Annual Guide 2019 I SOURCE TESTING ASSOCIATION


ing sampling the pitch and yaw and ratio of velocities should be considered flow measurements. The S-type pitot eration than the L-type pitot.


When conducting sampling the pitch and yaw and ratio of velocities should be considered when making fl ow measurements. The S-type pitot is more susceptible to alignment problems in operation than the L-type pitot.


g about alignment issues this applies as much to the angle of axial flow as to of the pitot tube to begin with. The graph below shows the effect of having the 15o and shows that the effect on velocity measurement is relatively negligible shows that the effect changes with gas velocity (i.e. the higher the velocity the or in flow measurement). Hence if the pitot is aligned with the axial flow within fect on velocity measurement is minimal. An ellipsoidal pitot tube is even less n-axial flow or alignment issues and at 15o to the flow the effect is <1%, but e prone to blocking in condensing gas steams.


If we are talking about alignment issues this applies as much to the angle of axial fl ow as to the alignment of the pitot tube to begin with. The graph below shows the effect of having the pitot twisted by 15o


and shows that the effect on velocity


measurement is relatively negligible (<6%). It also shows that the effect changes with gas velocity (i.e. the higher the velocity the greater the error in fl ow measurement). Hence if the pitot is aligned with the axial fl ow within 15o


measurement is minimal. An ellipsoidal pitot tube is even less sensitive to non-axial fl ow or alignment issues and at 15o


In talking about alignment, the effect which is frequently ignored is the pitch of the probe. This should be at 0o


ut alignment, the effect which is frequently ignored is the pitch of the probe. at 0o to the sampling plane in the duct, but this pitch angle can easily affect correct flow rates and hence setting the sample flow rate. Flow errors are tional to the pitch and so an error of 20o in pitch roughly corresponds to an flow at its worst case with an S-type pitot.


fl ow the effect is <1%, but these are more prone to blocking in condensing gas steams.


to the


sampling plane in the duct, but this pitch angle can easily affect obtaining the correct fl ow rates and hence setting the sample fl ow rate. Flow errors are roughly proportional to the pitch and so an error of 20o


in pitch roughly corresponds to an error of 20% in fl ow at its worst case with an S-type pitot.


then the effect on velocity to the


Leak Checking an S-Type Pitot is more susceptible to alignment


I have found that carrying out a leak check on an S-type is best achieved using a gas tight syringe and a piece of tubing. Pressurise the tube facing into the fl ow by at least the expected pressure and then hold it carefully. Don’t let the plunger move but equally don’t let the heat of the hand affect the air in the syringe. Then repeat on the other face of the pitot and apply a negative pressure. I have had the misfortune on several occasions to witness individuals cutting corners whilst conducting these types of tests. This is silly, and the ultimate culprit must be the company ethos which encourages a quick in and out, or certainly a feeling that too many tests in a day must be achieved.


But, where do leaks occur? They occur where quick fi t connectors are used to connect tubing and where pitots have been recently removed for calibration via SwagelokTM


fi ttings and


refi tted onto the probe assembly incorrectly. If a Method 5 piece of equipment is being used the O-rings on the quick connectors on the top of the dual inclined manometer, or a perished O-ring in the manometer fi ll port can all lead to leaks. O-rings are a common source of problems if not looked after and this usually involves grit getting on them and scoring the O-ring or a lack of lubrication of the O-ring and I always believe in applying a bit of spit on to the O-ring prior to making the connection as a natural lubricant. This has two effects: it removes grit and it lubricates the rubber O-ring. This leads to increased longevity of the O-ring and fewer failures of equipment.


Leak Checking an Ellipsoidal Type Pitot


he effect of Yaw and Flow Measurement using an S-type Pitot Test: Flow Measurements


The effect of Yaw and Flow Measurement using an S-type Pitot


is not checked for damage, non-alignment or if no leak check of the pitot tube hen the pitot traverse will be of dubious quality. The S-type pitot tube is ideal ay be high levels of dust, or droplets present, as the impact and static holes many companies pay lip-service to pitot leak checks, yet I have audited many d their ability to demonstrate a pitot leak check suggests a lack of familiarity dures. An isokinetic test without a valid pitot or system leak check is worthless!


Conducting a Test: Flow Measurements


g an S-Type Pitot


hat carrying out a leak check on an S-type is best achieved using a gas tight piece of tubing. Pressurise the tube facing into the flow by at least the sure and then hold it carefully. Don’t let the plunger move but equally don’t let hand affect the air in the syringe. Then repeat on the other face of the pitot


If the pitot tube is not checked for damage, non-alignment or if no leak check of the pitot tube is conducted then the pitot traverse will be of dubious quality. The S-type pitot tube is ideal where there may be high levels of dust, or droplets present, as the impact and static holes are large. Too many companies pay lip-service to pitot leak checks, yet I have audited many companies and their ability to demonstrate a pitot leak check suggests a lack of familiarity with the procedures. An isokinetic test without a valid pitot or system leak check is worthless!


The L-type or ellipsoidal pitot is more diffi cult to leak check. The small single hole can be pressurised easily and tested but the annular arrangement of the static pressure holes is more diffi cult. The easiest way to check this is to seal the holes with some insulation tape or Tefl on plumbers’ tape and provide a pressurisation port using a T-piece connector. Insert the T-piece connector in the line from the pitot to the manometer. Now apply a slight negative pressure with a syringe to the T-piece. Hold that for 15-60 sec and if the pressure remains constant then the pitot, connections and manometer are sound. Either remove or seal the T-piece to the syringe and the test is done. This can be set up as a permanent arrangement if required.


The probe tip nozzle, thermocouple and pitot tube arrangement need to conform with the standard being used. The distances between the pitot tube and the nozzle are important. If they are too close, then they will interfere with each other and cause disturbance of the dust fl ow patterns. Likewise, the nozzle tip, pitot tube openings and thermocouple need to be in the same plane in an integrated probe. Failure to do so will negate the accuracy of the isokinetic testing and invalidate the test. On several occasions teams have been audited with pitot tubes which are too long, sometimes by more than 25-30 mm. This is because the pitot tubes had been replaced and their lengths were not adjusted for the nozzles being used. This is because usually they are being used with Euro Style nozzles and the


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