Are you one of 99% engineers who size PSV fire case the wrong way? Sizing PSV fire case seems extremely easy on the surface. You use API 521 to calculate the fire heat input by calculating the wet area, then you get the latent heat from a process simulator, finally you get the reliving mass flow and you follow API 520 to size the PSV area. Sounds easy, right?
Common Mistakes
The calculation sequence seems easy, but it is actually extremely difficult when you get into the details. Common mistakes of 99% engineers are making:
Dynamic Simulation
A proper way to size PSV fire case is to use a dynamic process simulator, since the relieving process is dynamic, not steady state. In this example, VMGSim Dynamics is selected to do this demo.
As you see on the PFD, there is one vessel and one PSV. The PSV orifice size is E with a set pressure of 500 psia. The vessel is horizontal with a diameter of 5 ft and a length of 20 ft. Initially it is at 34.5 F and 400 psia, with a liquid level of 60%. Suddenly the vessel is exposed to a fire.
Let us start the simulator and see what's going to happen.
Strip Charts
Currently two strip charts are shown. On the vessel strip chart, the black line is the liquid level percentage in the vessel. The blue line is the wet area and the red line is the fire heat input. The liquid level first increases and then drops. The reason is as the vessel gets hotter, the liquid density gets lower and it results a bigger volume in the vessel. The wet area and the heat input is following the same shape of the curve, since they are directly related to liquid level.
The PSV strip chart is a lot more interesting than the vessel strip chart. The black line is the relief valve opening percentage. The blue line is the reliving mass flow. The red line is the reliving pressure and the pink line is the reliving temperature. As expected, the vessel pressure and temperature start to rise. At about 350 seconds, the vessel pressure reaches 500 psia and the relief valve starts to open. At about 900 seconds, the vessel reaches the highest pressure of about 530 psia. At the same time, the PSV reaches the highest opening of 67% and the relief flow reaches the highest of about 6900 lb/hr. After the pressure hits the peak, it starts to decline as expected. At the same time, the relief valve opening and the relief mass flow start to decline as well.
Very surprisingly at about 2400 seconds, the relief valves starts to open more and the relief flow is getting larger as well!!! The liquid level drops, thus the fire heat input drops as well. How come the relief mass flow is getting larger? The reason is the latent heat of the fluid is getting smaller and smaller. Even with smaller heat input, the relieving mass flow still gets larger, despite smaller heat input.
At about 4000 seconds, the relief flow arrives at another peak. This is where 99% engineers might get trouble. In some cases, the relief flow at the second peak is even larger than the first peak. The reason? It is because the latent heat is getting smaller.
At about 5200 seconds, the vessel has nearly 0% liquid, thus the vessel pressure drops very quickly while the temperature remain fairly the same.
After running about 7000 seconds, the vessel pressure is at about 500 psia, which is the PSV set pressure.
Since the highest vessel pressure is only about 530 psia, the vessel is protected by this PSV.
Thanks very much for watching this video. It is brought to you by Guofu Chen. More interesting topics can be found at showcase.GuofuChen.com
Common Mistakes
The calculation sequence seems easy, but it is actually extremely difficult when you get into the details. Common mistakes of 99% engineers are making:
- Assume constant wet area, which is not true. By the way, the wet area might increase at the beginning
- Assume an inappropriate latent heat. Latent heat is changing through the reliving period
Dynamic Simulation
A proper way to size PSV fire case is to use a dynamic process simulator, since the relieving process is dynamic, not steady state. In this example, VMGSim Dynamics is selected to do this demo.
As you see on the PFD, there is one vessel and one PSV. The PSV orifice size is E with a set pressure of 500 psia. The vessel is horizontal with a diameter of 5 ft and a length of 20 ft. Initially it is at 34.5 F and 400 psia, with a liquid level of 60%. Suddenly the vessel is exposed to a fire.
Let us start the simulator and see what's going to happen.
Strip Charts
Currently two strip charts are shown. On the vessel strip chart, the black line is the liquid level percentage in the vessel. The blue line is the wet area and the red line is the fire heat input. The liquid level first increases and then drops. The reason is as the vessel gets hotter, the liquid density gets lower and it results a bigger volume in the vessel. The wet area and the heat input is following the same shape of the curve, since they are directly related to liquid level.
The PSV strip chart is a lot more interesting than the vessel strip chart. The black line is the relief valve opening percentage. The blue line is the reliving mass flow. The red line is the reliving pressure and the pink line is the reliving temperature. As expected, the vessel pressure and temperature start to rise. At about 350 seconds, the vessel pressure reaches 500 psia and the relief valve starts to open. At about 900 seconds, the vessel reaches the highest pressure of about 530 psia. At the same time, the PSV reaches the highest opening of 67% and the relief flow reaches the highest of about 6900 lb/hr. After the pressure hits the peak, it starts to decline as expected. At the same time, the relief valve opening and the relief mass flow start to decline as well.
Very surprisingly at about 2400 seconds, the relief valves starts to open more and the relief flow is getting larger as well!!! The liquid level drops, thus the fire heat input drops as well. How come the relief mass flow is getting larger? The reason is the latent heat of the fluid is getting smaller and smaller. Even with smaller heat input, the relieving mass flow still gets larger, despite smaller heat input.
At about 4000 seconds, the relief flow arrives at another peak. This is where 99% engineers might get trouble. In some cases, the relief flow at the second peak is even larger than the first peak. The reason? It is because the latent heat is getting smaller.
At about 5200 seconds, the vessel has nearly 0% liquid, thus the vessel pressure drops very quickly while the temperature remain fairly the same.
After running about 7000 seconds, the vessel pressure is at about 500 psia, which is the PSV set pressure.
Since the highest vessel pressure is only about 530 psia, the vessel is protected by this PSV.
Thanks very much for watching this video. It is brought to you by Guofu Chen. More interesting topics can be found at showcase.GuofuChen.com
