Tetra in Action #5 - October, 2016

Assuring HRSG and Boiler Pressure Part Quality

Remember -Certification Does Not Always Equal Quality!

HRSGs and power boilers are built by assembling large numbers of components, mostly by welding.  Design choices in optimizing heat transfer against overall cost lead to configurations that aim to minimize the number of welds per square meter of heat transfer surface.  In addition, fabrication of the modules is frequently subcontracted by the OEM for manufacture in regions where manpower is cheaper.  The OEM becomes in effect an integrator of components built by others.  How can Product Quality for pressure parts be assured in that context?

Under the ASME Power Boiler Construction Code, the approach is to initially to certify a manufacturer by approving a suitable QC program, including the qualification of welding procedures and welders themselves.  Going forward, Quality would then be assured by the manufacturer’s adherence to their approved program and by the presence of the ASME Authorized Inspector on site. 

The Authorized Inspector has a number of compulsory checks that must be made in person but, in practice, a large portion of the welding continues without any further follow-up, except where the Code requires NDE.  If the approved QC program becomes a box-ticking exercise – for example when schedule constraints take priority – weld quality can decline resulting in unrepaired defects.  The problem for the Purchaser is to determine whether the manufacturer’s QC program can be trusted to guarantee a quality product.

Tetra’s experience is that product delivered by a number of OEM’s has on frequent occasions displayed unacceptable weld quality, despite the OEM and/or subcontracting fabricator having a valid quality certification in place.  The cause is systemic management focus on schedule and cost, with quality given a lower priority.  Many rejectable weld defects are not visible from the outside and only infrequently will the module fail the field hydro tests (although this has occurred).  The main concern is that a large population of weld defects present in a set of tube modules will affect pressure part longevity, particularly if the plant is in cyclic operation.
To assist clients in their pressure part procurement process, Tetra Engineering performs:

  • Project Vendor and Product Sourcing Quality Assessments
  • Expert and Enhanced Factory Fabrication Surveillance (see following example)
  • Warranty Risk Evaluation

A Recent Case History

Weld Inclusion

During a recent Tetra project thousands of still images of tube to header welds were taken using a borescope in several HRSGs at a plant site just after commissioning.  The images were thereafter reviewed independently by at least two engineers.  The tube to header welds were found to be characterised by a general lack of good workmanship with numerous defects or flaws apparent.  The initial inspection of one set of upper SH headers yielded 20 tubes that had weld defects serious enough to require repair.  It should be noted that repair can be difficult because of poor access.  Similar results were found in SH modules from the other units.


  Results on Header
Category Count %
Accept 100 53%
Substandard 68 36%
Fail/Repair 20 11%
Total 188 100%
Results: Borescope Inspection Campaign of 188 tubes 

For more information about this service send us an email (This email address is being protected from spambots. You need JavaScript enabled to view it. ).

A Successful First Boiler & Steam Cycle Knowledge Workshop in Dubai!

Tetra Engineering held its first steam Boiler & Steam Cycle Knowledge Workshop at the Media One Hotel in Dubai.  Five of the seven workshop topics were presented by a team of four instructors from our company, the other two were presented by external collaborating partners: Mr. Jamie Pollock of Mettek Ltd. and Messrs. Tord Forslund & Leif Andersson from CCI Valves. Attendees could sign up for topics “à la carte”, these ranged from how to meet B.31 requirements for piping inspections to particular problem issues such as waterside deposition or creep. See here for more information.

HRSG Workshop

HRSG Inspection Planning Guide Snapshots

One of the first applications of the borescope was for inspecting steam turbine rotor discontinuities in the 1920s.  Today a wide variety of commercial borescopes and videoprobes can be used for access within HRSG components.  These include both flexible and rigid borescopes and a variety of videoprobes and end effectors.   Mirror or prism attachments can be used to get right angle views of tubes or pipe connections.   Access can be via hand-holes or other penetrations.
Flexible video inspection probes employ high-resolution camera at the probe tip to capture detailed color inspection footage.   Interchangeable lenses bring subjects both near and far into sharp focus.  Variable-intensity fiber optic illumination floods hidden targets with cool, white light.  Knobs on the handset control the tip’s up/down and left/right articulation.


