A Standard for Dynamic Pressure Measurements in Gas Turbines

Foreword

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Introduction

If you want to insert an introduction, please refer to the ISO/IEC Directives Part 2:2011

Purpose

Measuring dynamic pressures within the varying environmental conditions of gas turbines is extremely complex due to the high-level understanding of the numerous pressure sensor technologies available, fluid mechanics and data acquisition and analysis required in order to collect accurate, reproducible dynamic pressure data. Selecting the appropriate pressure transducer to measure the desired dynamic pressure complicates the matter as there are numerous pressure sensor technologies available, such as piezoresistive, piezoelectric, optical, capacitive, resonant, etc., all of which have different advantages and limitations. The purpose of the Dynamic Pressure Standards Subcommittee is to establish a standard to compare gas turbine dynamic pressure instrumentation and a set of best practices for making accurate dynamic pressure measurements within gas turbines. In addition, the subcommittee aims to identify current and future dynamic pressure transducer needs for gas turbines.

Scope

• Standardize the specifications and validation/calibration testing methods used to define the dynamic performance of the various pressure transducer technologies to improve industry wide understanding of underlying technologies and capabilities.
• Provide an objective review of the fundamental sensor technologies used in the numerous dynamic pressure transducers available.
• Compile a set of best practices for making accurate, wide-bandwidth dynamic pressure measurements, including transducer packaging, installation, signal conditioning, data acquisition and analysis.
• Identify and address common problems encountered when making dynamic pressure measurements on gas turbines and recommend solutions to overcome such industry problems.

Dynamic Pressure Measurement Applications

Dynamic pressure measurements are critical to the proper function of gas turbines. Dynamic pressures are measured within the gas path for early detection of phenomena such as fan or turbine flutter, rotating stall, surge and acoustic resonances within the compressor to flow instabilities (rumble and screech) within the combustion chamber. Dynamic pressures such as these can contribute to catastrophic events as well as accelerated component ware and reduced turbine performance. Dynamic pressures are further present within liquid systems such as fuel, oil and hydraulic fluid power systems on gas turbines. Excessive dynamic pressures within these liquid-based systems can contribute to component fatigue and failure. Accurate dynamic pressure measurements can improve engine and system designs as well as identify component degradation ultimately contributing to improved reliability, performance, and longevity.

When making dynamic pressure measurements on gas turbines, there are numerous factors to consider; however, this standard focuses on the following critical factors:

1. Dynamic pressure probe types
2. Measurement location on the turbine, (specifically considering temperature requirements)
3. Measurement requirements, such as frequency response, accuracy, etc.
4. …

Normative references

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

Terms and definitions

For the purposes of this document, the terms and definitions given in ISA-37.1 Electrical Transducer Nomenclature and Terminology and ISA-37.16.01-2002 A Guide for the Dynamic Calibration of Pressure Transducers and the following apply.

Static and Dynamic Pressures

Dynamic Pressure A changing pressure where the rate of change is non-zero, (also referred to AC). NOTE: Dynamic pressure should not be confused with Vorlage:Math{2}}}

Steady Pressure (Also referred to Static Pressure) A DC, non-varying pressure, of 0Hz.

Unsteady Pressure See also Dynamic Pressure

Total Pressure The sum of Static Pressure (Steady Pressure) and Dynamic Pressure (Unsteady Pressure).

Surface Pressure Static Pressures (Steady Pressure), Stagnation Pressures, and/or Dynamic Pressures (Unsteady Pressures) on a selected surface

Stagnation Pressure The pressure when an incompressible fluid has come to rest (v=0 m/s). The stagnation pressure is defined as follows: P_stagnation=ρ v^2/2+P_static where ρ is the density of the fluid, v is the velocity of the fluid, and P_static is the static pressure.

Sensing Mechanisms

Sensing Mechanism Transduction mechanism through which the Measurand is transduced from one form of energy to another, e.g. pressure to electrical signal. Sensing Mechanisms include but are not limited to the following: Capacitive, Electromagnetic, Inductive, Piezoelectric, Piezoresistive and Optical.

Transducer A device which provides a usable Output in response to a specified Measurand [1]. Note: The term Transducer is preferred to “Sensor,” “Detector”, etc.

Sensor (See Transducer)

Sensing Element The portion of the transducer that responds to changes in the measurand [1].

Excitation The application of energy to a transducer such as with a static or dynamic electrical current or magnetic field [1].

