Vibration Issues in Boilers & Furnaces

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There’s a whole lotta shaking going on…. but why?
Vibration issues in boilers and fired heaters
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:
  1. Vibration can be caused by a single component of the system or the interaction of multiple components.
  2. Where vibrations manifest in the system may be far away from the source.
  3. “Identical” units do not exhibit the same behavior (i.e. one may vibrate and the other does not).
  4. Vibrations may come and go with almost imperceptible changes in things like wind, temperature, humidity, barometric pressure or a combination of these conditions.
  5. 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 150ppm 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 30ppm low NOx burner, which now must be kept between 10% and 40% excess air, or a 9ppm 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 cleanup 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 center 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 optimize 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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ERWIN PLATVOET
As CTO of XRG, Erwin is a true innovator, whose career spans more than three decades in heat transfer and combustion industries. Erwin is a graduate of Twente University in the Netherlands with a MS in Chemical Engineering. Erwin has served the industry around the globe in a variety of roles including Research and Development Engineer, Cracking Furnace Specialist, and Director of Engineering, and now CTO. Erwin holds eight patents in fired heat transfer and emissions control technology, has published numerous papers, and co-authored the John Zink Combustion handbook and Industrial Combustion Testing book. Erwin has been an active member of the API 560 and API 535 subcommittees and taken an active role in revising these standards.
BAILEY HENDRIX
Bailey graduated from Oklahoma State University with a Bachelor of Science in Mechanical Engineering. Upon graduation, she joined the private sector as an Applications Engineer in Tulsa, OK at a local combustion company where she managed the sales activities for the process burner refining market. She quickly accelerated her career, becoming the Refining Account Manager responsible for all business development and sales of process burners in North and South America. Her strong leadership skills and interpersonal qualities led her to a position as the Western Hemisphere Sales Director for the process burner business, leading a group of sales engineers in the areas of new equipment, retrofits and burner management systems. Her financial and commercial acumen drives the success of XRG Technologies’ business development.
ALLEN BURRIS
Allen’s background includes 10 years of experience in designing and selling process burners. Allen is a graduate of Oklahoma State University with a BS in Mechanical Engineering and is a licensed professional mechanical engineer in the State of Oklahoma. His knowledge and superior customer focus led him to a career change to process design, custom-engineered fired heater sales, and associated sub-systems for the petrochemical, refining and NGL industries. With more than two decades of experience in the combustion and fired heater industry, Allen has what it takes to overcome challenges associated with complex projects and possesses.
TIM WEBSTER
With over 25 years of experience in the combustion industry, Tim brings a wealth of industry experience and technical expertise to XRG. Tim graduated with a Bachelor of Science in Mechanical Engineering from San Jose State University and received a Master of Engineering from the University of Wisconsin. Tim began his career engineering custom combustion systems for a wide range of applications including boilers, heaters, furnaces, kilns, and incinerators. Tim is a licensed professional mechanical engineer in the states of California, Texas, Louisiana and Oklahoma, has authored numerous articles and papers, and has co-authored several combustion handbooks.
matt martin
As the Lead Scientist at XRG, Matt has over 30 years of experience in the combustion industry. He specializes in CFD of fired equipment, including UOP platforming heaters, burners in process heaters, thermal oxidizers and flares with over 300 simulations of installed, field-proven equipment. Matt received a Bachelor of Science in Computer Science with a minor in Mathematics from the University of Tulsa. He has written numerous publications, is listed as inventor or co-inventor on 27 patents and was awarded the title of Honeywell Fellow in 2011 for technical excellence and leadership.
gina briggs
Gina is a native Oklahoman and attended the University of Tulsa, graduating with a BSBA in Accounting. She is a Certified Public Accountant and Chartered Global Management Accountant. Gina began her career with the Tulsa office of Deloitte Haskins and Sells, providing audit and tax services. Since leaving Deloitte, she has held CFO positions with privately held companies in the manufacturing, construction and distribution industries. In 2013, she began a consulting practice providing contract CFO services to companies, one of which was XRG and joined XRG as CFO in 2019. Gina has always enjoyed working in the small business arena, helping business owners to profitably grow and manage their businesses.