HRSG Tube Failure Statistics

Introduction

Tetra Engineering Group, Inc. (TETRA) has assisted the owners of natural gas-fired combined cycle plants with effective remedies to HRSG thermal performance and integrity issues since 1993.  Over that time, TETRA inspected hundreds of HRSGs of all types and sizes from nearly every HRSG manufacturer and performed numerous failure analyses on HRSG components. 

Heat Recovery Steam Generators

Heat Recovery Steam Generator (HRSG) tubes are the interface where useful energy from the waste heat contained in gas turbine exhaust is used to generate steam.  HRSGs have undergone substantial changes in design over the past 20 years as the technology matured, starting from small single-pressure units driven by aeroderivative gas turbines to today’s large triple-pressure units with reheat running steam pressures in the high-subcritical region [1], not to mention the various once-through designs.  Besides fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications.  Baseload operation was normal for earlier HRSGs, but today’s deregulated electric power market imposes that many new large HRSGs cycle or operate in flexible duty.

The relatively simple design and less severe operating conditions than in radiant boilers running with coal or oil create a more benign environment for the HRSG heat exchanger tubes.  Nonetheless, failures do happen, and of particular concern are tube failures, which are not uncommon.  Modern demands on HRSGs has effectively driven them to see many of the tube failures that occurred on radiant boilers, with some differences.

When TETRA published the Tube Failure Diagnostic Guide in 2004, the authors anticipated the higher pressures and temperatures that emerging HRSG designs would impose on tubes.  The result would be greater risk of high temperature damage due to creep, corrosion fatigue and damage due to stresses that result from differential thermal expansion.  TETRA’s observations over the last 15 years have largely confirmed that prediction. 

Tube Failure Statistics 

Tube failures occur in all heat exchanger modules, from the cold-end economizers to the hot-end superheaters and reheaters, but the most prevalent types of damage mechanism in each area will be different.  The nature of service (base load or cycling), fuel type and water quality will also influence the frequency of occurrence for a given damage mechanism.  Mechanisms include: Creep, Fatigue, Creep-Fatigue, Underdeposit Attack, Flow-Accelerated Corrosion (FAC), Mechanical Erosion, Stress-Corrosion Cracking (SCC), Overheating and Mechanical (Tensile) Overload. 

So Which Failure Modes Are Most Common?

Very few comprehensive statistics have been published on failure locations and damage mechanism in modern HRSGs.  To gain insight into frequency and types of failures, Tetra Engineering has compiled statistics from internal company projects on tube failure locations in HRSGs from 86 different sites in North/South America, Europe, Middle East and Asia.  The data covers roughly 8 years of TETRA projects and is summarized in the Figures to the right.  Although it is not an exhaustive accounting of all projects in the period, it does provide a useful indication of the frequency of occurrence of failures by HRSG location and by mechanism.  Superheater and Reheater tubes are the most prominent tube types in our assessments, accounting for close to 50% of all the failures.  That’s probably influenced by the fact that these tube failures are the biggest ‘surprises’.  With steam tubes making up nearly half of the numbers, its no wonder that creep and fatigue (separately and combined) are the two most common failure mechanisms, representing 40% of all failures. Corrosion in its various forms (such as cold-end corrosion or under-deposit attack) represent 23% of the failures.  Flow-Accelerated Corrosion was the cause of failure in only 8% of the failures investigated (although failures sometimes continue to happen after the cause and solution has been identified). 

Conclusion

A sizeable sampling of the tube failure mechanisms and locations identified in projects performed by TETRA over recent years shows that hot-end components are the biggest problem area, with nearly half the failures investigated occurring in these tube locations.  This is consistent with the observed failure mechanisms where creep, creep-fatigue and fatigue account for close to 40% of the failures.  The occurrence of fatigue failures is most common in components subject to high thermal stresses and movements, i.e. such as the superheater and reheater tubes.  The relatively low frequency of FAC failures is somewhat surprising given the amount of attention it receives.  Unless a HRSG design is at fault, Tetra’s clients don’t often see repeat failures of FAC, because the remedies are generally well-understood. 

