rayz,
I'm sorry but I’m a little confuse here and maybe this will be a little bit out of discussion. First question came over of my head IS DESIGN TEMPERATURE ALWAYS THE SAME WITH MAWT?
Design temperature can be same as MAWT but shall not exceed.
A) MAWT
Here we must be a bit careful when we say MAWT.
MAWT for tank is subject to vessel/tank wall thickness... MAWT for a flange, instrument, attached to it may be much lower than the MAWT of tank.
We shall always refer MAWT for SYSTEM instead of a equipment / instrument.
Why design temperature ? Why MAWT ?
This main due the MAWT for equipment, instrument, piping, etc are different. However, ALL MAWT shall equal or more than the design temperature. Design temperature is what process demand, MAWT is what the equipment / device can take.
B) Coincident design pressure and pressure
Another concept we shall keep in mind is COINCIDENT design pressure & temperature.
In many cases, the fluid is operate at high pressure but low temperature (e.g. 16 barg with 40 degC). Process engineer may specify design condition of 18 barg @ 70 degC. However, the equipment shall be steam-out with LP steam (saturated steam 3.5 barg @ 150 degC). Thus the design condition shall be
Condition 1 : 18 barg @ 70 degC
Condition 2 : 2 barg @ 180 degC
The equipment, instrument, flange, piping, fitting, etc within the system shall be good for BOTH conditions BUT NOT necessary WORST of BOTH.
Carol2005,
Your system is exactly fit into the Coincident design condition case. (high P @ low T and low P @ high T)
BUT you shall always keep in that after regenaration, the vessel is still HOT. You SHALL NOT pressurize the system again before the vessel is cold below 500 F. Your system may experience coincident high P & T.
As mentioned earlier, for fire case, regardless what design temperature you set, the tank will eventual fail due to external fire. Additional protective measures shall be considered. Read more in
"Protective Measures against FIRE other than Pressure Relief Device (PRD)"
Hope this make sense.