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Fabric or metallic bellows boiler penetration seals can increase HRSG output and efficiency while reducing maintenance requirements.
By Drew Robb
When people are under pressure, it’s a good idea for them to take a break and let off a little steam; it’s unhealthy to keep things bottled up inside.
With a power plant, it’s just the opposite. Profitability depends on fully capturing all the available heat. Environmental regulations limit gas emissions and lack of generating capacity margin makes unplanned downtime a disaster for customers as well as for the generator’s bottom line. This makes it essential to go with proven technology.
“As a utility you don’t want to live on the edge of technology in hopes that it will work for you,” said John Bowater, Manager of PacifiCorp’s Currant Creek Power Plant in Mona, Utah. Regulators are not keen on that, either: “They want reliability and availability and this plant has had very high availability since we came on line.”
PacifiCorp selected a heat recovery steam generator (HRSG) from Vogt Power International of Louisville, Ky., to boost the output and efficiency of its Currant Creek plant. To ensure there was no leakage at the points where the piping penetrated the HRSG’s boiler shell, Vogt used metallic bellows-type seals from Expansion Joint Systems Inc.
“When we put in an HRSG and the unit is in the warranty period, the last thing we want to do is take care of leaking seals,” said Mark Schweikhart, Vogt’s associate project engineer who oversaw the Currant Creek installation.
Penetration seals are used when a tube or pipe penetrates a boiler casing. Often, a slip joint packed seal isolates the piping from the shell or casing, allowing the pipe to safely expand and contract with thermal changes. This type of compression seal prevents the escape of heat and gas from the boiler, resulting in higher efficiency and lower emissions (Figure 1).
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These seals, however, have problems. To begin with, the packing requires constant adjustment to reduce leakage, which adds to operational costs. Over time, however, stress on the tubes causes the seals to wear, resulting in increased leakage. And since these seals cannot be insulated, the escaping heated gas causes the area to become too hot for work crews to get close enough to adjust the seals. Thus unmaintained seals continue to degrade until the boiler is shut down and the area is cool enough to be worked on. Even worse, if the seal freezes or corrodes, or if the maintenance crew over-tightens the seal, the pipe’s free movement is reduced or eliminated, resulting in potentially costly damage to the piping.
For non-critical boilers, or ones that are shut down regularly, compression seals may be an acceptable approach, although costs still are associated with their ongoing maintenance. But given the current lack of excess generating capacity, most organizations cannot afford unscheduled shutdowns. Then there are the environmental issues of harmful emissions through the leaky seals as well as the lowered plant output in relation to the level of stack emissions. These economic, personnel and environmental factors combine to necessitate a better solution to boiler penetrations.
PenSeals Data
To reduce the maintenance, heat loss, environmental and noise problems associated with conventional slip joint packing, a fabric or a metallic bellows type boiler penetration seal can be used. Manufacturers in this area include Expansion Joint Systems, the James Walker Manufacturing Company and Senior plc’s Flexonics.
Unlike slip joints, which allow pipe motion through the seal as a way of dealing with vibration and thermal expansion, the bellows and fabric seals are welded to the pipe and the casing and move as the pipe does. Since they are welded to the pipe and casing, they provide a complete seal (although some gas may leak through the fabric material), eliminating heat and gas leakages associated with slip seals. Both types of seals allow three types of pipe movement: axial, angular and lateral.
Fabric seals are made of a variety of materials layered together to provide strength, flexibility and endurance (Figure 2a). Metal bellows seals either can be single or multi-ply, depending on how much pressure they must withstand. A single thin ply has greater flexibility but less strength than a single thick ply. Using multiple thin layers combines strength and flexibility. However, because HRSG boiler penetration seals are required to withstand only the relatively low gas pressure within the casing, a single ply may be all that is needed. That is the approach Vogt takes with seals it obtains from Expansion Joint Systems (EJS).
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“The only pressure the seal sees is the gas pressure inside the ductwork, which is about 15 to 20 psig,” said Schweikhart. “We require them to design it as a one ply.”
