Q & A > Question Details
Our crude fired heater is having trouble with positive draft. The heater is the balanced type. Our initial suspicion was fouling/leaks inside the air preheater. But when the ID fan was stopped, the draft inside the furnace increased even further. Chemical cleaning of the heater radiation zone was performed. The convection bank manway was opened and inspected. No significant clogging was observed during the inspection. Any clues as to what could be the possible reason for this?
04/08/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
If the ID fan is stopped but the FD fan continues to run, the furnace will almost certainly go to positive pressure unless the flue gas is bypassed around the air-preheater.

Potential problem areas are the convection section, flue side of the air-preheater, dampers and the ductwork itself…

Cleaning the radiant section can cause fouling problems in the flue gas system. You state that no significant clogging was observed from the convection bank manways, but often these manways don’t give the ability to see enough of the convection section. I have also seen situations where the foulants restricted the flue side of the air-preheater. Have you tried opening the flue bypass around the air-preheater? Another possibility is the damper – either the bypass damper or the main furnace damper(s). In some situations, the damper position can be different to the indicated position due to loose linkages, stretched / snapped cables or even rotation of the damper on the shaft within the duct work. Finally, in particularly dirty services (e.g. oil-fired furnaces, incinerators etc), it is possible that foulants build-up in the duct work to a level that starts to restrict flow. Locations where horizontal duct-work turns vertical are the most likely places.

I’d suggest a draught survey to try to narrow down the problem area. Sometimes it is possible to get a boroscope into the system to look at the convection section, air-preheater, dampers and some points of the ductwork.

If the problem is the convection section or air-preheater, on-line cleaning companies like CTP can be brought in to deal with the problem.

20/07/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
It is not clear whether the crude charge heater is oil fired or gas fired. If there is delayed combustion one could get a different pr (draft) profile within the heater. Check the length of flame. Ideally it should be short / bushy and not lean / longer. Further, visually inspecting connection banks may not be adequate. Check the improvement in heater efficiency immediately after soot blowing. Further, assess the draft profile during start up when no fuel was being fired. Also, check the low NOx burners health, if these types were installed.
20/07/2020 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
Positive draft may be due to multiple reasons. May be you are overfiring. May be your excess air is too high. I understand that you have checked the furnace operation in natural draft mode and still the problem persists. In that case, maybe the damper is stuck. Or maybe there are tramp air leakages. All in all, a detailed evaluation can resolve the issue.
20/07/2020 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
There are a few possibilities:
1. Finned tube fouling in the convection bank (may not be visible from the manway opening)
2. Stack damper malfunctioning (partially / fully closed but indicator showing OPEN position)
3. Explosion doors/pressure relieving doors remain open (allowing fugitive air into the fire box) apart from the APH leak.