PENG 001 Gas Flow Measurement By Pitot Static Tube.pdf

8/11/2019 PENG 001 Gas Flow Measurement By Pitot Static Tube.pdf

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PENG_001.DOC Revision 3 Page 1 of 9

BLUE CIRCLE CEMENT

PROCESS ENGINEERING STANDARDS

GAS FLOW MEASUREMENT BY PITOT STATIC TUBE

PENG_001.DOC Prepared Checked Date

R W Davies K Rabson 06/12/95

1 PURPOSE AND SCOPE

1.1 This WI outlines the technique to be employed when measuring gas flowsusing a Pitot static tube.

2 EQUIPMENT

Note: Care is needed to use equipment of known calibration as failure to do soleads to uncertainty of the results obtained.

2.1 Calibrated manometer, either electronic or fluid type.

2.2 Pitot static tube of KNOWN calibration factor, and conforming to the criteriaspecified in BS 1042 section 2.1, 1983. If at all possible, use an NPL Pitot tubeof known correction factor.

2.3 Measuring tape and measuring rod.

2.4 Calibrated digital thermometer.

2.5 Thermocouple.

2.6 Adjustable spanner.

2.7 Full appropriate safety equipment, i.e. safety shoes, safety helmet, coveralls,safety glasses, gloves, ear defenders, etc.


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3 FREQUENCY

3.1 On request, or as part of a routine process evaluation.

4 METHOD

4.1 Ensure the environment is safe.

4.1.1 Take particular care when the gas is hot and when the flow to be measured isat a higher pressure than atmospheric.

4.1.2 Make sure there is no risk that the Pitot itself can get caught in any adjacentfan blades, damaging the fan or causing injury to personnel.

4.2 Select sampling cross section

4.2.1 The sampling cross section should be located in a position where the Reynoldsnumber based on the diameter of the total pressure hole of the Pitot tube is inexcess of 200 1, and the local Mach number does not exceed 0.25 1.

4.2.2 The cross section selected for the measurement shall be located in a straightpipe length where the flow is substantially parallel to and symmetrical about theconduit axis. The sampling plane shall be perpendicular to the direction of theflow and shall be of a simple shape, such as circular or rectangular. Therefore,the sampling cross section needs to be far enough away from disturbances

that could cause swirl or turbulence. Typically located away from any flowdisturbance by at least:-

Downstream Upstreameight duct diameters 2 two duct diameters 2 six duct diameters 3 six duct diameters 3

4.3 Measure the dimensions of the sampling cross section

4.3.1 Circular Cross Sections 1

The mean diameter should be determined by calculating the arithmetic meanof at least four diameter measurements (including the traverse diameters),each measurement at approximately equal angles to each other. (i.e. 0 o, 45 o,90 o, and 135 o of the cross sectional plane; not 0 o, 90 o, 180 o, and 270 o of thecross sectional plane). Double the number of measurements if the differencebetween any two consecutive diameters is greater than 0.5%. It may not be


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practical to have four sockets at the specified angles; typically there are twosockets, one at 0 o and the other at 90 o. Therefore, it is suggested that a small5mm hole be drilled at the specified angles and a measuring rod be inserted forthe sake of measuring the duct dimensions. Once done the hole can be closed

by threading in a 5mm bolt.

4.3.2 Rectangular Cross Sections 1

The duct width and height should be determined by calculating the arithmeticmean of at least four equally spaced measurements at positions along each ofthe duct dimensions. Double the number of measurements if the differencebetween any two consecutive measurements is greater than 1.%

4.3.3 Use the above dimensions to calculate the duct cross sectional area A

4.4 Define the position of the measuring points in the cross section

The objective is to divide the sampling cross section area into a number ofsections of equal area. The measuring point per section should then bepositioned at the centre of each sectional area in order that the measurementwill be representative of that area. Two traverses can be achieved, one aninwards traverse and one an outwards traverse. This will result in a doubling ofthe number of measurements taken without a significant increase in effort.


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4.4.1 Circular Cross Sections

The minimum number of measurements should be six per duct diameter in aminimum of two perpendicular traverses. which if performed in an inwards and

outwards direction gives a minimum total of twenty four measurements percross section. The following table is calculated according to the "log-Tchebycheff" method.

