When John Cockerill talks, we listen. Global leader in HRSGs, John Cockerill (formerly CMI) hosted a webinar to discuss dry-run, vertical, Once-Through Boiler (OTB) technology, and why they’re poised to lead the market in offshore and land applications under 100MW.

Webinar Summary: Once-Through Boiler technology

Once-Through Boiler (OTB) technology is essentially a drumless HRSG. It runs in dry operation, eliminating the diverter damper, which contributes to a smaller footprint and lighter overall weight. Suitable to be used along with any gas turbine from 5-10 MW through 100 MW. Suitable for both land-based and offshore applications, with particular growth activity in 25-35MW range of gas turbines, used in platform, floating LNG, and FPSO. The dry-run Once-Through Boiler is lightweight, with a small footprint/design, and offers high reliability with simple maintenance and operation – a simple boiler to operate, with simplified controls.

Key Benefits

• Lowest boiler capital cost on a supply and install basis
• Allows operators to ramp up GT very, very, quickly and operate steam cycle independently of the GT
• Enables larger turn-down capability than drum systems
• Simple in terms of layout, with far less instrumentation, far less valving, and far less external piping
• Eliminates the bypass stack
• No need to maintain and operate diverter system
• Lighter weight, smaller footprint, make it ideal for 25-35 MW offshore and LNG applications
• Built using materials that can withstand corrosive attack of salt air or other contaminants.
• Significant operational benefits in terms of flexibility, and lifecycle cost reduction
• Boiler feed water (demineralized water) going into the unit comes out as steam, resulting in zero blowdown (compared to drum style units). Blowdown system is eliminated.

Simplicity of operation

The boiler is operated completely dry, with no water in the unit. All of the valves (drains and vents) are opened as the gas turbine fires. The boiler gets heated by the gas turbine. There’s a very slow ramp rate for the first section of the boiler start (5 or 10 minutes) to begin the process of steaming in the unit. Water flows in at a low-flow rate to stabilize the inlet rows and cool them. Once stable, operators can then ramp up through the start sequence. In shutdown, it’s the opposite: close off the feedwater valves, open up the drains and vents, and allow the boiler to completely boil out dry.

Speaker quote:

“When we consider Once-Through Boiler units for both offshore applications as well as land-based applications, they offer the lowest boiler capital cost on a supply and install basis that John Cockerill can provide. They also offer the lowest life cycle cost due to the simplicity of the product.

Once-Through Boilers provide real advantages in terms of your instrumentation and operation, offering a dramatic increase in flexibility by allowing the gas turbine to operate independently of the steam cycle, as well as significant turndown capability, and flexibility in terms of reacting to loads. Key for offshore applications, as well, is a small footprint and light weight, making it ideal for floating power plants and production platforms. And the last advantage (perhaps we’re tooting our own horn a little bit), but we think John Cockerill Energy is one of the strongest suppliers of heat recovery technology. We have a very, very broad breadth.”

Insights from the Q&A section:

The Once-Through Boiler is designed for oil and gas as fuels, but are there any requirements in the case of a heavier oil firing?

“We don’t require a blower or any in-operation cleaning of the tubes because the boiler dry-runs. What tends to happen is if you operate on a liquid fuel, you’ll have some soot build up on the exterior surface of your tubes. But each time the boiler is dry-run, as those tubes elevate in temperature, the build-up is burned off. It’s a bit like a self-cleaning oven where you turn your oven on, let it get hot, and it burns all this stuff out of your oven. The same example is for the OTB, let it get hot and the soot and potential buildup of material on the outside of your tubes will be burnt off and cleaned up. It’s a self-cleaning technology if you will.”

Can you clarify the demineralization system requirements?

We don’t require any specific demin system, we work with any water treatment system preferred by the vessel owner or the power plant owner/land-best based application. There are a variety of different technologies that can comfortably meet the requirement of 0.25 µS/cm. We try to stay within the standards most plant operators and vessel operators are comfortable with, and most know how to run a demin system, so we just say run your demin system but make it a little bit bigger so that you can run 100% of the condensate through it.

Can you tell us about performance/failure rates?

“There have been no issues around mean failure, repair rates, or any common failure issues. We encourage inspections each time the gas turbine is turned off during a scheduled outage, when you can inspect the two bundles of the OTB.

Most failure modes are typically related to water treatment. If you have bad water in the dry out zone of the unit, or some kind of contaminant that can create a galvanic node in the inside of your tubes, the most likely place we’ll find a failure is where there’s a transition from the evaporator to the superheater tubes. This is the most common area for failure as far as the tubes go. But it’s actually quite infrequent.”

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