[1]

D. Moelling, P. Jackson et F. Anderson, HRSG Tube Failure Diagnostic Guide - 2nd edition, Tetra Engineering Group Inc, 2004.

GT Upgrade Effects on HRSG Pressure Parts

Introduction

Tetra Engineering Group, Inc. (TETRA) has assisted the owners of natural gas-fired combined cycle plants with effective remedies to HRSG thermal performance and integrity issues since 1993.  Over that time, TETRA inspected hundreds of HRSGs of all types and sizes from nearly every HRSG manufacturer and performed numerous failure analyses on HRSG components. 

Heat Recovery Steam Generators

Heat Recovery Steam Generator (HRSG) tubes are the interface where useful energy from the waste heat in gas turbine exhaust is extracted and converted to steam.  HRSGs have undergone substantial changes in design over the past 20 years as the technology matured, starting from small single-pressure units driven by aeroderivative gas turbines at cogeneration facilities to today’s large triple-pressure with reheat units with steam pressures in the high-subcritical region [1].  Besides fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications.  Baseload operation was normal for earlier HRSGs, but today’s deregulating electric power market imposes that many new large HRSGs operate in cycling or flexible duty.  As with tubes in conventional radiant boilers, HRSG tubes are subject to a variety of service related damage.  Damage mechanisms vary in severity depending on the tube material composition, operating temperatures, magnitude and frequency of stresses and presence of corrosive products.

GT upgrades

OEM upgrade packages to gas turbines (GT) can significantly improve performance, operational flexibility and increase outage intervals. In combined cycle power plants, the impact of these upgrades should also be considered for the HRSG where the design was based on the original GT performance.

Common changes that impact the HRSG include :

  • More efficient operation of the GT at lower load (lower flue gas mass flows, higher temperatures)
  • Modifications to exhaust gas profiles at various loads
  • Duct burner firing characteristics
  • Changes to fuel fired
  • Increased outage intervals

The big question is how these upgrades affect the downstream pressure parts in the HRSG?

Thermal Hydraulic Analysis

By using a boiler simulation model of the HRSG one can simulate the conditions be for and after the change in GT performance. The details of the HRSG heat transfer surfaces with regard to tube details (wall thickness, material, fin height and density, etc.) are used as input.

At each measured operating point, key boundary data is used to set the conditions for the design model. They include:

  • GT Exhaust mass flow, temperature and composition computed from measured data
  • HP, CRH, HRH, LP steam pressures and temperatures from measured data.
  • Fuel Gas Heating water flow demand from measured data
  • Condensate feedwater temperatures to HRSG
  • LP Economizer/Preheater recirculation setpoint (temperature).

A direct comparison is then possible between measured and original design performance at the specific operating points. 

Effects of GT upgrade on HRSG Pressure Parts

The limiting HRSG issues during upgrades are:

  • Operating Pressures (below design limits)
  • Tube Metal Temperatures (below design limits)
  • Water/Steam Flow Velocities (below good practice limits)
  • Water/Steam pressure drops (reasonable values compared to design, adequacy of available pump head for feedwater).
  • Exhaust Gas Temperatures below local design limits
  • Steam Separator Performance
  • Safety Valve Adequacy

Conclusion

By the use of advanced thermohydraulic boiler simulations, the Heat Recovery Steam Generator can be assessed in detail and any potential issues can be addressed before the actual GT upgrade takes place. 


[1]

D. Moelling, P. Jackson et F. Anderson, HRSG Tube Failure Diagnostic Guide - 2nd edition, Tetra Engineering Group Inc, 2004.

HRSG Inspections - The Key to Reliability

HRSG Inspections - The Key To Reliability

Inspection is part of routine maintenance for any Heat Recovery Steam Generator (HRSG).  Visual inspections are performed at regular intervals in accordance with the requirements of regulatory bodies and insurers.  In the US statutory inspections are mandated typically every year although some jurisdictions allow justification for longer intervals.   Additional inspections are sometimes performed to establish the baseline condition of the HRSG (often early in life, but not always) or to perform less frequent special inspections to confirm component integrity. 

