The impact of geometric parameters of a S-type Pitot tube on the flow velocity measurements for greenhouse gas emission monitoring

In the monitoring of greenhouse gas emission from industrial smoke-stacks, the most common device used to measure the stack gas velocity is the S-type Pitot tube in South Korea, which is used to estimate the volumetric flow rate by what is termed the Continuous Emission Monitoring System (CEMS). The S-type Pitot tube installed in the stack is inevitably affected during velocity measurements by velocity changes, yaw and pitch angle misalignments due to the harsh environments. Various geometries of the S-type Pitot tube can affect the characteristics of the S-type Pitot tube coefficients, including the degree of sensitivity to velocity changes and yaw and pitch yaw angle misalignments. Nevertheless, there are no detailed guidelines pertaining to the S-type Pitot tube geometry considering accurate and reliable measurements in the ISO, EPA and ASTM international standards. In the present study, S-type Pitot tubes with various geometric parameters, in this case the distance between the impact and wake orifices and the bending angle of the orifices, were manufactured by a 3D printer. Wind tunnel experiments were conducted in the Korea Research Institute of Standards and Science (KRISS) air speed standard system to determine the optimal geometry of an S-type Pitot tube for the accuracy velocity measurements in actual smokestacks which undergo velocity changes and yaw and pitch angle misalignments. Particle image velocimetry was also used to understand the flow phenomena around an S-type Pitot tube under various geometric and misalignment conditions by means of qualitative visualization. The results indicate that S-type Pitot tubes with a long effective length have more constant distributions of the S-type Pitot tube coefficients when the velocity changes from 2 m/s to 15 m/s. The error indexes for yaw angle misalignments show that S-type Pitot tube models with large effective lengths are less affected by yaw angle misalignments. The S-type Pitot tube coefficients were mostly insensitive to the both positive and negative pitch angle misalignments regardless of the velocity and geometry of the various models tested.     

Experimental and numerical investigations of the factors affecting the S-type Pitot tube coefficients

In greenhouse gas emission monitoring from industrial stacks, the most common device used to measure stack gas velocity is the S-type Pitot tube. Various factors such as the Reynolds number and misalignment of the installation angle can be additional error sources for the S-type Pitot tube coefficients due to harsh environments. Manufacturing quality of the S-type Pitot tube is also a factor affecting on the measurement uncertainty of stack gas velocity. In the present study, wind tunnel experiments were conducted in Korea Research Institute of Standards and Science (KRISS) standard air speed system to examine the effects of various factors on the S-type Pitot tube coefficients. Numerical simulations were also used to understand flow phenomena around the S-type Pitot tube in the presence of misalignment and distortion of the geometry. The results indicate that misalignment of the pitch and yaw angle change within ±10° changes the S-type Pitot tube coefficients by approximately 2% compared with normal values. The manufacturing quality resulted in unstable values of the coefficients within 2%. However, variations of the Reynolds number (Re_D=3.0×10³–2.2×10⁴) had no significant effect on the S-type Pitot tube coefficients.     

Smokestack gas velocity measurements using 3D pitot tubes in a coal-fired power plant

On the path to carbon neutrality to reduce greenhouse gas (GHG) emissions, the Korean government has mandated legislation for controlling and monitoring GHG emissions emitted from smokestacks. A continuous emission measurement (CEM) method is considered to be the most reliable for determining CO2 emissions from stationary sources. In Korea, an S-type Pitot tube is the most popular technique to measure the gas velocity in a smokestack, but it will result in a certain error when the non-axial velocity components exist. To vanquish this limitation, Korea Research Institute of Standards and Science (KRISS) developed a nulling smokestack flow measurement (NSFM) instrument equipped with 3D Pitot tubes for taking on-site stack gas velocity measurements. 3D Pitot tubes used in this research, such as prism Pitot tube and sphere Pitot tube, are calibrated in the KRISS airspeed system. The instrument using 3D Pitot tubes with the nulling technique is expected to diminish the restriction on S-type Pitot tubes, and to enhance the quality of the GHG emission measurements in the smokestack. The 3D Pitot tubes can measure both axial and non-axial velocity components of a flow, whereas the S-type Pitot tubes can measure only the axial velocity component. The averaged axial velocity of the stack gas as measured by this instrument has expanded uncertainty of 3.3% (P = 95%, k = 2) for both prism and sphere Pitot tubes.     