Number of Pointsper traverse of

duct diameter D

10 Pointsper duct dia



1 0.0189 x D 0.0236 x D 0.0321 x D

2 0.0765 x D 0.1000 x D 0.1349 x D

3 0.1525 x D 0.1938 x D 0.3207 x D

4 0.2171 x D 0.3343 x D 0.6793 x D

5 0.3612 x D 0.6657 x D 0.8651 x D

6 0.6388 x D 0.8062 x D 0.9679 x D

7 0.7829 x D 0.9000 x D

8 0.8475 x D 0.9762 x D

9 0.9235 x D

10 0.9811 x D


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4.4.2 Rectangular Cross Sections

The minimum number of measuring locations shall be 25, their positionsbeing defined by the intersections of at least five straight lines running parallel

to each wall of the duct. The table below is calculated according to the "log-Tchebycheff" method. The table will allow for calculation of both the lengthand width spacing for a rectangular cross section, giving a range of 25 to 49measuring points, which if measured twice will yield 50 to 98 readings.

Often the sockets in a rectangular duct are in place. It is necessary to checktheir spacings. If they do not fit the above table, then it will be necessary todevise a measuring grid in order that each measuring point is representativeof an equal segment of the total area.

Number of Pointsper traverse of

duct dimension D

7 Pointsper traverse



1 0.053 x D 0.061 x D 0.074 x D

2 0.203 x D 0.235 x D 0.288 x D

3 0.366 x D 0.437 x D 0.500 x D

4 0.500 x D 0.563 x D 0.712 x D

5 0.634 x D 0.765 x D 0.929 x D

6 0.797 x D 0.939 x D7 0.949 x D

4.5 Measure the differential pressure at each measuring point P d

4.5.1 The Pitot tube used should be of diameter, d, where its ratio to the ductdiameter, D, given by d/D shall not exceed 0.02. The distance between theaxis of the head of the Pitot tube and the side wall should never be less thanthe head diameter, d.

4.5.2 As there is no flow of air through the Pitot tube, connecting hoses and themanometer, there will be no pressure drop. Therefore, for practical purposesthere is no limit to the distance between the Pitot tube and the manometer.

4.5.3 Once the Pitot tube is introduced then one needs to ensure that it is rigidlyfixed in each measuring position and that no inleaking air is allowed to enter


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the duct. The axis of the Pitot head needs to be in line with the axis of theduct and perfectly perpendicular to the flow as failure to do so will lead tomeasuring errors. Typically for a BS type tube a 20 o yaw will lead to a 2%error, whilst for an 's' type Pitot a 5 o yaw will lead to a 1% error. If at all

possible, use an NPL type Pitot tube and keep the s type for circumstanceswhere an NPL will not perform.

4.5.4 The Pitot tube tip facing the fluid flow measures Total Pressure of the fluid inthe duct, whilst the Pitot tube side holes, in the case of an NPL tube, and thereverse facing tube in the case of an 'S' type of tube measure StaticPressure. The two connection points of the manometer should be connectedseparately to the Total Pressure tapping and the Static Pressure tapping onthe Pitot tube. This will allow the manometer to display the DifferentialPressure between the two which is representative of :-

Total Pressure - Static Pressure = Differential Pressure

where it is accepted that

Total Pressure = Static pressure + Velocity Pressure

therefore

Differential Pressure = Velocity Pressure

Measure the differential pressure at each of the predetermined locations.

Allow sufficient time for the signal to stabilise. If the reading is unstable,consider implementing some damping on the manometer. However, if thereading is very unstable, then the gas flow is likely to be too unstable to yielda representative measurement. It is possible to measure an inward traverseand a outward traverse, thus doubling the number of readings obtained.

4.6 Measure the static pressure in the sampling cross section P st

Once all the traverse measurements are complete, place the Pitot tube in thecentre of the cross section and clamp firmly in place. In the case of the NPLPitot ensure the total pressure nose is facing the direction of oncoming flow.

