Outlet of PSV on shell-and-tube heat exchanger

[Orsiz]

Hi all,

I'm currently looking into the design of some relief valves in our plant. During a field visit, I noticed something I don't understand regarding the outlet of a PSV located on the tube side of a shell-and-tube heat exchanger. In the heat exchanger, water is heated to 55°C by 2 barg steam. The set-pressure of the PSV is 14 barg. In my opinion, the relief case is thermal expansion of the water in the fully filled tubes when the heat exchanger is blocked in.

The PSV has a 1" inlet and 2" outlet. According to API 521 STD, a 3/4" x 1" should already be large enough as only a very small amount of liquid has to be relieved in order to reduce the pressure back below the set pressure in case of thermal expansion. The strange thing is that the 2" outlet goes up vertically to leave the building. The piping is also insulated. This looks like the PSV is designed to relief steam.

I think only a very small amount of liquid water will be relieved upon thermal expansion and no steam because a small temperature increase will already increase the pressure to the set pressure. What is your opinion? The PSV is installed in the late 70's.

Orsiz

 

[Ehzin]

I believe it could be a decent indicator it was at least expected. It isn't exactly a great indicator because it's somewhat common practice to put low point drains in even for steam system relief. I've never really looked into whether that's to handle any condensate that's generated on low relief or to prevent rainwater building up on the outlet piping but the practice is fairly common.

Thanks,
Ehzin