A Pitot tube system for obtaining water velocity profiles with millimeter resolution in devices with limited optical access

An automated, miniature, S-type Pitot tube system was created to obtain fluid velocity profiles at low flows in equipment having limited optical access, which prevents the use of standard imaging techniques. Calibration of this non-standard Pitot tube at small differential pressures with a custom, low-pressure system is also described. Application of this system to a vertical, high-pressure, water tunnel facility (HWTF) is presented. The HWTF uses static flow conditioning elements to stabilize individual gaseous, liquid, or solid particles with water for optical viewing. Stabilization of these particles in the viewing section of the HWTF requires a specific flow field, created by a combination of a radially expanding test section and a special flow conditioner located upstream of the test section. Analysis of the conditioned flow field in the viewing section of the HWTF required measurements across its diameter at three locations at 1 mm spatial resolution. The custom S-type Pitot tube system resolved pressure differences of <100 Pa created by water flowing at 5–30 cm/s while providing a relatively low response time of ~300 s despite the small diameter (<1 mm) and long length (340 mm) of the Pitot tube needed to fit the HWTF geometry. Particle imaging velocimetry measurements in the central, viewable part of the HWTF confirmed the Pitot tube measurements in this region.     

Investigation of the pressure response of different Pitot tubes

Pitot tubes are commonly used to measure gas flow in ducts. The integration of the velocity profile which allows the calculation of the gas flow is described in several international standards such as ISO 3966 or ISO 10780.The common working principle of Pitot tubes is based on the measurement of the differential pressure between the two different pressure taps. The gas velocity is related to this differential pressure through a flow coefficient depending on the Pitot tube type.In case of stable flow, in a pressurized duct, fluctuations of the in-line pressure, even low, can occur. If the response times of the two pressure lines (static and total) between the Pitot tube head and the differential pressure sensor are not equal, these fluctuations can be seen as fluctuations of the measured differential pressure and then of the calculated velocity.This phenomenon is investigated for different design of Pitot tubes and the difference in behaviour of the two pressure lines is highlighted.     

The effect of turbulence on a multi-hole Pitot calibration

Accurate calibrations of multi-hole Pitot tubes require thousands of measurements spanning ranges of the fluid's velocity, and the pitch and yaw angles. When calibrating a commercially-manufactured multi-hole Pitot tube in NIST's low-turbulence wind tunnel, we found hysteresis in certain ranges of airspeed, pitch angle, and yaw angle. In the worst case, the hysteresis caused a calibration error of 30%. We demonstrate that the hysteresis was caused by a flow instability associated with flow separation. A turbulence intensity of only 1% removes the hysteresis; however, the calibration depends on the turbulence intensity over the entire range of our measurements (0.25–2%). Therefore, multi-hole Pitot tubes should be calibrated and used at the same turbulence levels.     

Air velocity and flow measurement using a Pitot tube

The accurate measurement of both air velocity and volumetric airflow can be accomplished using a Pitot tube, a differential pressure transducer, and a computer system which includes the necessary hardware and software to convert the raw transducer signals into the proper engineering units. The incorporation of sensors to measure the air temperature, barometric pressure, and relative humidity can further increase the accuracy of the velocity and flow measurements. The Pitot tube measures air velocity directly by means of a pressure transducer which generates an electrical signal which is proportional to the difference between the pressure generated by the total pressure and the still air (static pressure). The volumetric flow is then calculated by measuring the average velocity of an air stream passing through a passage of a known diameter. When measuring volumetric flow, the ‘passage of a known diameter’ must be designed to reduce air turbulence as the air mass flows over the Pitot tube. Also, the placement of the pitot tube in the passage will influence how accurately the measured flow tracks the actual flow through the passage. Calibrating the measurement system in a wind tunnel can further increase the accuracy of the velocity and the flow measurements. This objective of this paper is to provide the field engineer with single, concise source of information on flow measurement using a Pitot tube.     

