Boiler Blowdown Buried Piping
Boiler Blowdown Buried Piping
(OP)
At my plant we have 250 horizontal feet of 6"nps Boiler Blowdown steel pipe that is suspended in 14"nps transite (cement sewer) pipe, surrounded by calcium silicate insulation and buried under 5 feet of soil between manholes.
I say 'suspended' because 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 (from under insulation corrosion during periods of wet boiler outages).
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 say 'suspended' because 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 (from under insulation corrosion during periods of wet boiler outages).
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?





RE: Boiler Blowdown Buried Piping
Independent events are seldomly independent.
RE: Boiler Blowdown Buried Piping
We have continuous blow-off that maintains the steel pipe at +212 degF and intermittent blow-off every 4 hours at 110 lbs/sec, which could raise pressure and temperature to +300 degF, which should drive off humidity in the annulus between transite and steel pipe. In this very dry environment, would you have different concerns?
RE: Boiler Blowdown Buried Piping
Independent events are seldomly independent.
RE: Boiler Blowdown Buried Piping
RE: Boiler Blowdown Buried Piping
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" getting a foothold. The transite pipe is in excellent shape and only localized, external wall thinning has been detected on the steel pipe.
I would appreciate hearing from anyone familiar with the technology.
RE: Boiler Blowdown Buried Piping
Independent events are seldomly independent.
RE: Boiler Blowdown Buried Piping
If supports are damages these have to be replaced and I would recommend to address a pipe-support supplier with this question and also let him make necessary loading calculations to determine suitable type of support.
With regards to moisture under insulation (CUI risk). If also calcium silicate insulation is damaged you need to consider replacement with an insulation system that allows water/moisture drainage. E.g. drainplugs. Remember water ingress can’t be avoided. Under the conditions you describe you never get a 100% vapour tight insulation system.
Regards,
Johan Sentjens
www.linkedin.com/in/johansentjens