Using Turbine Flow Meters for Aerospace Test and Measurement Applications
Turbine flow meters have long been a preferred technology for obtaining precise measurements of fluid flow in the aerospace industry. In addition to their high accuracy, they are recognized for exceptional turndown, repeatability and speed of response.
With recent design enhancements, instrumentation manufacturers have expanded the advantages turbine flow meters offer in a host of demanding aerospace applications. Indeed, the turbine remains one of the most accurate and reliable transducers for today's critical flow measurements.
In the aerospace and defense industry, testing of fuel system components is key to ensuring final vehicle or aircraft performance. By pairing a flow meter with a flow computer or smart transmitter, aerospace equipment manufacturers can perform precise metering of fuel flow in engine test cells.
Experience has shown that turbine flow meters are the sensor-of-choice for test and measurement applications requiring the best accuracy available with the benefits of high resolution, extended turndown across wide flow ranges, fast speed of response, proven reliability, and compact size. These meters also feature a small footprint for ease of installation on both fixed and portable test stands.
Turbine flow meters incorporate a time-tested measuring principle. They contain a freely suspended rotor, and the flow against its vanes causes the device to rotate at a rate proportional to flow velocity. A sensor/transmitter is used to detect the rotational rate of the rotor; when the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid in either forward or reverse direction.
Common Flow Applications
Available in compact and lightweight packages with rugged materials of construction, turbine flow meters are used to measure diverse fluids ranging from aircraft fuel to hydraulic fluid, lubricant, cryogenic fluid and coolant.
Common turbine meter applications include:
Hydraulic system verification
Qualification and acceptance testing
Maintenance, Repair, and Overhaul (MRO)
From monitoring the fuel consumption of rotary and fixed wing aircraft, missiles and drones, to evaluating the performance of hydraulic fluid and lubricants on test stands, aerospace applications place high demands on flow sensing.
Latest Technology Advancements
In recent years, instrumentation manufacturers have expanded the traditional advantages offered by turbine flow meter technology. Turbine meters now have unprecedented mechanical linearity, resulting in minimizing, or negating, temperature-induced viscosity influence. Meters equipped with sophisticated electronics also provide total compensation to enhance measurement accuracy, while extending linearity over their repeatable range.
The following improvements enable turbine meters to satisfy application requirements in ways once considered infeasible:
Hydraulically coupled rotors
With the latest advancements in technology, turbine flow meter accuracy has improved. Increased sensitivity allows for the precise measurement of even extremely low flow rates in some models. Manufacturers also continue to develop blade and rotor materials that are highly sensitive, durable and less vulnerable to drag and other factors that have traditionally decreased accuracy.
Important Application Considerations
Today's versatile turbine flow meter systems can be configured to achieve the best solution to measure liquid or gas in the most challenging environments. For instance, flow meter suppliers provide an assortment of electrical pickoffs to meet temperature and signal transmission distance requirements, which are complemented by an assortment of electronic processors and indicators. And, selecting from either standard or custom end connection designs can optimize meter installation.
For users with demanding fuel system measurement requirements, the following application consideration guidelines will make turbine flow meter configuration simple, while presenting alternatives to maximize accuracy and minimize cost.
Fluid Parameters: Fluid properties vary from one flow measurement application to another and need to be defined in order to properly develop the correct meter configuration and calibration specification. Fluid parameters include:
Affects the flow meter's wetted parts
Defines filtration requirements
Determines water, solvent or oil blending calibration
Provides fluid density information, required for inferred mass flow
Operating Fluid Temperature (minimum and maximum)
Defines fluid viscosity range
Identifies the number of calibrations required to develop a Universal Viscosity Curve (UVC) calibration
Required to select pickoff type (from cryogenic to high-temperature applications)
Determines if remote electronics are required
Static Line Pressure
Over 1000 psig changes viscosity and density properties
Determines the permissible range in combination with the pressure drop and UVC capability
Meter Type: There are different types of turbine flow meters, and some are more specialized to certain applications. Depending on the particular needs, one style of meter may be preferable to another. Users can choose from four basic configurations, depending on their requirements:
Low-flow axial meters
Precision single-rotor meters
Original Equipment Manufacturer (OEM) specialty meters
Electrical Pickoffs: Because test and measurement applications vary so greatly, most turbine flow meter manufacturers offer multiple electrical pickoff choices to meet specific end user requirements. A pickoff is mounted on the meter body and is used to take the output of the device.
