Abstract:
A flow meter tube is provided having improved performance in the measurement of the flow rate of erosive material. The internal wall surface of the flow meter tube has a smooth erosion-resistant coating thereon to protect the flow meter wall from erosion by the erosive fluid flowing there through. The coating has a Rockwell C hardness of 56 to 70 and a surface roughness of less than or equal to 250 microinches rms.

Description:
[0001]    The present invention relates to flow meters, particularly those used to handle gases and/or fluids having entrained erosive material therein. 
       BACKGROUND OF THE INVENTION 
       [0002]    Venturi flow meters are used to measure the flow rates of fluids (gas, liquids and their mixtures with each other and particulates). Where the fluid has an erosive characteristic, it rapidly wears the venturi, changing its dimensions and shape, leading to inaccurate measurements of flow rate. In the past, this problem was attempted to be addressed by applying a weld overlay material to the interior surfaces of the venturi meter which are subject to wear. These materials (which typically contain chromium carbide or tungsten carbide), while having higher wear resistance than the underlying steel or cast iron of the venturi meter walls, have a very rough surface. The rough surface is caused by the method of coating application, a series of weld beads, resulting in a bumpy surface. In addition, in order to produce beneficial improvements in wear lifetime, these materials had to be applied in average thicknesses of about 0.250 to 0.375 inch. These relatively thick and very rough coatings also adversely affected the accuracy of the flow rate measured. Roughness produces inaccuracy in the flow measurement. The high thickness of the coating is required to get to reasonable lifetime because of the wear rate of the coating. As the coating wore, again the accuracy of the venturi was affected since the wear is nonuniform and the shape of the venturi is altered. 
         [0003]    A diametric cross section through an example of a prior art venturi flow meter tube  10  is shown in  FIG. 1 . It has an erosive fluid flowing through it along axis AA in the direction shown by arrows B. The fluid enters from pipe (not shown) into the inlet section  12  of the venturi  10 . It then flows through the throat section  16  of the venturi into the outlet section  14 , which has a constant diameter section  15  followed by a diverging section  17 . The venturi is formed by a wall  22  (typically cast iron or steel). Formed at both ends of the venturi are flanges  23 ,  25  for attachment to piping (not shown) at the inlet and outlet. In order to measure fluid flow through the venturi flow meter tube, pressure taps  18  and  20  for measuring pressure are provided through the wall  22  and weld overlay  24  of the venturi  10  at its inlet section  12  and outlet section  16 . 
         [0004]    When the flowing fluid is highly abrasive or erosive, the wall  22  may be coated with a weld overlay  24  to increase the wear resistance and therefore the life of the venturi tube flow meter  10 .  FIG. 2  shows a partial cross section through the venturi flow meter tube of  FIG. 1 . It can be seen that the weld beads  26  cause the surface  28  of the weld overlay coating  24  to be very rough. 
         [0005]    For example, in the mining and refining of oil sands, the oil sand is first crushed and then transported via hydrotransport to a bitumen extraction facility. In hydrotransport, the crushed oil sand is mixed with hot water to form a slurry which is pumped through a pipe to the extraction facility. This slurry is highly erosive, particularly to the flow meters in the pipeline. Similar erosion conditions are also encountered in other industrial applications such as in coal slurry pipelines (See: Brolick, “Black Mesa Pipeline—25 Year Success Story,” Annual Meeting of the Canadian Institute of Mining, Metallurgy and Petroleum, Apr. 29, 1996). Further examples of highly erosive fluids requiring flow rate monitoring include slurries of ores, and/or minerals, and slurries of mine tailings, and fluids containing other waste products. 
         [0006]    There is therefore a need for a flow meter capable of accurately measuring the flow rates of erosive fluids. 
       SUMMARY OF THE INVENTION 
       [0007]    The inventors have provided a flow meter tube having a wall with an internal coating of a wear-resistant material having a surface roughness of less than or equal to 250 microinches rms (root mean square) and a Rockwell C hardness of 56 to 70. Preferably, the coating has a thickness in the range of 0.015 to 0.120 inch. 
         [0008]    Preferably, the coating has an erosion rate of 0.041 mm 3 /g or less and more preferably 0.025 mm 3 /g or less (per ASTM G76). 
         [0009]    Preferably, the coating has an abrasion resistance factor of at least 100 and more preferably at least 140 (1/volume loss (cm 3 ) (ASTM G65). 
         [0010]    These and other aspects of the invention will become more apparent upon review of the drawings, which are briefly described below, in conjunction with the detailed description of the invention. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a cross section, along a diameter, of an embodiment of prior art venturi flow meter tube having a weld overlay hard facing on its inside wall. 
           [0012]      FIG. 2  shows a partial longitudinal cross section taken in plane containing axis AA of  FIG. 1 . 
           [0013]      FIG. 3  shows a cross section, along a diameter, of an embodiment of a venturi flow meter tube in accordance with the present invention. 
