Flashing thermal relief PSV

[fdomin]

I have to protect the tubeside of a heat exchanger for tubeside thermal expansion scenario. Tubeside fluid is crude at 400ºF and shellside fluid is atm. tower bottoms at 590ºF.

PSV set pressure is 425 psig and at that pressure saturation temperature is 565ºF.
Do I need to calculate the PSV to handle vapor-liquid mixture? The valve will open before the crude reaches ATB temperature but if trapped crude heating continues will it vaporize and a liquid service PSV would be insufficient?

 

[djack77494]

I would anticipate heating your crude to 565F for a blocked in case. When that happens, your PSV will open and you'll discharge a flashing liquid. As the trapped inventory decreases, you'll switch to discharging vaporized crude. So, the short answer to your inquiry is that you must consider both flashing liquid and vapor flows.

 

[fdomin]

djack77494,

thanks for your help.
Should I also consider 10% overpressure?
In that case pressure would raise to 468 psig and crude saturation temperature at that temp is 600F, then the relief would be liquid only.
As you can see the case is in the border of liquid only/two phase relief.

 

[djack77494]

fdomin,
My answer remains the same - you must consider both flashing liquid and vapor flows. In describing what physically happens during a relief event, yes, your tubeside pressure might build up to 110% of the design pressure (and still be within code limits). If your system is built so that this could happen, then your worst case relieving condition would be at 468psig & 600F. Heat of vaporization, coefficient of thermal expansion, and physical properties would then (always) be calculated at those relieving conditions.

 

[JimCasey]

Agree with djack. Evaluate for flowing pressure of set + 10%.

You probably know more about crude than I do, but I would operate under the assumption that the volatiles would be flashing at practially any temperature. Crude is not a single chemical but composed of some fraction of everything from CH4 to C99+