JP2006162417A - Total pressure/static pressure measuring venturi system flow measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure flow velocity from a static pressure and a total pressure as a pitot tube system by suppressing a contraction ratio in a small range and reducing a pressure loss in a large flow rate area, while expecting pressure recovery as a venturi tube, and to enlarge a measuring range of the static pressure or a differential pressure capable of measuring a flow rate highly accurately. <P>SOLUTION: A short measuring straight pipe 12 is arranged between a contraction pipe 11 having a smooth sectional area change and an enlarged pipe 13 having a mild sectional area change, and the differential pressure between the total pressure and the static pressure, an absolute pressure of the static pressure, its differential pressure and a temperature of fluid are measured at a part of the measuring straight pipe 12, and a fluid density is determined from a gas constant or the like, to thereby measure the flow rate from the flow velocity in the measuring straight pipe 12 and the sectional area thereof. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is used in a wide range of industrial fields for measuring the flow rate of fluid such as air and gas.

In general, a method for measuring the flow rate by using a Pitot tube to obtain the flow rate is widely used. However, the measurement pipe often requires a length of more than 10 times the pipe diameter with a constant cross-sectional area, and the flow velocity distribution. The total pressure, the static pressure, and the pressure difference such as the differential pressure are measured (see Patent Document 1). In this case, as the flow rate increases, the resistance due to the pipeline increases in a square manner. This resistance is often a pressure loss in the fluid conduit. Moreover, in the flow measurement by a normal venturi pipe, although the static pressure of an inlet pipe line and a throat part is measured (refer patent document 2), throttle area ratio 1 / (beta) ² (inlet pipe sectional area / throat sectional area) is small. In this case, the change of the flow coefficient due to the flow velocity becomes relatively large, and generally the measurement accuracy in a region where the flow velocity is small is deteriorated. When the aperture ratio is increased, not only the resistance loss becomes extremely large when the flow rate is large, but also the differential pressure increases, so the measurement accuracy in the low speed region tends to be insufficient due to the measurement range of the pressure sensor.

JP 2004-117261 A JP 2001-269604 A

  In the measuring device of the present invention, pressure recovery can be expected as a venturi tube, while the flow rate measurement from static pressure and total pressure as a Pitot tube method is achieved by suppressing the throttle ratio to a small range and reducing the pressure loss in a large flow rate region. It is a problem to increase the measurement range of static pressure or differential pressure that enables high-precision flow measurement.

  The venturi type flow measuring device according to the present invention has a short measurement straight pipe disposed between a throttle pipe having a smooth cross-sectional area change and an enlarged pipe having a smooth and gentle cross-sectional area change. Measure the absolute pressure of each of the total pressure and static pressure of the fluid, the differential pressure and temperature of the fluid, and further determine the fluid density from the gas constant, etc. Take the configuration.

In general, the venturi throat diameter ratio β and the area ratio β ^{2 with} respect to the inlet pipe diameter in the venturi pipe are related to the flow coefficient, but when β ≧ 0.5, the ratio of resistance loss is small, but the flow coefficient is 1.0 or more. The influence of the proximity velocity of the flow becomes large. Here, the pressure loss can be reduced by setting β≈0.6 in consideration of the pressure ratio, error limit, and the like. The Pitot tube method was selected as the means for measuring the flow velocity, and the total pressure and static pressure were measured. For static pressure measurement with a particularly large change range, the static pressure is taken out from a plurality of static pressure take-out holes or annular slits so that the average of the entire circumference of the pipe can be measured, and the pressure sensor is switched to two or more stages and used. The method can be adopted. For the total pressure measurement, a measurement hole is arranged at a position near the center of the pipe diameter that can correspond to the average flow velocity of the pipe in turbulent flow at one or more locations so as not to disturb the flow. An appropriate straight pipe portion can be secured.

