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| With TYPE K, the damper drive arrives pre-engineered and assembled so it can be bolted into the ground and hooked up to existing air supply lines, electric instrument connections, and then the linkage. TYPE K handles all the engineering and assembly-work at their facility, which simplifies field labour and field supervision. |
Repeatable and accurate positioning of dampers improves thermal efficiency of furnaces, helping to lower nitrogen oxides (NOX) emissions as much as 80% and increasing profit margins by 2-8% through fuel savings. Thermal NOX results from the thermal fixation of molecular nitrogen and oxygen present in the combustion air. NOX emissions increase rapidly at peak flame temperatures exceeding 1,540°C (2,800°F) and with the increasing duration of time that reactants remain within the area of peak flame.
Because of recent, more stringent government regulations, petroleum refineries throughout Canada and the US are now forced to examine every aspect of their process-heating operations in order to reduce cumulative NOX emissions from their plants. While aging furnace burners are already rapidly being replaced with newer low NOX burners, attention is now being directed toward the furnace and heater stack damper drives.
Changing the final damper control elements to more reliable and accurate damper drives greatly improves the combustion of air and flue gases and reduces tramp air from entering furnaces. The resulting efficiency, depending on the type of low NOX burner selected and on how "tight" the furnace is for leaks, can lower emissions as much as 55-80% when used in combination with other low NOX strategies.
In 1998, the US Environmental Protection Agency established national emission standards for Hazardous Air Pollutants from Petroleum Refinery Vents, which cover catalytic cracker, catalytic reformer and sulphur plants. Some areas of the US have until 2008 to meet emissions targets, but the EPA has already teamed up with the US Department of Justice to force multi-million dollar emission-reduction deals with major oil producers throughout the US.
A Canadian damper control success story
For some consolation, US refineries can consider the plight of Canadian petrochemical operators, where air quality regulations are even more stringent. Ontario is committed to reducing NOX emissions by 50% as of 2006, according to Ian McLellan, a mechanical engineer for SNC-Lavalin Group. McLellan has worked with dampers since 1973.
"Some 90% of the drives in North America will need to be replaced because none of the old systems were designed with precise control in mind," he says. "Now we're talking about fractions of a percent control, so you can't use the old stuff." Currently working on projects at oil refineries in Ontario, McLellan explains why damper control became an important element at one site.
"Plant management did an upgrade on a large furnace, but the furnace was never able to demonstrate a stable O2 level, partly because they didn't have good repeatability control over the furnace air," he said. "Yet we couldn't manually fool around with the original system because it was controlled by some "old unreliable" actuators. They're large and they have a bit of play in them. Repeatability was probably on the order of 1.5%,hardly enough to qualify for precise control."
"The other problem was that all three traditional actuators were jamming when the towers would heat up. The refractory was falling out, hitting the damper and stopping it. The old arm-connected pneumatic actuators couldn't stand up to this. The damper shaft actuator arms were actually bending." The chief stationary engineer at the plant was already familiar with the TYPE K damper drives and asked McLellan to specify them for this project.
Installation at this plant posed a particularly difficult challenge, as a special coupling was required.
The damper drive had to attach to a duct that got up to 1,500°F. A special drive coupling was needed that would reach out into the existing damper coupling and not unduly end load the damper shaft as it expanded from the heat. The manufacturer's engineers designed a customized bracket and coupling that would allow for significant axial shaft expansion. They came up with a sliding coupling utilizing a square key-way and a heat sink. This allows the shaft to expand sideways about one inch on each side as the air stream heats and cools. Pillow block bearings were also installed to maintain support while still permitting the sliding action inside the shaft coupling. This also helped prevent the refractory from getting in the works and jamming the damper blade.
Installation of the first two damper drives was completed by the middle of December 2001. McLellan and his staff immediately noticed an improvement in damper control.
"With the TYPE K damper drives, the stability of NOX emission reductions improved tremendously," reports McLellan. "Before, the process used to wander around a lot. Essentially, we run at approximately 7% excess air, so plus or minus 1.5% meant you could swing from 5.5 to 8.5%. But now, repeatability is on the order of 0.1 to 0.25% -- a big improvement. I also expect the TYPE K to perform well for a long time, as it has a lot of robust features. For instance, it has an energized seal with stainless steel backing fingers and a low-friction coated interior, so there will be very low wear on the actuator. Additionally, air pressure is applied to both sides of the vane, so there is no backlash. The positioner basically holds the damper at desired set point without drifting."
Improved damper drive performance is significant for refineries
The successes realized at some plants across North America mean that other refineries can meet their emission targets by improving their damper drive elements. More important, the retrofitting of other NOX control strategies doesn't necessarily have to be a financial drain on petrochemical operations.
With NOX reduction deadlines drawing ever closer, operators of refineries and petrochemical plants can derive some benefit from their efforts to reduce emissions. The installation of modern damper control can actually improve product quality and safety levels, while lowering maintenance expenses and on-site man hours, even for those plants located outside of non-attainment zones. This type of "payback" casts the whole issue of emissions reduction into a much better light.
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