Saturday, May 30, 2009
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A pressure vessel or system expose to external fire, entire system may be isolated automatically by a plant wide emergency shutdown system (ESD) and emergency depressuring system will be initiated to depressure system to safe level and evacuate the inventory from high risk area (expose to external fire) to disposal system. Depressuring system is known as one of the most effective measures against external fire risk. Besides, there are others measures as discussed in "Protective Measures against FIRE...".
Pressure relief device (PRD) may not protect pressure vessel or system from external fire. Having said that PRD may serve in some circumtances to "buy time" for operator action. Besides, design code and/or local regulation demand a PRD in pressure vessel or system as ultimate protection.
One may have process simulation for normal production system. The stream for vessel or system expose to fire is at normal operating pressure and temperature. How shall one can adjust the process simulation to bring it up the relieving pressure ? One may consider a constant density method.
Constant density method
Pressure vessel expose to fire will be isolated and settled-out. Read more in "Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS". Assuming no credit for automatic depressuring system and operator intervention, the inventory trapped in the pressure vessel will remain as trapped vapor mass (MV0) with vapor volume (VV0) and liquid mass (ML0) with liquid volume (VL0) at normal pressure (P0) and temperature (T0) when ESD is just initiated. Total trapped mass (MT0) will be MV0 + ML0 and total trapped volume (VT0) will be VV0 + VL0. Mix denstiy will be MT0/VT0.
Trapped inventory will be heated up with external fire. Heat added into the trapped system will cause temperature rise, more liquid vaporise and pressure rise upto relieving pressure, before pressure relief device is popped open. At relieving condition (relieving pressure and temperature), trapped vapor mass (MVr) with vapor volume (VVr) and liquid mass (MLr) with liquid volume (VLr). Total trapped mass (MTr) will be MVr + MLr and total trapped volume (VTr) will be VVr + VLr. Mix denstiy will be MTr/VTr. As there is no inventory evacuated from the system, total trapped mass (MT0) and volume (VT0) at normal condition will be same as trapped mass (MTr) and volume (VTr) at relieving condition.
MT0 = MTr
VT0 = VTr
VT0 = VTr
Similar mix density (MT0/VT0) will remain same.
MT0/VT0 = MTr/VTr
This is commonly known as constant density method.
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