Many plant managers and engineers never get started with a reliability programme because the task seems so challenging. This integrated three-step process includes a System Review, Historical Review and Budget Review. Each step has multiple key components that must be considered when implementing a unit specific strategy, writes Shridhar Nambi, MD, Greenesol Power Systems.
STEP-1: SYSTEM REVIEW
Age of the unit Facilities built in the 1960s and ’70s experience damage related to the obvious number of hours of operation; however, they were designed with heavier wall thickness in both tubing and header components. As a result, these units tend to have longer life expectancies than some of the newer facilities. Facilities that were built in the 1980s pushed the limits with the do more with less approach. Tubing and headers were supplied with thinner walled components, conserving costs on construction, but ultimately reducing the service life of the critical components. Modern facilities are being constructed to adapt to the thermal cycling that has become a part of the energy culture of today and are experiencing earlier than expected failures. Many of these failures are the result of exotic materials that are being used which have not been in service long enough to know the true behavior of the material under the thermal and mechanical stresses of cycling a unit.
Design of the unit
Some boiler units clearly have inherent design flaws. Various design flaws include the placement of the burners in the furnace, how the tubing/headers are supported and/or the use of water guns or soot blowers. Understanding the inherent design flaws of a specific unit will help a company become proactive in their approach to preventative maintenance and ascertain areas to target for remaining useful life determinations.
Materials
Understanding the materials specific to a unit and recognising the inherent concerns of those materials (weld ability, resistance to elevated temperatures and pressure, heat transfer ability) will enable facilities to be more progressive in their pursuit to preventing service related damage.
STEP – 2: FAILURE ANALYSIS
The ability to identify and track the locations of a tube failure and its root cause is essential to comprehensively reducing forced outages. Once the root cause of the failure is properly identified, a long term plan can be implemented to ensure the failures/leaks have been rectified. Proper and current documentation is critical to managing failures and leaks and can be done in real-time with the use of a data management program such as the 4-SYTE System Strategy. Most common causes of failures have been seen to be stress rupture, water-side corrosion, fire-side corrosion, erosion, fatigue, The most likely failures can occur in water wall, economiser and superheater or reheater tube circuits.
Primary failure mechanisms are the processes that degrade the tube and produce a failure. Each failure mechanism may include several circumstances such as poor fuel quality, equipment malfunction or improper operation. Each would be considered a root cause since they have created the conditions for a failure mechanism to exist. Verification of the root cause is a vital activity in a failure investigation and is necessary to assure the correction of a failure problem. Secondary failure mechanisms such as adjacent tube washing or adjacent tube impact can produce a tube failure and are always a concern after an initial failure.
Modifications
At times inherent deficiencies of a unit design will be identified. As a result, the unit may undergo design modifications which can resolve the original design flaw concerns, but ultimately can create other issues such as steam flow restrictions and temperature excursions etc. Additionally, as part of the clean air initiatives currently underway, many units are being modified to burn alternative fuels. Recognising what modifications have transpired in a specific unit can lend perspective into potential side effects which may be occurring as a result of those modifications.
Replacements
As an aging plant begins to experience repeated failures, sections of tubing and other critical components will require replacement. These replaced sections will have fewer hours of operation and therefore will not need to be considered for inspection on the same schedule as original equipment within the unit. This observation is particularly unit specific and is a major basis for why a cookie-cutter approach to inspection/maintenance is ineffective and can lead to squandering of precious budget funding inspecting equipment that has not yet reached a point in its life cycle to require examination.
Operational changes
Most power generation facilities were designed on the assumption that they would be operated in a base-load mode or infrequently cycled. However, in response to local power market conditions and the terms of their power purchase agreements, many plants are now cycling their units more frequently than designers had intended. This results in greater thermal stresses, more pressure cycles, and therefore more cyclic fatigue damage and overall faster wear and degradation to the critical components due to both mechanical and corrosion processes.
As a general comment, cycling service has an adverse effect on the life expectancy of a unit. This is due to the fact that cycling results in fatigue loading (alternating cyclic stresses); whereas base load operation results in creep (sustained stresses). Depending on the severity of the stresses, and the number of cycles, fatigue loading can result in cracking, particularly at restraint locations.
Upsets
When a unit trips and is brought offline suddenly or experiences a water hammer event, an immense amount of thermal and mechanical fatigue can be introduced to the involved components. It is beneficial to understand if a unit has experienced any major upsets during its life cycle in order to determine if evaluating areas that wouldn’t normally come under the microscope is necessary. This is similarly unit specific and would be comparable to the considerations you would evaluate if you were purchasing a used car. Just as the purchaser would investigate any past maintenance troubles or collisions of the vehicle prior to purchasing, plant managers must consider the history of their units prior to determining the inspection prioritisation of their critical components.
Operational training
Often, the operators of units are responding to directives from a senior authority to bring the unit online or offline to meet the load requirements and capacity. Understanding the effects of ramping constraints in both unit commitment and economic dispatch is imperative. Operators can have a tremendous effect on the life expectancy of a unit simply by recognising the effects of proper ramp rate execution. Operators have direct control of the temperature of the unit; therefore proper unit specific training can add years to the life of the unit.
STEP – 3: DETERMINING REMAINING USEFUL LIFE
Determining the remaining useful life of critical components/tubing will allow for proper budgeting for replacements. Additionally, as systems begin to reach the end of their life cycle, more failures will inevitably begin to occur. Understanding when to cut your losses and replace sections will improve reliability. Many factors can affect the life expectancy of key components in a boiler including water chemistry, fuel type and quality, thermal cycles, materials, temperature excursions, inadequate heat transfer and flow rate. Understanding key factors associated with a specific unit that can ultimately contribute to shortening the life expectancy is paramount to predicting remaining useful life of critical components.
Prioritisation: inspection, repairs, replacement
The ability and necessity to develop a plan of action that includes prioritisation for inspection, repairs and/or replacements established from the unit specific design and historical operation will dramatically improve the budgetary process. Allotted funds will be used in an effective manner and outage planners will have the ability to provide back- up documentation required to warrant the necessity for such funding during the company fiscal budget planning process.
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