Planned Outage Protocol
When personnel at Wheelabrator Inc.’s 53 MW waste-to-energy (WtE) plant in Falls Township, Penn., are ready to commence a scheduled plant outage, meticulous planning has been underway for many months, including procurement of materials and agreements with contractors.
That’s the case with all of the company’s plants, according to Art Posey, Wheelabrator senior manager of maintenance, because a high level and detail of planning is the key to successful execution of an outage.
Like coal and other types of power plants, WtE and biomass power facilities undergo several outages a year—both major and minor—and plans are dictated by several factors, including weather, electrical demand and trash flow, which is unique to WtE plants. If all goes well with maintenance, unplanned shutdowns are minimal.
For the plant maintenance team at Covanta Energy, the job is never done. When a major outage is complete, planning for the next one begins immediately, says Frank Miller, vice president of maintenance technology. Deciding the right time for an outage isn’t just a matter of choosing a random date, however, as there are many factors that need to be taken into account. “Your commitment on the power side of things is taken into consideration—whether your permit or agreement with the local utility agrees with the optimum time to do an outage—as well as economic drivers such as fuel opportunities and cost in the case of biomass plants, and also practical issues, meaning the condition of the boiler,” Miller says.
During a boiler’s years of operation, various components will need to be replaced, and their condition from outage to outage is heavily scrutinized. This includes auxiliary equipment or equipment external to the boiler such as fans, motors and conveyors, and internal boiler components including pressure parts, water wall tubes, super heaters, and various parts needed to generate steam in the boiler. “They all have a finite life and replacement has to be planned out,” Miller says.
Major maintenance done at an WtE plant or biomass plant may vary for numerous reasons, one of which may be the long-term replacement plan for a particular part or piece, and its year-to-year condition. “We monitor the boilers when they come down, perform nondestructive testing, measure wall-to-wall thickness, and the thickness of the tubes through extensive parts of the boiler,” Miller says. This helps maintenance answer the question of what immediate repairs need to be done, versus repairs integrated into the long-term plan.
Prior to the boiler being brought down, Covanta requires completion of a unique set of internal technical safety standards that must be completed, most of which are inspections.
At that point, the plant is ready to undergo an outage.
Cooling and Cleaning
Accurately deciding when to cease trash flow to the boiler can influence whether the outage begins at the agreed-upon time, Posey says, as trash has to be fully processed through the unit and the ash run out, followed by a cooling process. That takes some time, as a typical WtE plant can reach temperatures above 2,500 degrees Fahrenheit. How fast a boiler cools greatly depends on the size and configuration of boilers, according to Miller, but typically it takes anywhere from 6 to 12 hours.
WtE plants’ cooling rates vary greatly on the amount of refractory in the boiler, a cement or concrete-like material that is often sprayed onto boiler walls with a gun apparatus, or applied in the form of blocks or various-shaped tile. “Its primary purpose is to protect the tubes in the combustion zone where it does a couple of things, one of which is to protect tubes from impingements or excessive heat,” Miller explains. By capturing the heat energy from combustion, refractory works as part of the temperature profile throughout the boiler. Distribution of heat in the boiler isn’t uniform, Miller adds, as it happens more intensely in some areas than others.
Aside from the refractory, a lot of heat is still trapped in the boiler water and steam, so cool water and air are circulated through the boiler to get it down to a proper temperature for entry. Then, the outage crew begins an hour-by-hour plan that maps out the sequence of activities that must occur from the time the boiler is down to when it is generating power again, beginning with an initial inspection from outside of the boiler to gauge the amount of deslagging work required.
“You don’t enter a boiler unless it’s properly locked and tagged out, with safe entry permits,” Miller says. “These initial inspections are done from doorways to get an idea of the condition of the boiler.”
Cleaning may begin with the use of high-pressure, industrial water spray equipment to remove ash and soot, according to Posey. Standard practice at a WtE plant is to then dynamite the boiler in various locations. “The reason we do that is because fly ash is very sticky, and depending on the configuration of tubes and their shapes, it tends to stick and accumulate in various places—in the realm of tons—forming clinkers,” Miller explains.
Once clinkers are cooled off, they sometimes fracture and fall or dislodge on their own, but in many cases personnel have to help them along.
Although biomass power plants have some slagging issues, they aren’t to the extent of WtE plants, and rather than in the combustion zone, they occur in the superheater sections of the boiler. “The reason for that is because many boilers are designed as big boxes made of water wall tubes with an open path for the flue gas to pass through, but various components of the boiler may hang down in that path—sometimes called pendants—intentionally getting in the way of the flue gas to absorb heat,” Miller says.