Exposed Sensing Element Sensing element is in direct contact with the medium being sensed.

Isolated Sensing Element A sensing elment that employs design characteristics that isolate the transducer element from the fluid being measured (Measured Fluid).

Electromagnetic Susceptability The apparent susceptability of a sensor or sensor and signal conditioning to external electromagnetic energy. It is expressed as “unit of pressure per unit field strength at a specified frequency for a specific installation.”

Measured Fluid The fluid which comes in contact with the Sensing Element, also referred to as Working Fluid or Media [1].

Capacitive Converting a change of Measurand into a change in capacitance [1].

Electromagnetic Converting a change of Measurand into an Output induced in a conductor by a change in magnetic flux, in the absence of Excitation [1].

Electrostatic Converting a change of Measurand into a change in charge.

Inductive Converting a change of Measurand into a change of the self-inductance of a coil or coils [1].

Piezoelectric Converting a change of Measurand into a change in the electrostatic charge or voltage generated by certain materials when mechanically stressed [1].

Piezoresistive Converting a change of Measurand into a change in resistance.

Optical Converting a change of Measurand into a change in light intensity, phase, polarization, wavelength or transit time.

Microwave Converting a change of Measurand into a change in intensity, phase, polarization, wavelength or transit time.

Table 1: Comparison of Sensing Mechanismns [2]

Sensing Mechanism	Advantages	Disadvantages
Capacitive	Low power, low noise, hermetic package	Complex measurement circuitry
Electromagnetic
Inductive
Optical	EMI insensitive	Hermetic package
Piezoelectric	Self-generating charge sensor consumes no power; hermetic package	No static measurements (under some circumstances, quasistatic measurements)
Piezoresistive	Simple resistance measurement, inherently shielded structures, hermetic package	high electrical noise, temperature dependency, high power consumption

Capacitive Specific Definitions

Dynamic Capacitive Transducer A capacitive transducer that can only resolve the time-varying component of the measurand but does not resolve the steady component of the measurand.

Dynamic and Static Capacitive Sensor A capacitive transducer that can resolve both the time-varying component of the measurand and the steady component of the measurand.

Quiescent Capacitance The capacitance of a Sensing Element plus associated capacitance of electrical connections between the transducer element and the first stage of signal conditioning electronics when no or default measurand is applied. Quiescent Capacitance=Parasitic Cap + Sensing Element Cap.

Parasitic Capacitance The portion of the quiescent capacitance that is due to the capacitance of aspects of the transducer circuit other than the transducer element. This capacitance does not change due to changes in the measurand, but may change do to other inputs such as temperature changes, vibration, etc.

Capacitance (Sensing Element) The capacitance of the Sensing Element.

Transducer Capacitance The total capacitance of the sensing element, sensor package and integrated cabling.

Electrical Excitation Waveform The time versus amplitude characteristics of the excitation signal. (e.g. Time varying or DC waveform).

Electrical Excitation Frequency The instantaneous frequency of the excitation waveform.

Excitation Peak-to-Peak Amplitude The peak to peak variation of the excitation waveform.

Relative Permittivity of Measured Fluid The dielectric constant of the medium being sensed, which can vary with pressure, temperature, humidity, etc.

Conductivity of Measured Fluid The conductance of the medium being sensed, which can vary with pressure, temperature, humidity, etc.

Capacitive Response to Pressure (CRP) The relationship between pressure and the change in capacitance of the pressure transducer.

Capacitive Response to Temperature (CRT) The relationship between temperature and capacitance change of the pressure transducer. Temperature response may be compensated for by signal conditioning electronics, and possibly with the addition of a temperature transducer integrated into the pressure sensor system.

Piezoelectric Specific Definitions

Internal Resistance The resistance measured across the Sensing Element when a specified DC voltage is applied at Room Conditions unless otherwise stated.

Dielectric Withstanding/Strength The capability to withstand dielectric loading across the Transducer casing and electrically shorted sensing element leads when a specified AC or DC voltage is applied at Room Ambient Conditions for a specified time frame unless otherwise stated.

Transient Temperature Error (See ISA 37.2, Section 6.8) or Thermal Shock Capability to withstand a change in temperature within a specified time frame, defined as a % of unamplied full scale output (voltage, charge, etc.) shift in output from the pressure transducer when exposed to a thermal transient event quantified in degrees per second. . QUESTION: Is this covered in thermal gradient?

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  Sensitivity, Linearity