Revisit the Pitot static tubes in the standards

A Pitot tube is a popular device used for the measurements of flow fields. To control the accuracy of the Pitot tube coefficient, the international standard organization (ISO), the American Society for Testing and Materials (ASTM), and the Japanese Industrial Standards (JIS) issued guidelines that recommended the shape and working conditions of these devices. However, many Pitot tubes on the market do not follow these guidelines. In the present study, various types of Pitot tubes in the market were tested at the National Metrology Institute of Japan (NMIJ) to determine the effects of the geometry and flow characteristics. The results revealed certain limitations in the existing ISO and JIS standards, specifically with regard to the recommended design parameters of the AMCA Pitot tube, the reference coefficient value for the JIS Pitot tube, and the redefinition and limitation of Reynolds numbers pertaining to Pitot tube working conditions.     

Evaluation of hot-film,dual optical and Pitot tube probes for liquid–liquid two-phase flow measurements

An experimental study of kerosene–water upward two-phase flow in a vertical pipe was carried out using hot-film, dual optical and Pitot tube probes to measure the water, kerosene drops and mixture velocities. Experiments were conducted in a vertical pipe of 77.8 mm inner diameter at 4.2 m from the inlet (L/D=54). The tests were carried out for constant superficial water velocities of 0.29, 0.59 and 0.77 m/s (flow rates = 83, 167 and 220 l/min) and volume fractions of 4.2%, 9.2%, 18.6% and 28.2%. The Fluent 6.3.26 was used to model the single and two-phase flow and to reproduce the results for the experimental study. Two methods were used to evaluate the accuracy of the probes for the measurement of the velocities of water, drops and mixture for two-phase flow: (i) comparison of measured local velocities with predictions from the CFD simulation; (ii) comparison between the area-averaged velocities calculated from the integration of the local measurements of water, drops and mixture velocities and velocities calculated from flow meters’ measurements.The results for single phase flow measured using Pitot tube and hot-film probe agree well with CFD predictions. In the case of two-phase flow, the water and drops velocities were measured by hot-film and dual optical probes respectively. The latter was also used to measure the volume fraction. These three measured parameters were used to calculate the mixture velocity. The Pitot tube was also used to measure the mixture velocity by applying the same principle used for single phase flow velocity. Overall the mixture local velocity measured by Pitot tube and that calculated from hot-film and dual optical probe measurements agreed well with Fluent predictions. The discrepancy between the mixture area-averaged velocity and velocity calculated from flow meters was less than 10% except for one test case. It is concluded that the combined hot-film and optical approach can be used for water and drop velocity measurements with good accuracy for the flow conditions considered in this study. The Pitot tube can also be used for the measurement of mixture velocities for conditions of mixture velocities greater than 0.4 m/s. The small discrepancy between the predictions and experimental data from the present study and literature demonstrated that both instrumentation and CFD simulations have the potential for two-phase flow investigation and industrial applications.     

Methodology for the application of velocimetry by molecular tagging of hypersonic flows

A laser-induced NO fluorescence technique was applied to measure velocity in a hypersonic shock tunnel nozzle exit. For the application of the technique, a detailed study of the density and fluorescence lifetime of the tracer radical, flow velocity and effective test time is proposed, resulting in a methodology for the application of the technique in hypersonic pulsed facilities. The study has demonstrated that it is necessary to jointly evaluate the flow velocity, the fluorescence lifetime of the radical and the width at half height of the laser beam, resulting in a kind of indicator for the feasibility of the technique. The variation of the laser incidence time with respect to the Pitot signal showed that it is not enough to select a stable Pitot pressure signal region to define the laser incidence time, preliminary trial and error analysis are necessary for each device used. Furthermore, the analysis of the velocity values calculated from the linear fit method shows that the adoption of such a method eliminates the effect of the systematic error of the measurements.     

PC-based gas-solids two-phase mass flowmeter for pneumatically conveying systems

The development and performance of a PC-based instrumentation system for measuring the mass flow rate of gas-solid flow in a pneumatically conveying system have been discussed in this paper. A laboratory scale pneumatic conveyor incorporating facilities for calibration has been fabricated for this purpose. The test section is a vertical plexiglass tube of diameter 12.7 mm with an upward flow direction and sand has been used as the solid phase. The principle of measurement is based on inferential technique. Instantaneous velocity and concentration of the bulk solids are measured using a pair of electrodynamic sensors and a capacitance sensor, respectively. Several modifications of the existing technique of measurement have been suggested.     

