Abstract:
The present invention provides a method for calculating flow rate of a fluid using a pressure differential device, based on detected pressure and temperature upstream of and detected pressure adjacent a flow constriction of the pressure differential device, and calibration coefficients calculated from the results of a flow calibration performed on the pressure differential device. By incorporating the results of the flow calibration in the computation, a non-iterative method for obtaining mass flow rate is realized.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the determination of mass flow rate and, more particularly, to an accurate, non-iterative method of calculating mass flow rate using a pressure differential device, such as a venturi flow meter. In an exemplary application, this non-iterative method can be implemented within the controller of a combined-cycle power generation system for computing steam flow rate for each venturi in the steam cooling system. 
     The steam cooling system for a combined cycle plant incorporates multiple venturis for steam flow rate control and protection. These venturis must provide accurate flow rate information over a range of steam pressures and temperatures to ensure successful operation of the system. 
     With reference to FIG. 1, a venturi  10  is a pressure differential device which is inserted in a conduit and is used to determine the rate of flowing fluid within the conduit. In FIG. 1 a conduit  12  is illustrated having a longitudinal flow path through which a fluid may flow as shown by the flow arrow. The upstream pressure P 1  is sensed by a fluid pressure sensor  14 . A temperature probe  16  is provided to detect fluid temperature upstream. Pressure P 2  is detected in the throat  18  of the venturi. A flow computer or processor  20  receives pressure P 1 , pressure P 2  and the temperature T. Based on this information and predetermined information, the processor calculates the flow rate. The measurements are shown referenced to upstream conditions only as an example. 
     Discharge coefficient is a variable in the computation of venturi mass flow rate. Reynolds number is a measure of the ratio of the inertial to viscous forces that the flowing fluid experiences within the venturi. A flow calibration performed on the venturi will reveal how the discharge coefficient varies with Reynolds number. A typical plot of flow calibration data is illustrated in FIG.  2 . Note that the discharge coefficient drops off quite rapidly for low Reynolds numbers. 
     The current approach to obtaining venturi mass flow rate involves either an iteration upon mass flow rate or an assumption of constant discharge coefficient. 
     In accordance with the iteration method, since both discharge coefficient and Reynolds number are a function of mass flow rate, which is unknown, a guess is first made at the discharge coefficient. From this discharge coefficient, a mass flow rate, q m , is computed as follows, using the ASME definition of venturi mass flow rate, See e.g., “Measurement of Fluid Flow in Pipes Using Orifice, Nozzle, and Venturi,” ASME MFC-3M-1989: 
     
       
           q   m =0.09970190 CY   1   d   2 ( h   w ρ fl /(1−β 4 )) 0.5   (1) 
       
     
     wherein: 
     q m =mass rate of flow, lbm/sec 
     C=venturi discharge coefficient, dimensionless 
     Y 1 =expansion factor based on upstream absolute static pressure, dimensionless 
     d=venturi throat diameter at flowing conditions, inch 
     D=upstream internal pipe diameter at flowing conditions, inch 
     h w =differential pressure, inches of water 
     ρ fl =density of the flowing fluid based on upstream absolute static conditions, lbm/cuft 
     β=diameter ratio at flowing conditions, β=d/D, dimensionless 
     Reynolds number, R d  is then computed from mass flow rate as follows: 
     
       
         R d =48 q   m /(π d μ)  (2) 
       
