Increasing Draft from a Natural Draft Stack

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By Joe Tleimat 

Is your heater failing to generate enough draft? Here are few easy things to check!

Tramp air flows into the heater through unintended openings, like sight doors, casing seams, and pressure relief doors. Tramp air increases flow through the convection box and stack, while lowering flue gas temperature. The greater pressure-drop and lower flue gas buoyancy reduce performance of the stack.

  1. Eliminate as much tramp air ingress as possible.
    • Understand how much tramp air exists:
      • Compare the radiant section exit O2 to the stack O2. In a tightly sealed heater, the values should be the same.  The stack O2 being higher than the radiant O2 is an indication of tramp air ingress into the convection section.
      • Determine the radiant O2 level at which the heater starts to show signs of incomplete combustion (high CO or combustibles). In a heater with properly balanced and clean burners in good condition with heater arch temperature above 1200°F, it should be less than 1.5% for most common process heater burner technologies.  Higher values are an indication of tramp air ingress into the radiant section.
    • Visually inspect the heater casing for signs of leaks. During a turnaround, smoke testing can identify leak points. Common areas include:
      • Where tubes pass through the heater casing or header boxes
      • Where coil guide pins pass through the heater casing
      • Observation doors that do not seal against the heater casing
      • Header box covers that do not seal against the heater casing
      • Any flue gas ducting expansion joints or slip joints
      • Bolted joints where bolting is missing or loose
      • Burners that are out of service with the air register open
      • Heater casing panels with joints that are not seal welded
      • Pressure relief doors that are not sealed
    • Look inside the radiant section while the heater is in operation. Note dark spots or seams in the wall refractory. Dark spots indicate tramp air ingress is cooling the refractory.
    • Seal all leaks. A few methods include:
      • Engineered tube seals block air leakage where tubes pass through the casing.
      • High temperature silicone sealant / caulking (rated for 500°F),while not permanent, is effective at sealing small leaks in the casing seams, where doors meet the casing, or in headerbox joints. This sealant will typically last several years.  On larger seam leaks, if welding on the casing is allowed, consider packing the seam with a ceramic fiber blanket and welding a steel cover plate to hold the blanket in place.
      • Observation doors often need worn or missing hinge pins replaced. Power tool cleaning of any sealing surfaces may also be needed.
  1. Confirm that the convection section tube fins are not plugged.
    • During a turnaround the fins can be visually inspected to identify signs of fouling or fin oxidation.
    • During operation the flue gas pressure drop across the convection section can be measured and compared to what a thermal and hydraulic model of the heater would predict.
    • Plot trends of crossover temperature and/or stack temperature over time. A crossover temperature that trends lower and a stack temperature that trends higher may indicate convection fouling.
    • Clean the fins if necessary. Possible methods include:
      • During a turnaround: water washing (with foaming detergent), miniaturized robotic hydroblasting, grit blasting, and CO2 pellet blasting.
      • During operation: CO2 pellet blasting if sufficient access doors exist. Sootblowing or dry chemical cleaning helps with light soot or ash deposits as might occur with oil firing.

Each method has benefits and limitations and should be considered on a per-case basis.

  1. Determine if there is a velocity cone on top of the stack. If so, review whether it is needed.  Some have been designed to create a 50 feet-per-second stack exit velocity.  These can consume 0.1 – 0.3 inH2O of pressure drop. It might be possible to eliminate the cone.  A dispersion analysis may be needed to determine whether the change in exit velocity has an unacceptable impact on the environment.

If the basic checks above do not provide adequate relief, consider the following to increase draft:

  1. Add stack height. This sounds easy…. until the civil and structural analysis is carried out.  Frequently the heater structure and foundation cannot accommodate the extra overturning moment associated with the wind load on a taller stack.  The dead load also increases.  In some cases, the stack height needs to be added at the bottom of the stack (larger diameter and more weight than at the top of the stack) to address the stack rigidity requirements.  This in turn has implications for ladders and platforms, instrument, and damper actuator connections, and more.  The cost to make this modification can be high.
  2. Install a steam lance in the stack, pointing the steam jet straight up. The momentum from the steam jet causes an entrainment effect.  One example involved a 60 MMBtu/hr heater with a 5’6” diameter stack. Injection of 3,000 lb/hr of 125 psig steam (with 100°F of superheat) generated 0.5 inH2O extra draft.  Switching to 650 psig steam reduced steam consumption by 30%.  The steam lance was installed while the heater was in service and a single nozzle was cut into the stack. The capital cost of this option is relatively low, but the cost of steam consumption must be considered.Some refiners use this draft as the motive force to air purge natural draft heaters before start-up.  This reduces the potential for refractory damage associated with steam purging in the radiant section.For expert analysis and recommendations on this topic, contact XRG Technologies and partner with the industry leaders in fired heater efficiency.
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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.