The high costs associated with unexpected forced outages for merchant units or for units in deregulated markets encourage efforts to anticipate problems in HRSG components that are susceptible to service related damage. Damage mechanisms vary in severity based on materials selected by the HRSG OEM, local operating stresses and the interaction between vibration and corrosion mechanisms.  In addition to fundamental design differences, modern HRSGs are also subject to a wider range of duty in power applications.  Baseload operation was normal for smaller HRSGs, but today’s deregulating electric power market requires many new large HRSGs to operate in cycling duty.  This has resulted in many units experiencing damage much sooner than would be expected with baseload operation.  Some cycled units have experienced tube failures and leaks after only a few thousand hours of operation.  The more severe duty from cycling operation reveals design weaknesses earlier at many newer combined cycle plants, as well as not infrequently fabrication-related deficiencies

At Combined Cycle Power Plants the Heat Recovery Steam Generator is an easily-overlooked, but key component, providing steam to the steam turbine and to attached process steam hosts.  While base-loaded HRSGs tend to be the most reliable - cycled or two-shifted plants suffering from accelerated damage - the importance of allocating sufficient resources to maintenance of the HRSG is often only realised in hindsight. 

Even a simple visual HRSG Inspection during an outage can provide useful information about the how the HRSG is aging and what maintenance should be planned to ensure ongoing reliability.  Many have experienced the sudden drop in reliability that comes when a neglected HRSG begins having tube failures in one area, causing frequent shutdowns that later aggravate other incipient problems that could have been avoided by simple performing a Visual Inspection, and taking remedial action early on. 

Tetra Engineering has been performing HRSG Inspections for over fifteen years - over 750 Inspections to date! - and provides an integrated service that supports Owners and Operators throughout the full HRSG life-cycle.  In addition to the HRSG Inspections we also perform HRSG Condition Assessments, in which we review not only the present condition of the HRSG Unit but also how historical operation has affected its integrity. 

As part of its HRSG Life Management services, Tetra also draws on its unique skill set, including extensive field experience and engineering analysis, to assess the HRSG's susceptibility to certain damage mechanisms (e.g. Fatigue, Creep, FAC, Corrosion) and produced customised HRSG Inspection Plans. Those plans can be used by inspectors and NDT-teams during plant outages to target HRSG Inspections where damage is most likely to be found.

Developing an Inspection Plan

The HRSG inspection is part of the process to maintain high reliability and efficiency in the combined cycle plant operation.   As such the inspection planning process must accommodate several differing requirements:

  • Statutory or other mandatory inspection requirements
  • Preventive maintenance requirements
  • Problem tracking and resolution requirements

When developing the plan one must try to answer the following questions:

1)      When to inspect?

When to perform inspections is a combination of several factors:

·         Statutory requirements on inspection frequency

·         Timing of unit outages for scheduled maintenance

·         Urgency of known problems in the HRSG (cracking, underdeposit corrosion, FAC, erosive wear, cold end corrosion, etc.)

·         Availability of inspection support

2)      Where to inspect?

The scope of inspection locations in and around the HRSG will depend on a number of factors

·         Available time for inspection (including cooldown/heatup time)

·         Effort required to gain access such as draining drums, removing baffles, installing scaffolding/skyclimbers (if required)

·         Estimate of potential damage risk to key components

·         Status of known problems

3)      What to look for in the inspection?

A pre-inspection review of operating and maintenance data can have great value in answering this question.  For example, an analysis of water chemistry logs over the period leading up to the outage is invaluable in determining if there is an increased potential for certain types of degradation within the HRSG. 

4)      What inspection techniques should be used?

The choice of appropriate inspection techniques will depend on allowable time, accessibility and the type of problems under consideration. 

5)      What are the post inspection requirements?

Post-inspection requirements are as important as the inspections themselves.  These include reporting, recordkeeping, input to further repair actions or engineering assessments.  

