Harry McArthur, P. Eng. Senior Staff Engneer

Flowcare Engineering Inc. Cambridge Ontario

A Comparison of Flow Testing Results where Measurements

were Taken by both a Standard Pitot Tube and a Directional Probe

AMCA International Engineering Conference Las Vegas, NV, USA 2 – 4 March 2008

ENGINEERING PAPER5254-08

Vern Martin P. Eng. Flowcare Engineering Inc.

Cambridge Ontario

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A Comparison of Flow Testing Results where Measurements were Taken by both a Standard Pitot Tube and a Directional Probe

Harry McArthur, P.Eng, Senior Staff Engineer

FLOWCARE Engineering Inc., Cambridge, Ontario 1.0 Background Fan testing requires the determination of the volume flow rate. One of the most common methods to obtain this measurement is the velocity traverse method. The velocity traverse method consists of subdividing a duct cross-sectional area into a number of elemental areas. The velocity pressure normal to each elemental area is then measured, using a suitable probe at the centroid of each elemental area. The average velocity normal to the duct cross sectional area is then calculated using a formula that recognizes the contributions of each elemental area in conjunction with the gas density. This method of testing generally requires that the following conditions hold true: • All the flow in the test plane is in one

direction (there is no flow reversal.) • The flow has a relatively uniform velocity

profile across the test plane. • The flow is predominately normal to the test

plane, not skewed. Skewed flow is defined as flow that is in a direction other than normal to the test plane. Only the component of velocity normal to the test plane is pertinent to the calculation of flow. If the flow is skewed, the test method must be able to quantify that portion of the flow that is normal to the test plane.

Recognizing the requirements of field testing where varied velocity distributions and/or skewed flow may occur, performance standards have been developed in an effort to reduce or eliminate the uncertainty and errors associated with these factors. These standards either discourage testing at locations where questionable conditions may occur or specify the use of probes capable of indicating both flow direction and velocity pressure. The AMCA Publication 203 ‘Field Performance Measurements’ and the AMCA Standard 803 ‘Industrial Process/Power Generation Fans: Site Performance Test Standard’ are a test guide

and standard based on the use of non-directional probes. These documents stipulate specific requirements for both the traverse test plane and the test results that are designed to reduce or eliminate problems associated with flow uncertainties. The American Society of Mechanical Engineers (ASME) Performance Test Code No.11 (PTC-11) is a test code that addresses the uncertainty of field testing by specifying the use of a directional probe at a relatively large number of points for the velocity traverse. In this way, the actual flow vectors can be measured and the uncertainty due to a lack of uniformity reduced by testing at a large number of points. Testing done by FLOWCARE has shown that velocity traverses with Pitot-static tubes can give relatively accurate test results for test conditions that would be considered unacceptable based on AMCA 803 or even AMCA 203 test criteria. These are test conditions with significantly skewed flows. • This paper will discusses the AMCA and

PTC-11 test standards and procedures used to conduct a velocity traverse and the specific requirements of these standards as they pertain to testing in conditions where there may be skewed flow in the velocity traverse test plane.

• A case study detailing the results of fan performance testing done on two 6,000 hp, ID fans on Unit 2 of PacifiCorp’s Jim Bridger Generating Station will be presented. The test plane chosen for these fans does not meet AMCA requirements due its geometry and the flow angles. The velocity traverses were done following both PTC-11 code requirements and AMCA 203 guidelines with equal results.

• The performance and accuracy of a Pitot-static tube in a skewed flow stream will be examined.

• The paper will discuss the implications of the test results on the accuracy and reliability of the two test probes.

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2.0 Field Performance Test Standards for Acceptable Velocity Traverse

Probably the single greatest factor behind the specification of test standards and codes as they pertain to the velocity traverse involves the direction of the flow stream through the test plane. A concern is fuelled by the reality that errors in the assumption of the flow direction can lead to significant errors in the calculation of the volume or mass flow rates. AMCA addresses this issue by the stipulation of specific requirements for both the traverse test plane and the test results. These are designed to eliminate problems associated with flow uncertainties. ASME addresses this issue in PTC-11 by the use of a directional probe and a large number of traverse points. 2.1 AMCA 203 - Criteria for the Velocity

Traverse AMCA Publication 203 ‘Field Performance Measurements’ is the simplest and least costly test procedure. AMCA 203 recommends the following criteria for the traverse test plane and the resulting velocity measurements: • The cross-sectional shape of the airway

(duct) should not be irregular. Specifically the test plane should be rectangular or circular in cross-section.

