Can a Back Pressure valve be designed on a condensate return line to prevent flashing in the line. There is a hotwell downstream on the valve at 0 psig. The condensate is generated by 150# steam @ 360 deg.F .

Steam traps are essentially just automatic valves - they open when the see air or condensate, and close when they see steam. The amount of fluid they'll pass is dependent upon the orifice size, and the differential pressure. The orifice size is pretty much fixed. If you increase the downstream pressure, you'll lose differential across the traps, so they won't pass as much air or condensate as they once did. I think you'll have a lot of problems if you install a backpressure valve in your condensate line, especially on any applications with a temperature control valve on the steam supply, such as heat exchangers. If there is concern about the amount of steam venting from the hotwell, make sure that your existing steam traps are in good repair, are the correct style for the application, and are sized properly. A great many traps, like a lot of things in steam systems, are GROSSLY oversized.

To answer your question formally:  Yes, you can install a back-pressure control valve (or similar device) on your condensate line.  However, the obvious result will be just like TBP states: you've going to reduce your condensate capacity (if not halt it altogether) and probably cause condensate flooding upstream of the steam trap - thereby ceasing operation of some of your heat exchangers and coils.  In other words, the system will not work as designed and probably cause you all kind of process upsets.

What you're trying to achieve -condensate flashing- is laudable, but not practical.  In order to have condensate flow you must (as in all fluid flow) establish and maintain a driving force (in this case, a pressure drop).  Without a driving force, you can't have condensate flow - flashing or not.  Unfortunately, in this case of steam condensate, you will flash the instant you release the saturated liquid to a lower pressure.  That's a price we have to pay if we are not going to subcool the condensate and just deal with it as saturated.  In most all cases we don't subcool the condensate because this defeats the economies of maintaining the heat content so the boiler room won't have to consume more fuel.

What makes your proposal even more impractical is that you are going to flash anyway, as soon as the back pressure control valve opens to a lesser pressure down stream!  So what have you achieved?  Instead of flashing at the steam trap outlet, after you spend money and effort and suffer reduced capacity, you still flash at the control valve outlet.  I wouldn't do it because I don't think it will work as you would want it to.

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