Cycle Chemistry Influenced Failure and Damage Mechanisms

Part 3 - Analytical Tools to Address Cycle Chemistry Influenced Failure and Damage Mechanisms
Authors:

1.0 Introduction

The cycle chemistry treatments and control on fossil and combined cycle plants influence a high percentage of the availability, reliability and safety issues experienced on these plants worldwide. As this is a very large and important area for fossil and combined cycle plants, Structural Integrity decided to describe it in three parts. The first part, (N&V 2016, Volume 40) introduced the equipment and materials of construction and how reliability depends on various protective oxides, the formation of which relates directly to the cycle chemistry treatments that are used in the condensate, feedwater, boiler / HRSG evaporator water, and steam. These optimum chemistry treatments were also described in the first article. The second part (N&V 2016, Volume 41), delineated the damage and failure mechanisms influenced by not operating with these optimum treatments which results in the protective oxides breaking down. This third article describes the key analytical tools which have been developed by Structural Integrity and used in over 200 plant assessments worldwide to identify whether these failure and damage mechanisms will occur by identifying the number of Repeat Cycle Chemistry Situations (RCCS). These same tools are used to optimize fossil and combined cycle chemistry control to proactively prevent failure and damage.

2.0 Developing An Understanding of Cycle Chemistry Influenced Failure/Damage in Fossil and Combined Cycle/HRSG Plants Using Repeat Cycle Chemistry Situations (RCCS)

As described in the first article, the understanding of the cycle chemistry influenced failure and damage mechanisms in the steam/water circuits of conventional fossil and combined cycle/HRSGs is very advanced, and has been known and documented for more than 30 years. Despite this, and as described in the second article, chemistry influenced damage and the associated availability losses due to deficient chemistry practices are often enormous. Damage and component failure incidents persist, in both fossil units and combined cycle units, and in the case of Flow-Accelerated Corrosion (FAC) can create a safety problem for plant operating staff. It is thus very clear that the approaches taken by organizations operating fossil and combined cycle plants to prevent such damage are frequently unsuccessful. Similarly, fossil industry usage of the response methodology by which chemistry-related damage events are reacted to (identification of the mechanism, assessment of the root cause, and implementation of actions to stop the mechanism) is often ineffective.

Analysis by Structural Integrity in 2008 of past cycle chemistry assessments and damage/failure investigations of over 100 organizations worldwide at that time lead to a very interesting new concept to prevent damage/failure proactively. This involves identifying Repeat Cycle Chemistry Situations (RCCS). The RCCS which can be regarded as the basics of cycle chemistry, are allowed to continue by the chemistry or operating staff or are imposed on the plant/organization as a consequence of inadequate management support for cycle chemistry.

The first sub-section (2.1) introduces the reader to RCCS while the second (2.2) provides information on the application of the RCCS analysis to 185 plants worldwide since 2008. This analysis in total from over 280 plants worldwide confirms that the process can be used proactively to identify cycle chemistry deficiencies, which if not addressed will lead to future failure/damage of the types delineated in the last section (3.0). The RCCS analysis is also used in root cause analysis to identify the cycle chemistry features responsible and which can be addressed through Action Planning.

Corrosion Products. Categories include: corrosion product levels are not known or monitored by plant staff; the levels are too high and above international guideline values (examples: could be >1ppb total iron at the economizer inlet for supercritical units on Oxygenated Treatment (OT), or > 2 ppb in the feedwater of sub-critical fossil plants or combined cycle/HRSG plants); inadequate and/or insufficient locations being monitored; sampling conducted at the same time /shift each time; using techniques with incorrect detection limit; a most common feature is monitoring the soluble part only by not digesting the sample in a laboratory before using a spectrophotomer. A key easy-to-observe verification aspect of this RCCS is black deposits in the steam and water sampling troughs for units on AVT(O), or red deposits for units on AVT(R).