• The cross-sectional shape and area of the duct in the vicinity of the traverse plane should be uniform. Divergences and convergence of the duct should be moderate so that uniform flow conditions exist.

• The velocity distribution should be uniform with more than 75% of the velocity pressure measurements greater than 1/10 of the maximum measurement.

• The angle between the flow stream and the traverse plane must be within 10 degrees of a right angle.

The first two criteria are relatively straightforward and easy to assess. The third criteria can not be established until after the test is completed while the fourth criteria would require the use of a directional probe to be accurately assessed. The last two criteria are those most likely to be affected by the presence of skewed flow. There is no requirement that individual flow vectors fall into any range but only that the angle between

the flow stream and the traverse plane must fall with 10 degrees. There are no guidelines given as to how this flow stream angle is established. It appears that these aspects are left up to the subjective experience and judgment of the tester. It should be noted that only velocity pressure measurements are required at each elemental point. Static pressures can be measured at each point or obtained by static pressure taps on the duct side panels. 2.2 AMCA 803 Criteria for the Velocity

Traverse AMCA 803 test standard has significantly more stringent requirements then the guidelines provided by AMCA 203. This standard requires velocity pressure and static pressure measurements be taken at each point in the traverse plane. It also has a number of requirements that specifically address the geometry of the duct at the traverse test plane, the directionality of the flow through the traverse test plane and the resulting velocity measurements. Failure to meet any of these criteria invalidates the results from qualifying as an 803 test. These criteria are as follows: • The cross-section of the duct at the

measurement plane shall be circular or rectangular with no irregularities.

• The measurement plane shall be free from any accumulation of dust or debris.

• The measurement plane shall not intersect any internal stiffeners, supports, splitters, vanes. It shall clear such internal obstructions by at least 0.5 times the duct diameter or equivalent diameter for a rectangular duct sections.

• Any measurement plane shall be at least 0.5 D upstream and 1.0 D downstream of any bend or change in cross-sectional area.

• Standard deviation of the velocity variation must be less than 10% of the mean velocity.

• AMCA 803 considers the Pitot-static tube as the primary probe type. The Pitot tube shall be parallel to the axis of the duct within ±7.5°.

• The angle of flow at each measurement point may not exceed 15° from a right angle to the measurement plane. This criterion may require the use of a directional probe to measure the flow

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angle. The use of a directional probe is not a mandatory requirement. The directional probes recommended by AMCA 803 are the Fecheimer probe, wedge probe, three hole cylindrical probe or other angle-sensitive measuring device. All of these probes with the exception of ‘other’ are probes designed to sense the flow angle in only a single plane.

• The number of points which may exceed 10° from a right angle to the measurement plane shall not exceed 10% of the total number of traverse points.

• Ultimately, an agreement must be reached between the tester and the customer as to the acceptability of the test plane and test results.

The first few criteria dealing with the duct geometry are relatively straightforward and easy to assess. That is not to say that meeting these criteria is always possible. The requirements pertaining to the distortion of the velocity profile and/or skewed flow are often difficult or impossible to meet and can only be assessed after a test is completed. Therefore, costly and time consuming tests possibly involving multiple traverses with multiple sensors may end up being invalidated due to a single test point vector that exceeds 15° from the axis of the duct. There is also the possibility of error due to the fact that the recommended directional probes only address flow in one plane. 2.3 PTC-11 Test Criteria PTC-11 is primarily a fan test code. As such, the test planes are at the fan inlet(s) and fan outlet. PTC-11 states ‘Due to the highly disturbed flow at the fan boundaries and the errors obtained when making measurements with probes unable to distinguish directionality, probes capable of indicating gas direction and speed, hereinafter referred to as directional probes, are generally required. Only the component of velocity normal to the elemental area is pertinent to the calculation of flow.’ While allowing other flow measurement methods, the velocity traverse is considered the primary method by PTC-11. One of the most common types of directional probes is the prismatic 5-hole probe. Figure 1 shows a sketch of the tip of a 5-hole prismatic probe and the location of the pressure taps (holes) on the probe tip.

