A detailed look at HRSGs common failures also how does condition monitoring improves reliability and flexibility of the components. At the end some on-line condition monitoring methods have been mentioned.

1. HRSG condition monitoring methods
Presented by
Mahdi Esfahani
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2. Introduction
• Heat recovery steam generators.
• Failure mechanisms and statistics .
• Condition monitoring definition and standards.
• On-line condition monitoring methods.
• Recent ideas and innovations.
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3. 3/24

4. The basic HRSG starts at the exhaust of the gas turbine and ends at the exit of a stack that releases exhaust
gas to the atmosphere. The HRSG contains in its most basic form ductwork and casing (enclosure),
economizers that heat water to near saturation, evaporators and steam drums that convert water from the
economizers to steam and separate the steam from water, superheaters and reheaters that heat steam
beyond saturation, and a stack that exhausts to the atmosphere. A substantial amount of piping, valves,
controls and platforms and stairways are necessary to complete the HRSG.
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5. Failure mechanisms
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N Damage type Usually found in Occasionally found in Best location to first inspect
1
Flow accelerated
corrosion
Economizer, low
pressure steam
generating sections
Thinning of first few rows of tubes
in low pressure parts
2 Corrosion fatigue
Economizers,
Evaporators,
Preheaters
Superheaters,
Reheaters
Selected tube-to-header weld areas on
economizer or preheater row that see greatest
thermal transient when during operation
(typically startup/shutdown cycle)
3
Creep or Creep
Fatigue
Superheaters,
Reheaters
Selection of hottest tubes that might
have exceeded creep threshold
temperature, particularly if tubes are
bowed
4 Graphitization
Superheaters,
reheaters
Selection of tubes running hot
(>800°F/425°C ) for long period (>100K
Hours) and only if material is carbon
steel
5
Deposition &
Underdeposit
Corrosion
Evaporators
Selection of horizontal or bent tube
sections

6. Failure mechanisms
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N Damage type Usually found in Occasionally found in Best location to first inspect
6
External Corrosion
and Oxidation
FW Preheaters, LP
Economizers
Superheaters,
Reheaters
Selected gas side surfaces, particularly
toward colder end of unit.
7
Acid Dewpoint
Corrosion
FW Preheaters, or LP
Economizers
Coldest Tubes Near Stack
8 Fatigue (LCF)
Superheaters,
Reheaters
Economizers, FW
Preheaters
Vibration-induced (high cycle fatigue) in first
row tubes or near gas baffles;
Thermal transient induced (low-cycle
fatigue) at tubes on inlet header of bundle
on cycling units
9 Pitting
Drums, Economizers,
FW Heaters, Drain
Lines
Evaporators,
Superheaters,
Reheaters, Vents
Drum interior surfaces at each outage.
Inspect selected locations on tube or header
interiors if:
· frequent or long layups have occurred prior
to outage
· feedwater dissolved oxygen was 2 or 3 times
above plant chemistry limit for extended periods
· drum(s) show significant pitting
10 Thermal Overstress
HP Superheater,
Reheater
Check all tubes for any sign of bowing
or further bowing since last outage

7. Failure mechanisms
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N Damage type Usually found in Occasionally found in Best location to first inspect
11
Stress Corrosion
Cracking
Economizers, FW
Heaters
Superheaters,
Reheaters, Evaporators
Systematically inspect significant sample of
tubes if a failure has occurred or if there is
reason to believe (history at similar plants)
that SCC may be occurring in a particular
region
12 Tensile Overload
HP Superheater,
Reheater
Bowed tubes or all tubes in area if
failure has occurred
13 Thermal Quench
HP Superheater,
Reheater
Bowed tubes or all tubes in area if
failure has occurred
14 Wear All tubes
Tubes near supports having bent
fins or where support is damaged
15
Erosive Wear and
FAC
Economizers,
FW Heaters (FAC); LP
Evaporator Tubes (FAC
and EW); LP and IP Drum
Internals (FAC or EW)
LP & IP Evaporators
(FAC or EW)
Sizeable sample of accessible bends in carbon
steel tubes, jumpers and risers, particularly if
operating at temperature range of 200 °F –
400 °F, (100 to 200°C);
Baffles in LP or IP drums above risers

8. Consequence factors: CF Redundancy factor: RF Downtime factor: DF Cost of replacement factor: CrF
Severity factor: SF Loading factor: LF Restoration/service factor: SF
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9. 10%
22%
22%
10%
10%
8%
13%
5%
Tube failure statistics
creep
fatigue
corrosion
fabrication defect
tensile overload
FAC
other
creep-fatigue
24%
24%
8%
5%
12%
8%
5%
14%
Failure locations
Superheater
Reheater
HP economizer
LP economizer
HP evaporator
LP evaporator
Preheater
other tubes
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10. CM definition
maintenance
Planned
(preventative)
maintenance
Condition based
maintenance
On-line monitoring
(fired)
Off-line
monitoring
(unfired)
Scheduled
maintenance
Unplanned
maintenance
Breakdown
maintenance
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11. Objective of condition monitoring
• To identify changes in condition of an equipment
that will indicate some potential failure.
• Prediction and diagnosis of fault.
• To avoid further damages.
• Outage of equipment can be better planned.
• Enhance the equipment endurance limit.
• Less down time and more productivity.
• Greater safety and reliability to work force.
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12. CM standards
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S.N Standard title description
1 ISO 17359
Condition monitoring and diagnostics of machines, General
guideline
2
ISO 13373
Mechanical vibration and shock – vibration monitoring of
machine
3 ISO 29821
Condition monitoring and diagnostics of machines —
Ultrasound
General guidelines
4 API RP 573 Inspection of fired boilers and heaters
5 ASME PTC 4.4
Gas Turbine Heat Recovery Steam Generators
Performance Test Codes