  In a state where the compressibility is almost negligible, the flow velocity of the fluid increases and decreases in inverse proportion to the cross-sectional area of the pipe. Then, the flow rate can be obtained by measuring the temperature, absolute pressure, and flow velocity of the fluid in this part. As a simple device for measuring the flow velocity, a Pitot tube method using total pressure and static pressure is possible.

  In this invention, the static pressure and the total pressure are measured according to the Pitot tube method, and the density is obtained from the known gas constant by measuring the temperature, and the volume flow rate and the mass flow rate are measured from the cross-sectional area and the average flow velocity. it can. In particular, by measuring each pressure and temperature with a fast response within 1 to 10 ms, it is possible to measure the flow rate with a fast response corresponding to a rapidly changing flow rate.

  According to the present invention, the pressure loss caused in the flow rate measurement by the throttle is suppressed to be considerably small by the Venturi type pipe configuration, and it is possible to measure up to a range where the differential pressure between the total pressure and the static pressure is large. Ordinarily, the pulsation can be attenuated by using a surge tube at the entrance of the measurement section even under conditions where the measurement error increases due to the pulsation of the flow rate or pressure, and measurement can be performed over a wide flow rate range with extremely high accuracy. In particular, when the flow rate is relatively small, a highly sensitive pressure sensor can be used, and highly accurate measurement can be realized.

  A specific embodiment of the present invention will be described with reference to a measurement unit having a venturi structure in FIG. 1 and a system configuration diagram in FIG. As shown in FIG. 1, the measuring section 2 of the total pressure / static pressure measuring venturi type flow meter 1 of the present invention is connected to pipe lines 5 and 6 through which fluid flows through an inlet pipe 3 and an outlet pipe 4. The pipe line 5 is a pipe line connected to the fluid generation source 9. Normally, a surge tube 7 is inserted between the inlet pipe 3 and the pipe line 5 to smooth the pulsating flow and pulsating pressure waveform of the inlet gas. The inflow 8 is configured to flow from the inlet pipe 3 to the measuring unit 2. The surge tube 7 is a device that is attached to the pipeline system and suppresses or reduces the pulsation of the pipeline system due to a volume change caused by expansion and contraction of a rubber film or the like. Such a surge tube 7 is disclosed in Utility Model Registration No. 3085960. Has been. The measuring unit 2 includes a throttle pipe 11, a measurement straight pipe 12, and an enlarged pipe 13 in a continuous manner. The throttle pipe 11 is on the upstream side of the flow, has a smooth cross-sectional area change, decreases in diameter toward the downstream side, and is connected to the inlet pipe 3 at the upstream end. The measurement straight pipe 12 is a straight pipe having a constant diameter and is connected to the downstream end of the throttle pipe 11 at the upstream end. The enlarged pipe 13 is on the downstream side of the flow and has a smooth and gradual cross-sectional area change. The diameter of the enlarged pipe 13 increases toward the downstream side, and is connected to the measurement straight pipe 12 at the upstream end and the outlet pipe 4 at the downstream end. Connect to. A thin temperature sensor 14 and a total pressure measuring pipe 21 placed in parallel with the flow are arranged at appropriate positions in the measurement straight pipe 3, and a static pressure take-out hole or slit 18 is provided in the pipe wall. The pressure in the total pressure measuring tube 21 is guided to the total pressure sensor 24. The total pressure is measured with responsiveness including a high-speed responsive sensor within 1 to 10 ms, and within about 10 mm from the static pressure take-out hole or slit 18 in the tube axis direction and within about 0.8 of the tube diameter. The total pressure at a position close to the average flow velocity of the measurement straight pipe line 12 at one or more measurement points at a position close to the center of the tube is taken out by an appropriate total pressure measurement tube 21 and a pressure conduit 16 to correspond to the flow velocity. The total pressure to be obtained is determined with a fast response within 1 to 10 ms. For the static pressure measurement, one or a plurality of static pressure extraction holes or slits 18 are provided in the circumferential direction of the inner surface of the tube wall at 1 mm or more and 3 mm or less, and the static pressure of the entire circumference is averaged. The pressure conduit 15 leads to the static pressure sensor 25.

