A: You should not use a steam radiator valve on a water radiator or water on steam. They look the same from the outside but the seal design and material is different. Many plumbers will tell you it doesn't matter but thet two products are different. If you for example to to web site of Legend Valve and look a the specification sheet for the two different styles of vavles, you will see the interal designs are different. I assure you that if the valve industry could get away with one vavle that would work in both appliacaitons they would not make two different one. If you are going to go through the trouble of changing one which involves getting the spud out of the old radiator you might as well do it right.
A: That is highly unlikely and further would say it could only happen if you are changing your valve with the exact same valve from the same manufacturer. Valve designs may look the same but vary manufacturer to manufacturer. In fact you would also need to change the spud that goes into the radiator that attaches to the valve, as that is not a standardized pattern. Getting the old ones out can be very difficult as the steam can often cause that portion of the valve to fuse to the decaying iron. Typically you need a spud wrench and a long pipe wrench. Often I heat the brass spud with my acetylene torch causing it to expand. Then when it cools it comes out much easier.
A: Your decimal points are in the wrong place. Residential steam heating boilers and systems should operate at .2 PSI (point 2 PSI) and have a maximum pressure of .5 PSI (point 5 PSI which can also be expressed as one half pound of pressure) They should NOT operate at 2 or 5 PSI. That is wrong and dangerous. As a point of reference, the steam pressure in the empire state building is only 1.5 PSI. If one and a half pounds of pressure can get steam to the top of a 102 story building you would not need nearly anything close to the pressures you are indicating to move steam around a residential home. In fact a well maintained residential steam heating systems do not build pressure at all. The pressures you are suggesting are dangerously high and would only be used under the supervision of a qualified licensed plumber or stationary plant engineer for the purpose of cleaning and decontaminating the system. The system should not be allowed to operate at those pressures unsupervised. I will assume that this was just a simple description mistake. However if not, I would turn your pressure down and call in a qualified licensed professional. Requiring pressures that high would suggest that you might have a blockage; this could be caused by scale contaminate, trapped returning condensate, a bad main vent or a grossly oversized boiler. Steam moves extremely fast even under no pressure what so ever. If you are building pressure something is blocking its path. This could be a physical obstruction or that the air in the pipes can not get our out of the way. "Hight Pressure" residential steam heating systems, were phased out in the 1860's with the creation of IBR, the Institute of Boilers and Radiators and HSB the Hartford Steam Boiler Inspection and Insurance Company. If you have been erroneously operating at those pressures it is not the pressure that will therefore ruin the vents or valves, it will be the contaminant and condensate that those excessive pressures have driven into them. Steam moves incredibly fast even with no pressure you do not need to build pressure for it to move if the system is running correctly.
A: The outside diameter versus inside diameter thing gets very confusing. Plumbers will refer to pipe by its inside diameter and HVAC professionals will call it tube and refer to it by its outside diameter. This means that what a plumber will call 1/2" copper pipe an air-conditioning contractor will call 5/8" tubing. The difference between the 1/2" and the 5/8" is the cumulative wall thickness. You have measured the outside diameter of what is called black iron pipe. Its actually steel pipe, but for some reason the name black iron has historically been used and stuck. This product is referred to by its inside diameter. Therefore the pipe that measures 13/4" outside dimeter is what we call 11/2" and would need an 11/2" valve, The pipe that has an outside diameter of 11/2" would be what we refer to as 11/4" and would require an 11/4" valve. Chris 978 651 3301