13. Developing a monitoring Plan
• Timing of unit outages for scheduled maintenance
• Urgency of known problems in the HRSG (cracking, underdeposit corrosion, FAC,
erosive wear, etc.)
• Availability of inspection support
When to inspect?
• Estimate of potential damage risk to key components
• Status of known problems
• Available time for inspection (including cooldown/heatup time)
Where to inspect?
• Allowable time
• Accessibility
• Type of problem under consideration
What inspection
techniques should be
used?
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14. HRSG on-line condition monitoring methods
• Leak detection methods
- Acoustic leak detection monitoring.
- leak detection through micro vibrations.
• Overheating monitoring methods
- Infrared methods for temperature monitoring.
- Advanced fiber optic methods
for temperature monitoring inside the boiler.
- Internally applied thermocouples.
• Corrosion monitoring methods
- Conventional and advanced corrosion-coupon probes.
- Electrical resistance corrosion probes.
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15. Acoustic leak detection monitoring (air-borne noise)
• Detection through microphones, engineered to withstand
harsh condition.
• Receiving acoustic signals generated by the leak.
• Choosing proper filter to select the most favorable
frequency window.
• Maximum leak signal/background noise ratio to define
the proper listening frequency window.
• Covering whole volume by means of 10-20 sensors.
• Coupling sensors to the inside boiler volume by means of
minimally intrusive acoustic waveguides.
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16. leak detection through micro vibrations (structure-borne noise)
• High-frequency piezoelectric sensors coupled to the metal
structure (typically to the SH and RH headers in the
penthouse) by means of waveguides, in the form of thin
short metal rods externally welded to the component wall
according to an appropriate welding procedure.
• Signal processing essentially consists of filtering and taking
the signal root mean square (RMS).
• Compare the current RMS noise level to pre-set threshold
levels, established during the system setup.
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17. Overheating monitoring
Optical fiber thermometers :
• Tubes surface temperature measurements.
• Long-term stability, quick response and high
sensitivity.
• No need to calibration of individual probes.
Internally applied thermocouples :
• temperature profile measurement across the
tube wall.
• average wall temperature and the
temperature at the outer fireside surface of
the tube can be obtained.
• 10 to 20 instrumented tube inserts are
installed in critical areas of the boiler to
provide an overall picture of the ongoing heat
transfer process.
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18. Electrical resistance corrosion probes
• rectangular matrix of electrodes welded to the external,
cold-side, tube surfaces, typically 1m apart.
• Installations of 100–200 sensors are reported, covering an
area of about 100–200m2 of waterwall.
• Pairs of electrodes inject current and measure the
equivalent circuit resistance, based on the same well-
proven principle used in simple corrosion probes.
• The collected set of data is processed, taking into account
the influence of current path geometry and temperature,
to obtain maps of corrosion thinning over relatively large
areas.
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19. Ideas and innovations
• Leak detection in HRSGs utilizing radioactive tracers .
• 3D Point Location Method.
• Intelligent pigs/in-line inspection devices.
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20. Leak detection in HRSGs utilizing radioactive tracers
In a typical tracer process, a small amount of radioisotope is injected into
the high-pressure side of the HRSG. Two radiation detectors are used to
monitor its movement through the HRSG. The inlet detector mounted on
the high-pressure side measures peak and time, whilst the output
detector mounted on the low-pressure side detects the radiotracer that
has infiltrated into the lower pressure side. The selection of a suitable
radioactive transmitter is crucial to the success of a radiotracer technique.
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21. 3D Point Location Method
• The position of a leakage source in a two dimensional plane can be
estimated by using more than three sensors and their RMS magnitude
ratio (RMS3 > RMS2 > RMS1) as show in Figure(1).
• In order to apply Boiler Tube Leakage Detection to a coal-fuel power
plant, a 3D model like Figure (2) was made using a left hand coordinate
system after reviewing the drawings of the boilers and visiting the field.
• If leakage occurs, the sensor closest to the leakage will respond most
sensitively and show a considerable change. It is expected that the
leakage source will be around these sensors. Figure (3)
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23. INTELLIGENT PIGS/IN-LINE INSPECTION DEVICES
• Utilize immersion-based ultrasonics to
measure tube inside diameters in both
ferritic and austenitic heater tubes.
• Ultrasonic transducers measure the tube
wall thickness.
• Outfitted with multiple ultrasonic
transducers (typically between 50-200
sensors) to permit high density sampling.
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24. INTELLIGENT PIGS/IN-LINE INSPECTION DEVICES
• useful in addressing creep, corrosion, erosion or
pitting-type damage mechanisms.
• capable of inspecting tubes which contain
changing diameters throughout the length, and
varying thicknesses/schedules.
• propelled through the tubes using a liquid
medium (i.e. water, soda ash solution, diesel,
glycol, etc.) to act as both couplant and hydraulic
vehicle and allow a thorough, complete
inspection of a tube pass.
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