Those obstructions cause a mixture of fly ash and sand from the biomass boilers to accumulate in the superheater areas, typically cleaned out with dynamite detonation cords, which are wound with much smaller amounts of dynamite and then wrapped around the boiler tubes. “When it’s ignited, it causes the tubes to shake violently, instead of resulting in the concussion you get from a dynamite blast,” Miller says.
In the case of both WtE and biomass boilers, employees or contractors usually finish the deslagging process by knocking out loose bits of slag and ash with long sticks. The resulting slag may be run out on the grate system or manually removed from the boiler, depending on where it is.
Overall, a good deslagging experience takes around 8 hours, Miller says, and a not-so-good experience requires around 20 hours of work. Usually, it’s completed during a 12-hour shift.
Since a boiler is considered a confined work area by the Occupation Safety and Health Administration, there are safe operating procedures that have to be followed in order to achieve a permit necessary for entry. This includes locking shut all of the steam and water valves leading to the boiler so work can take place without any chance of cutting into pressurized lines, Posey explains, as well as locking out electrical supply to fans, pumps and conveyors. The boiler is drained and vented, moving equipment including ash and grate systems are shut down, and the air in the boiler is sampled to ensure it is safe for sustained work. Once that’s been completed, permits are issued for entry and work can begin.
Permits and Inspection
The extent of scaffolding, which is typically erected in the major passages of a boiler, largely determines whether the outage is major or minor. Once it’s erected, the operations group controlling the outage issues permits to various work groups, which may be working in or outside of the boiler.
Shift supervisors usually sign off on the permits, but sometimes there are separate outage supervisors, according to Miller. “Typically, it’s dictated by how much work is going on, but operations always controls the boiler and plant environment, and maintenance performs the work, first complying with the safe work permit process.”
Permitting is not a simple process. Each work group leader has to understand how a permit is issued, what safety precautions need to be taken and what has to be locked out. Every Covanta employee or contractor working on the site is required to put a personal lock on a lock box, augmented with a key that will unlock the boiler. “Sometimes, you may have upward of 300 people working in an outage, and every employee locks himself on that permit, so it cannot be closed out until all of the locks are off,” Miller says.
Once again, planning is key, as the plan must include what permits come out first, what has to be done in what order, and what time contractors are to show up. “Permits become a major boiler activity—making sure the planner or maintenance department knows exactly the sequence of events becomes very important, as well as the estimation of time it will take to walk down the permits.”
Once permitting to access the scaffolding is obtained, typically the first internal work done is cursory inspections, or boiler walkthroughs to gain an idea of their condition. It may be what was expected, but occasionally is not. “We’ll also do extensive nondestructive testing, which can be done in many different ways,” Miller explains. “Typically in our industry, we use ultrasonic metal thickness testing, which measures boiler tube and boiler wall thickness. It’s a direct way of measuring the condition of the tube and the rate of metal loss, the primary causes of which are erosion and corrosion.”
Erosion and corrosion occur to different degrees in biomass power plants and WtE plants. At a WtE plant, anything can make its way into the waste stream, including plastics and items containing lots of chlorides and sulfides. At high temperatures, salts become very aggressive in regard to corrosion, according to Miller. “You don’t really see salt in the fuel stream at biomass plants, but they have other, less-aggressive things going on,” he says. ”[Biomass plants] have sticky and sand-like fly ash, and typically use a bed of sand in the combustion zone, so that sand also goes through the flue gas stream until it is separated. Operating for thousands of hours at high temperatures while being sand blasted causes boiler tubes to have aggressive erosive metal loss, but not to the extent of a WtE plant of a similar size.”
A key to preventing boiler tube erosion and corrosion is proper application of Inconel, a stainless steel alloy that was invented a couple decades ago. “I’ve been in the business since they first started building plants, and we fought this aggressive corrosive and erosive environment for many years trying to understand what was causing it,” Miller says. “Over time, numerous things improved, such as how we were combusting trash and our combustion controls, but one thing we continued to notice is that regular boiler tubes, which are typically just carbon steel, were very susceptible to that double whammy of corrosion and erosion.”
Using Inconel, which is welded on the outside of the tube internal to the boiler as an overlay, has extended boiler tube life significantly. “In some cases 20 years,” Miller says, adding that Covanta has a dedicated Inconel crew.
The initial erosion and corrosion inspections and overall boiler evaluation should result in a good map of pressure part conditions, most of which are typically anticipated. “Operating these boilers for 20 years, we have a pretty good idea of metal loss rates,” says Miller, who has a total of 35 years in the energy industry. “We try to predict where particular components or parts of the boiler will be at a certain time and plan accordingly, but we also have to be ready to respond to as-found conditions. They might be more aggressive than we expected, so we have to be ready for that with two plans of action: anticipated work, and emergent work or as-found conditions.”
