Plugging Air Pre-heater tubes

[philruh]

We have a furnace with a Hamon air preheater, with 5 banks of tubes,
They are planning to blind both ends of a cast iron "tube" bank in an existing air pre-heater. The tube bank is 2'x11'x10' and they are assuming half of this volume is being blinded. The concern is that if the tube is not leaking, that the pressure will build up inside during heat up, and they want to vent the space. What size hole is needed, to keep the pressure inside the "box" under 1 psi. The Flue gas max temp is 750 deg F. Is this the same size hole required, for preventing a vacuum from forming when it cools down? Or should they just pressurize the tube?

 

[unclesyd]

Are you going to blind each tube or the whole bank with one piece of metal?
The safest procedure if you plug both ends of a tube in any heat exchanger is to pierce each tube to insure no pressure build up. If pressure does build up in the closed tube this is very hazardous to anyone who later works on the Hx. As your HX is a Air heater (gas/gas) the hole doesn’t need to be very large 1/8" is large enough for an individual tube. If you are covering the whole bank a 1/2" hole in the plates should work and don’t rupture each tube.
If someday you plan reuse of this bank of tubes and plan to plug each tube you can always drill one plug on each tube. If you do plug your tubes watch the type and use of the plugs.

 

[philruh]

The plan is to weld in plates to seal each row.
We are not sure if any are leaking at this time.
They want to be able to reverse the process if possible.

 

[unclesyd]

Watch where you are welding as some of the older Hx had Duriron (Hi Silicon Cast Iron) tubes. They will not weld in any way. The 1/2" hole should be enough.
If your project requirs welding to Cast Iron, be careful and make sure you have a good CI welder even if you have to go outside the fence.

Good luck with your project.

 

[philruh]

The tubes are bolted to a channel, and they show seal welding the plate to the channel using angle iron. (the logic for the seal weld, is so that air does not leak into the stack and condense high sulphur flue gas

What type of problems have you seen on plugging C.I. tubes.

 

[unclesyd]

If the tubes happen to Duriron they would be considered non-weldable. The intent of my post was to keep you from welding on any CI if possible. The reason for the caution on welding CI is that the most important factor is the Welder. Not all welders can weld CI even on a good day. Just a little heads up on possible traps in your project.

 

[philruh]

one of the first thoughts was to "bolt" the blanking plates to the Cast Iron, drill and tap I guess.
Then they thought about filling tubes with refractory, and decided that this would be hard to reverse, and the weight might be prohibitive.
Then the plates welded to the channels, and they went though a series of: refractory lined, Pyro-block lined, vapor barriers, coal tar epoxy, and finally settled on plain c.s. / seal welded , no internal refractory/insulation, but external insulation to keep it hot.(above the dew point)
The concerns were: a. blank the tubes, since too many tubes, were causing condensation and pluggage. b. make it reversible c. what if the tubes leak, air would leak in and cause condensation and pluggage. d. if hole in tube, flue gas would condense on the plates therefore external insulation. e. what if the tubes don't leak, heat up would cause overpressure.

Have they misssed anything.