Feb-2023

Increasing draft from a natural draft stack

Easy methods to check if your heater is out of draft.

XRG Technologies

Article Summary

Is your heater out of draft? Here, we have outlined a few easy things to check.

1. Confirm as much tramp air ingress as possible has been eliminated.
a) Gain an understanding of 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 burners that are clean and in good condition and with a heater arch temperature above 1,200°F, this 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.
b) Visually inspect the heater casing for signs of leak points.  During a turnaround, smoke testing the heater can identify leak points. Some common areas include:
= Where tubes pass through heater casing or header boxes
= Where coil guide pins pass through the heater casing
= Observation doors that do not seal against the casing
= Header box covers that do not seal well against the heater casing
= Any ducting expansion joints or slip joints
= Bolted joints where bolting is missing or not tight
= 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 well
c) Look inside the radiant section while the heater is in operation. Note dark spots or seams in the wall refractory.  The dark spots are an indication of air ingress cooling the refractory.
d) There are multiple methods available to seal identified leaks. A few common ones include:
= Engineered tube seals – available to block air leakage where tubes pass through the casing
= High temperature silicone sealant/caulking (rated for 500°F) – while not permanent, it is effective at sealing small leaks in the casing seams, where doors meet the casing, or in headerbox joints. On larger seam leaks, if welding on the casing is allowed, consider packing the seam with ceramic fibre blanket and welding a steel cover plate over it hold the blanket in place.
= Frequently, observation doors need to have worn or missing hinge pins replaced. Power tool cleaning of any sealing surfaces may also be needed.

2. Confirm the convection section tube fins are not plugged.
a) During a turnaround the fins can be visually inspected to identify signs of fouling or fin oxidation.
b) 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.
c) There are several methods for cleaning the fins, including:  
= During a turnaround – water washing (with foaming detergent), miniaturised 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 its limitations as well as pros and cons.  None are perfect. Consideration is needed for each case.

3. Check to see if there is a velocity cone on top of the stack.  
If so, review whether it is really needed. Some have been designed to create a 50 feet per second stack exit velocity.  These can consume 0.1-0.3 in wc 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 surrounding environment.

If the basic checks above do not provide adequate relief, the following are two options to consider for increasing draft:
υ 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, etc. The cost to make this modification can be high.
ϖ    Install a steam lance in the stack, pointing the steam jet straight up. This creates an ejector effect in the stack. One example involved a 60 MMBtu/hr heater with a 5 ft 6 in diameter stack. And, 3,000 lb/hr of 125 psig steam (with 100ºF of superheat) generated 0.5 in wc extra draft. Switching to 650 psig steam reduced steam consumption by 30%. The steam lance was installed while the heater was in service. A single nozzle was cut into the stack. The capital cost of this option is relatively low, but the cost of steam consumption needs to be considered.

This effect works in a cold firebox. Some refiners use this draft as the motive force to air purge natural draft heaters before start-up. It reduces the potential for refractory damage associated with steam purging in the radiant section.

If you need help with assessing any of the above, please contact XRG Technologies. We can help evaluate any of the above items.