A confounding problem: Vibration issues in boilers and furnaces

There’s a whole lot of shaking going on – but why? Identifying industry trends that have contributed to this issue and what can increase the likelihood of vibrations.

Tim Webster
XRG Technologies

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Article Summary

A conversation with a long-time colleague at an industry trade show got me thinking about one of the most confounding problems that occur in boilers and furnaces: vibration. Our discussion revolved around the fact that systems today seemed much more prone to vibration problems than in the ‘good old days’. Now, I’m thinking about industry trends over the last several decades that have contributed to this issue.

Vibration in combustion systems has been studied for hundreds of years, from Dr. Higgins in the 1700s to Lord Raleigh in the 1800s and even NASA as part of the Apollo program in the late 1900s. Despite all that research, it is still difficult to predict when vibration problems will happen and how to solve them when they do. The issue is so complex for several reasons:
υ    Vibration can be caused by a single component of the system or the interaction of multiple components.
ϖ Where vibrations manifest in the system may be far away from the source.
ω ‘Identical’ units do not exhibit the same behaviour (one may vibrate and the other does not).
ξ Vibrations may come and go with almost imperceptible changes in things like wind, temperature, humidity, barometric pressure or a combination of these conditions.  
ψ Solutions that have worked in the past don’t work on similar problems.

Despite the difficulty in determining up front if a system will experience vibration problems, we have identified qualities that increase the likelihood of vibration occurring. 

The combustion process serves as a major participant in most vibrational problems. Flames generate noise across a wide range of frequencies and introduce a large amount of energy to these systems. This means that they can act as either sources of vibration, or as amplifiers for vibrations from other parts of the system. The drive to reduce NOx emissions has further increased the role that burners play in vibration problems. Gas is now the most common fuel used and, unlike coal or oil where most of the NOx was formed by the nitrogen that was part of the fuel, most of the NOx formed during its combustion is a function of the temperature within the flame (aka ‘thermal NOx’). Burners now employ a variety of techniques to lower flame temperatures, but this also negatively impacts some operational characteristics. 

Because lower NOx burners also tend to have narrower windows of operation for excess air level, they can cause vibration when operated outside of these windows. Take a conventional burner from the 1970s which may have produced 150 ppm of NOx. Although high in NOx, this burner may have reliably operated with excess air levels anywhere between 5% and 60%. Compare this with a 30 ppm low NOx burner, which now must be kept between 10% and 40% excess air, or a 9 ppm ultra-low NOx burner, which must operate between 15% and 25%. Failure to keep these newer burners in the proper ranges can result in ‘rumble’ (vibrations between 10 and 60 Hz) when the excess air is too low or ‘panting’ (vibrations less than 10 Hz) when excess air levels are too high. Systems with high degrees of variability, due to changes in fuel composition or rapid load swings, may have trouble finding control devices that respond quickly and accurately enough to keep them within this required window. In these cases, higher NOx burners and the use of back-end clean-up systems like Selective Catalytic Reduction (SCR) might be a better operational choice.

When sound waves travel inside a vessel, their speed is a function of the temperature of the gases through which they travel. When you have a very hot burner flame in the centre of the furnace and cooler flue gases near the walls, the sound waves within the furnace travel at different speeds. When low NOx or ultra-low NOx burners try to lower these peak temperatures to reduce thermal NOx formation, they often spread the heat out more effectively in the furnace making the temperatures more uniform. This creates optimal conditions for a ‘standing wave’ to form inside the furnace as all the sound waves travel at the same speed. This is often seen as a high frequency multiple of the fan speed. For example, a system with a fan operating at 60 Hz might experience a loud ‘whining’ noise at 180, 240, or 300 Hz. While this high frequency vibration may not be as damaging to components as rumbling or panting, it can still make the area around the furnace a loud and unpleasant place to be. 

The competitive forces of the market push manufacturers to continuously optimise designs and try to reduce costs, and when it comes to boilers and furnaces this generally means making them smaller. The smaller the furnace section for a given capacity, the higher and more uniform the temperatures in the furnace tend to be. Both conditions increase the potential for vibration, especially when coupled with lower NOx burners. One simple way to correlate this is by taking the total heat release of the burners and dividing it by the volume of the radiant section. For example, compare two boilers with a burner heat input of 100 MMBtu/hr. Boiler A has a radiant furnace section that is 6 feet wide by 9 feet tall by 28 feet long and boiler B has a radiant furnace section that is 7 feet tall by 7 feet high by 20 feet long. This give a furnace heat release for boiler A of 100,000,000/(6x9x28) = 66,137 Btu/ft3 and for boiler B it is 100,000,000/(7x7x20) = 102,041 Btu/ft3. While both units could be capable of meeting the performance requirements of a project, boiler B has a much higher probability of developing a vibration problem.

Historically, systems that employ forced draft fans to supply the combustion air were commonly supplied with a damper on the outlet of the fan that controlled the airflow. To improve electrical efficiency, especially when operating at lower load, these outlet dampers are now often replaced by inlet vane dampers or variable speed drives. However, the outlet damper acted like a resistor in the system, helping to prevent vibrations from being transferred from the fan to the burner, and vice-versa. Eliminating it removes a critical tool that prevents or mitigates vibration problems. Because vibration problems occur when the driving forces in a system exceed the dampening forces, the more dampening you can build into a system the better. Another area where this can be helpful is having draft control dampers in the stack or breeching. Often, these are only used when multiple units share a common stack or on very tall stacks, however, when present they can be another key tool to tune out vibration issues. 

Many of these tools, like the installation of fan outlets and stack dampers, are expensive to add in the field. Additionally, some of the selection decisions, like burner type, NOx control strategy, and furnace or boiler selection, are even more difficult to change once the equipment is in the field. Therefore, it is best to include this analysis up-front in the project. Spending a little extra to include a couple of dampers, or not choosing the smallest and cheapest unit may save you from much higher costs and numerous headaches down the line. 

A drop of prevention might be worth a barrel of cure!

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