Boiler/Evaporator Deposits. Categories include: boiler waterwall or HRSG HP evaporator samples have not been taken; there is no knowledge of deposits and deposition rate; samples have been taken but not analyzed comprehensively; deposits excessive and exceed criteria to chemical clean; in HRSGs the HP evaporator deposits are not linked with chemistry in the lower pressure circuits or to the levels of transported total iron; the boiler/evaporator has been sampled and needs cleaning but management delayed or cancelled the actual clean.

Drum Carryover. Categories include: not conducted since commissioning; not conducted even on units with steam turbine Phase Transition Zone (PTZ) problems; not aware of simple process to measure carryover; saturated steam samples not working or nonexistent; samples taken are not isokinetic.

Continuous On-line Cycle Chemistry Instrumentation. Categories include: installed and operating instruments is at a low % compared to International Standard (a normal level is between 58 and 65%); too many instruments out of service, not maintained or calibrated; instruments are not alarmed for operators and many are shared by multiple locations and not / never switched; plant relies on grab samples to control plant (1 – 3 times per day/shift); the instrumentation most often missing is CACE (cation conductivity) and sodium on main or HP steam and conductivity (specific conductivity) on makeup line to condenser

2.1 Development of Repeat Cycle Chemistry Situations (RCCS)

The analysis which we did in 2008 identified two key features which related to why and how cycle chemistry influenced failure/damage occurred in fossil and combined cycle/HRSG plants. From the mechanism aspect, the first shows that cycle chemistry influenced failure/damage involves the breakdown of the protective oxide which grows on all fluid-touched surfaces. This could involve cracking, fluxing, dissolving, and solubilizing of the oxide layers as well as transportation and deposition of corrosion products (oxides) on the heat transfer surfaces. From the viewpoint of organizational or management aspects of the cycle chemistry and its control, it became clear that every cycle chemistry failure/damage incident can be related backwards in time to multiples of RCCS which were not recognized or properly addressed and allowed to repeat or continue. In some cases, the chemistry staff had not recognized the importance of the situation and allowed it to continue. In other cases, the chemistry staff recognized the importance, but was not successful in convincing the management (either plant or executive) that action was required to eliminate the RCCS. In many cases the management has delayed action or has not provided the necessary funds to resolve the situation. In doing this type of retroactive analysis, it very quickly became obvious that plants/organizations can get away with having one or two RCCS, but once this number increases then failure/damage was a certainty.

In 2008, the following ten RCCS were identified which were very commonly associated with preventable cycle chemistry related damage in fossil and combined cycle plants:

  • High levels of corrosion products
  • High boiler/HP evaporator deposition
  • Non-optimum chemical cleaning
  • Contaminant ingress (with no reaction by the operators)
  • Drum carryover
  • High level of air in-leakage
  • Lack of shutdown protection
  • Inadequate on-line alarmed instrumentation
  • Not challenging the status quo
  • No Action Plans for any of these repeat situations

After using the RCCS analysis at 185 plants worldwide since 2008, the categories have remained the same but it has become clear that there are multiple sub-categories for each. To assist the readers in understanding the concept of RCCS and whether they exist in their plants the following subsections provide a few notes on some of the most important categories. Some examples of a few case studies are provided later to further illustrate this concept.

This RCCS analysis is very powerful in assisting with root cause analysis, in identifying where cycle chemistry failure/damage will occur in the future, and where improvements should be made. The compiled statistics of RCCS have also been used internationally to identify where international research and guidance is necessary.

Challenging the Status Quo. Categories include: no change in chemistry since commissioning; using incorrect or outdated guidelines; continuing to use reducing agents in combined cycle/HRSGs and in fossil plants with all-ferrous feedwater systems, and thus risking or experiencing single-phase FAC; continuing to use the wrong phosphate treatment (usually not using only tri-sodium phosphate); not having a chemistry manual for the unit, plant or organization; incorrect addition point for chemicals (most often reducing agent with AVT(R)); not questioning use of proprietary chemical additions (phosphate blends, amines, FFP) and therefore not knowing the composition of chemicals added to the unit / plant; not determining through monitoring the optimum feedwater pH to prevent/control two-phase FAC.

