Vertical natural draft API hot oil heater - installing chimney cap on stack

[Stefan2211]

Dear All,
We have a 16 MW vertical API heater operated with 4 upwards firing Zeeco gas burners (natural draft). Our plant is in Indonesia near the seaside and we are facing corrosion problems due to the saline environment. The stack has a diameter of 1.7 m and rain water enters the stack in case the furnace is stopped. That caused already visible corrosion in the convection heater (Can be seen from inside the furnace as rusty round shade on the convection tubes).
I am planning to prevent rain water entering the stack by installing a chimney cap on top of the stack. The open side cross-sections of the cap will have the same area as the cross section of the stack so there should be no flow restriction for the flue gases.
Any objections or comments to install such rain cap on a natural draft heater?

Thanks
Stefan

 

[LittleInch]

Any sketch or your proposal / cross section would help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Stefan2211]

Still in construction, will follow asap. Thanks LittleInch

 

[Stefan2211]

Good day LittleInch,
attached is the preliminary sketch of the rain cap. The open area is basically double than the stack outlet area. So there will be no back pressure effecting the combustion of the natural draft burners.

 

[FacEngrPE]

Check your air permit. It may have requirements for plume height. That would be incompatible with the proposed rain cap.

Suggestion - Look at a stack outlet like this:

Reference UFC 3-410-04 Industrial Ventilation For your stack the coaxial extension would be about 6.8m. On smaller vents detail A does good job of keeping water out of the exhaust duct. I have not worked on one this big.

Or install a door that can be closed when the heater is off line. A door might impact space entry ventilation requirements.

Fred

 

[Stefan2211]

Thanks Fred, plant is already in operation so modification of stack is not possible anymore. Our plant is near Singapore and we facing heavy monsoon rains. If the furnace is not in operation water enters the stack and causes corrosion in the convection heater. The plant is 100 m from the seaside so we have corrosive environment. The attached pic is from inside the furnace where you clearly see the water impact. It is basically the stack footprint. I need to study the plume height - never came across it so far.
Thanks for your input.

 

[LittleInch]

You really need to talk to a furnace vendor, but my comments are:

1) I would also build a downward pointing cone maybe a little smaller radius but maybe twice the height of your upward pointing cone. You want to do everything you can to reduce any exit losses on the end, but 2 x stack area should be OK

2) make your vertical channels about three time thicker. They will be subject to high temps, wind rain etc and will corrode much faster than the rest of the stack. Also in the event of any accidental ignition of a vapour cloud in the burners you could find your hat blow offs...

A natural vented water bath heater suffered this a few years ago when the burners didn't ignite properly, they didn't leave enough time before attempt no 2 and the resultant "bang" blew the top hat off the stack and it landed about 150m away....


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FacEngrPE]

This water problem also occurs on barge mounted boilers, and probably lots of other places. The yellow stain hints at H2SO4, likely from sulfur in fuel oil. We can not burn that in the US anymore. If you are burning high sulfur fuel, you should not expect good service life in furnace exhaust from stainless steel.

The barge boiler is where the idea for a door came from.
I know nothing about environmental emissions in your area, plume height may be a non issue.

 

[Stefan2211]

LittleInch, I was also thinking of a downwards cone for a smoother exhaust gas flow and I appreciate your comments. We still will talk to the furnace supplier once I have all your helpful advises collected. Thanks

 

[Stefan2211]

Hi Fred, we just burning natural gas,basically without any sulphur. I havent been able to set up a scaffloding yet to have a closer look to the fin tubes. It is a 6" carbon steel fin tube. Thanks for your input

 

[georgeverghese]

Have seen another design which prevents rainwater from getting in - involves a downward pointing cap followed by an upriser of a larger dia. So the flue gas first takes a dip and rises up straight through the outer annulus. And it has drain pipes at the bottom of the outer annulus to drain away rainwater. Risk of these drain holes bunging up though. Hence I would prefer the 3rd sketch in FacEngPe's post.

 

[Stefan2211]

Thanks George, we will not be able to modify the stack as it is a vertical natural draft furnace. We still will get a feedback from furnace designer.