Figure: Typical Flexible Fiberscope (Courtesy, InterTest, Inc.)

If you are interested in the full version of the HRSG Inspection Planning Guide (2nd edition) contact This email address is being protected from spambots. You need JavaScript enabled to view it.

Tetra Article in Power Magazine

A recent article published in Power Magazine explains the continuing importance of Condition Assessments of HRSGs in optimising Operational and Capital Expenditure. See this link for more details. 

Tetra In Action #4 - July, 2016

Come see us at the PowerGen Asia conference in Seoul!

Chief Engineer David Moelling of Tetra Engineering is presenting the Tetra/R&R Consult paper Root Cause Failure Analysis of Superheater Tubes Using Computational Fluid Dynamics Simulations at the PowerGen conference in Seoul on the 20th of September at 2PM. 

“As the electricity market has evolved with the addition of renewables to the generation mix, Heat Recovery Steam Generators (HRSG) that were originally designed for base load conditions are now frequently forced to operate in a cycling and/or low-load regime. This can lead to front end tube-to-header fatigue, creep or creep-fatigue failures, often induced by GT flow imbalances causing locally-elevated tube temperatures and/or bending stresses on joints due to large temperature differences between tube rows. This paper focuses on the use of Computational Fluid Dynamics (CFD) as a tool to analyze the root cause of Superheater tube failures. Exhaust gas flow profiles were analyzed for base load and low load conditions for a vertical Heat Recovery Steam Generator (HRSG). The plant suffered from tube failures that could be shown to be directly related to low-load (and start-up) exhaust gas flow patterns. The contribution of CFD to identifying these conditions that lead to the failures are presented and mechanisms to reduce future risk of failure, such as addition of flow-conditioning devices, are investigated and presented."

More details about the Tetra presentation and the conference can be found here and here.

Boiler Preservation Engineering

OEM or industry guidelines for Boiler or HRSG waterside and gas-side preservation (or “layup”) are useful but typically don’t go further than general advice on the different available options (wet vs. dry etc.). The difficulty is in moving from the general to the specific for a given plant. Tetra can define viable preservation options and help you choose an optimal strategy from among these that fits your budget and operational constraints. If desired, Tetra will then develop a detailed engineering specification for your preservation system.   For more information about the service send us an This email address is being protected from spambots. You need JavaScript enabled to view it..

HRSG Inspection Planning Guide Snapshots

The following HRSG locations are affected by external corrosion and oxidation:

  • External Corrosion (Evaporators, Economizers, Feedwater Preheaters)
  • Acid Dew Point Corrosion (Feedwater Pre-Heaters)
  • Oxidation on High Temperature Tubes (Superheaters, Reheaters)

External corrosion, as the name implies, occurs on the exterior surface of steel components in the HRSG. It affects both pressure parts and structural elements and requires the presence of corrosive agents to drive the reaction with the metal.  The most common agents are oxygen and water.  When shutdown these are present in the air and when running are present as components in the gas stream.  

If you are interested in the full version of the HRSG Inspection Planning Guide (2nd edition) contact Ms Christine Vallon at This email address is being protected from spambots. You need JavaScript enabled to view it..  


HRSG Tube Failure Guide, 3rd Ed.

Pre-Publication Discount Available on HRSG Tube Failure Guide, 3rd Ed. 

The latest updated edition of Tetra Engineering's HRSG Tube Failure Diagnostic Guide will be published August 15, 2016.

Thousands of combined cycle professionals are familiar with Tetra's HRSG Tube Failure Diagnostic Guide; first published during the 'bubble' in construction in the early 2000s.

The latest edition includes additional material on waterside deposits, creep fatigue cracking, fireside corrosion and others.  Many new photos and illustrations now make it easier to use.

Each copy of the Guide is offered at $195 USD after August 15.  The special pre-publication price is $165 USD.  Inquire for volume discounts.  Shipping is in addition to the per copy charge.