Shutdown/Layup Protection. Categories include: Unit/plant has no equipment for providing shutdown protection for boiler, HRSG or feedwater heaters; equipment present but not used or inoperable / not maintained; poor / no operator procedures; only partial protection applied (boiler/HRSG vs, feedwater); no dehumidified air (DHA) provided for the steam turbine shutdowns.

Contaminant Ingress. Categories include: no assessment of risk; inadequate instrumentation and alarms (especially for seawater cooled plants); operators allow exceedances of control and shutdown levels; chemists and/or operators compromise limits to plant ability (make high readings acceptable), or make up (invent) normal and action levels which have no technical relevance; no comprehensive procedures to deal with contaminant ingress.

2.2 Using RCCS to Identify Deficiencies in Cycle Chemistry Control

Between 2008 and 2016 Structural Integrity has applied the analysis of RCCS during 185 plant assessments. 117 of these were at fossil plants and 68 at combined cycle plants involving HRSGs from 18 manufacturers. The work involved a large range of assessments which included: boiler and HRSGs tube failure mechanism and root cause assessments; fossil and combined cycle FAC and Air-Cooled Condenser (ACC) assessments; cycle chemistry assessments and chemistry optimization; cycle chemistry treatment conversions to OT and Phosphate Treatment (PT); Plant Transmitter Zone (PTZ) blade and disk failure/damage root cause analyses in fossil and combined cycle plants; copper deposition on fossil plant HP turbines; development of shutdown/layup and preservation procedures for all types of plants; and combined cycle plants with desalination equipment interface problems.

Table 1. Analysis of Repeat Cycle Chemistry Situations (RCCS) in Conventional Fossil and Combined Cycle/HRSG Plants. (Notes: The numbers in the table represent the percentage of plants where the RCCS was identified. Action Plans were developed for each RCCS at each plant).

Table 1 shows the data for these fossil and combined cycle/HRSG plants. Table 1 clearly shows a ranking order of RCCSs with monitoring corrosion products and on-line instrumentation being the most often cycle chemistry processes not being applied properly. These are followed by not challenging the status quo and measuring carryover. General shutdown procedures for plants is relatively high on the list with the sub category of applying / using DHA most often missing. It is expected that the application of Film Forming Products (FFP) will over the next 5-10 years start to provide this shutdown protection.

3.0 Case Studies

This section provides four case studies as examples of applying the RCCS methodology to make assessments on failure / damage and its proactive use to assist fossil and combined cycle / HRSG plants in determining if failure / damage will occur in the future.

Case Study 1. This L-0 blade cracking occurred in a 700MW 2×1 combined cycle / HRSG plant after about 90,000 operating hours. The cracking emanated from pits on the blade surface. The plant had two gas turbines and a steam turbine (HP/IP and LP), and triple-pressure HRSGs with HP drum pressure of ~10.3 MPa (1500psi). The condenser had titanium tubes which had experienced numerous condenser leaks of the brackish cooling water. The cycle chemistry condensate/feedwater treatment included a proprietary amine blend (ETA / MPA) and a reducing agent (Carbohydrazide), and a proprietary phosphate blend be added to all three drums.

During the root cause analysis the following seven RCCS were identified with the last five being directly related to the PTZ cracking:

