Shell and Tube X changer- Automated valves 1

[Kevm99]

Guys,
I have a question which I hope you all may throw some light on.

There is a hot hydrocarbon gas at high pressure being cooled via cooling water in a shell and tube exchanger. The water is on the shell side and high pressure gas on the tube side. The cooling water system is a closed system comprising of expansion tank, circulation pump and other users.

It will be normal to install a PSV on the shell side for the tube rupture case as the design pressure of the shell is less than 2/3 of the tube. However, it is normal to also install automated shutdown valves on the inlet and outlet of the shell side (cooling water side) that closes on activation of the shutdown system (ESD or PSD). Wouldn't trapping water in the shell side be more of a problem in the event that there is hot gas flowing in the tube side (due to the possibility of vapourising the water)? Or am I missing something?

Appreciate any comments on this.

 

[Montemayor]

Kevm99:

I don’t believe you’ve missed anything at all. In fact, I would complement you for your insight and engineering practical sense. You’re doing exactly what is expected of a Chem Eng: challenge the safety logic of your process in a constructive and intelligent manner.

I have never applied automated block valves on my water cooling systems. Therefore, I do not agree with the statement that "it is normal to also install automated shutdown valves on the inlet and outlet of the shell side (cooling water side) that closes on activation of the shutdown system". Of course, one has to specifically look at the actual, real process – not just generalize – and decide if automated valves are called for in the sake of safety. I would not install the automated CW block valves unless I had to – for safety reasons. And if I had to install them, I would arrive at the same logical conclusion that you’ve deduced: the heat exchanger is converted into a process steam generator – except that it is totally blocked-in and MUST BE RELIEVED in accordance with the maximum rate of heat input from the hot hydrocarbon gases flowing in the tubeside of the exchanger.

As a side comment, perhaps there is something special about your process that requires CW block valves. Perhaps the original designers took the process decision that the worst possible tube rupture scenario would be: the tube(s) ruptures and the water side is blocked (either by accident or by mistake), therefore requiring that the PSV be sized for handling not only the gas volume – but also the accompanying steam produced by direct contact with the hot gases.

Additionally, I presume you are applying the customary 2-phase flow analysis on this application. I fear that 2-phase flow (which, as I’m confident you know) will predominate and call for an even larger PSV than normal in order to meet your pressure constraints.

I hope this experience is of some help.

 

[121202]

Automated shutdown valves are often installed on the cooling water supply and return lines of shell and tube exchangers for the simple reason that they limit the migration of hydrocarbon vapor into the cooling water system in the event of a tube rupture. Obviously the impact on plant safety has to be evaulated on a case by case basis. However, preventing migration of hydrocarbon gas between hazardous and non hazardous systems is normally viewed as a basic safety requirement and is obviously desireable.