Work and Unplanned Outages
With such a range of different maintenance and repair work to do, having a do-it-all person in-house for most waste-to-energy or biomass power companies isn’t realistic. At Wheelabrator, specialty repair contractors are utilized to perform most of the work done during an outage, Posey says. “Our employees are involved in all aspects of the outage, but are vastly outnumbered by the contractors brought in to perform the bulk of the work.” While all plant maintenance and operations personnel are involved in an outage, as well as employees from other Wheelabrator facilities and regional outage teams, contractors for water washing and/or blasting, scaffolding, testing and inspection, mechanical repairs, certified boiler repair, refractory, insulation and valve repair may all be brought in, as well as representatives from equipment manufacturers.
For a company like Covanta Energy, which owns or operates more than 40 plants across the U.S., it’s a different story. “Covanta Field Services’ primary job is to perform boiler work in most of the facilities we own and operate,” Miller explains. “It’s a 148-employee, five-crew team that travels to various plants when they are down for repair to perform the actual, physical repair work on the boilers.”
Covanta has 200 to 210 outages per year with so many plants, and CFS performs approximately 80 percent of those outages. Each plant typically undergoes two outages a year, one major and one minor. “Since we’ve pursued a very aggressive pressure part program during the last five to six outages, we’ve been able to go to one major and one minor outage a year, usually about six months apart,” Miller says. “Having a good boiler reliability program allows us to do a lot of preplanning and preparation of components on a planned basis, rather than on an as-found basis, which is very costly and time consuming.”
Still, unplanned down time does happen. “Unscheduled outages occur several times a year in most boilers,” Posey says. “Tube failures are, by far, the leading cause of unscheduled boiler outages, which typically last about 24 hours.”
Tube failures can affect other tubes in their vicinity, causing them to fail as well, so when a water or steam tube is repaired, it’s important to perform a wide inspection of the area to determine what else might have been affected. “Nobody likes to fix a tube leak and then have to bring the boiler back down after discovering a nearby tube is leaking,” Miller says.
Unplanned outages can also be caused by other component failures such as a motor on an important fan or a pressure part leak, and while they are turned around as fast as possible, any active work orders are also attended to.
Overall, outage costs vary with the size and design of the plant, and in the case of WtE plants, the annual refuse throughput. Typically, according to both Posey and Miller, around 75 percent of a plant’s annual maintenance budget is spent during outages. As plant operators and maintenance personnel continue to achieve innovation and expertise, however, that number is going down. “We have improved our boiler availability so much that the historical reasons for boilers to come down for repair—tube failures—is really dwindling,” Miller adds. “That’s really allowing us to focus on the secondary problems.”
Author: Anna Simet
Contributions Editor, Biomass Magazine
Overcoming Unplanned Outages
By Luke Geiver
The best way to handle unplanned outages or severe weather events is to be prepared before they happen. The team at ReEnergy Holdings Inc., a New York-based biomass operations company that runs 12 plants throughout the Northeastern U.S., has the experience to prove it. In the fall of 2011, the team had to deal with a record-breaking winter storm system, and earlier this year, the team endured Hurricane Sandy.
Jim White, vice president of operations, says all of his plants establish guidelines and procedures to address emergency conditions for different types of events. For weather related events, each facility’s staff will perform a safety check before a storm hits, securing loose material or equipment, clearing walk downs to avoid flying debris and verifying that shift personnel have the appropriate supplies in case they are required to stay past their shift. In some instances, a plant may have to purchase extra feedstock, backup equipment or storm clean-up supplies prior to the peak of a weather event.
“We try to minimize any potential damage to equipment,” White says, in addition to the company’s main goal of plant personnel safety. “Most of the equipment has a redundancy; there will be two of everything. As we start to see some concerns, we can energize another pump or take one offline to protect the equipment.”
When a storm is approaching, each plant team will provide input on what to expect based on past events, White adds. But that isn’t the only important communication line that should be in place. Most biomass plant managers need to be in contact with the regions Independent System Operator, the electricity grid’s governing body. “We have an obligation to make sure we are communicating with them [ISO] as far as what issues we may be having onsite,” White says. In some cases, an ISO may require a plant to decrease generation due to grid issues or the loss of transmission lines. At times, he says, a plant may have to cycle the equipment off proactively to prevent any significant damages, an action that might allow the plant to go online sooner after an unscheduled shutdown. In other cases, a plant manager may have to inform an ISO that strong winds or heavy rains are impacting production. “This is where interaction with the ISO is critical,” White says. “When we come out of these events we look at what we can tweak to our systems to make them better.”