Tetra Engineering will have a booth next month at the upcoming PowerGen Middle East in Abu Dhabi on October 12-14. We will present our latest service offerings including Field Services, Engineering Analysis, 3rd Party Engineering, and Training. Come see us at the N21 booth, and enter your business card for your chance to win a Wilesco Steam Engine Generator!
Announcing Tetra Engineering's Fall 2014 USA Training Seminar: Effective Operation of HRSGs on November 12 & 13 at The Simsbury Inn, Simsbury, Connecticut USA. See this link for more information on the syllabus.
The sudden contact of low-temperature water droplets on high temperature steel leads to what are commonly called thermal impingement stresses. These tensile stresses can be very, very high and lead to yielding of the steel; resulting in visible, local bulging of the pipe and/or cracking of the internal wall. Frequent incidences of this problem with associated failures are observed in thermal power plants; they often occur in Main Steam Bypass and Hot Reheat Bypass Power Piping or in attemporator sprays.
Good control logic and operator awareness is crucial to ensure that spray water doesn't build up when valves are closed, that sprays are not exceeding their proper flow range or that there is over-spraying into the steam.
Inspection techniques vary depending on access but may include Ultrasonic Testing (UT) and Remote Visual Inspection (RVI) by videoscope, as well as hardness testing of sensitive materials. Tetra prepares an Inspection Plan for O&M staff to follow to ensure that the condition of the relevant components is sufficiently monitored to mitigate risks.
Tetra Engineering provides a full-scope service for the modelling and operation analysis of all types of natural or forced circulation Power Boiler and HRSGs, including supercritical and once-through designs. This can be integrated into a full plant model to incorporate the steam turbine and steam cycle balance of plant.
The level of detail can be adapted to focus on local effects (e.g. row-to-row temperature differences) and can be steady-state or time-dependent, the latter to model transients such as startups. The state-of-the-art Power Plant Simulator and Designer™ (PPSD) software package developed by KED software is the primary tool for this work. Frequent applications at Tetra Engineering include:
- Calculating steam cycle efficiency (heat balance) according to ASME PTC or other codes
- Determining natural circulation rates in support of FAC risk assessment
- Generating metal temperature profiles for inputs to finite-element stress calculation
- Assessing the impact of design changes or modification to original plant operating regime
The modern power generation market is very different to what it was ten or even five years ago. The requirement to provide network load flexibility is falling on combined cycle plants with HRSGs, they are having to fill the supply gap between base loaded nuclear and large coal plants and the highly variable renewable energy generators. The result of operating at lower loads means the HRSG operating conditions may be very different from what was envisaged in the design case. Equally the increased number of startups and shutdowns presents problems for components designed for baseload. The problems faced by plants that are cycling or operating at off-design conditions can include:
- Water chemistry instability due to transient conditions
- Quenching problems in superheater and reheater tubes during condensate formation
- Attemporator and Bypass Spray problems handling increased demand
- Economiser shock
- High temperatures at superheaters and reheaters leading to creep failures
- Higher heat flux in the HP Evaporator leading to increased deposit loading
- Increased FAC risk due to increased heat at the LP Evaporator
Tetra develops inspection plans to address these and other issues using:
- Visual Inspection of the HRSG hot gas path
- Borescope/videoscope inspection of waterside conditions of lead high-pressure (HP) evaporator tube panels and other areas
- Ultrasonic testing (UT) of wall thickness for selected (high-risk) tube, header, and riser components
- Magnetic particle testing of all accessible tube-header weld connections in the HP steam and reheater tube panels
- UT shear wave or UT phased-array inspection of Grade 91 components
- Visual inspection of accessible HRSG waterside components
- External walkdown and thermography
Flow-Accelerated Corrosion (FAC) or Erosions-Corrosion remains a common cause of failure in HRSGs and conventional boilers, yet the underlying causes and solutions have been well understood for more than 25 years. The two factors influencing the susceptibility to FAC are: a) Unit Design and b) Process Conditions. It is important to know how FAC-prone your HRSG or boiler design are, this will help you optimise your operating conditions to suit that design.
When assessing design factors, the primary question is whether the HRSG/boiler tubes and pipes' geometries and materials are suited to their flow profiles across the range of operating load cases. Operational factors are primarily concerned with assuring good control of water chemistry parameters (pH, dissolved oxygen or ORP, iron levels).
Higher demands on outlet steam conditions has led to increased use of modified low alloy materials (9-12% Cr), such as Grade 91. The superior high temperature material properties of these types of steels (strength, fatigue resistance, creep resistance, improved steam oxidation resistance) gives the designer advantages with respect to boiler reliability and thermal performance. The advantages of using this type of materials are many; however, the long-term integrity has been a concern to the industry ever since the first installations.
A number of failures have occurred in recent years, some catastrophic; type IV creep cracking in the Heat Affected Zone (HAZ) has been the dominant failure mechanism. Advantageous mechanical properties can only be assured if heat treatment is properly performed through the entire fabrication and construction cycle. Improper welding or heat treatment will result in a product not fit for its purpose, increasing the risk of premature failures. Tetra performs risk assessments encompassing all key aspects that can contribute to component failure:
- Manufacture and Assembly
Finite Element methods are used to analyse large piping and boiler components in order to accurately determine the geometrical effects on the nominal stresses present. The results are used in a creep analysis to determine damage up to date and the remaining life of the components. If the plant is planning to change the operating process conditions, a boiler simulation study can be used to predict the local temperature and pressure conditions in tubes and piping in advance. Key components to be checked are identified and integrated into an Inspection Plan, for use by plant staff and NDT personnel.
Since 2007 the ASME B31.1 Power Piping Code has included Chapter VII, establishing mandatory requirements for Operation and Maintenance. The key elements of this program are discussed here, and require that Operators outline a detailed maintenance plan for their Covered Piping Systems (CPSs). The Code also requires that operators perform regular Condition Assessments of their CPS ensuring that relevant degradation mechanisms are being covered.
At the 2014 ASME Power Conference in Baltimore, David Moelling of Tetra Engineering presented the recent research paper titled: Role of Boiling Mode and Rate in Formation of Waterside Deposits in Heat Recovery Steam Generator Evaporator Tubes, written by David Moelling and James Malloy.
The paper discusses results from research in the theory of boiling mode and deposit formation, employing advanced thermal simulation of evaporation in real boiler models, and compares results with actual experiences in recent years.
See the abstract to the paper below: