Boiler Blowdown Buried Piping

[Bambie]

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?

 

[BigInch]

I would suspect that they serve as supports, so that there is no metal to concrete contact induced corrosion. Concrete pipe should always be assumed to have it's void space (4-5%) filled with water, thus being a ready source of water to drive crevice corrosion should it come into direct contact with metal. A metal support would probably do no harm to the pipe, although some corrosion at the support's contact point with the concrete should be expected.

Independent events are seldomly independent.

 

[Bambie]

BigInch,

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?

 

[BigInch]

Yes. If that's the case, I'd be worried a lot more about restraining expansion than corrosion. I'd also avoid welding things onto the pipe. I'd also be worried about how the pipe got damaged. Put some warning markers visible on the surface above it, a side walk, or at least some warning tape buried 12" below surface level and hopefully at least 2 ft above the top of the pipe.

Independent events are seldomly independent.

 

[rconner]

Hearing the history and depending on something as fragile (and now as hard to work around for other reasons) as asbestos cement pipe for casing would give me the "Heebie-Jeebies". Any way to say parallel or otherwise replace at some point (even if by "trenchless" means)?

 

[Bambie]

Gentlemen,

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.

 

[BigInch]

So there is corrosion after all.

Independent events are seldomly independent.

 

[sentjoh]

It appear that the support were designed to avoid thermal bridges and allow axial movements.
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

http://www.linkedin.com/in/johansentjens