Ultrasonic pulse-Doppler flow meter application for hydraulic power plants

At hydraulic power stations, Pitot tubes have commonly been used to measure flow rates in steel penstocks for performance testing of hydraulic turbines. Due to the difficulties of Pitot tube installation, transit-time ultrasonic flow meters are becoming a popular replacement, but their accuracy is sensitive to velocity profiles that depend on Reynolds numbers and pipe surface roughness. Ultrasonic pulse Doppler flow meters have recently gained favor as suitable tools to measure flow rates in steel penstocks because they can measure instantaneous velocity profiles directly. Field tests were conducted at an actual hydraulic power plant using an ultrasonic pulse Doppler flow meter, and it was found capable of measuring velocity profiles in a large steel penstock with a diameter of over one meter and Reynolds number of more than five million. Furthermore, two ultrasonic transducers were placed on the pipe surface to validate the multi-line measurement of asymmetric flow. Each transducer recorded the velocity profile simultaneously from the pipe centerline to its far wall during plant operation. Velocity profiles were obtained from three-minute measurements to improve the accuracy of flow rate measurements.     

The effect of low Reynolds number flows on pitot tube measurements

An investigation on the low Reynolds number effect on hemispherical-tipped Pitot tube measurements was performed by measuring the center-line velocity during the laminar flow of a Newtonian fluid in a 25 mm (1 in.) diameter vertical recirculating pipe loop. The primary objective of the study was to reconsider the available low Reynolds number Pitot tube data in the literature with modern instrumentation.Using the results of this experimental study, a correlation that accurately predicts the low Reynolds number Pitot tube behavior has been developed. The correlation accounts for an additional viscous term in the relationship for the pressure coefficient (C_p) which is not accounted for in Bernoulli's Equation. The correlation is semi-empirical and accurately fits experimental data gathered in this study, as well as a significant body of experimental data available in the literature. The correlation, which is based on a Pitot tube Reynolds number calculated using the opening diameter (d), has been shown to be provide more accurate predictions of C_p for a wide range of opening diameter to outer diameter ratios (0.22≤d/D≤0.6) than available correlations based on outer diameter.The transition Pitot tube Reynolds number, below which Bernoulli's Equation is no longer appropriate, was predicted to be approximately 35, compared to a value of 79 obtained from fitting data collected by Barker. The correlation developed in this study provides smoother transitions at both ends of the low Reynolds range. At the low end (Re<10) it converges with a Stokes Law’ analogy, while at the critical transition (Re~35) it converges asymptotically with Bernoulli's Equation. The correlation also accurately predicts the behavior of the pressure coefficient with Reynolds numbers between these ranges.     

A mathematical model of the self-averaging Pitot tube: A mathematical model of a flow sensor

Flowmeters with self-averaging Pitot tubes are more and more often applied in practice. Their advantages are practically no additional flow losses, usability in the case of high temperature of fluids and simplicity of fitting. A mathematical model of a self-averaging Pitot tube including the influence of the probe shape, selected constructional features and flow conditions on the quantity of differential pressure gained has been given in this paper. The values and ranges of variations of the coefficients established for the model have been assessed on the basis of the numerically computed velocity and pressure fields around and inside the probe. Velocity and pressure fields were calculated by means of solving conservation equation and turbulence models. The characteristics linking values of the flow coefficient with values of the Reynolds number have been presented. The conclusions have been formulated taking flow metrology needs into account.     

