At my plant we have 250 horizontal feet of 6"nps Boiler Blowdown steel pipe that is surrounded by 2-1/2” thick calcium silicate insulation, supported every 12’ inside 14"nps transite (cement sewer) pipe and the whole assembly is buried under 5 feet of soil between manholes.

This buried blowdown configuration has provided 30 years of trouble free service, which would have continued if insulation wetting had not occurred during boiler outages resulting in UIC "under insulation corrosion" gaining a foothold. The transite pipe is in excellent shape and only localized, external wall thinning has been detected on the steel pipe.

The supports are fabricated from several compressed fibre circular plates bolted together with semi-circular cut-outs around the perimeter. Some of the transite pipe sections and supports have been damaged during excavation to repair external wall loss of the steel pipe.
Continuous blow-off maintains the steel pipe at +212 degF and intermittent blow-off every 4 hours at 110 lbs/sec, which raises pressure and temperature to +300 degF, producing a very dry environment in the annulus between transite and steel pipe.

My question is whether to fabricate replacement compressed fibre supports or simply weld steel saddles to the replacement pipe sections.

Are these non-metallic, electrically isolating supports intended to prevent galvanic corrosion or just to prevent fretting of the transite pipe?

I would appreciate hearing from anyone familiar with this technology.