Boiler Fitness Survey for Condition Assessment of Industrial Boilers

Introduction

For many years boiler condition assessment and life extension programs have been common for the electric power utility industry. Components such as steam outlet headers, main steam and reheat steam piping, and steam turbines are all subject to eventual material failure from operating at high temperatures and stresses. As a general class, industrial boilers typically operate at much lower temperatures and pressures. As a result, the life of these boilers is not necessarily defined by a finite material life. There are numerous examples of boilers which are more than 50 years old which are still in operation. Often, these older boilers are retired for reasons other than reliability or safety.

Industrial versus Utility

Condition assessment programs for industrial boilers are similar to the approach used for electric utilities, except for the primary mechanisms of failure and the inspection methods that are used. For purposes of condition assessment classification, industrial boilers are those units designed for outlet steam temperatures of 900 F (482 C) or less with design pressures that typically do not exceed 1500 psig (104 bar).

Condition Assessment Considerations

In the planning of a condition assessment program, it is important to consider the objectives for the boiler. Typical questions to be addressed include:

• How many years of service are required from the boiler?
• At what capacity will it be operated?
• Will it be base loaded or cycled regularly?
• Are upgrades under consideration and, if so, what systems may be upgraded or replaced?

Planning the Scope of the Boiler Fitness Survey

The scope of a boiler fitness survey depends on several factors, such as the boiler design type, design temperatures and pressures, materials, fuels, age, unit history and future plans for the boiler or plant. B&W follows a multi-level approach in the planning of the survey scope. Typical activities for Level I and Level II evaluations are as follows.

Level I

• Evaluate past operating and maintenance history.
• Identify any critical components on basis of history, experience with similar boilers, and objectives for future of unit.
• Perform complete visual inspections of all accessible areas of the boiler and/or auxiliaries; photo-document problem areas as needed.
• Identify the root cause of damage found to the limits of Level I survey.
• Develop a final boiler fitness report with recommendations.

Level II (in addition to items in Level I)

• Establish the outage inspection and testing plan.
• Define support requirements for the inspections including any materials that may be needed.
• Assess operation – may include walk down of boiler and piping, and data gathering to evaluate performance of auxiliary equipment.
• Implement inspection and testing plan. May include tube samples for basic condition assessment analysis.
• Perform preliminary life estimates and provide recommendations for immediate action as needed.
• Prepare for follow up operational testing as required/planned.
• Estimate remaining life – analyze data and inspection results.
• Implement operational testing if required.

Depending on results of Level I and II analyses, additional material testing, engineering studies and/or more extensive and specialized testing may be recommended.

Critical Systems and Components

The critical components or systems can be prioritized by the impact they have in the following categories.

1. Safety – anything with the potential to injure personnel or damage property.
2. Reliability – failures that could lead to forced outages and loss of production.
3. Performance – effects on unit efficiency, emissions, etc.

In a comprehensive boiler evaluation, components such as the safety valves, burners, flame safety system and combustion controls should be given high priority because they are directly related to the safe operation of the boiler. These systems and components must be maintained, calibrated and tested as part of the normal operation of the boiler.

Generally, the critical components that are the focus of inspections are those whose failure will directly affect the reliability of the boiler. Boiler pressure parts are given special attention since failure leads to forced outages and lost steam production. These critical pressure parts include:

• Drums – steam, lower, uptake, downtake, etc.
• Headers – both steam and water
• Tubing – superheater, boiler or generating bank, waterwall, economizer
• Piping – steam and feedwater
• Deaerator – may have special safety concerns
• Attemperators – sometimes called desuperheaters

Components that are more likely to have adverse effects on boiler performance as they deteriorate with age include:

• Air heaters – recuperative (tubular), regenerative (Ljungstrom), steam coil
• Fans – induced draft, forced draft, primary air (pulverized coal firing)
• Burners
• Fuel preparation equipment (especially coal firing, i.e., pulverizers)
• Boiler settings such as casing and BRIL (brickwork, refractory, insulation and lagging)
• Structural supports

Nondestructive Examinations (NDE)

NDE can be an important part of the boiler fitness survey. NDE is done to obtain sufficient data to allow proper assessment and decision-making regarding the integrity of the component. The choice of NDE methods will depend upon location and type of potential damage as well as the limitations caused by the arrangement and geometry of the component itself. A variety of NDE methods have been used by B&W on industrial boilers including visual examination, magnetic particle testing, liquid dye penetrant testing (PT), and ultrasonic testing (UT) to name a few.

Remaining Life

Various methods have also been developed for assessing the remaining useful life (RUL) of key boiler components. For industrial boilers the most common tool for RUL assessment is analysis of corrosion and erosion rates and comparison of actual component wall thicknesses versus American Society of Mechanical Engineers (ASME) Boiler Code calculated minimums. Since tube life and tube failure tend to be the major cause of forced outages in aging boilers, RUL of low-temperature tubes is a large part of industrial tube remaining life assessment.

In 1985, B&W developed A guideline that boiler owners could use for setting a flag or benchmark thickness in assessing tube wall measurement data was developed by B&W in 1985. The guideline, released as B&W Plant Service Bulletin PSB-26, Tube Thickness Evaluation Repair or Replacement Guideline, was intended to help boiler owners by giving them guidance, while not being overly conservative.

Summary

Extending the life of industrial boilers which operate at lower temperatures and pressures is a viable option to support plant steam production needs. The boiler fitness survey has proven to be an effective program for determining the current condition and remaining useful life of aging industrial boilers. Extensive visual examination by an experienced B&W Field Service Engineer complemented by NDE allows comprehensive assessment and provides the owner with the information needed to make long range decisions regarding the boiler and steam systems.

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