Figure 1 – Sketch of Prismatic 5-Hole Probe - Port P1 is used to measure total pressure. - Ports P2 and P3 are used to establish the

yaw angle. - Ports P4 and P5 are used to measure the

pitch angle. Figure 2 shows the relationship of pitch and yaw to the probe tip.

Figure 2 – Yaw and Pitch Referenced to Probe Tip

• The traverse plane suggested by PTC-11

assumes that the traverse plane contains skewed flow. The skewed flow is accounted for by using a directional probe.

• The directionality of the flow vector at individual elements is measured in terms of yaw and pitch angles. The yaw angle is determined by rotating the probe about its axis until the pressure (P1 – P2) is zero. The angular rotation of the probe is the yaw angle. The pitch angle is generally

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determined by measuring the differential pressure across the pitch test ports and then interpolating the pitch angle from calibration curves.

• All pertinent pressures must be interpolated as a function of the pitch and/or yaw angles and velocity from calibration curves. Each calibration curve is for a specific velocity and covers a range of pitch angles generally from 0° to ±40°. A number of calibration curves covering the anticipated velocity range of the test plane are required.

Finding the yaw angle at each traverse point greatly increases the amount of time required to perform the test and thus keep the system at a constant operating point. PTC-11 requires that five different test values be recorded at each test point. These parameters are the yaw angle or yaw pressure differential, pitch pressure differential, the velocity pressure, total and/or static pressure and the temperature. Testing to this code is generally costly, time consuming and onerous to complete. The accuracy of the measured data is very dependent on proper use of the probe in the field as well as accurate calibration over the measured velocity range. The time required to log and record all required field test parameters has resulted in heavy

reliance on electronic instrumentation and data logging technology. Once field data collection is complete, the flow calculation procedure is also complex and onerous.

3.0 Description of Actual Field Test

Procedures Used for Case Study

The selection of a test plane on the ID fans at PacifiCorp’s Jim Bridger Generating Station presented a difficult challenge. Figure 3 shows the configuration of the fans and surrounding ducts. It also shows the location of the test plane in one of the fan inlet boxes. The fans are double width drawing flue gas from an electrostatic precipitator. The flue gas feeds into a crossover header that in turn feeds into the transition above the fan inlet boxes from three different directions. The pants split the flow from the three converging ducts into the inlet boxes. The pants have both converging and diverging sides. Two sides converged on the inlet box at 30 degree angles while the opposite two sides diverged at approximately 15 degrees.

Figure 3 – Fan and Duct Arrangement and Location of Test Plane

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The velocity traverse test planes chosen for these fans were in the fan inlet boxes at a plane approximately 18 inches below the inlet box flange. Consideration for a test plane in the discharge evase was rejected as the evase was diverging in all planes, had an unknown internal geometry due to a false bottom, terminated in a 45° degree elbow and was difficult to access. None of the potential test planes met AMCA 803 requirements in terms of the duct configuration. It was also anticipated that there would be significantly skewed flows at the inlet test plane that would also disqualify an AMCA 803 test. Due to the poor traverse plane location and the anticipation of significantly skewed flows, a velocity traverse with a directional probe had been specified. The inlet boxes have a cross-sectional area of 121 ft2. A total of 64 velocity pressure readings were obtained in each box. The velocity traverses were carried out using a prismatic 5-hole probe. For three of the inlet

boxes, data were first collected using the 5-hole probe first and then using the Pitot tube. Two fan tests were carried out using both the 5-hole probe and the Pitot tube. Table 1 lists the tests and operating conditions in terms of the Variable Inlet Vane (VIV) damper setting and the gross megawatt production rate over the test period. It should be noted that for Test 21A, both probes were only used on one inlet box. • When using the 5-hole probe, difficulty was

encountered in establishing the yaw angle at some traverse points as turbulence resulted in fluctuations of up to 15 degrees. A 15° error in the yaw angle can result in a 5% or greater error in establishing the actual velocity pressure.

• Flow measurements with the Pitot tube could be carried out in approximately 2 hours while the tests with the directional probe took up to 8 hours.

Fan Test No Date Description VIV

(% Open) GrossMW

21 21A Nov. 6, 2007 Test of fan with motor fully loaded 69 540 22 22A Nov. 7, 2007 Test of fans at MCR conditions 80 540

Table 1 – Fan Performance Test Schedule, Nov. 6 to Nov. 8, 2007

For the Pitot tube measurements, data readings were averaged over a period of 10 to 20 seconds. With the 5-hole probe, due to the time necessary to zero the yaw and the greater number of parameters that had to be recorded, data were obtained using a shorter averaging period. This period could be as short as 3 seconds. The flow distribution across the inlet boxes was relatively uniform. This is illustrated in Figure 4 which shows a comparison of the flow traverses in the north inlet box of Fan 22 for both the 5-hole probe and the Pitot tube. The velocity profile measured by the pitot tube appears smoother than that obtained by the 5-hole probe; probably because the pitot tube measurements were averaged over a longer time period.

Table 2 shows a comparison between data obtained using the 5-hole probe and that obtained using the Pitot tube for Test 21 of north inlet box.

Table 3 shows the maximum, minimum and average pitch, yaw and absolute angles of the 5-hole probe. All the angles are referenced to the axis normal to the traverse plane. The sign on the yaw and pitch indicate the quadrant. The absolute angle is the angle between the velocity vector and the axis normal to the test plane.

Table 4 shows a comparison of the flow and pressure data between the two types of probes used for Test 22 of ID Fan 22.

Table 5 shows the maximum, minimum and average pitch and yaw angles of the 5-hole probe for test 22 of ID Fan 22.

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Figure 4 - Fan 22 North Inlet Box Velocity Profiles Measured by a Pitot Tube and 5-Hole Probe

North Inlet Box Fan 21A Nov. 6, 2007 5-hole Probe Pitot Tube %Difference Static Pressure (in.wg) -18.71 -18.17 2.9

Velocity Pressure (in.wg) 1.11 1.06 4.4 Total Pressure (in.wg) -17.61 -17.11 2.8

Volume (acfm) 687,162 678,793 1.2

Table 2 – Test 21, Comparison of 5-Hole Probe and Pitot Tube Data

North Inlet Box Pitch Angle

(Deg.) Yaw Angle

(Deg.) Absolute Angle

(Deg.) Maximum 15.2 17.8 41.1 Minimum -13.8 -41.0 0.2 Average 0.8 -5.5 12.1

Table 3 – Test 21, North Inlet Box, Max., Min. and Average Pitch, Yaw and Absolute Angles

North Inlet Box

Fan 22A Nov. 7, 2007 5-hole Probe Pitot Tube % Difference Static Pressure (in.wg) -18.69 -18.50 1.0

Velocity Pressure (in.wg) 1.07 1.13 -5.9 Total Pressure (in.wg) -17.62 -17.37 1.4

Volume (acfm) 692,574 716,421 -3.4 South Inlet Box

Fan 22A Nov. 7, 2007 5-hole Probe Pitot Tube % Difference Static Pressure (in.wg) -18.53 -18.54 0.0

Velocity Pressure (in.wg) 1.09 1.11 -1.6 Total Pressure (in.wg) -17.44 -17.43 0.1

Volume (acfm) 686,657 694,562 -1.2 Average/Total

Fan 22A Nov. 7, 2007 5-hole Probe Pitot Tube % Difference Static Pressure (in.wg) -18.61 -18.52 0.5

Velocity Pressure (in.wg) 1.08 1.12 -3.7 Total Pressure (in.wg) -17.53 -17.40 0.7 Total Volume (acfm) 1,379,231 1,410,983 -2.3

Table 4 – Test 22, Comparison of 5-Hole Probe and Pitot Tube Data

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North Inlet Box South Inlet Box Pitch

Angle (Deg)

Yaw Angle(Deg)

Absolute Angle (Deg.)

Pitch Angle(Deg)

Yaw Angle(Deg)

Absolute Angle (Deg.)

Maximum 15.6 39.7 40.1 11.5 25.0 25 Minimum -24.3 -25.0 0.86 -12.0 -13.3 1.2 Average -0.5 0.3 11.7 0.1 -1.8 8.6

Table 5 – Test 22, Maximum, Minimum and Average Pitch and Yaw Angles

The test results from the 5-hole probe and the standard Pitot tube showed very close correlation in spite of the fact that the pitch and yaw angles covered a wide range of values. 4.0 Effects of Pitch and Yaw on Pitot-static Tube Pressure Measurements The Pitot-static tube is the primary sensing instrument for the AMCA 203 and the AMCA 803 test standards. One of the concerns in using the Pitot-static tube is the direction of the flow vector impacting on the tube. This concern is evidenced in the criteria given for acceptable traverse plane configurations and the limitations placed on the allowable angularity of the flow stream in the AMCA test guide and standards. Figure 5 shows a graph illustrating the errors in static, total (stagnation) and dynamic pressures when a Pitot tube is subjected to increasing yaw.

The graph in Figure 5 was created from data in NACA Technical Note No. 546 ‘Comparative Tests of Pitot-static Tubes’ By Kenneth G. Merriam and Ellis R. Spaulding published in 1937. Part of the analysis carried out by Merriam and Spaulding was the effect of yaw on the Pitot tube. This paper presents a recommended design for Pitot tubes that is the basis of many Pitot tubes sold today. The percentage errors shown in Figures 5 pertain to the error between the actual total, static and dynamic pressures of the flow stream as compared to what the pressure gauges attached to the Pitot tube will be indicating. The errors are shown as a percentage of the dynamic pressure. They are not

Pitot-static Tube Percent Error in Dynamic Pressure

-25

-20

-15

-10

-5

0

5

0 5 10 15 20 25Yaw Angle Degree

Per

cent

age

Err

or

Static PressureTotal PressureDynamic Pressure

Figure 5 – Relation between Percent Error and Degrees of Yaw

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absolute errors. It is interesting to note that the dynamic pressure error will result in slightly higher than actual dynamic pressure readings for angles of yaw up to approximately 18° of yaw. This is a interesting observation due to the fact that Section 8.3 of AMCA 203 states ‘the angle of the flow stream in any specific location is indicated by the orientation of the nose of the Pitot-static tube that produces the maximum velocity pressure readings at that location’. It can be seen from Figure 5 that this exercise may result in the Pitot tube pointing approximately 10° to 12° off of the direction of the flow stream. Figure 5 shows the errors as a function of yaw angle. In reality, skewed flow will have both pitch and yaw components. Figure 6 shows a Pitot tube in a skewed flow stream where the flow stream impacts the Pitot tube in a plane that does not fall on either the pitch or yaw planes.

Figure 6 – Pitot-static Tube in a Skewed Flow Stream

The absolute value of the flow stream (V) impacts the Pitot tube at an angle phi that is relative to the axis (Z) normal to the test plane. For flow testing purposes, only the velocity normal to the test plane (Vz) is of interest.

Therefore, if V and Phi are known, Vz can be calculated as follows: Vz = V * cos(Φ) This is the rational behind the use of the directional probe. The velocity pressure of vector V is Pv and the equivalent velocity pressure of velocity component normal to the test plane (Vz) is Pvz. Pv = Density * (V/C)2 V = C * √(Pv/density) Pvz = Density * (Vz/C)2 Pvz = Density * [(V * cos(Φ) )/C]2 Pvz = Pv * cos2 (Φ) Where: C = Constant 4.1 Effect of Skewed Flow on Measurement