Visual Inspection 

The predominant technique for inspecting HRSGs is visual.  Visual inspection is performed at nearly every opportunity.  Generally, a visual exam encompasses a walkdown of the interior to view accessible components within the gas path as well as the adjoining accessible compartments.  These are located above and below the gas path on horizontal gas path (HGP) units and at the sides on vertical gas path (VGP) units.  These compartments contain interconnecting piping such as jumpers, risers, drains and vents.  The walkdown should also include drum internals and the HRSG casing with supporting structures.  The external power piping on the unit and piping supports might be added given sufficient time.  Deaerator tanks and condensers are also subject to opportunistic visual inspection and periodic nondestructive inspection as required by insurers. 

The following areas should be included in the HRSG Visual Inspection scope:

  • Gas Path
  • Attic
  • Basement
  • Steam/Water Side
  • Exterior
  • Drums & Tanks

Typical Damage and Failure Mechanisms

Early identification of damage and degradation is the focus of inspection programs.  Some damage mechanisms can be identified by visual inspection, others only by non-destructive techniques or by removal of sections of affected components.  Because a large portion of the HRSG pressure boundary consists of finned tube, economic inspection of large amounts of surface area is difficult. Below is a list of common damage mechanisms that can affect HRSG reliability. The list is not exhaustive but covers the majority of failures seen.

Gas Path

  • Corrosion (Uniform, Local, Acidic Deposits)
  • Fatigue (Corrosion Fatigue)
  • Creep
  • Thermal Expansion
  • Fretting
  • Overheating

Waterside and Steamside

  • Corrosion (Uniform, Local, Flow-Accelerated Corrosion or FAC, Under-deposit attack, Pitting)
  • Stress Corrosion Cracking
  • Erosion
  • Creep
  • Fatigue
  • Thermal Shock

External

  • Insulation, Cladding and Casing Penetration Damage
  • Heat damage to casing
  • Concrete Pad deterioration
  • Malfunctioning supports and hangers

The ultimate purpose of any inspection is to maintain the reliability and longevity of the equipment and systems under inspection.  To do this, inspection should help the operator determine what is damaging the HRSG, how fast is the damage occurring and where the damage is located.   With this information the operator can plan for corrective actions and repairs.  An effective inspection plan often requires input from HRSG design documentation, actual plant operating conditions, and in some cases component life prediction tools to properly prioritize inspection effort and scope. 

Pressure Boundary Inspection Quick Guide

Pressure Boundary Inspection Quick Guide

Effective HRSG inspections focus on damage mechanisms that could affect each of the specific components; not all components are susceptible to all damage types.  Focusing an inspection only on components affected at other plants (scope defined by “industry experience”) has in some instances resulted in wasted resources or ineffective utilization of resources.  This is because high-risk locations for some mechanisms (for example, FAC) vary from plant to plant.  Blind inspections that do not consider plant specific conditions often yield sub-optimal results. The table below shows is a quick summary guide to the damage mechanisms affecting pressure parts and the most likely locations where one might look first for that type of damage, if time is limited and/or in the absence of any clear indication that this damage might have occurred in any particular area of the HRSG.  Once the pressure boundary is breached then water and steam will leak out.  If the leak is large enough, it will be detected by excessive makeup water consumption and/or by noting steam or water exiting the casing during operation. During an internal inspection, the location of leaks and the pressure boundary breach are typically identified by the presence of water traces or staining on the inside of the unit.  Nevertheless, it is important to note that the source of most staining or discoloration is not pressure boundary leaks.  Other common sources include: rainwater or vented steam/water leakage from roof, water leaks during testing and tube exposure to elements during construction

Damage Type

Usually Found In

Occasionally Found In

Best Location to First Inspect

1

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)

2

Creep or Creep Fatigue 

Superheaters, Reheaters,

 

Selection of hottest tubes that might have exceeded creep threshold temperature, particularly if tubes are bowed

3

Graphitization

Superheaters, reheaters

 

Selection of tubes running hot (>800°F/425°C ) for long period (>100K Hours) and only if material is carbon steel

4

Deposition & Underdeposit Corrosion 

Evaporators

 