When specifying a pickoff, there are many factors to consider. The following list outlines the consideration process:
RF carrier pickoff (requires carrier amplifier)
Magnetic pickoff (no power required)
Pickoff fluid temperature ranges
Embedded temperature sensor (RTD or Thermistor)
Transmission distance (when amplified)
High-vibration pickoff coils
FM, CSA, CE and ATEX approvals for EMI, explosion-proof and intrinsically safe applications
In some cases, an RTD thermowell temperature probe can be inserted into a flow straighetner to provide improved temperature monitoring in place of an embedded pickoff temperature sensor.
End Connections: End connections are determined by the pipeline size and pressure, ease of removal, and other specific application criteria. Equally as important is the adjoining pipe and end connection pressure rating. High temperature will reduce the pressure rating on all fittings.
Flow Straighteners: Flow straighteners are recommended on single-rotor turbine flow meters to negate swirl from influencing the accuracy of the meter. Some flow straighteners employ a bladed insert to prevent swirl and minimize pressure drop. In addition, they can be paired pressure and temperature taps.
When flow straighteners are impractical due to space limitations, a turbine meter can be calibrated in the same piping as found in the installation to compensate for fluid swirl. Generally, dualrotor meters can be used without flow straighteners.
Packaging: Turbine flow meter packaging options are available to allow for integral or remote mounting. Remote mounting provides a solution when space is limited or when environmental temperatures are excessive. OEM meters are commonly designed with an embedded flow processor, allowing for complete interchangeability of the meter system.
Calibration: Turbine meters are highly repeatable, however, care must be taken to choose a calibration that will maximize their accuracy. The meters are viscosity-sensitive and may need a calibration that corrects for temperature/viscosity effects on the output. This type of calibration is accomplished by blending solvent and oil to simulate the kinematic viscosity of the fluid at a given temperature. Wide temperature variations might require multiple calibrations to develop a UVC. A flow processor uses this data to provide a fully temperature compensated precision flow output.
The quantity of calibration data points, over the usable flow range, will determine the resolution of the calibration curve. More data points result in a higher degree of accuracy. Because of the predictability of the turbine meter, 10 data points are generally sufficient. For master meters, 20 to 30 data points are recommended.
Service Provider: Since a turbine flow meter's performance is highly dependent on the quality of its calibration, it is wise to utilize primary standard liquid and gas calibrations performed by NVLAP-accredited calibration facilities. The calibration criteria at these sites are based on the ISO/IEC 17025 International Standard, which is used to evaluate the competence of calibration laboratories throughout the world. The standard specifically assesses factors relevant to the ability to produce precise and accurate calibration data including:
Correct equipment – properly calibrated and maintained
Adequate quality assurance procedures
Proper correlation practices
Appropriate testing methods
Traceability of measurements to the National Institute of Standards and Technology (NIST)
Accurate recording and reporting procedures
Suitable testing facilities
Technical competence of staff
Leading turbine flow meter manufacturers provide knowledgeable technical assistance for specifying the correct instrument and calibration for a given application. Their experience and know-how can guide users in selecting the proper meter electronics based on a wide range of power and output configurations.
In many cases, precision turbine flow meters are designed and manufactured to provide a building block approach to satisfy the most difficult applications requirements. This approach takes into consideration fluid temperature, environmental conditions, vibration, shock, bi-directional flow, and a host of tube and pipe connections, which solves a multitude of challenges.
This article was written by Mike McCoy, Technical Sales Manager, Badger Meter (Milwaukee, WI). For more information, visit here .
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