of Velocity Pressure Normal to test Plane

The errors shown in Figure 5 are the errors relative to the actual flow stream velocity vector V. However, only the velocity component normal to the test plane is of interest. Therefore, the velocity pressures that the Pitot tube indicates should be compared against the equivalent velocity pressure (Pvz) of the velocity vector (Vz) normal to the test plane. Figure 7 shows the velocity pressure indicated by a Pitot tube and the actual equivalent velocity pressure (Pvz) of the Vz velocity component. Also shown on Figure 7 are the errors between the indicated velocity pressure and the actual (Pvz) and the error in the velocity Vz calculated from the indicated velocity pressure. It can be seen from Figure 7 that the error in the calculated velocity is approximately 5% for skewed flow angles of 20°. Tests done by FLOWCARE under non-laboratory conditions indicate that the error in the calculated velocity is approximately 10% at angles up to 40°. The error will result in the calculated velocity being higher than the true velocity normal to the test plane.

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Comparsion Between Measured and Actual Velocity Pressures Normal to Test Plane

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

0 5 10 15 20 25Angle of Skewed Flow

Uni

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Pre

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10%

15%

20%

25%

30%

35%

Perc

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rror

Measured VpActual Vpz Normal to Test PlaneError in Measured Velocity Error in Vp Measurement

Figure 7 - Comparison Between Measured and Actual Velocity Pressures Normal to Test Plane Table 6 shows the results of the ID fan tests at PacifiCorp’s Jim Bridger Generating Station. The table shows average angle of skew, the percentage error in the measured flow rate as compared to the 5-hole probe and the predicted

error based on the data in Figure 7. This table is based on the assumption that the velocity pressure measured by the 5-hole probe is more accurate than that obtained by the Pitot tube.

Fan Average

Angle of SkewPercentage Error in Flow

Predicted Error From Figure 7

Fan 21 North Inlet Box 12.1 1.2% Low 3.0% High Fan 22 North Inlet Box 11.7 3.4% High 2.9% High Fan 22 South Inlet Box 8.6 1.2% High 1.7% High

Table 6 – Comparison of Actual Results to Predicted Results

It can be seen from Table 6 that the predictions based on Figure 7 are close for two cases. The lack of agreement between the results for the test on Fan 21 and Figure 7 can potentially be explained by the time lapse between the 5-hole test and the Pitot test. The 5-Hole probe test was carried out over an eight hour period. When the 5-hole probe test was complete, it was followed by the Pitot tube test. The fan was ostensibly operating at a fixed point over this whole period of time but there undoubtedly was some variation in the actual volume flow rates.

4.2 Effect of Skewed Flow on Measurement of Total and Static Pressures with a Pitot Tube

It appears from Figure 5 that the errors in the total and static pressures increase rapidly and exponentially with increasing degrees of yaw. The errors are such that indicated total and static pressure readings will be lower than the actual flow stream pressures. However, as was previously stated, the errors shown in Figure 5 are errors as a percentage of the velocity pressure. They are not absolute errors. Figure 8 shows a graph of the total pressure error as a function of velocity pressure.

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Total Pressure Error as a Function of Velocity Pressure

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

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110%

0% 500% 1000% 1500% 2000% 2500%

Total Pressure as Multiple of Vp

Max

imum

Pos

sibl

e To

tal P

ress

ure

Erro

r (%

)

0 Vp 5xVp 10xVp 15xVp 20xVp 25xVp

Figure 8 – Total Pressure Error as a Function of Velocity Pressure

For flow vector angles between phi = 0° and phi = 90°, the error in the total pressure reading of the Pitot tube will range from 0 at 0° to approximately (Pt – Pv)/Pt at phi = 90°. At the 90° position, the total pressure tap will be oriented perpendicular to the flow stream and will be functioning essentially as a static pressure tap. If the velocity pressure is a small fraction of the total pressure, the absolute error in the total pressure indicated by a Pitot tube in skewed flow will be minimal. For example, the maximum total pressure error in a duct with a velocity pressure of 1 in.wg and a total pressure of 20 in.wg is 5%. This error will occur with the Pitot tube oriented 90° to the air stream. If the flow is at 25° to the Pitot tube, the total pressure error as indicated by Figure 5 is approximately 25%. This translates into an error in the total pressure indicated by the Pitot tube of only 1.3%. However, for ducts with high velocities and low total pressures, the error due to skewed flows can become very significant. For a duct with both the velocity and total pressures equal to 1.0 in.wg, the error in total pressure reading due to skewed flow can be as high as 100%. Therefore, when assessing the possible error in a field test, the relationship between the velocity and total pressure must be taken into account. The indicated static pressure from a Pitot tube in skewed flow is limited by the magnitude of the

