too low or too high, the paper web will not remain homogenous and the quality of the finished paper will not be as good as possible.  
           [0005]    At present, the gas content and especially the air content of a fluid or a liquid substance are measured mainly by means of methods and devices based on the ultrasound and the measurement of density. In the ultrasound measurement, the ultrasound is transmitted through the fluid to be measured and the attenuation of the ultrasound is measured. The attenuation of the ultrasound is a function of the gas content of the fluid: the higher the gas content the greater the attenuation of the ultrasound. In the paper industry, the gas content of papermaking pulp with a consistency below 2% is typically measured by means of the ultrasound. The quality of the final product, i.e. paper, depends on the quality of the liquid pulp, which, in turn, is partly dependent on the gas content thereof.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    The purpose of the invention is to provide a method and an apparatus implementing the method with which the aforementioned problems can be solved. This is achieved with a method of the type described in the introduction, which is characterized in that at least two of said measurements are carried out substantially from the same measurement point in the fluid by utilizing one integrated measuring equipment comprising units for measuring at least said two properties for the transmission and reception of electromagnetic radiation.  
           [0007]    The arrangement according to the invention is characterized in that at least two measuring units, intended for the transmission and reception of electromagnetic radiation, from the units for measuring the consistency, flow and/or gas content are integrated into one measuring equipment, which is attached to the pipe and arranged to perform at least two of said measurements substantially from the same measurement point in the fluid.  
           [0008]    The method and the arrangement according to the invention provide several advantages. It is possible to measure with one measuring equipment several properties of a fluid, such as a suspension or slurry, from one measurement point in the process, which reduces the costs of mounting the measuring equipment. Further, one measuring equipment requires less cabling than two or more separate measuring equipments. Particularly air mixed in the papermaking pulp also affects the measurement of production that is based on the consistency and the flow, since with a high air content the process pipe is not full as it is assumed to be in the measurement of the production.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]    In the following, the invention will be described in greater detail in connection with preferred embodiments with reference to the accompanying drawings, in which  
         [0010]    [0010]FIG. 1 shows a prior art measuring arrangement,  
         [0011]    [0011]FIG. 2 shows a measuring arrangement according to the invention,  
         [0012]    [0012]FIG. 3A shows the placing of transmitters and receivers in a process pipe,  
         [0013]    [0013]FIG. 3B shows the placing of transmitters and receivers in a process pipe,  
         [0014]    [0014]FIG. 4 shows the placing of transmitters and receivers, and  
         [0015]    [0015]FIG. 5 shows the placing of transmitters and receivers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The arrangement according to the invention is particularly applicable for use in the paper industry without being limited thereto, however.  
         [0017]    [0017]FIG. 1 shows a prior art arrangement for measuring papermaking pulp. A pump  100  makes the pulp flow in a pipe  102 . The air content of the pulp is measured such that a sample is taken from the flowing pulp via a thin tube  104  to an air measuring equipment  106 , which operates, for example, by means of the ultrasound. The air content can be measured with the ultrasound if the original consistency of the flowing pulp is less than 2%. The flowing pulp is often thicker, wherefore its consistency must be reduced for the measurement in the sample line. The air content can also be measured by means of density measurement, wherein the density of the flowing pulp is measured from the pulp in the sample line also without reducing the consistency. The consistency of the pulp is measured from a different point in the pipe with another separate measuring equipment, i.e. a consistency measuring equipment  108 . Further, the flow of the pulp is measured with a third measuring equipment, a flow measuring equipment  110 , which is separate from the other measuring equipments and attached to the pipe  102  via a separate measuring pipe  103 .  
         [0018]    The arrangement according to the invention combines the different measurements of a fluid. In the present application, a fluid refers to any fluid substance which may flow in a pipe, in particular, such as a suspension, slurry, phase mixture or papermaking pulp. The measurements are combined such that at least the transmitters and receivers for the measurement signal are integrated into one unit of the measuring arrangement, which, in turn, is a part of the process pipe. FIG. 2 shows a measuring arrangement according to the invention comprising a pump  100 , a pipe  102 , a measuring tube  202  and an integrated measuring equipment  200 . The integrated measuring equipment may measure the consistency, flow and/or gas content of a flowing fluid. Further, it is possible to integrate into the measuring equipment  200  the measurement of the temperature, pressure and conductivity of the fluid. The measuring equipment  200  can be connected to a computer  204  by means of a known connection, such as RS-485, so that the measurement results provided by the measuring equipment  200  can be processed in an automated manner by means of mathematical statistics, for example. The computer  204  is thus able to form a general view of the conditions of flow of the fluid in the process pipe  102  and of the volume of the flow. Further, on the basis of this data the computer  204  can determine the amount of production of the process particularly by means of the flow velocity and the consistency. The amount of production can be determined more accurately by also utilizing the gas content. The volume flow V can be calculated by multiplying the cross-sectional area A of the measuring tube  202  by the velocity v, in other words V=A*v. The amount of production T 1  can be calculated by multiplying the volume flow V by the consistency of the fluid s, in other words T 1 =V*s. The unit of the production T 1  is thus kg/s. The air content is preferably taken into account by multiplying the amount of production T 1  by the concentration of the solids and the liquid (1−I), wherein I is the relative gas content, the value of which is in the range [0, . . . ,1]. The actual amount of production T 0  is therefore T 0 =T 1 *(1−I). The amount of production T 1  is often rather close to value T 0 .  
