With more-urgent issues frequently demanding staff attention, it is easy to overlook corrosion and minor structural damage on coal-conveying systems. If ignored for too long, however, small problems can develop into costly fixes. A regular inspection and repair process can help.
Material-handling systems at coal-fired power plants operate nearly continuously in harsh conditions for decades. Temperatures, moisture, overload conditions, and collisions from equipment can take a toll on structural framing. Plants can have more than a mile of conveyor structures that consist of thousands of members and connections. Keeping track of every component on a monthly or yearly basis is practically impossible, but it is possible to implement a program to identify damage before a failure occurs, and prioritize repairs to address the greatest risks.
An Often Neglected Activity
An ideal process to reduce the risk of failure in material-handling structures and maximize the life of the system may include:
■ Performing a comprehensive structural condition assessment every five years on a thoroughly cleaned system with access to all members and connections.
■ Addressing severe damage immediately.
■ Prioritizing remaining deficiencies and designing repairs in accordance with the prioritized list.
■ Estimating repair costs based on repair drawings.
■ Performing maintenance with crews working during non-outage periods.
■ Eliminating the conditions that led to the structural damage.
■ Hiring an independent testing agency to examine welded connections and coatings.
■ Having experienced painters prepare the steel surface and coat the steel according to the paint manufacturer’s instructions.
Many factors can disrupt the ideal process and raise questions, such as these:
■ Comprehensive structural condition assessments can be separated by more than 10 years. What should the plant monitor more often to avoid serious structural issues?
■ Some structures cannot be completely cleaned and will often start getting dirty as soon as the cleaning is finished. How can the need for observation be balanced with the reality of a dirty environment?
■ Not all areas can be accessed by existing platforms, reached by manlift, or viewed from the ground; therefore, scaffolding or a crane with a man basket may be required. Is the cost commensurate with the importance of the area to be observed?
■ Sometimes even severe damage cannot be addressed immediately, because of the need to develop a strategy and mobilize a team. How can the plant reduce the risk while planning the repair?
■ It is usually cost effective to group repairs in the same area of the structure instead of only addressing the highest priority repairs, because access and mobilization is such a large part of the cost of structural repairs. Can the budget support grouping high- and low-priority tasks?
■ Repairs will often seem inexpensive and simple until the cost of access, abatement, and surface preparation for painting is considered. In addition, once wall coverings and members are removed, more damage is often found. Are contingency funds available for previously unidentified conditions?
■ Conveyor systems often have no redundancy when the plant is running at peak levels, so some repairs cannot be made until an outage. Will labor be available, and at what cost?
■ Leaking pipes and conveyors that spill material can be fixed, but frequently the cause of the damage is repeated exposure to water from rain and equipment washing, which cannot be eliminated. Can repairs be designed to accommodate conditions that will occur during the remaining life of the structure?
■ Construction materials’ testing is an important way for owners to verify that the investment in repairs is not being wasted. Is there a cost-effective way to include quality assurance into the project?
■ Paint is often applied in the field with no surface preparation, making the repaired steel susceptible to rapid coating failure and corrosion. Can alternatives be found to minimize the risk?
Prioritizing the Work
When prioritizing areas to observe that may need attention, it is beneficial to consider the most common types of structural damage at outdoor material-handling structures, including:
■ Severe corrosion, leading to steel material loss.
■ Damage of members due to collision, especially columns near roads.
■ Poor drainage around the structure that causes either undermined foundations due to erosion or soil and lignite buildup that buries the base of the steel structure.
■ Buckled members, including local flange buckling.
■ Members damaged when used as rigging points.
■ Lifting lugs or other appurtenances added to structure, but not checked by a structural engineer.
■ Damage to concrete pedestals or foundations.
■ Beams or other members cut to allow access for maintenance.
Severe corrosion is most likely to occur where steel is exposed to moisture and granular material for long periods of time. Locations that need to be high priorities are partially enclosed structures, such as take-up or transfer towers, where many steel members are out of sight (Figure 1). Members damaged due to collision are often found near roads or yard areas where vehicles operate. Areas with problematic grading are easy to identify by looking for erosion problems (Figure 2). Buckled members can occur anywhere, but the members that are most critical will be those with no or little redundancy, which can be identified by a structural engineer (Figure 3). Finally, asking the operators on staff where they have seen damage is an excellent way to isolate the highest priorities.
