I have a safety relief valve on a water pipe tested to 200 psi being heated by steam at 525 F, 300 psi, and 6000 pph.  I would like to determine the capacity and the pressure setting required for the valve.  The safety valve will protect against thermal expansion of the water if the flow in the water pipe is stopped but is still being heated by the steam. Does anyone have a suggestion on how best to approach this problem.

Depending on heat transfer efficiency and probable heat loss from the pipe to ambient or somthing else, maximum temperature reachable of the pipe can be determined. We are interested in the case where pipe flow is blocked and steam is still flowing, of cource. Maximum reachable temperature of pipe should be determined, but the calculation will not be simple or the result will not be very exact. If pipe is strong enough to withstand the same pressure and same temperature as the steam, you wouldn't need this calculation, you wouldn't need a safety valve.

I think contributions are very good, taking into account some very important points that are basic in engineer's heat transfer calculation; but when we take into account two-phase flow and heat transfer, things get very complicated. I'm not sure do we really need it; it can be justified only if accuracy gives you some tangible benefit.

I will try to approch from total practical side. This problem is safety problem. Safety is the most important, all other things come after that.
1.  Not only thermal expansion of water, but water pressure. When water expand thermally in this case its pressure rises because it is confined by pipe. The water pressure is what we need to control!
2. You must have some data for working pressure of water pipe. In this case you simply set safety relief valve to 1,1-1,15 x working pressure and that's it! Pressure must not come beyond it anyhow. But this is SECOND safety measure!
3. FIRST safety measure is: you must block steam inflow when water side pressure reaches some point that is above working pressure but bellow relief valve set pressure (let us say 1, 05 x water working pressure). This way real purpose of safety relief valve is to protect water pipe only from accumulated heat. It means when FIRST safety measure block the source of trouble (steam), there will be further water heating and pressure rise for a while because of thermal accumulation. This can be one moment or "little more", depending on water and steam volumes and contact area, but this moment is critical for safety because pressure still rises although you switch off everything
4. Not earlier then now thermal expansion comes into play. You calculate critical thermal expansion to find minimal opening area for relief valve. This is where you must consult standards. In my little country there is mandatory standard for this (it is called "Safety Relief Valve Calculation"; there must be some equivalent ANSI stuff).
Regards

MORE NEWS