In such a configuration, the inflow 8 is increased to a flow velocity V ₁ in accordance with the reduction of the cross-sectional area of the throttle pipe 11 of the measurement unit 2. In the measurement, the cross-sectional area of the measurement straight pipe is known as A, and the flow rate is obtained from the flow velocity V ₁ . At this time, the gas constant R is used as a known constant in addition to the fluid temperature measurement value T ₁ , the static pressure P _s, and the total pressure P _t . Instead of the gas constant R, a gas density ρ _o (kg / m ³ ) or a specific weight γ _o (kgf / m ³ ) at a reference state, for example, 20 ° C. and 101325 Pa can be used. In this type of venturi, unlike a general venturi that uses the inlet absolute pressure and the throat static pressure as the basis of the flow rate calculation, the flow velocity is obtained from the total pressure P _t and the static pressure P _s as in the Pitot tube, and the static pressure at that time The flow rate is calculated based on the absolute pressure P _s corresponding to.

Normally, the total pressure is mainly caused by the flow path resistance on the downstream side from the measurement part, and there is little fluctuation unless there is a restriction or the like after the enlarged part of the venturi. The static pressure varies considerably with the square of the flow velocity. If the average flow velocity V ₁ can be measured from the diameter D ₂ and the cross-sectional area A of the measurement straight pipe 12, the temperature T _1, and the density ρ _o of the gas in the reference state, the flow velocity Q _s can be obtained. The flow velocity measurement with the Pitot tube is basically based on the following equation.

V ₁ = [{2 (Pt−Ps) / ρ}] ^1/2
Here, the density ρ is determined according to the fluid constant from the temperature and the absolute pressure. The volume flow rate Qv is obtained from the product of the flow velocity V1 and the cross-sectional area A, and the mass Qm is obtained by multiplying by the density ρ. Of course, the density ρ is a function of the temperature T ₁ and the absolute pressure Ps. That is, Qv = A · V ₁ and Qm = ρ · Qv. Actually, the calculation is performed by a mathematical formula obtained by multiplying a flow coefficient determined by calibration or the like, but a signal from the pressure sensor or thermometer is converted into a flow signal by the calculation circuit 17 and output at an appropriate time interval.

The flow of the measurement unit 2 is reduced in flow rate in the gentle and smooth expanded pipe 13 to restore the static pressure to some extent. Actually, the static pressure changes according to the flow rate and the pipe diameters D ₁ , D ₂ , and D ₃ , but when the flow rate is large, the pressure recovery is possible by about 80%, and the overall pressure loss in the measuring section 2 Can be less. The fluid that has flowed out of the expansion pipe 13 flows out of the system as an outflow flow 23 from, for example, an exhaust gas suction duct 22 (see FIG. 2).

FIG. 2 shows, as an example, the configuration of a system in which the total pressure / static pressure measurement venturi type flow meter 1 is applied to the measurement of the exhaust gas flow rate of the internal combustion engine 26 that is a fluid generation source. The surge tube 7 is connected to the 5 outlet 27 to attenuate the pulsation, and the measurement unit 2 of the total pressure / static pressure measurement venturi type flow meter 1 is connected to the downstream side thereof. Although not shown in FIG. 1, a pressure sensor for static pressure measurement is attached to this measuring unit by attaching a plurality of low ranges and high ranges by branching the pressure conduit as required, and high accuracy at low flow rates. It is possible to measure a large difference between total pressure and static pressure even in a high flow rate range. For this purpose, a multi-stage pressure measurement system in which the differential pressure is obtained with an accuracy within 1% of the measured differential pressure may be provided. Flow rate operation static pressure Ps in the calculation circuit 17 (absolute pressure), and executes the total pressure P _t and the temperature T ₁ of the real-time substantially calculated by entering for example every 1 ms, and outputs the instantaneous flow rate value. Alternatively, it is output by calculating and averaging at a time interval of about 10 ms. Depending on the pressure resistance of the pressure sensor, measures are taken such as preventing a large negative pressure from being applied to the solenoid valve in order to protect the highly sensitive static pressure sensor.