     
     wherein: 
     R d =Reynolds number referred to d, dimensionless 
     q m =mass rate of flow, lbm/sec 
     d=venturi throat diameter at flowing conditions, inch 
     μ=absolute viscosity of the flowing fluid, lbm/ft-sec, based on upstream temperature. 
     In the same reference (ASME MFC-3M-1989), an equivalent expression for mass flow rate based on downstream conditions (pressure and temperature) is given. 
     Since the venturi flow calibration is typically presented as a curve relating discharge coefficient to Reynolds number (see, for example, the typical calibration curve of FIG.  2 ), a new discharge coefficient can then be computed from the Reynolds number. From this new discharge coefficient, a new mass flow rate is then computed using Equation 1. This process is repeated until the change in computed mass flow rate from one iteration to the next is insignificant. 
     In accordance with the constant discharge coefficient method, discharge coefficient is assumed to be constant, which eliminates the need to iterate. However, this method limits the ability to accurately compute mass flow rate, especially in the low Reynolds number region where the discharge coefficient can vary quite dramatically. 
     BRIEF SUMMARY OF THE INVENTION 
     A non-iterative method for obtaining mass flow rate using a pressure differential flow meter is provided by the invention. More specifically, a non-iterative routine has been developed to compute mass flow rate quickly and accurately by incorporating the results from a flow calibration performed on each venturi directly in the computation. 
     Accordingly, the invention is embodied in a method for determining mass flow rate of a fluid flowing through a conduit having a first flow passage area, comprising the steps of: providing a pressure differential device comprising a flow constriction defining a fluid passage having a second flow passage area; flowing fluid through the pressure differential device; sensing a fluid pressure P 1  at a first pressure sensing location in the conduit remote from the flow constriction; sensing a fluid pressure P 2  at a second pressure sensing location downstream of an entrance of the flow constriction; and determining the mass flow rate based on sensed values of the fluid pressure P 1  and the fluid pressure P 2 , and an expression of discharge coefficient C as a function of Reynolds Number R d  determined from flow calibration data obtained by performing a flow calibration on the flow constricting member. In the presently preferred embodiment, the functional expression is a polynomial expression, and the mass flow rate is determined based on the polynomial coefficients of the polynomial expression. In the presently preferred embodiment, furthermore, a fluid temperature T in the conduit is also sensed and the sensed temperature is used in the determination of mass flow rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of a presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view of a venturi flow meter; 
     FIG. 2 illustrates a typical discharge coefficient versus Reynolds number calibration curve; 
     FIG. 3 is a schematic illustration of a fluid conducting conduit having an orifice plate pressure differential device; and 
     FIG. 4 is a schematic illustration of a fluid conducting conduit having a flow nozzle pressure differential device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A flow calibration can be performed on a venturi to reduce the percentage of uncertainty of the discharge coefficient. Typically, as illustrated in FIG. 2, the result of a venturi flow calibration is presented as a plot of discharge coefficient, C, versus Reynolds number, R d . To accurately compute venturi mass flow rate, discharge coefficient must be represented as a function of Reynolds number. Using standard regression analysis (see any textbook on statistics or numerical analysis, e.g., “Statistics for Experiments,” E. P. Box et al., New York, Wiley-Interscience, 1978), a quadratic (2 nd  order polynomial) equation can be fit to this flow calibration data. Although the method described herein below refers to a quadratic expression of discharge coefficient as a function of Reynolds number, it may be applied to a cubic (3 rd  order polynomial) expression as well. However, for all cases tested to date, the quadratic expression appears to adequately represent discharge coefficient as a function of Reynolds number. 
     A general quadratic relationship between C and R d  is presented in Equation 3. Specific values for the polynomial coefficients a 0 , a 1 , a 2  are determined from the regression analysis on the flow calibration data, as mentioned above. 
     
       
           C=a   2   R   d   2   +a   1   R   d   +a   0   (3) 
       
     
     Multiplying both sides of Equation 3 by q m /C yields: 
     
       
           q   m   =a   2   R   d   2 ( q   m   /C )+ a   1   R   d ( q   m   /C )+ a   0 ( q   m   /C )  (4) 
       
     
     Subtracting q m  from both sides of Equation 4 yields: 
     
       
         0 =a   2   R   d   2 (q m   /C )+ a   1   R   d ( q   m   /C )− q   m   +a   0 ( q   m   /C )  (5) 
       
     
     The following equation can be obtained by rearranging Equation 5: 
     
       
         0=( a   2   R   d   2 /( q   m   C )) q   m   2 +( a   1   R   d   /C− 1) q   m +( a   0   q   m   /C )  (6) 
       
     
     There exists two roots to Equation 6, but only one has a positive value. Mass flow rate can be solved for by selecting the root with the positive value. 
     