         [0019]    In the inventive arrangement, the transmitters and the receivers are integrated together. Especially in the case of microwave technology, the antennas must be integrated into combinations which enable easy measurement of particularly the consistency and the flow velocity of a fluid. The antennas can also be replaced with optical transmitters and receivers. FIGS.  3  to  5  show the placing and the shapes of the antennas. FIGS. 3A and 3B show the cross-section of the measuring tube  202  to be connected to the point of measurement in the pipe  102  with the inside of the tube  202  visible. The wall of the measuring tube  202  is provided with a support structure  300 , which is a part of the measuring equipment  200  and comprises three antennas  302 A- 306 A or  302 B- 306 B. Antennas  302 A and  302 B, and  304 A and  304 B measure the flow of the fluid in the measuring tube  202  preferably by means of correlation. The measurement is carried out, for example, in the following manner. Antenna  302 A transmits a signal to the fluid, and antenna  302 B situated on the opposite side of the tube receives the signal which has passed through the tube and which is subjected to interference caused by the flowing substance. Antenna  304 A, which is placed at a distance D from the previous antenna, also transmits a signal to the fluid, and antenna  304 B placed on the opposite side of the tube receives the signal which has passed through the tube and which is subjected to interference caused by the flowing substance. In order to determine the flow velocity, the signals received by antennas  302 B and  304 B are compared to each other at different moments. The signals correlate best with such a time difference between the moments of measurement that is required by the flowing fluid to travel over the distance D between the antennas. The computer  204  or the measuring equipment  200  itself forms the correlation C(τ) generally according to formula (1), as follows  
           C (τ)=∫ x ( t )· y ( t −τ) dt   (1)  
         [0020]    wherein x(t) is the signal of antenna  302  and y(t−τ) is the signal of antenna  304  which is delayed by τ. The value of the correlation is calculated with several delay values such that τε[t 0 , . . . , t n ], wherein to is the shortest possible delay and t n  is the longest possible delay. The shortest and the longest possible delay determine the highest and the lowest flow velocity that can be measured over the distance D between antennas  302  and  304 . The interval between the measurements τε[t 0 , . . . t n ] is determined specifically in each case. The correlation is calculated in the inventive arrangement either analogically (e.g. in the integrated measuring equipment  200 ) or digitally (e.g. in the computer  204 ).  
         [0021]    Antennas  306 A and  306 B measure the consistency of a fluid. Particularly in suspensions the travel time, phase and attenuation of a microwave signal are also dependent on the consistency of the fluid. The consistency is preferably measured by means of the same microwave antennas  306 A and  306 B as the gas content. The microwave measurement requires frequency modulation according to the FMCW (Frequency Modulated Continuous Wave) technique. In the FMCW technique, the frequency of the oscillator is swept linearly over a broad band, as in a radar. The receiver detects the difference between the travel times of the reference signal and the received signal, which is dependent on the consistency, for example. In all liquid and suspension processes the consistency typically changes more slowly than, for example, the pressure variation caused by the pump  100 . Therefore the consistency can be measured as an average over a long period of time on the basis of the travel time, whereas the gas content can be measured at considerably shorter intervals (typically less than 1 s) without frequency modulation, for example in the measurement of the signal phase based on the travel time.  
         [0022]    [0022]FIG. 4 shows an arrangement which corresponds otherwise to FIG. 3 except that antennas  402 - 406  are positioned at even intervals on the surface of a support structure  400 .  
         [0023]    In FIG. 5, antennas  502 - 506  are placed similarly as in FIG. 3, but the shape of antennas  502  and  504  measuring the velocity is elongated and not round. The advantage of such a shape is that the same point in the fluid probably travels past each antenna. An additional advantage is that even though the flow propagates obliquely at an angle α, the correlation measurement automatically measures the velocity of the flow in the direction of the tube.  
         [0024]    The inventive arrangement employs microwave antennas. The antennas are preferably ceramic since the area of such antennas can be made smaller than that of plastic antennas. The diameasuring equipment of a round antenna may be, for example,  24 mm instead of  85 mm, as in prior art arrangements.  
         [0025]    In the inventive arrangement, the measurements can be carried out also in the form of optical measurements in addition to or instead of microwave measurements. In an optical measurement, antennas  302 A- 306 A and  302 B- 306 B of FIGS. 3A and 3B (correspondingly also the antennas of FIGS. 4 and 5) are replaced with optical transmitters and receivers. In such a case, the transmitter may be any optical power source, such as a lamp, a LED (Light Emitting Diode) or a laser. The receiver, in turn, may be any optical detector, such as a PIN diode or avalance diode. Since the actual optical power transmitter and receiver should not be placed in direct contact with the flowing fluid, antennas  302 A- 306 A and  302 B- 306 B of FIGS. 3A and 3B (correspondingly also the antennas of FIGS. 4 and 5) can be replaced with optical fibres or fibre bundles.  
         [0026]    Even though the invention is described above with reference to the examples according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in various ways within the scope of the inventive idea disclosed in the appended claims.