velocity pressure. However, unlike the total pressure error, the error in the static pressure does not increase as quickly as the total pressure error. The static tap in the Pitot tube consist of 8 ports evenly spaced around the perimeter of the Pitot tube at a location 8 tube diameters downstream of the Pitot tube tip. With flow axial to the Pitot tube, these static ports are perpendicular to the flow stream and only sense the static component of the flow. As the flow is skewed, the ports facing the flow stream will see an increase of pressure as flow begins to stagnate on the upstream side of the tube. Meanwhile, the ports on the sides and downstream side of the tube will either see just the static or a pressure below static as flow starts to separate on the downstream side of the tube. Figure 9 shows part of the results of a Computation Fluid Dynamics (CFD) analysis done by FLOWCARE on a Pitot tube subjected to various degrees of skewed flow. The Pitot tube in Figure 9 is positioned 45° to the flow stream. While the condition shown in Figure 9 is an extreme case, it illustrates the conditions around the Pitot static ports. It can be seen from Figure 9 that the area around the static pressure ports is subjected to a wide range of pressures. The net result will be flow into and out of these ports.

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Figure 9 – Flow Past Static Ports with Pitot 45° to Flow Stream

The net effect will be measured static pressures below the actual static pressure in the duct. The CFD analysis, while not conclusive or definitive, indicates that as the skew angle increases, the error in the static pressure reading will reach a limiting value. This limiting value will be dependent on both the flow stream velocity and flow angle. 6.0 Conclusions • Due to concerns regarding the directionality

of the flow, a 5–hole probe was used to measure both the velocity pressure and flow direction at each traverse point. The results of the 5-hole probe confirmed the presence of skewed flows. A Pitot-static tube was also used to measure fan performance. A comparison between the two test methods indicated that the Pitot-static tube gave comparable results. Indications are that a Pitot-tube can be used over a wider range of skewed flows than originally thought.

• Errors in the total and static pressure measurements obtained by a Pitot tube in skewed flows are dependent on the actual values of these pressures and the velocity pressures in the duct. The error in the total pressure will be less for ducts with higher pressure and lower velocity pressures.

• It is not possible to obtain the true static pressure at each traverse point in a skewed flow using just a Pitot tube. Comparing the Pitot static pressure with static pressure measurements obtained by some other means such as duct wall taps should yield reasonably good results. A total pressure can

then be calculated using Pvz and the static pressure obtained from side wall taps. The total pressure calculated in this manner will not be the true total pressure of the flow stream as it will not contain the velocity pressure component of the flow parallel to the test plane. However, the error will be relatively small.

• Calibrating Pitot-static tubes over a range of Yaw/Pitch angles may allow for the determination of the angle of a flow stream relative to the Pitot tube axis. This determination would be accomplished by recording indicated Pt, Ps and Pv pressures and comparing the error in the values obtained by the Pitot tube against Ps and/or Pt values measured by wall taps or total pressure from a total pressure probes unaffected by skew. Using the error between the calculated and measured values and the calibrated error curves it may be possible to estimate the angle of the flow relative the axis of the Pitot tube.

References: Merriam, Kenneth, G., Spaulding, Ellis R. ‘Comparative Tests of Pitot-static Tubes’ NACA. Technical Note No. 546, 1937. Folsom, R. G., ‘Review of the Pitot tube’, ASME Fluid Meters Research Committee, ASME, 1955 IP-142 AMCA Publication 203. Field Performance Measurement of Fan Systems AMCA Standard 803, Industrial Process/Power Generation Fans: Site Performance Test Standard ANSI/ASME PTC 11-1984, ‘Performance Test Codes, Fans,’ ASME PTC 19.5-2004, Performance Test Codes Flow Measurement