Pros and cons of Al insulation cladding 2

[robsalv]

Hi ya folks.

We have two approx 50m tall cyrogenic distillation columns which we've been recladding on a 10-15yr life cycle with galvanised sheet material because of corrosion. These towers bear the brunt of the prevailing wind.

We're some 4 - 5kms from a bay with the prevailing wind being from the bay, and the towers are down[prevailing]wind of 4 boiler stacks [4 smallish industrial D type 15m high boilers]. The boilers burn a concoction of natural gas and byproduct stream fuels resulting in probably highish SO2/SO3 emmisions. The stacks are some 100m from the towers.

We're in Melbourne Australia, so we have a moderately variable environment.

I've recommended that our towers be recladded in Aluminium this time on the assumption that this will result in a much longer life. My feeling is that the prevailing corrosive environment will be related to the airborne salt and not the stack emmisions. The cladding contractor has raised concerns about the sulphurous emmisions from the boiler stacks and has said that in his experience, Al cladding reacts vigorously to sulphur.

I can't find any reference to agressive attack on Al in an environment that includes ppm levels of sulphur dioxide/trioxide UNLESS the aluminium has certain alloying elements. Still he is convinced that we're making a poor choice and is recommending stainless cladding. Thing is, everything I read tells me that stainless is about as resistant to the sulphurous environment as Al cladding.

The only real precaution that leaps out at me in regards to Al cladding is the material of the fixing screws. It needs to be galvanically compatible, or else we'll have problems in no time flat.

So, I guess I'm looking for your considered thoughts and opinions on the above. Fire away folks.

Cheers. TIA.

Rob

 

[unclesyd]

We have a very large plant in a similar enviroment that you are in. We had used both Aluminum and S/S in about equal amounts due environments . All screws and banding were S/S on both coverings.
All new are replacement coverings are S/S. We were beginning to see a small corrosion problem on the Aluminum covers. Even though a lot of this could be from poor product or installation the decision to change to S/S was encouraged by other factors.

In the 80's I was privileged to a report of large fire an another large chemical plant in Texas. The fire was a minor hydraulic oil fire that progressed into a major loss. The expansion of the fire was attributed to Aluminum coverings melting and exposing expanded Calcium Silicate pipe insulation banded with S/S straps to the fire. The insulation failed and exposed the line to the fire and it failed.

As all our new construction was using Al coverings and S/S bands we deemed it prudent to change the coverings to S/S.

We had done quite a bit of work with Calcium Silicate insulation as were large consumers. The Calcium silicate insulation had no strength when heated above 400̊F. We duplicated the results of the report at our fire training grounds. The Aluminum Covering did as expected and S/S held up even when we hit it with a fire hose.

The only place that Al coverings are used now is on the primary Steam Lines were we are cover the existing insulation with mastic/chicken wire lagging.

We also have two areas where Al had corrosion problems.
The areas involved the use of HNO3 and HCl. In both area the corrosion was slow to start but progressed quite rapidly one it started. The emissions were very low but enough to start the corrsion.

The clincher for the use of S/S coverings and strapping was a fire we had. We duplicated the results of the Texas fire. We didn’t have the same line failure as the insulation was asbestos with both AL and S/S coverings. The Al melted along with the mastic, but the S/S tie wire held the clam shells on the piping. The S/S coverings were not phased.

I think S/S coverings, bands, and screw are the best and in the long run be the most economical.