           [0014]      FIG. 4  shows a partial longitudinal cross section taken in a plane containing axis AA of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 3  shows a diametric cross section through a preferred embodiment of venturi flow meter tube  60  in accordance with the present invention. It has an inlet section  62 , adjoining a throat or converging section  66 , followed by an outlet section  64 . Outlet section  64  has a first or upstream section  65  with a constant cross sectional area along its length, and a second or downstream section  67  whose cross sectional area increases in the direction of flow B. Pressure taps  68  and  72  for determining flow rate are respectively located at the inlet section  62  and the outlet section  64  where they pierce wall  72 . Wall  72  defines the shape of the venturi flow meter tube  60 . The wall is typically made of steel or cast iron. It may be made in one piece or sections which are welded together. The inside surface of wall  72  has an erosion-resistant coating  74  thereon, which is more clearly shown in the  FIG. 4  cross-sectional view. The coating  74  is a relatively thin and smooth coating compared to the prior art thick, bumpy weld overlay coating  24 . It may be in thickness of 0.015 to 0.120 inch, depending on the severity of the erosive environment. Preferably, it has a thickness of 0.030 to 0.090, more preferably 0.030 to 0.080, and most preferably 0.045 to 0.075 inch for oil sand applications. It has improved abrasion and erosion resistance, compared to weld overlays used in the prior art. The roughness of the surface  76  of the coating  74  has a roughness no greater than 250 microinches rms. The preferred coating is autogenously metallurgically bonded to the wall  72  at the bond line  78  between the wall and the coating. The coating  74  is preferably comprised of 42 to 68 weight percent tungsten carbide (a mixture of cobalt cemented tungsten carbide particles and tungsten carbide particles) and has a Rockwell C hardness 56 to 70. These hard particles are uniformly distributed and embedded in a corrosion-resistant matrix of an alloy including nickel and chromium. Preferably the Rockwell C hardness of the coating is 64-70 and the tungsten carbide loading is 58 to 66 weight percent. A preferred coating material is Conforma Clad® WC 200 brazed cladding in a nominal thickness of about 0.060 inch. WC 200 has a nominal composition of about (in weight percent): 65 tungsten carbide, 30 nickel, 5 chromium, 1.5 boron, 3 cobalt, and 0.3 iron. WC 200 cladding has the following nominal properties:
       Density: 0.44 lb/in 3      Thermal Conductivity: 230 (BTU·in/h·ft 2 · 0 F)   Metallurgical Bond:
           Strength: &gt;70,000 psi   Porosity: &lt;3%   
           Rockwell C Hardness: 64-70   Abrasion Resistance Factor:
           (ARF)=1/volume loss (cm3): 157   
           (Dry Sand Abrasion Test:
           ASTM G65)   
           Erosion Rate (mm 3 /g): 0.023   (ASTM G76
           45° impingement angle,   83 m/s, Alumina&lt;63 μm)   
                 
         [0030]    In comparison, tungsten carbide and chromium carbide weld overlays typically have an abrasion resistance factor of 70-100 and 40-50 (ASTM G65), respectively. Chromium carbide weld overlay has an erosion rate of 0.063 to 0.079 (ASTM G76), respectively. (See: Conforma Clad Inc., “Technical Bulletin—Standard Tungsten Carbide Cladding Formulas,” GN-001 [2003].) The method of application of the wear-resistant coating is described in U.S. Pat. No. 3,743,556 to Breton et al. The &#39;556 Breton et al. patent discloses a process for applying a wear-resistant coating that first applies a cloth that contains particles of tungsten carbide to a surface that requires protection against wear. Second, another piece of cloth that contains particles of a braze alloy is positioned over the cloth that contains the carbide particles. The substrate with the two layers of cloth is placed in an inert-atmosphere furnace and then heated to the brazing temperature of the braze alloy. The braze alloy infiltrates down into the carbide particles and brazes them to each other and to the substrate. (See also: U.S. Pat. Nos. 4,194,040; 5,164,247; and 5,352,526.) Alternatively, the hard particles and braze material may be painted onto the wall and then brazed as described in U.S. Pat. No. 6,649,682. In the application of Conforma Clad cladding to the interior of a venturi flow meter tube, the pressure taps should not penetrate to the interior surface of the wall  72  prior to coating. After coating, the openings for the pressure taps are EDM machined through the coating  74  and the wall  72 . 
         [0031]    Where the venturi flow meter tube is made in sections, if necessary, the coating may be applied prior to welding the sections together. In that case, any gaps in the cladding in the welded areas will be subsequently protected with hard facing. 
         [0032]    The flow meter tube  60 , in accordance with the present invention, is connected into a system or pipeline through which an erosive fluid is flowing via upstream flange  80  and downstream flange  82  which may be bolted or welded onto the pipes in the line (not shown). The erosive fluid flowing through the flow meter tube  60 , in direction B, has a significantly reduced impact on the accuracy of the flow meter since the coating  74  is smooth and highly erosion resistant and therefore wears at a much reduced rate compared to the prior art. In addition, because of its smoothness and uniform metallurgical structure, it can wear relatively uniformly compared to the prior art, thus minimizing unpredictable changes to the internal shape of the flow meter tube. 
         [0033]    The patents and other documents identified herein are hereby incorporated by reference herein. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or a practice of the invention disclosed herein. It is intended that the specification and examples are illustrative only and are not intended to be limiting on the scope of the invention. The true scope and spirit of the invention is indicated by the following claims.