- International Symposium on NDT Contribution to the Infrastructure Safety Systems, 1999 NOV 22-26 Torres, published by UFSM, Santa Maria, RS, Brazil

TABLE OF CONTENTS

Abstract Introduction Role of NDE Techniques Video probing using fibreoptics In-situ Replica Technique Advanced Ultrasonic Techniques Trends for 2000

Abstract

Assessing the condition and remaining life of power plant components operating at high temperatures and at high stresses is necessary to optimize inspection and maintenance schedules, to make RUN, REPAIR, REPLACE decisions and to avoid unplanned outages. While two different approaches are available for Residual Life Assessment of Power Plant components, one using data analysis based on operational history and the other based on periodic examination of critical components, the latter method is widely adopted as it is more accurate since it does not rely on standard material data with their associated uncertainities and does not necessarily require knowledge of the operational stress-temperature data.

Methods based on periodic examination involve :

A.	Various NDE techniques for detection of cracks, effects of corrosion / erosion etc. in addition to commonly adopted techniques such as Ultrasonic thickness gauging, Ultrasonic flaw detection, Penetrant testing, Flourescent magnetic particle testing, Specialised techniques such as assessment of hydrogen damage by Ultrasonics, measurement of steam side oxide scale by in-situ ultrasonics, boresonic inspection of rotors, eddy current examination of rotor blades and root, video probe examination of critical components are employed.
B.	Metallurgical tests such as in-situ metallography using replica method, in-situ chemical analysis by metal spectroscope / X-ray flourescence method, in-situ hardness measurement etc.
C.	Sampling of component specimens for detailed laboratory analysis. This paper deals with the periodic inspection approach adopted by Industrial Quality Concepts for Residual Life Assessment of power plant components.

Introduction

Many utility systems in the world have power plants operating with fossil fuel with service life crossing 100,000 hours of operation. These units are subjected to time dependant degradation phenomena such as :

a.	high temperature tensile failures
b.	creep and rupture failures
c.	low cycle fatigue at elevated temperatures
d.	hydrogen embrittlement
e.	hot corrosion / erosion failures

It is essential to identify the critical areas where failures are likely to occur and select suitable NDE techniques for detection of such failures. Based on design criticality, past experience and previous failure information, suitable approach in inspection methodologies is adopted. Table - 1 gives typical failure mechanisms and the associated inspection methodology for different components of power plant.

COMPONENT

DAMAGE MECHANISM

INSPECTION TECHNIQUES

Creep

Fatigue

Erosion

Corrosion

HTT

VT

UTG

UTF

PT

MT

ET

HT

REP

FOT

DT

CA

SPL

BOILER & AUXILIARIES

1. Drums

´

´

´

´

´

´

´

´

´

2. Steam Headers

´

´

´

´

´

´

´

´

´

´

3. Water Headers

´

´

´

´

´

´

´

´

´

´

4. Steam Piping

´

´

´

´

´

´

´

´

´

´

5. Superheater & Reheater Tubes

´

´

´

´

´

´

´

´

´ a

6. Waterwall tubes

´

´

´

´

´

´

´

´

´ b

7. Ductings

´

´

´

´

´

8. Precipitator

´

´

´

´

´

9. Structures

´

´

´

´

´

´

TURBINE

1. Rotor

´

´

´

´

´

´

´

´ c

2. Shell

´

´

´

´

´

´

´

´

3. Steam Chest

´

´

´

´

´

´

´

´

´

4. Casing

´

´

´

´

´

´

´

´

5. Blades

´

´

´

´

´

´

´

´

´

´ d

6. HT Bolts

´

´

´

´

´

´

´

´

GENERATOR

1. Rotor

´

´

´

´

2. Retaining rings

´

´

´

´

´

CONDENSER & HEAT EXCHANGER

´

´

´

´

´

´

´

´

Table 1: Damage mechanisms and inspection techniques for Power Plant components

LEGEND : X - APPLICABLE; HTT - High Temperature Tensile Failure; VT - Visual Inspection & Dimensional Check; UTG - Ultrasonic Thickness Gauging; UTF - Ultrasonic Flaw Detection; PT - Penetrant Testing; MT - Magnetic Particle Inspection; ET - Eddy Current Testing; HT - Hardness Measurement; REP - In-situ Replica Metallography; FOT - Fibreoptic Examination; DT - Destructive Tests on Samples; CA - Chemical Analysis In-situ; SPL - Special Test Methods; a - Oxide scale measurement by special Ultrasonic technique; b - Hydrogen damage assessment by special Ultrasonic technique; c - Boresonic Inspection; d - Special Eddy Current Test of Roots

Role of NDE Techniques

NDE techniques adopted in the residual life assessment of power plant components can be broadly classified as conventional techniques and specialised techniques. Conventional techniques include Visual examination & Dimensional measurement using appropriate tools, Ultrasonic thickness gauging, Penetrant testing, Magnetic particle inspection using wet flourescent method and Eddy current test on non ferromagnetic tubing.