Pitometry as a validation tool for water flow measurement in large diameter pipelines

Accurate measurement of water flow rates in large diameter pipelines is a challenge for water companies that need to produce, transport and distribute increasing quantities of water. To a large extent, this challenge results from the impossibility of recalibration of the flow meters within the periodicity established in the metrological regulations since the removal of a large size flow meter from its site of operation in the field and its transport to a calibration laboratory is in most cases technically and economically impracticable. Because of this scenario, this paper presents the pitometry technique as an interesting alternative to solve problems related to the validation of water flow measurements performed by flow measurement systems installed in large diameter conduits. The technique is based on the determination of the water flow rate by mapping the velocity profile of the water flow inside the pipe by means of Cole type Pitot tubes. The water flow rate is determined in a cross section of the pipe located near and in series to the flowmeter to be evaluated. Based on the results obtained in a great number of water flow measurements already performed by applying the pitometry technique in large diameter pipelines in the field, it is possible to conclude that this methodology is perfectly applicable in the validation of the performance of flow meters installed in these conduits solving satisfactorily the issues related to its operation.     

Calibration of the Lorentz force flowmeter

The Lorentz force flowmeter (LFF) is a measuring device utilizing localized static magnetic fields for the non-contact measurement of the flow rate in electrically conducting fluids. In case of highly aggressive or high-temperature conductive fluids, it is difficult to reproduce in a laboratory the conditions typically met in application. Hence, calibration becomes a difficult task. For the LFF used for open channel flow measurement in liquid aluminum, we adapt a robust calibration method. This method utilizes reference device calibration data to calibrate other devices. In a first step, liquid calibration involving the fluid flow is performed in a reference open channel with a given reference LFF device. In a second step, the liquid calibration characteristic is transferred from the reference LFF device to the test LFF device during a dry calibration procedure, in which liquid metal flow is replaced by the motion of solid metal bars through the magnet systems of both reference and test devices. Hence, the liquid calibration is needed once for the reference device only. This combined calibration strategy may also be applied to similar measurement devices.     

皮托静压管的流量测量及不确定度评估

本文介绍了皮托静压管的结构原理及使用场合,讨论了使用皮托静压管进行流量测量时的测量计算方法,分析了实际测量条件下不同因素对流量测量结果的影响,并对测量结果的不确定度进行评估.     

Reproduction of air velocity in the entrance region of the test pipe in a wind tunnel

The airflow development in the pipe, in the entrance region of the wind tunnel located in the Lithuanian Energy Institute, the laboratory of Heat Equipment Research and Testing is investigated to analyze the conditions for the reproduction of air velocity values. The analysis is performed to reveal undeveloped flow conditions where the calibration of the devices is usually made, the entrance region of the pipes, or free stream from the nozzles. In this study, different flow regimes have been investigated using different air velocity measurement methods. Experimental and numerical results clearly show the features of the developing flow. They both demonstrate the stable core of the velocity profile up to 5 D in the pipe and ≤1 D from the entrance into the free stream in the testing chamber. Ultrasonic anemometer (UA) installed in the aerodynamic test facility shows reliable and highly comparable results with another non-intrusive device – laser Doppler velocimeter (LDA) in a range of velocities from 0.05 m/s up to 30 m/s. UA integrated into the wind tunnel is not found to be used for metrological issues for air velocity. Due to the fast response, they both enabled to analyze fluctuations in the flow. Local vortices identified in the flow have influenced the low-frequency fluctuations and the scatter of measurement results. Moreover, high-frequency fluctuations found in the flow originated from the flow turbulence and might be due to the electronic or acoustic noise. The stabilization of the entrance region in the pipe influences the mean value of air velocity, the transversal distribution of velocity and the development of axial velocity in different test sections of the pipe in a wind tunnel. Along with the recirculation zones in cavities of ultrasonic transducers, these factors are essential that make an impact on the reproduction of air velocity value.     

流致振动差压式测振装置的研究

为解决管束流致振动试验中,现有接触式振动测量方法的应用局限性,提出一种基于测量受激对象换热管两侧压差脉动频率原理的测振装置,即差压式测振装置。差压式测振装置解决了传统接触式传感器对换热管外部流场的干扰问题以及安装空间受限的问题。为了验证差压式测振装置的有效性,分别进行了单管和换热管束两类流致振动测试试验。试验结果表明,差压式测振装置能在复杂的流动环境中准确捕捉到换热管受到的各类激振形式的频率,而且单管流致振动试验结果表明差压式测振装置所测得的周期性涡激振动频率与理论值偏差在5%以内。因此,差压式测振装置的研制与应用对于流致振动试验的测试以及换热管振动的长期监测均有很重要的意义。