       
           q   m =(− b− ( b   2 −4 ac ) 0.5 )/2 a   (7) 
       
     
     where: 
     a=a 2 R d   2 /(q m C)=a 2 (R d /q m )(R d /q m )(q m /C) 
     b=a 1 R d /C−1=a 1 (R d /q m )(q m /C)−1 
     c=a 0 (q m /C) 
     Equation 7 reveals that mass flow rate, q m , can be computed without the need for iteration. All three variables, a, b and c in Equation 7 are directly computed from either known or measured parameters. The venturi flow calibration coefficients, a 0 , a 1 , a 2 , are known as a result of the flow calibration performed on the venturi and subsequent regression analysis and values for (q m /C) and (R d /q m ) can be computed from measured parameters as presented in the following Equations 8 and 9, respectively. Equations 8 and 9 are simply rearranged versions of Equations 1 and 2, respectively. 
     
       
         ( q   m   /C )=0.09970190Y 1 d 2 ( h   w ρ fl /(1−β 4 )) 0.5   (8) 
       
     
     
       
         ( R   d   /q   m )=48/(π d μ)  (9) 
       
     
     wherein: 
     q m  mass rate of flow, lbm/sec 
     R d  Reynolds number referred to d, dimensionless 
     C (venturi) discharge coefficient, dimensionless 
     a 0 a 1 a 2  venturi flow calibration coefficients, lbm/sec 
     D upstream internal pipe diameter at flowing conditions, inch 
     d flow constriction minimum (venturi throat) diameter at flowing conditions, inch 
     Y 1  expansion factor based on upstream absolute static pressure, dimensionless 
     h w  differential pressure, inches of water 
     ρ fl  density of the flowing fluid based on upstream absolute static conditions, lbm/cuft 
     β diameter ratio at flowing conditions, β=d/D, dimensionless 
     μ absolute viscosity of the flowing fluid, lbm/ft-sec, based on temperature. 
     Referring again to FIG. 1, wherein a venturi as shown generally at  10  is disposed in conduit  12 , the upstream pressure P 1  is detected at  14 , i.e. upstream of the flow constriction defined by the venturi. Further, pressure P 2  is detected downstream of the entrance to the constricted passage. Where the flow constriction device is a venturi, pressure P 2  is detected at the throat passage  18 . Temperature probe  16  is provided for measuring temperature T of the fluid flowing through conduit  12 . Generally, it is contemplated that such a temperature probe would be included due to the density and viscosity variations caused by changes in temperature. The density value and viscosity value for the respective detected temperature may be determined from stored data or other suitable method for use in the above-described computation. Detected pressure P 1  and P 2  are utilized for example to calculate differential pressure h w . The other variables used to ascertain flow rate, such as the upstream internal pipe diameter and the venturi throat diameter, are predetermined. Based on the predetermined information and the measured data P 1 , P 2 , and temperature T, as well as the venturi flow calibration coefficients a 0 , a 1 , a 2 , the processor  20  can calculate flow rate as described above. 
     Although the computation of mass flow rate in accordance with the invention has been discussed in detail with reference to computing venturi mass flow rate, this routine could also be used to accurately compute orifice and nozzle mass flow rate. In that regard, orifice and nozzle pressure differential devices are shown respectively in FIGS. 3 and 4 wherein components that are the same as or replace components shown in FIG. 1 are labeled with corresponding reference numerals indexed by 100 and 200, respectively, but are not discussed in detail herein. Similar to the venturi  10 , the orifice  110  and the nozzle  210  are pressure differential devices which can be inserted in a conduit and used to determine the mass flow rate of flowing fluid within that conduit. The standard equation for computing flow through an orifice  110  or a nozzle  210 , as illustrated in FIGS. 3 and 4, respectively, is identical to Equation 1 presented hereinabove. Therefore, the determination of orifice mass flow rate and nozzle mass flow rate corresponds to the discussion above with respect to venturi mass flow rate. 
     Completely analogous to the description given above, the mass flow rate can be calculated based on pressure and temperature measured downstream of the device. The ASME reference (ASME MFC-3M-1989) can be consulted for the expression for mass flow rate for downstream conditions corresponding to equation (1) discussed hereinabove, from which an expression of q m /C and be derived. That expression and the expression for R d /q m  can then be used for the determination of mass flow rate without iteration as detailed hereinabove. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.