For more information please contact Natalie Vasa at This email address is being protected from spambots. You need JavaScript enabled to view it. or by phone at +1 860 651-4622


Tetra In Action #3 - June, 2016

Dubai: September, 2016 - Boiler & Steam Cycle Knowledge Workshops

Tetra Engineering announces a series of targeted workshops to provide thermal and process plant O&M engineers, managers and operators with a working knowledge in several aspects of the boiler and steam process technology. Drawing on our field experience working at power and process plants throughout the world, the objective is to provide attendees with clear instruction on the operator actions and design issues that can impact reliability, efficiency and operating life.  The programme comprises a set of focused, half-day training modules intended to leave attendees with a practical working knowledge on these issues.  A number of outside experts will join Tetra's senior staff in delivering the workshops.

The course syllabus can be found here.

Newly Published Technical Papers by Tetra Engineering Collaborators

An engineering consultant needs to stay up to date in his knowledge and try to contribute to furthering that knowledge.   Therefore, the staff engineers at Tetra don’t just work on commercial projects, they also take the time to do research and development on a variety of boiler and steam cycle engineering issues.  This is actively encouraged by the company as it only increases the level of skill that our collaborators can bring to bear for solving a given client’s problem. 

Over the years Tetra engineers have published many papers and attended numerous technical conferences and forums.   In the past 2 years the following papers were presented or published with collaborators from the various Tetra offices as authors or co-authors:

  1. Technical Paper and Presentation:  Impact of Startup Purge Credit on Combined Cycle Plant Operation. Authors: Andreas Fabricius, Mark Taylor, Dave Moelling (all Tetra Engineering); Proceedings of the ASME 2015 Power and Energy Conversion Conference PowerEnergy 2015, June 28-July 2, 2015, San Diego, California; (Link)
  2. Technical Presentation:  Chemical Cleaning of the HRSG Waterside: When, Where and How? James Malloy (Tetra Engineering) and Steve Wynne (Chemical Cleaning Consultants), European HRSG Forum, Munich May 11-13 2015. (Link)
  3. Technical Paper:  Premature Grade 91 failures - Worldwide Plant Operational Experiences, Authors: Andreas Fabricius and Dr. Peter Jackson (both Tetra Engineering); Journal of Engineering Failure Analysis, August 2016, Volume 66, Pages 398-406. (Link)
  4. Technical Paper and Presentation: Understanding Variations in Flow-Accelerated Corrosion Wear Rates in HRSG Evaporator Tubes,  Authors: James Malloy and Mark Taylor (Tetra Engineering) Jan Rusaas (R&R Consult, Denmark); Conference on Flow Accelerated Corrosion 24-27 May, 2016; Lille, France. (Link)
  5. Technical Article:  Covered Piping Systems O&M Programs for CCGT Plants, Authors:  Robert Rosario and Dr. Peter Jackson, Power Magazine, April 2016, (Link)

An additional paper by Tetra collaborators Dave Moelling and Andreas Fabricius (and third co-author Jan Rusaas of R&R Consult) has been accepted for presentation at the upcoming Powergen Asia (Tuesday Sept 20th, see details here):  Root Cause Failure Analysis of Superheater Tubes Using Computational Fluid Dynamics Simulations.

Technical Tidbits from Tetra’s HRSG Inspection Planning Guide

HRSG Structures are exposed to weather, hot turbine exhaust gases, steam and water leaks and emissions control chemicals.   Deadweight, wind, thermal and seismic stresses also are applied to the HRSG.  These components are typically inspected on an opportunistic basis, for example during annual statutory inspections by the insurance company’s boiler inspector or during scheduled work on other major components such as the GT.  

Early HRSGs had frequent problems with the duct and casing/liners.  HRSG OEMs have responded to these problems with improved rials, improved bolting patterns and related improvements to designs of other components such as duct burners that have been responsible for damage to liners in certain configurations.

Casing liner plates are often found damaged in the transition duct at the exit of the gas turbine.  This is particularly common in ducts with abrupt geometry changes such as 90° angles in the duct or other sharp angled transitions.  Another common location for liner damage…

Casing Liner Plate Washer Nut Missing

If you are interested in the full version of the HRSG Inspection Planning Guide (2nd edition) contact  Christine Vallon (This email address is being protected from spambots. You need JavaScript enabled to view it. )  or Natalie Vasa (This email address is being protected from spambots. You need JavaScript enabled to view it. )