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Sealing Gaps on Process Furnaces and Boilers

How do gaps exist on fired heater systems and how does air ingress occur?

Furnaces or process heaters as well as boilers will have interfaces through which the process or water tubes will enter and exit the fired heater system


Most process furnaces or heaters operate the fireside under a slight vacuum


This provides an avenue for uncontrolled air ingress into the fired heater system when there are significant gaps in these interfaces

Air Infiltration in Furnaces

Which furnaces are susceptible to air ingress?

Essentially all process furnaces that operate their fireside under vacuum conditions are susceptible to uncontrolled air ingress into their systems. These would include the following:

  • Crude Distillation Furnaces or Crude Charge Heaters, Hydrotreater Furnaces, Hydrocracker Furnaces, Platformer Furnaces, Reformer Furnaces, Ethylene Cracking Furnaces, Hydrogen Reformer Furnaces, Thermal Oil Furnaces or Heaters
  • Boilers : Utility Boilers, Waste Heat Boilers, Co-Generation Boilers

Why do these gaps exist allowing for air ingress?

  • Poor initial design allowing for large or irregular protrusions and gaps.
  • Damage of the seals that were initially installed to seal these interface gaps.
  • Poor materials selection (e.g. unable to withstand the flue gas temperatures or velocities).
  • Inadequate provision for movement of tubes between the hot and cold positions. Whereby gaps are perfectly sealed when the unit is under maintenance and in a cold position, but these seals are unable to cope with the movements of the tubes when the unit is fired up and hence fail resulting in gaps appearing in these locations.

What are the key downsides of uncontrolled air ingress ?

  • Additional fuel consumption Cold ingress air that enters the fired heater through these gaps require additional fuel to be heated up. Although some of this additional heat that was put in can be recovered in the downstream sections of the heaters (e.g. convection banks, superheaters, economisers, air pre-heaters), this relies on there being sufficient surface area in the heat recovery sections to absorb the additional heat that was put in. As a minimum, the heat lost to stack in the additional air that was introduced into the system via uncontrolled air ingress is irrecoverable.
  • Sub-stoichiometric firing Most operators and automated control loops rely on feedback from the installed Oxygen (O2) analyzers as one of the inputs to optimise the air to fuel ratio mix going into the furnace or fired heater or boiler. If there is air ingress into the fire box, but above the flame and before the flue gas O2 analyser, the excess O2 detected from the ingress air causes the controls to reduce the amount of combustion air being fed to the burners and this can eventually lead to sub-stoichiometric firing in the system.
  • Excessive cooling of the Firing chamber Quenching of the temperatures in the firebox requires additional fuel to be fired to ensure the process side outlet temperatures are maintained consistently. This is more pertinent if the unit operates at varying loads, which would in turn cause the vacuum in the firing box to fluctuate as well. During periods of low firing loads, the vacuum in the fire box goes lower, allowing for a relatively greater quantity of uncontrolled air ingress into the furnace leading to relatively higher inefficiencies in the system.
  • Rapid quenching of tubes at the interfaces At the interface between the hot furnace flue gasses and the much colder ambient air that is sucked into the furnace through these gaps , a very steep temperature gradient exists on the metal surface leading to high localised mechanical stresses. In extreme cases, metal dusting is known to occur at these temperature interfaces especially in furnaces operating at extremely high temperatures, such as reformer furnaces where tube skin temperatures can exceed 800 °C.

How to overcome air ingress in Furnace / Process Heater / Boilers ?

KLAY EnerSol together with our partner Insulcon B.V. provide a number of unique and cost effective seals such as Quilted Sleeves, Multi Tube Bellows and Expansion Bellows that can mitigate the bulk of the fired heater gap issues faced by most plant owners.

Key advantages of our products are :

  • We have good knowledge of the relative movement of the tubes at the interfaces between the hot (on load) and cold (shut down/turn down) positions and how to compensate for this.
  • Our seals are fabricated with sufficient flexibility to provide a continued seal throughout the operating cycle.
  • Proper materials selection to ensure the seals can withstand the extremes of temperatures it is likely to experience throughout the operating cycle.
  • Cost effective with very fast payback on investments

We tailor make each solution for our customers making sure the most optimum and fit for purpose solution is provided. Please feel free to contact us to get more information on our innovative and cost effective seal solutions. With a well sealed furnace or fired heater system, ingress air can be kept at a minimum allowing for more energy efficient and stable operation.