Structure explanatory drawing which shows the measurement part of a total pressure and a static pressure measurement venturi type | formula flow measuring device. The system structural example of this measuring apparatus connected with the exhaust system of the internal combustion engine.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Total pressure / static pressure measurement Venturi type flow meter 2 Measurement part 3 Inlet pipe 4 Outlet pipe 5 Pipe line 6 Pipe line 7 Surge tube 8 Inflow flow 9 Fluid generation source 11 Throttle pipe line 12 Measurement straight pipe line 13 Expansion pipe line 14 Temperature sensor 15 Pressure conduit 16 Pressure conduit 17 Calculation circuit 18 Static pressure take-out hole or slit 21 Total pressure measuring tube 22 Exhaust gas suction duct 23 Outflow flow 24 Total pressure sensor 25 Static pressure sensor 26 Internal combustion engine 27 Outlet

Claims (6)

A short measurement straight line is placed between a throttle line with a smooth cross-sectional area change and an enlarged pipe with a smooth and gentle cross-sectional area change. A total pressure / static pressure measurement venturi system that measures the flow rate from the flow velocity and cross-sectional area in the measurement straight line by measuring the differential pressure from the pressure and the absolute pressure and temperature of the static pressure, and determining the fluid density from the gas constant, etc. Flow measurement device. 2. The flow rate measuring apparatus according to claim 1, wherein the measurement range of the static pressure or the differential pressure is switched to two or more steps corresponding to the measurement range of the pressure sensor to measure a large static pressure even in the measurement range range of the small static pressure or the differential pressure. A total pressure / static pressure measurement Venturi type flow measurement device equipped with a multi-stage pressure measurement system that allows the differential pressure to be obtained with an accuracy within 1% of the measured differential pressure even in the range range. 2. The flow rate measuring apparatus according to claim 1, wherein a slit or a pressure extracting hole having a diameter of 1 mm or more and 3 mm or less is provided around the pipe for static pressure measurement, and an average static pressure of the pipe is a pressure conduit having an appropriate length and volume. The pipe diameter is close to the center within 0.8 mm of the pipe diameter within 10 mm in the pipe axial direction from the static pressure extraction hole. The total pressure at a position close to the average flow velocity of the measurement straight pipe at one or more measurement points is taken out with an appropriate measurement pipe and pressure conduit, and the total pressure corresponding to the flow speed is high speed within 1 to 10 ms. Venturi type flow measurement device that measures total pressure and static pressure so that it is required to have high responsiveness. 4. The flow rate measuring device according to claim 1, wherein a surge tube for attenuating the flow pulsation is arranged upstream of the inlet pipe to reduce the influence of the flow pulsation, and in particular to eliminate the reverse flow. Venturi type flow measurement device that measures total pressure and static pressure by detecting pressure in only one direction. 5. The flow rate measuring device according to claim 1, wherein the pressure loss is reduced by a smooth throttle line between the inlet pipe and the outlet pipe, a straight measuring pipe, and a gentle and smooth enlarged pipe. In addition, the pressure recovery in the expanded pipe is made more efficient, and the outlet pipe diameter is made equal to or greater than the inlet pipe diameter to reduce the total pressure loss in the flow measurement device. Method flow measurement device. A measurement pipeline is provided between the upstream pipeline and the downstream pipeline, and the measurement pipeline is continuous to the downstream end of the throttle conduit and the downstream end of the throttle conduit, the diameter of which decreases toward the downstream side. Consists of a constant measurement straight line and an enlarged pipe that is continuous with the downstream end of the measurement straight pipe and increases in diameter toward the downstream side. In the measurement straight pipe, A flow rate measuring device comprising a measuring device for measuring a differential pressure from a pressure, an absolute pressure of a static pressure, and a temperature.

Images