Tube rupture causing incompatible materials mixing 2

[acewolf187]

Hi all,

I am considering a shell and tube heat exchanger with water in the tubes at 4 barg and hot oil in the shell at 1.5 barg. In case of tube rupture, I know water will flow from the tubes into the shell containing hot oil at around 250 degrees celsius. For pressure relief of the shell in this scenario, I feel like simply accounting for inflow from a higher pressure source is not enough, and the water would most likely boil. So I will account for the relief of steam equal to the ingress rate.

What are yor thoughts and experiences on what would happen in the scenario I´ve described please?

Best Regards,
Acewolf187

 

[met11]

If you're considering this a credible scenario (full guillotine rupture), be prepared for an enormous relief device to relieve the steam equal to the ingress rate. That is how I would size this as well if I had to, just assume there's enough heat capacity on the shell to vaporize the water at a rate equal to the ingress rate, which would be worst case.

However I doubt any relief device would be able to handle the flowrate without the pressure surge damaging the equipment. A rupture disc may be able to, but it's hard to say. I'd fight on this scenario for one of two other options:

1. Just consider a small pinhole leak, which may be more credible. Consider the relief of initial oil/liquid volume equal to the volume of the steam being generated, followed potentially by the flow of steam.

2. Rate the heat exchanger and nearby piping shell side to at least 565 psig (vapor pressure of water at oil temperature). For carbon steel, this would be 300# @ 500°F. For stainless you'd likely have to go up to 600#. Depending on the size of the exchanger, this could add a little bit of cost, but it would be an inherently safer design.

 

[acewolf187]

I agree, the pin-hole leak is a far more credible scenario.

Thanks Met11.

 

[don1980]

Sizing the PSV for this scenario, even assuming just a pin-hole leak, implies that this failure is regarded as tolerated. My suggestion is that you clearly classify this as an intolerable failure. A water leak will cause a "steam explosion". Theoretically, one can defend against this by using a rupture disk rather than a PSV, but relying on a disk in a hot oil system introduces a range of other intolerable hazards.

The bottom line is that you need to assess the risks and implement the necessary preventative measures to ensure that it doesn't happen. That could include: 1) using a different (barrier) fluid rather than water, 2) performing thorough NDT on the tubes so that repairs are made before failures occur, 3) enhancing the mechanical design of the oil side. Sizing the PSV for this scenario can divert attention from the more important task of making sure it doesn't happen.

 

[georgeverghese]

What do you have as process design pressure of the HX shellside? And the process design pressure of the rest of the hot oil system ?