|1. Swiss cheese. A severely corroded beam can lose much of its load-carrying capability. Courtesy: JQ|
|2. Washout. Soil erosion around conveyor structures can undermine the foundation. Courtesy: JQ|
|3. Buckling. Beams can be damaged when used as a rigging point to lift a conveyor belt or counterweight, for example. Courtesy: JQ|
Members that are subject to high loads, or have little redundancy, and are also out of sight during normal operations should be priorities for observation. An area that meets almost all of the criteria for a high-priority location is the connection between a conveyor truss and a transfer tower (Figure 4).
|4. High-risk areas. The intersection of a conveyor truss and a transfer tower is always a high priority for inspection. Courtesy: JQ|
It is important to note that the most severe damage does not always become the highest priority repair. An experienced structural engineer can determine, either by inspection or through analysis, the areas that must be addressed immediately and also work with the plant to establish a repair priority for areas that are not immediate concerns. An engineer can also advise the plant about how to shore or secure the area until repairs can be made.
Factors Affecting Costs
Many of the repairs required after observation are straightforward. Cover plates are often used to reinforce thinned members, and adding supplemental members can be either a permanent fix or a temporary solution until a member is replaced. Although material costs may be low, at some older plants, lead paint abatement can add significant cost to a project.
Frequently, the main cost factor for observations and repairs is access. Areas that can be accessed on foot (near the ground or on an elevated platform) are easy, and even a manlift is not very expensive and can provide observation access up to 140 feet. On the other hand, when areas must be accessed by scaffolding or crane with a man basket, the cost of access increases significantly (Figure 5). On two recent projects at lignite power plants in Texas, the cost of the highest priority repairs was $500,000 to $1,000,000, with more than half of the project costs going toward transfer towers and the connections between conveyor trusses and transfer towers.
|5. Hanging out. When a crane with man basket is required for observation access and repairs, it is usually a major factor in the cost of the project. Courtesy: JQ|
If it is known before the observation that an area will require repairs, scaffolding works well for observation access because it will usually be required for the repairs. When scaffolding is installed for the observations, the plant and engineer should consider expediting the repair plan to take advantage of the scaffolding while it is in place. It is also preferable to address lower-priority repairs in the vicinity of a critical repair while the scaffold is in place.
Clean and Preserve
For the majority of observed corrosion, no repair is required, and the next step is to clean and paint the steel. Many of the recommended repair projects will also involve the cleaning, surface preparation, and painting of steel members. What does it mean to clean and paint? How should the steel be cleaned, and what type of paint should be used at repair locations?
Depending on the exposure of the steel, one of two procedures is recommended in the Society for Protective Coatings Standards. For interior steel members not subject to significant moisture and outdoor steel away from the ground, a “standard surface preparation and painting” process will be recommended. The steel surface preparation for these conditions is SSPC-SP 2 “Hand Tool Cleaning” or SSPC-SP 3 “Power Tool Cleaning.” The recommended coating is a standard industrial-grade oil-based paint.
For interior steel members subjected to significant moisture and outdoor steel members near the ground (such as column bases), sandblast surface preparation and coal tar or epoxy coating are recommended. The steel surface preparation should follow standard SSPC-SP 10/NACE-2 “Near-White Metal Blast Cleaning” over a designated area to be painted at a time when there is no dust in the air. Painting requires a two-part process of primer and outer coat.
Dust shields are often placed over the bottom flange of beams in lignite-handling facilities to reduce buildup of explosive dust. When beam shields are not sealed well, water and lignite dust can accumulate and corrode the steel over time (Figure 6).
|6. Festering wounds. Beam shields can contribute to corrosion if they are not sealed properly against water and dust infiltration. Courtesy: JQ|
Beams that are most susceptible are ones that are frequently washed down or below areas that are washed down. If an area is frequently washed, then the benefit of dust shields is questionable, especially considering the risk of corrosion damage. On the other hand, in areas that stay dry, beam shields usually do not present problems.
Focusing on Quality Throughout the Project
After paying for observation and repairs, it is important to ensure that the repairs will last. Hiring a testing lab that is independent from the contractor to inspect steel, concrete, welds, bolted connections, surface preparation, and coatings is a way to verify that the installation meets the engineer’s specifications. Proper surface preparation is critical for coating performance.
Maintenance of coal-handling system structures is a daunting responsibility. With thousands of members and connections subjected to harsh conditions, and many of the components out of sight during normal operations, it is important to prioritize the areas to observe. This allows part of the project budget to be allocated to providing access to areas most likely to be damaged. An experienced structural engineer can assist the plant in prioritizing the observations. ■