  • Total iron corrosion products not measured at any location around the cycle.
  • No HP evaporator tubes had been removed to assess internal deposits.
  • Instrumentation at low level compared to international standards. The level of instrumentation (about 50%) was inadequate for identifying contamination quickly. There was no sodium at the condensate pump discharge or in HP superheated steam (HPSH), pH in feedwater, no Conductivity After Cation Exchange (CACE) in steam, and no combination of CACE/pH in the HP drums.
  • Carryover had not been measured. Unknown levels of carryover into steam as the operators / chemists had failed to monitor carryover on a regular basis and during contamination events exceeding the shutdown limit suggesting that steam contamination levels had been higher than the plant guideline limits on multiple occasions.
  • Shutdown protection had not been not applied. There was inadequate shutdown protection for the plant and no DHA applied to the LP steam turbine despite frequent contamination events which exceeded the plant shutdown limits.
  • Repetitive contaminant ingress. The operators continued to operate when contamination exceeded the unit shutdown limits multiple times, and continued to operate attemperation during these contaminant periods.
  • Not challenging the Status Quo. Plant continued to operate with inadequate and out of date chemistry guidance, and kept changing (increasing) the shutdown limit to allow the plant to keep operating. But the operators continued to ignore the shutdown limits and action levels that they had developed. They also continued to use a reducing agent despite the clear international guidance for combined cycle / HRSG plants that this chemical should not be used.
  • It can easily be seen that this represents a “full house” of RCCS. Singly, by themselves, each RCCS would (probably) not have caused failure / damage, or be viewed as the plant operating out of control. But together, these are commonly the basis of PTZ failures and damage worldwide. The other important observation is that operating with seven RCCS in total is rare but is a clear indicator that some other failure / damage mechanism, such as hydrogen damage, will occur in the future.

Case Study 2. The unit in this assessment was a 650MW 2×1 combined cycle plant with about 93,000 operating hours. The plant had two gas turbines and a steam turbine (HP and IP/LP), and triple-pressure HRSGs with HP drum pressure of ~10.3 MPa (1500psi). The condenser had SeaCure tubes which had experienced condenser leaks of the cooling water (~200 ppb Cl and ~400 ppb SO4). The cycle chemistry condensate / feedwater treatment included a proprietary amine blend (ETA / MPA). The reducing agent (hydroquinone) had been eliminated a few years before the assessment. A proprietary phosphate blend was added to the HP drums.

During the cycle chemistry / FAC assessment for this plant the following seven RCCSs were identified:

  • Total iron corrosion products not measured.
  • No HP evaporator tubes removed to assess deposits.
  • Instrumentation at low level compared to international standards. The plant had no operational on-line continuous instrumentation and was “controlled” by grab samples.
  • Carryover had never been measured.
  • Shutdown protection not applied to HRSGs and there was no DHA for the steam turbine.
  • Air In-leakage was a continuing problem.
  • Status Quo. Plant guidance had not been updated for 6 years.

By comparing this listing with that from the first case study, the similarities will be noted, and the risks for PTZ cracking and Under-Deposit Corrosion (UDC) were assessed to be high illustrating the power of the RCCS methodology.

 

3.1 Case Studies 1 & 2: Damage / Failure in the Phase Transition Zone (PTZ) of the Steam Turbine in Combined Cycle/HRSG Plants

Protection of steam turbines from chemistry influenced damage as indicated in the second article (N&V 2016, Volume 41) has long been recognized as an integral key aspect of effective cycle chemistry programs for fossil and combined cycle/HRSG plants. Equipment manufacturers and research organizations have performed extensive investigations of damage mechanisms and determined that most are related to the chemistry, both during operation and when the unit is out of service. Experience has shown that many organizations continue to experience contamination of the steam, leading to various consequences. In some instances, a developing problem is identified during service through monitoring of carryover. But in most cases, the existence of steam purity issues only becomes apparent when blade or disc cracking is observed during an inspection conducted as a scheduled maintenance activity or as a consequence of a failure incident. This sub-section includes two combined cycle/HRSG Case Studies which illustrate a pattern observed worldwide in conventional fossil and combined cycle plants. The first case was a failure incident where the last stage blades were found cracked during a maintenance inspection. The second in a plant 8,000 km from the first was not a failure situation but part of a combined cycle/HRSG plant cycle chemistry assessment where the analysis of the RCCS was almost identical to the first case study, so suggested proactively that future failure was a possibility.

