For a long time, the accuracy of ultrasonic flow meters was found to be insufficient. Turbine meters and vortex meters were the norm. But ultrasonic is on the rise. With the well-known advantages: no moving parts and good resistance to pollution. But they are also very accurate. AAR overhauls aircraft components. For tests and calibrations they make testrigs where Transus ultrasonic flowmeters are used. What considerations play a role in the choice of the flow measurement principle?
A testrig for aircraft components
AAR carries out maintenance on active components of aircraft. They work a lot for defense: starter motors, gearboxes and air valves of F16's for example. Or cabin pressure regulators for passenger aircraft. Real centipedes work in AAR's engineering department. They devise test cabinets and testrigs for testing and calibration components. These consist of a combination of pneumatics, electronics and mechanics. When a component is taken into maintenance, a testrig is designed that often functions for decades.
Important requirements for a testrig:
- Preferably suitable for the calibration of several types of components: this means that a test rig must be able to handle a wide range of specifications.
- Traceability of a test. In the aircraft industry, everything must be recorded so that it is possible to trace exactly what happened to a component afterwards. The engineers write their own software for this purpose. In this way they register the entire test and link it to a unique component ID.
- Reproducibility: the tests must take place under the same conditions each time. This makes good flow measurement important.
Air valve from the turbine motor
Large spread of specifications
The test bench we are going to look at today is suitable to test different types of air valves. Many components are tested at low pressure, such as components related to cabin pressure control and cabin temperature control. Other components require a high pressure of about 20 bar and a temperature up to 600°C. These are valves that are part of the turbine engine and components of the de-icing system.
The air supply therefore varies from 0-100 m3/min, at a temperature of 0-600°C and a pressure of 0-20 bar. To generate this there are 3 compressors with a combined power of 650kW.
Previously, AAR turbine flow meters were used, but when the entire air supply system was upgraded, these were replaced by Transus' ultrasonic meters. The problems encountered with the turbine flow meters were:
- The calibration went up to 250pnd pressure, but the pressure was sometimes higher than that, so the meter was not accurate.
- There were problems with reading the meter.
- Downtime as a result of periodic calibration (10 days).
- Maintenance because the meter had moving parts.
The new air street
The measuring line consists of 10 diameters of pipe (3", DN80), a flow conditioner, again 10 diameters of pipe, ultrasonic flow meter. The flow conditioner provides a good flow profile after about 8 diameters, so some margin is maintained.
Next to the measuring line, the supply line contains a shutoff valve and a bellows valve for regulating the pressure. The latter turned out to produce a lot of noise, after which the entire installation was provided with sound insulation.
The choice for an ultrasonic flow meter in a measuring line
Vortex flowmeters, turbine flowmeters, differential pressure flowmeters and venturi flowmeters were also considered when choosing the ultrasonic flowmetering technique. We list below the considerations for each type for this application.
Ultrasonic flow meter:
- Accuracy 1% of the measured value. If the flow meter including inlet length, flow conditioner and drain elements is calibrated, an accuracy of 0.5% is achievable.
- Transus also offers 4 channel custody transfer flowmeters, but this was not necessary for this application, thus saving costs.
- Because of the 3 measurement paths, the flow meter has built-in redundancy.
- Real-time diagnosis of the measurement signals by comparing the measurement values of the different channels.
- High turndown, which is very important for this application because the measuring line has to function at a flow of 0-100 m3/min and a pressure of 0-20 bar.
- Can not be used at low flow rates
- Accuracy of 1.5% of measured value, which is in principle sufficient
Turbine flow meter:
- Highly accurate
- The disadvantage is that it has moving parts
- Can over spin, causing unplanned maintenance to take place
- Must be calibrated regularly resulting in downtime
Differential pressure flow meter:
- Accuracy is 1% of full scale, but approximately 10% inaccuracy at 1/10 of the scale
- Turndown is low
Venturi flow meter:
- Also only accurate at full scale, making it inaccurate in the lower area of the measuring range
- May not be used on aircraft
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