Selection of horizontal or bent tube sections

5

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

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 

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

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

11

Thermal Overstress 

HP Superheater, Reheater

 

Check all tubes for any sign of bowing or further bowing since last outage

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

Weld & Fabrication Defects 

All areas

 

Full survey of similar welds as accessible upon failure occurring

 

Inspection Technology for Heat Recovery Steam generators (HRSG)

Inspection Technology for Heat Recovery Steam generators (HRSG)

In addition to selecting inspection technology, the group responsible for inspection must have a clear understanding of what they are looking for and what actions will be required depending on inspection outcomes.  For example, if video inspection of connecting piping welds such as downcomers or related piping is planned, there must be a clear plan as to how to treat indications of “potentially bad welds”.  That is, should a review of plant radiograph records be triggered by such a situation?  Will it be necessary to mobilize mechanical contractors and personnel to erect scaffolding to provide access to the area?  What additional inspection techniques can be performed to determine the significance of this indication? 

Inspection of the interior of the HRSG gas path also has access issues.   In horizontal gas path HRSG’s, easy access is often only possible on the internal casing floor.   Spacing between tube bundles is often tight.   Good lighting is important for both near and remote areas.   Flashlights, halogen lights, drop lights and high-powered searchlight beams are helpful.  The high-powered beams help get visibility to the top of the tube bundles, which can be 60 feet (20 meters) higher than the floor.   Access to upper areas of tube bundles is often possible through hatches in the top of the boiler casing into upper “Penthouse” areas.  The extent of access varies by HRSG OEM and by design.  While providing access to upper headers, vents and riser piping, these areas are typically low, often 3 ft (1 m) or less above the upper headers, with limited access and difficult mobility for inspectors.  Access to the upper tube-to-header weld joints is usually very limited although the 180° return bends typical of some designs can often be accessed. 

While access is difficult, these spaces provide an important opportunity for critical inspections such as of the upper bends in the IP and LP Evaporators or 180° return bends in the HP Economizer.  These areas are often at high risk for erosive wear and FAC damage.  The upper bends in the evaporators are subject to a two-phase form of FAC that results in more rapid wear rates and therefore loss of metal wall thickness than occurs at the bottom of the unit where conditions are single phase in all HRSG modules.  Access to lower headers and drains is often via the removal of baffle plates that allow entry into lower basement areas.  In vertical gas path HRSGs the tube faces between modules are usually accessible, allowing the whole face to be inspected. Depending on the OEM the headers and antlers may be directly accessible from the gas path, or may require the removal of internal baffles or doors. Some OEMs have separate header compartments with their own access doors, allowing detailed inspection of the headers, antlers and 180° returns for all the tubes.  The interior of steam drums is usually via direct entry to the drum.  The drum internals including steam separators, baffle plates, feed piping, and nozzles can be inspected.  Removal of baffle plates may be required to gain access to risers and tubes for internal inspection.  As noted previously, baffle plate wear damage in the drum is often a good indicator for FAC damage in inaccessible components in the IP and LP Evaporator.  Generally, the HP Evaporator/drum operate at a temperature that is not favorable for FAC damage. In all cases good naked eye visual inspection should try to be within 24 inches (60 cm) of the surface, with good lighting.

The table below shows the principal applicable HRSG Inspection methods.

Principal HRSG Inspection Methods

Abbrev

HRSG Inspection Method(s)

VT

Visual Test

-

      Borescopy, Videoscopy

-

      Surface Replication

-

      Laser Profilometry

UT

Ultrasonic Test; wall thickness gauging and crack detection

RT

Radiographic Test

PT

Liquid Penetrant Test

MT, WFMT

Magnetic Particle Test, Wet Fluorescent Magnetic Particle Test

ECT

Eddy Current Test

AE

Acoustic Emission Test

IR

Thermography

MET

Destructive Examination; Metallography

-

Chemical Analysis

-

In-Situ Materials Testing

About Us

Established in 1988, Tetra Engineering has more than 25 years experience providing solutions to the power industry. We specialize in solutions for HRSGs, conventional boilers & steam-cycle balance of plant.

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