In the case of the s type Pitot turn the tube through 90o

, in order that the openapertures of the tube are at 90 o to the direction of gas flow. Disconnect boththe hoses, and then reconnect the static pressure hose to the positivemanometer connection in order to display the

Static Pressure - Atmospheric Pressure = Differential Pressure


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Don't be surprised if the reading is negative as a suction in the duct willalways have a lower pressure than atmospheric. The reading therefore is theStatic pressure relative to the Atmospheric pressure. Note pressure readings

are typically taken as a head of water gauge. Allow the manometer tostabilise before a reading is taken.

4.7 Measure the temperature in the duct T act

Allow the thermocouple to stabilise before a reading is taken. Use themethod outlined in PENG_002.DOC. Determine the local absolutetemperature

Tabs = T act (F) + 459.4 in Rankin units

Tabs = T act (C) + 273.2 in Kelvin units

4.8 Determine the local atmospheric pressure P atm

If a barometer is not available on site, the local airport usually provides aweather reporting service and will provide the atmospheric pressure onrequest.

4.9 Determine the volume flow rate in the sampling cross section Q act

4.9.1 Determine the absolute pressure of the fluid. P abs

Note pressure readings are typically taken as a head of water gauge.

4.9.2 Determine the actual fluid density E act

4.9.3 Determine the average root differential pressure P r

Take the square root of each of the differential pressure readings, then totalthem and divide by the number of readings to derive the average rootdifferential pressure. P d is measured typically as in H 2O or mm H 2O.

P + P = P st atmabs

) P ( ) P ( .

)T ( )T ( . E = E

n

abs

abs

nnact


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4.9.4 Determine the corrected average root differential pressure P corr

Due to the variances in the calibration of the various Pitot tubes in use then acorrection factor is needed. The correction factor for an ellipsoid head tubefrom a reputable supplier will be close to unity, however each tube needs tobe checked against a known standard. Once the correction factor is knownfor the tube in use it is suggested that the factor be engraved of the tube forfuture reference.

Check that the application of the correction factor is used in the correct sensein the calculation used.The correction factors documented in the table are CF 2. These factors willneed to be square rooted for derivation of CF, which then should be used inthe above equation and also in the spreadsheet calculation.

4.9.5 Determine the velocity of the fluid in the sampling cross section Vact

4.9.6 Determine the volume of the fluid Q act

Readingsof No P = P

d r

)CF . P ( = ) P ( 2

r 2

corr

E .C E . ) P ( . g .2 =V

act

w2

corr act

A.V =Q act act


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4.10 Nomenclature

Equation Description Imperial Unit Metric Unit

P atm Local atmospheric pressure in H 2O mm H 2OP abs Local absolute pressure in H 2O mm H 2OP st Local static pressure in H 2O mm H 2OP n Normal pressure 406.8473 in H 2O 10333.92 mmH 2OP d Differential pressure in H 2O mm H 2OP r Average root differential pressure root in H 2O root mm H 2OP corr Corrected average root differential pressure root in H 2O root mm H 2OCF Pitot tube correction factor

A Cross sectional area of duct ft 2 m 2 En Density of fluid under normal conditions lb/ft

3 kg/m 3 Eact Density of fluid at actual conditions lb/ft

3 kg/m 3 Ew Density of water at normal conditions 62.41 lb/ft

3 999.83 kg/m 3 Temperature of fluid at normal conditions 32 oF 0 oC

Tn Absolute temperature at normal conditions 491.4oR 273.1 oK

Tact Temperature of fluid at actual conditions oF oCTabs Local absolute temperature

oR oKg Acceleration due to gravity 32.17 ft/s 2 9.81 m/s 2 Vact Velocity of gas at actual conditions ft/s m/sQ act Gas flowrate under actual conditions Aft

3/s Am 3/sC Constant 12 in/ft 1000 mm/m

5 REFERENCES

1 British Standard 1042, Measurement of fluid flow in closed conduits, Part 2Velocity area methods, Section 2.1 Method using Pitot static tubes, 1983

2 Bureau of National Affairs USA 54 FR 46235 November, 19893 Airflow Developments handout on Pitot static tubes

6 APPENDICES

1 Pitot Traverse Calculation Spreadsheet SPITOT.XLS

Documents

PENG 001 Gas Flow Measurement By Pitot Static Tube.pdf