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Underground Steam Vault

Underground Steam Vault

Underground Steam Vault

I am seeking advise about what material to use on an underground steam vault

An underground steam vault (valve pit) that is made of concrete is irreparably damaged from steam and condensate leaks.  Due to the short duration that the steam and condensate lines can be down during construction, I would like a prefabricated (factory piped) vault.  Typically these are made of carbon steel that are galvanized and coated with a mastic coal tar material or are made of concrete.  A factory piped concrete vault would have to be shipped in two pieces and delivery times are not acceptable; so this is not an option.  A cast in place concrete vault will take to long to form and cure.

The owner has rejected the carbon steel vault option as they have had failures due to corrosion.  When there is a leak, it often cannot be repaired for several months and the hot wet environment corrodes the steel vaults from the inside rapidly.  Typically the environment inside the vault will be moist and ~170 degrees F.  I do not have a soil analysis but the site is not located near salt water but there are undoubtedly some chlorides in the soil.

The vault will be rectangular so welds will be required along the edges and near pipe entries.  The piping will not directly contact the walls of the vault.

RE: Underground Steam Vault

Simply use 316 grade material. Easy,to fabricate,low on corrosion and maintenance and can stand high temperatures and pressures..

RE: Underground Steam Vault

You might want to check local building codes related to this project. Some building codes mandate concrete piping vaults.

RE: Underground Steam Vault

I think that you'll find prefab concrete manhole structures will have an asphaltic type of gasket between the sections. I can tell you from direct personal experience, that these gaskets DO NOT like hot environments, like those in by steam manholes. Any kind of plastic or asphaltic coating on anything to do with steam will die a quick and ugly death. If you're trying to work in a downtown area, you'll routinely hit old, unlocated services that are still IN service. A prefab structure leaves no real way to jockey around these. Poured in-place structures provide quite a bit of play.

We found that serious spalling of the concrete manhole structures was due to corroding rebar. We minimized the rebar, and kept it back from the inside surface of the structure as much as possible.

We took over a very old & neglected steam system, and rebuilt a number of structures "on the run", without many problems. Typically, it was a mechanical "do-over" as well. We'd have the new expansion joints, valves, etc on hand, and prefabbed as much as possible. The digging contractor would arrive and jackhammer the old structure out of the ground, put in the shoring, and pour the floor (with a sump - the old ones had no sump). We'd shut down the steam at 11 PM Sat night, and get the piping work done by 7 AM Sun. We'd temporarily anchore the expansion joints, and bring the steam back on. The digging crew would arrive early the following week, build the forms, and pour the walls & top. We'd tidy-up the mechanical, he'd set the manhole rings, finish backfilling, and pave it.

If all we had to do was the structure, the steam was never off.

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