AlbertaMecchie
Mechanical
Our client has a 2-1/2" threaded carbon steel 200 WOG rated valve on a piping system that they want to use for low pressure utility steam. The system has not been used since installed. Maximum steam pressure in the system would be 15 psig at 250 deg F, and it's downstream of a small 700 lb/hr gas fired steam boiler - piping is protected by the 15 psig boiler outlet relief valve. I am assessing the piping system to the requirements of B31.1 Power Piping code. The 200 WOG rated valve as installed is not sufficient, as I don't know if that particular valve has any steam pressure rating. The only replacement 2-1/2" valve I have found locally that could work is a Toyo Bronze or Brass 206A threaded gate valve that is rated to 125 psi for saturated steam at 353 deg F. So the valve would be sufficiently rated for the steam pressure and temperature, however because this would be for installation into a carbon steel piping system (A53 Gr. B), I was wondering if there would be any potential issues with galvanic corrosion. Section 124.7 Nonferrous Metals of B31.1 code states that nonferrous metals may be used in piping systems under the following conditions - 124.7(b) The designer shall consider the possibility of galvanic corrosion when combinations of dissimilar metals, such as copper, aluminum, and their alloys, are used in conjunction with each other or steel or other metals in the presence of an electrolyte. My understanding is that saturated steam is not considered an electrolyte. The threaded areas of the connection would not be exposed to moisture other than normal ambient air humidity. Any experience / suggestions on this would be appreciated. I would consider installing a threaded brass valve, but if experience is that this a problem to put on a carbon steel system, I would advise them to replace this with a 2-1/2" Class 150 flanged carbon steel (A105N) valve rated for steam use.
