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[Bill B]

I made a similar post on another forum; I apologize for the possible redundancy, but this is critical to me.

I have a dozen or so thermal relief cases that have me confused. The basic question is "For a liquid service, if the heat source is great enough to increase the liquid temperature to the saturation conditions at the flowing RV pressure, what is the correct basis for calculation of the orifice size"
In each case the beginning liquid is subcooled.

Examples include: (1) blocked in heat exhanger with inlet process temperature greater than the saturation temp at relief, (2) deadheaded system with blocked in pump.

Based on section 5.14.3, p.33, "if blocked in liquid has a vapor pressure higher than the relief-design pressure, then the pressure -relief device should be capable of handling the vapor generation rate. If discovery and correction before liquid boiling is expected, then it is not necessary to account for vaporization in sizing the device".

So in the most conservative case, one would compute the rate of steam generated (heat duty/Hvap) and calculate the orifice area based on the steam equations. Would another "model" to consider be flashing, saturated liquid water? This gives a much lower orifice size.

And if this flashing liquid case is the appropriate model, then would the mass rate to the RV be based on the thermal relief (cubical expansion) equations, or on the vaporization rate value?

Your comments are appreciated.

[djack77494]

I have a dozen or so thermal relief cases....
And if this flashing liquid case is the appropriate model, then would the mass rate to the RV be based on the thermal relief (cubical expansion) equations, or on the vaporization rate value?

Bill,
I have seen and contributed to several recent postings where there have been cases of "thermal expansion". My opinion is that we have become overly focused on labelling the case, and not sufficiently concerned with doing the calculations. (General rant that doesn't particularly apply to your post.)

Sizing a PSV for vapor is quite different than sizing one for liquid flow. Besides just correctly identifying the orifice size, the process or project engineer (whoever does the calcs) should also inform the PSV supplier or the expected phase(s) of the fluid.

You seem to have correctly identified the approach. Based on your description, you would have an initial liquid release due to expansion of the liquid as heat enters your closed system. After some time, the liquid's temperature will have risen to the point where it begins to vaporize. Beyond this point, assuming your PSV is mounted at/near the high point of the system, you will be relieving vapors. Note that the volumetric flowrate should be MUCH higher for vapor flow. So you can select the governing case (vapor flow) and present your data in a PSV datasheet, or, if you are more cautious, then you can provide both sets of data to the vendor, allowing him/her to choose the appropriate PSV. I favor the latter approach since it clearly indicates the phase change.