By Drew Robb, Freelance Writer
Heat recovery steam generators (HRSGs) are an excellent way to capture more value from the fuel budget. But achieving the most from an HRSG requires that they be operated as efficiently as possible. One potential weak point, for example, is where pipes enter and leave the boiler casing, resulting in hot spots, thermal loss and even unplanned shutdowns.
“We have experienced several cracks caused by overheating in our bottom panels and excessive thermal growth,” said Neal Holden, combustion turbine engineering manager for Progress Energy in Raleigh, N.C. “That is a constant battle: limiting energy loss and keeping the casing temperature down to a reasonable level.”
Figure 1 Seal Detail
Seals can be disassembled and repaired without cutting metal piping or welds, eliminating the need for specially trained welders to install replacements.
This area is particularly troublesome as units designed for base load operation are now cycled frequently.
“Startups and shutdowns can be very detrimental to the life of the plant,” said Tony Thompson, director of engineering for Vogt Power. “The difference in the thermal growth rates between two materials can cause leakage in the joints and seals and could cause customers to shut down.”
To address those issues, Progress Energy, which owns 21,000 MW of generation capacity in the Carolinas and Florida, recently installed a new generation of boiler penetration sealsslider sealson three of its HRSGs. The result has been significantly lower energy loss, while lessening the maintenance load and risk of unplanned outage.
“Before, we were constantly worrying about the area, viewing it with thermography and continually maintaining it,” Holden said. “Now it is less of a worry. You still have to keep an eye on it, but you can plan it into your scheduled maintenance intervals.”
Cutting Cycling Stress
HRSG manufactures are making numerous changes to their designs to help them deal with the stresses of frequent cycling. Vogt has developed methods to retain heat in the boiler to minimize thermal stress and reduce startup time. Rather than the more than two hours it would take to go from cold start to full load, for example, keeping the water hot cuts that time in half. Alstom, on the other hand, has a once-through design that eliminates the high pressure drum, allowing for a 25-minute cold start and also a design called the OCC (optimized for cycling and constructability) horizontal HRSG, which uses single-row harps and thin-walled header pipes.
“By using thin pressure parts in the header areas, thermal stress is less than half of what it would be in a typical HRSG so it survives frequent shutdowns and cold starts,” said Tom Mastronarde, Alstom’s chief engineer for Global Technology Development.
Both companies are helping their customers eliminate flow assisted corrosion caused by water impurities and water temperature issues. Some operators are running their GTs at minimum capacity during low demand periods, rather than fully shutting them down to reduce start-up time.
“The most common operating problem in the last two to three years has been customers trying to turn the GTs down as low as possible,” said Thompson. “The boilers were never intended to be turned down that low.”
Excessively low temperatures can cause pitting of the headers and tubes, as well as impact damage to the blades in the steam turbine if there is a high level of dissolved silica in the spray water. Thompson said these problems can be addressed by redesigning the boiler or adding an additional attemperator. New boilers, he said, are capable of operating down to 50 or 60 percent of load.
Closing the Gaps
Ensuring safe, reliable HRSG operation also requires minimizing temperature gradients around the boiler casing. Internally, the exhaust gas will evenly heat the metal. The problem occurs at those points where a break in the casing allows the water pipes to enter and exit. In the case of the Nooter/Ericksen HRSGs used by Progress Energy, this meant eight 10” to 12” penetrations for a single column. Thermal expansion results in up to 7” vertical movement and 3/4” to 2” lateral movement. As a result, the penetration seals need to be able to accommodate that amount of movement within the limited space available underneath the HRSG.
For such boiler penetrations, traditional solutions include using a slip joint packed seal, a fabric or a metal bellows seal. Each has its advantages and disadvantages. Slip joint seals allow the pipe to expand and contract and do a good job initially sealing off leakage. However, they also require constant adjustment. What’s more, leakage increases as the seal wears. Furthermore, since they cannot be insulated, they can get too hot to be adjusted except during shutdowns.
Figure 2 Diagram of a Sliding Seal
Source: Expansion Joint Systems
With fabric and metal bellows seals, the seal is welded to the casing on one end and the pipe on the other. This means the seal is flexible enough to accommodate any thermal growth or vibration and provide a more complete seal than the slip joint seals while also accommodating lateral, axial and angular movement. With bellows seals, there is a tradeoff between strength and flexibility. The thicker the material used, the stronger it is; but it is also less flexible. Fabric seals are more flexible than metal bellows, but are less resistant to heat damage, especially when operating close together, as is typical with HRSGs. In such cases, especially when temperatures rise above 500 F, metal is the only option. Initial installations can use a single-piece metal bellows seal, since these can be slid into place over the end of the pipe. Repairs and retrofits typically incorporate seals with a two-piece “clamshell” design so the pipes don’t have to be cut to install the new seals.
The problem with clamshell seals, however, is that the two halves need to be welded together once the seal is in place. This requires using thicker material than is optimal for long life.
“You want to make the bellows very thin for cycling,” said Holden. “But when you retrofit them in the field in the form of clamshells, the metal has to be sufficiently thick for a field welder to install it. That hurts the number of cycles before failure.”
Easier Seal Maintenance
Metal bellows are also susceptible to damage during the welding process. To address these issues, two companiesExpansion Joint Systems (EJS) and HRST Inc.have developed slider penetration seals. Both incorporate a clamshell design that can be clamped rather than welded in place. The HRST seals come in sizes from 3/4” to 24” and can accommodate up to 2” lateral movement on the larger seals.
Vogt Power’s engineering team worked with EJS to develop the Penetration Slider Seal (PS Seal) which comes in standard sizes ranging from 2” to 24” and has unlimited axial and 1” lateral movement. The PS Seal uses a floating ring design with a stainless steel mesh seal. Over time, particulates get caught in the mesh, reducing the amount of gas flow.
Progress Energy started using slider seals about two years ago, hiring Titan Contractors to do the installation. The energy provider now has 48 slider sealsa mix of EJS and HRSTon three of its HRSGs. Holden said the slider seals are doing a better job of sealing off the heat than the previous seals.
“Where it welds to the casing the temperature is 200 to 300 degrees and the hot zone on the casing is very small, which is a reasonable expectation,” he said. “Previously we had temperatures on the casing as high as 600 to 700 degrees and the casing was in waves, deformed by the thermal growth.”
He said the gaskets require some maintenance on the cycling units. They could last for more than a year, but he decided to replace them annually to be safe.
“One thing that we like about the EJS design is that it was maintainable, you can disassemble it and repair the gasket,” said Holden. “I wish we didn’t have to replace them, but the crew gets better at it each time, so it is getting easier.”
And routine replacement is better than the unplanned outages previously caused by cracked casings.