Pressure Reliefs for Heat Exchanger 2

[spud22]

I have a shell and tube heat exchanger, designed to handle a glycol solution at 200 Deg. F on the shell side, and water at 190 Deg. F on the tube side. Both flow paths have isolation valves on their inlet and outlet piping, so they could be isolated and thermal expansion of the liquid come into play.
My question is whether a pressure relief valve is required, and if so, what formula to use in its sizing?
Thanks

 

[BigInch]

Anything subject to thermal expansion that can be isolated should have a relief valve.

If it is only to be for thermal expansion, the expanded volume (liquid, vapor, or both) must be relieved within the time taken for the temperature rise to occur.

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[davefitz]

first of all, shell and tube HX's may also need a shell side pressure relief valve sized for the case of one tube breaking and discharging thru 2 ends of the failed tube, and this is likely to be a larger relief than a thermal relief.

Secondly , one could estimate the size of a thermal releif valve as follows:
volumetric capacity, dV/dt =m*{dv/dT},p * dT/dt
where m= mass of liquid initially in tubes at lowest practical temperature

{dv/dT},p = partial derivative of specific volume with respect to temperature at constant pressure- see thermo book

dT/dt= max rate of change of fluid temp due to heat transfer from shell to coldes practical fluid in tubes= q,max/(m*Cp)

 

[vpl]

Several years ago, I was doing an in-plant inspection. The utility was in the process of replacing some heat exchangers, because the old ones had rusted away. I noticed that the new design had eliminated the relief valves. When pointed out to the utility, their reply was "oh, these will never be isolated." A couple weeks later, they suffered a loss of power event (remember the big blackout in the northeast USA and parts of Ontario?); some automatic valves closed and the utility's brand new heat exchangers looked like a bunch of pretzels. Mega bucks and time to fix them. Of course, the utility decided they still didn't need any relief valves.



Patricia Lougheed

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[spud22]

Thanks for the responses. Regarding VPL's comments on the heat exchangers, did they fail because of pressure or vacuum? With liqiod to liquid heat exchangers and limited temperature available, it seems that release of a small amount of liquid would relieve the pressure. Not good, but not a pretzel condition. Anyway, thanks for the comment. Life's definitely a learning experience.