A Standard for Dynamic Pressure Measurements in Gas Turbines

Foreword

1. The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2. The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3. IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user.
4. In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5. IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies.
6. All users should ensure that they have the latest edition of this publication.
7. No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8. Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication.
9. Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

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