Specialised NDE techniques include video probing using fibreoptics to assess the damage on the internal surfaces especially for corrosion, erosion, cracks and the presence of foreign materials, in-situ replica technique to study the material degradation and the presence of microcracks, ultrasonic testing using high frequency pulser and transducer for measurement of oxide scale on the steam side, ultrasonic attenuation measurement to detect hydrogen damage, boresonic inspection using multi probes with special attachment to detect cracking in rotor bores and special eddy current technique to detect turbine blade root cracks. A brief description of the specialised NDE techniques are discussed.

Video probing using fibreoptics


Fig 1:

The internal condition of many power plant components need examination. High temperature headers and turbine rotor bores may develop cracking due to creep or fatigue or a combination of both. Video probing using fibreoptics can detect the presence of these cracks in time for further evaluation. In addition, the presence of corrosion pittings, hydrogen damage and foreign particles may also be detected by this examination. Figure 1 shows the presence of thick deposits inside a boiler tube which can reduce the flow of the medium and builtup of temperature and can cause failure. Based on the above finding, chemical cleaning of the boiler is recommended.

In-situ Replica Technique


Fig 2:

Power plant components which operate at high temperatures such as boiler headers, steam pipes, turbine casings, valves, etc. creep is a major cause of cracking especially on the highly stressed brittle regions. Creep damage occurs in different stages and the first sign in the formation of microscopic cavities at grain boundaries. Application of an NDE technique using a plastic film replica to topograph the metal surface can assess the creep damage and the presence of microcracks. Figure 2 is a micrograph taken by replica NDE technique which reveals the presence of grain boundary microcracks in a superheater header which has operated for 1,98,000 hours at a temperature of 515°C. The findings of this examination has recommended replacement of the header at the first available opportunity.

Advanced Ultrasonic Techniques
Failures of superheater and reheater tubes can be considered as one of the most prevalent causes for the unforeseen outages of a thermal power plant. The failures are often attributed to creep deformation which can lead to rupture. Such premature failures are caused by the steam side builtup which can cause an increase in the operating temperature of the tube. Using a high frequency Pulser-Receiver and using specially designed high frequency transducers operating in the frequency range of 20 - 50 MHz, it is possible to measure the oxide scale thickness on the steam side and in turn can calculate the operating temperature and also the remaining life.

Hydrogen damage failures of waterwall tubes are generic for some utilities where condenser leaking occur for prolonged periods. Since the damage is initiated from the waterside surface of the tubes, the same cannot be revealed on the outside surface. Ultrasonic examination using a 5 MHz longitudinal wave transducer and measuring the attenuation characteristics can indicate the regions where microfissuring due to hydrogen damage has occurred. Replacement of tubes can be done in time to avoid unplanned outages.

Fig 3:

Cracking on the boreside of turbine rotors is a common problem and due to the geometry, ultrasonic examination from the outer surface may not give detailed information of the bore. Hence multiaxial examination from the rotor bore using special attachments with multiple probe head is employed. The location of the cracks and their sizes are plotted for detailed analysis and further action. Figure 3 shows a typical setup for turbine rotor bore examination.

Turbine blades in thermal power plants suffer from metal fatigue as a result of vibration. The problem is aggravated by other mechanisms such as creep in the case of high pressure turbines or corrosion and embrittlement in the case of low pressure turbines. The failures usually occur at the root area and is difficult to examine by normal NDE techniques. Special ultrasonic and eddy current techniques have been developed for the timely detection of the same.

Trends for 2000

As more and more power plant equipments are reaching their designed life, utility owners are forced to take vital decisions on RUN, REPAIR, REPLACE for different components. Innovative NDE techniques are developed continuously and coupled with on-line monitoring and special computer programs, the decision making process has become more realistic and cost saving.

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