Although no fabric material is 100 percent gas tight, fabric seals provide greater flexibility than metal seals, accommodating a wider lateral movement in the pipes. They also can be replaced without factory trained technicians. Heat, however, poses a problem for fabric seals, particularly if the seals are grouped close together. When the spacing is too tight, the fabric seals tend to react against each other or against other radiant surfaces nearby, causing premature failure. When used at temperatures above 500 F, therefore, care must be taken to provide adequate air flow to prevent overheating (Figure 2b).
“In our applications,” said Schweikhart, “certain locations are not well-suited for fabric because of the temperatures in those areas, so they don’t last nearly as long as the metal seals.” Some customers specify that they only want metallic-no fabric-seals, he said.
Types of Seals
While metallic boiler penetration seals have a similar bellows-type design, design differences exist depending on whether they are used for the original construction or as part of the aftermarket. Vogt Power, for example, uses three different types of metallic bellows seals for different parts of the boiler market. All models typically have carbon steel plates for attaching to the casing wall and the pipe-though they can use stainless steel or another alloy as appropriate-and they have 304 stainless steel bellows.
“There are some plants out there that have had some problems with stress corrosion cracking on the bellows,” said Schweikhart. “EJS has come up with a new bellows material in the last couple of years, and we feel this new material has addressed the problems we experienced with the cracking.”
All three types of penetration seals are compatible in terms of size and function. What distinguishes them is when and how they are installed, and by whom. Vogt uses the one-piece PenSeal Plus penetration seals for newly manufactured HRSGs. It uses clamshell seals for repairs and maintenance (Figure 3).
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“In the aftermarket, we primarily use the clamshell seal. Because it comes in two pieces, we don’t have to cut into the large expensive piping, but can just put them around the pipe and weld them in the field,” he said. “Whereas on capital projects-the brand new boilers that we design-we use the one piece seal that can be welded in place as the piping is installed.”
Clamshell units (Figure 3) are also used when the original seal is damaged during HRSG erection.
“EJS supplies a comparable clamshell type two piece seal to go in its place so we don’t run into any problems if something is damaged,” he said.
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Finally, Vogt helped EJS develop a new type of replacement penetration seal called the Penetration Slider Seal (Figure 4). These use a floating ring design and stainless steel flow reduction seals. While they do not provide as complete a seal as the PenSeal Plus design, leakage declines over time as particulates build up in the seal and reduce the gas flow. The advantage these seals have over the clamshell seals is that they don’t require a specialist welder to install or maintain the seals.
All Quiet
Proper boiler penetration seals reduce the noise level caused by escaping gas; perhaps more importantly, they quiet operator complaints. In fact, in most cases, the operator really shouldn’t care what type of penetration seals are on the boiler, any more than someone should care what brand of head gasket is in his car’s engine. In fact, a person can spend years driving a car without ever thinking about the head gasket.
And so it should be with penetration seals. If the seals operate properly, the operator can put his attention on other aspects of running the plant, rather than worrying about leaks or keeping the seals properly adjusted.
Vogt tries to achieve trouble-free boiler penetrations by using metallic bellows seals. As Schweikhart said, they don’t want to be called back to the site to replace defective seals. These seals were used for the two HRSGs that Vogt installed at PacifiCorp’s Currant Creek Power Plant. The 525 MW facility, located about 80 miles south of Salt Lake City, contains two gas turbines, a steam turbine and two Vogt duct fired HRSGs. The facility came on line as a simple cycle GT plant before the 2005 summer season, with the HRSGs and steam turbine added the following spring. The seals have worked as expected. Plant manager Bowater says that the HRSGs have been so trouble free that his staff can ignore them and concentrate on other aspects of plant operations.
“I could talk to you about (other systems) but not the boiler penetrations,” said Bowater. “We haven’t even been back in the boilers yet to have a look around.”
And, that is how it should be.
Drew Robb, a freelance writer based in Southern California, writes about engineering and technology.
