Outlet of PSV on shell-and-tube heat exchanger
Outlet of PSV on shell-and-tube heat exchanger
(OP)
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
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





RE: Outlet of PSV on shell-and-tube heat exchanger
RE: Outlet of PSV on shell-and-tube heat exchanger
However, external firecase relief cannot be excluded even in the case of the tubeside in most plants, since the HX channel / bonnet can still be exposed to fire case heat flux. Note heat exchangers are classified as pressure vessels and not piping.
If you can confidently say that flammable liquids with a flash point temp exceeding minimum ambient cannot accumulate beneath this HX ( from within the common kerbed area), then you could justify asking for a dispensation from having to accomodate firecase at this HX - check your national codes also in case there are some rules re firecase relief on heat exchangers also.
RE: Outlet of PSV on shell-and-tube heat exchanger
Dont know if your Company has such rules imposed / established, else my previous comment would be still applicable.
RE: Outlet of PSV on shell-and-tube heat exchanger
I agree that 1" x 2" sounds too large for a thermal expansion scenario. I'm curious if it's actually intended to provide relief for a tube-rupture scenario.
RE: Outlet of PSV on shell-and-tube heat exchanger
No flammable liquids are present near/below the HX. The only surrounding fluids are hot air, low pressure steam and water. Is a tube rupture scenario possible hear as a scenario? The tube (water) side is the HP side, the shell (steam) side is the LP side.
RE: Outlet of PSV on shell-and-tube heat exchanger
You should be able to retrieve and study the sizing documentation that led to the selection of the installed PSV, and why it is the size it is. Pressure relief device documentation should be kept and maintained for the life of the facility. Auditors ask for ours from time to time.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Outlet of PSV on shell-and-tube heat exchanger
These documents should be indeed available but they aren't for our plant (I looked in the original design documents of the plant, but no success). I could only find a datasheet with specified set pressure, relief area and construction materials.
RE: Outlet of PSV on shell-and-tube heat exchanger
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Outlet of PSV on shell-and-tube heat exchanger
From how you describe this HX, does look like tube rupture is not applicable to this tubeside PSV.
RE: Outlet of PSV on shell-and-tube heat exchanger
IMO, it seems that the PSV of 14barg set pressure could be used to protect the water tube from overpressure due to the discharge blockage.
RE: Outlet of PSV on shell-and-tube heat exchanger
In your blocked in case surely you could have superheated water at atmospheric pressure flashing of to steam as it leaks out?
Could the HX be fed with water greater than 14 bar? Without a fuller description of e system is difficult to say much more.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Outlet of PSV on shell-and-tube heat exchanger
Regarding the flashing of to steam. How can you know that the water temperature will be above 100°C at the set pressure of 14 barg? Only then, you will have flashing I assume?
RE: Outlet of PSV on shell-and-tube heat exchanger
Even if it's just v ghot water you don't want it in the building.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Outlet of PSV on shell-and-tube heat exchanger
RE: Outlet of PSV on shell-and-tube heat exchanger
Is there an alternative closed drain system?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Outlet of PSV on shell-and-tube heat exchanger
Also, what's the gpm or m^3/s, whatever units work, of the pump normally? Is there a control valve directly downstream of the exchanger? Does the steam work off a simple steam trap or level control or does it have some other form of temperature control? If you have a thermal data sheet for the exchanger, what's the designed flow of both the steam/condensate and water along with temps in/out? And lastly, how much piping is there between the pump to the exchanger and then from the exchanger to the control valve (Assuming there is a valve)? Oh, and line size?
May be a bit of information to provide but I believe there exists a scenario where the exchanger could continuously exhaust out flash steam. Depends on the design of the exchanger. If that's the case then the pipe routing makes more sense and the only issue to correct would be possibly routing of a drain line.
Thanks,
Ehzin
RE: Outlet of PSV on shell-and-tube heat exchanger
The normal water flow is 26 m3/h. There is a control valve upstream of the HX which regulates the steam flow depending on the water temperature. There is no control valve on the water side. The steam works off a simpele steam trap.
Steam (130°C) design flow: 1500 kg/h
Water (15°C -> 55°C, 4" piping, approximately 30 m from pump to HX inlet): 26 m3/h
RE: Outlet of PSV on shell-and-tube heat exchanger
If you're curious on what scenario could probably cause continuous flash steam then we could go in detail on that. Depends on if you're interested.
Thanks,
Ehzin
RE: Outlet of PSV on shell-and-tube heat exchanger
I still have to check the presence of a bleed hole in the outlet piping of the PSV. If this would be the case, then I indeed believe the original designer assumed liquid relief in the beginning. After some time (HX discharge still blocked), steam would be relieved. Do you agree?
Orsiz
RE: Outlet of PSV on shell-and-tube heat exchanger
Thanks,
Ehzin