3.2 Case Study 3: Under-Deposit Corrosion – Hydrogen Damage

Although the understanding for hydrogen damage was developed over 50 years ago (N&V 2016, Volume 41), hydrogen damage is still prolific in fossil and combined cycle / HRSG plants worldwide. Structural Integrity continues to conduct metallurgical analyses and root cause investigations multiple times each year and continues to identify the same suite of RCCS in the plants that experience this UDC mechanism. In brief, these include:

  • Excessive feedwater corrosion products.
  • Non-monitored feedwater corrosion products.
  • Measuring only soluble corrosion products (no digestion).
  • No boiler waterwall or HRSG HP evaporator tubes taken for deposit analysis.
  • Excessive deposits on tube ID surfaces.
  • Delayed/postponed chemical cleaning.
  • Repetitive contamination above Action or Unit Shutdown Levels.
  • Contaminant ingress above Shutdown limit.
  • No operational or managerial support to shutdown with low pH.
  • Inadequate on-line instrumentation below the international standard.
  • High level of air in-leakage
  • Not challenging the cycle chemistry status quo including the following categories: the feedwater and boiler water treatments and control limits were not optimal; the specification of chemical treatments and guidance were largely determined by a chemical supplier and thus plant personnel are not fully aware of the active chemical composition of the products they were feeding to the HRSG. No cycle chemistry manual is available for the unit / plant.
  • No Action Plans to address any of the above repeat situations. This is because very often the plant staff had accepted these situations as “normal and allowable” under the culture but in other cases were ignored for various reasons.
  • 3.3 Case Study 4: Understanding Deposits in HRSG HP Evaporators

Deposition in HRSG HP evaporators is the precursor for any UDC mechanism as discussed in the second article (N&V 2016, Volume 41), and Table 1 illustrates that not having a comprehensive understanding of these deposits and the deposition rate is key to a number of HRSG failure mechanisms. A new deposit map for HRSG HP evaporator deposits was provided in the second article. The deposit levels also provide an indirect indicator of FAC in lower pressure parts of the HRSG.

4. Summary

The optimum cycle chemistry control of fossil and combined cycle / HRSG plants is of paramount importance in achieving and maintaining the desired availability, reliability and performance. There are a number of key basic features which need to be adopted and addressed to achieve this highest level of operational performance. These have been introduced in the three News and Views articles and involve primarily ensuring that the cycle chemistry drivers for the main damage mechanisms are comprehensively understood and addressed in developing and monitoring the cycle chemistry for fossil and combined cycle/HRSG plants. The previous two articles provided information on the optimum cycle chemistry treatments and control for fossil and combined cycle/HRSG plants as well as an overview of the most important cycle chemistry influenced failure and damage mechanisms that occur in these plants. This third article has introduced a very powerful assessment methodology developed by Structural Integrity to identify proactively if any of these mechanisms will occur in a plant and how they are influenced by the cycle chemistry. It has illustrated how these Repeat Cycle Chemistry Situations (RCCS) can identify how operating outside of optimum treatments and without adequate cycle chemistry control systems (monitoring, instrumentation, analysis, etc) will lead to failure / damage of the plant.

A couple of case studies have been included to illustrate how to address and ultimately prevent the major cycle chemistry influenced mechanisms. Specific programs should be developed to ensure that RCCS are not allowed to occur or continue. Addressing each RCCS with an Action Plan to eliminate the situation has been shown to address future failure and damage. The assessment methodology has also been used in many root cause analyses studies.

5. Bibliography

There are a plethora of international guidelines available in many countries of the world for the reader: IAPWS (International), EPRI (US), VGB (Germany), JIS (Japan), Russian, Chinese, Manufacturers of major fossil and combined cycle / HRSG equipment (International), Chemical Supply Companies (International). Structural Integrity uses the Technical Guidance Documents (TGD) of the International Association for the Properties of Water and Steam (IAPWS) in all the cycle chemistry related plant assessments and root cause analyses conducted. These are freely downloadable on the IAPWS website (www.IAPWS.org). These have been used as the reference materials throughout this article and series and full attribution is given to IAPWS.

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