Rigorous Method for Fire Case

In previous article, I have explained that relieving load for fire case can be calculated as heat input divided by latent heat of vaporization. (W = Q/Hv). The heat input have been discussed before, see fire case –heat input rate. The latent heat of vaporization is rather difficult to be determined. During fire, liquid in the vessel will be vaporized, but the amount of vapor formed is not fixed, because the liquid composition is change overtime. In this article, I will explain you step by step the rigorous method for fire case

Let imagine the system consist of a three phase separator below, the fire case calculation procedure is as follow;

1. Determine the inventory volume of isolated system, both for condensate, water, and gas based on actual size of the vessel and pipe.

2. Calculate the wetted area.

3. Perform simulation:

a. Define stream for WATER, CONDENSATE, and GAS. Input composition, pressure, and temperature as operating condition of the three phase separator.

b. Adjust mass flow for those streams, to achieve actual volume for each stream in accordance with inventory volume calculation (1). Mix those streams to be STREAM OPERATING

c. (a) and (b) is model of the separator at normal operating condition based on actual condition.

d. Install three phase separator, V-0. The STREAM OPERATING is as inlet stream to the separator. Adjust Q-100 (heat input to vessel) to achieve pressure RELIEVING CONDITION

(e). The temperature and pressure will increase to initial RELIEVING CONDITION due to fire. (This simulation assumes the PSV instantly reach relieving condition after opening). The outlet stream from separator is mixed.

e. Balance the mixing stream (d) to stream RELIEVING CONDITION. Input the pressure of relieving pressure value (=1.21 x set pressure).

f. (d) and (e) is model of the separator at relieving condition as initial stage which is PSV will start open

g. Install a three phase separator (V-1). The stream RELIEVING CONDITION (e) is as inlet stream to the separator.

h. Heat input due to fire case is calculated by increasing temperature with interval 10 F. Thus, set the temperature outlet separator (V-1) of 10 F higher than the RELIEVING CONDITION.

i. The outlet stream is CONDENSATE 2, WATER 2 and VAPOR. The VAPOR will be divided into two streams, LOAD STAGE 1 and VAPOR 2

j. Mix CONDENSATE 2, WATER 2 and VAPOR 2 to be INLET STAGE 2.

k. Adjust mass flow rate of VAPOR 2 to achieve actual volume flow of INLET STAGE 2 is equal with initial actual volume STREAM OPERATING.

l. The stream LOAD STAGE 1 is relieving load fire case

m. The heat input to V-1 in HYSYS simulation is Q-1 BTU/hr, whereas the actual heat input is Q API = 21.000 F A ^0.82 BTU/hr

n. Calculate the required time of increasing temperature at stage 1, that Is (Q-1) / (21.000 F.A ^0.82)

o. Install a three phase separator V-2. The stream INLET STAGE 2 is as inlet stream to the separator.

p. Repeat the procedure from (g) to (n), for stage 2, stage 3, stage 4 ....with temperature is increased by 10F in every stage. We can stop the procedure if the cumulative time is one hour (it’s assumed that the fire has been suppressed during 1 hr, you can use more than 1 hour if you want)

q. Calculate the required area for each stage, (with the relieving load are LOAD SATGE 1, LOAD STAGE 2, LOAD STAGE 3, …)

r. Use the stage requiring the maximum orifice area as basis datasheet of the PSV.

finish :D

Good morning my friend, That's all I can share today, … I’m very happy today because I find a new spirit for better life. Don't regret what was loss; it didn't disappear actually, just in use by more appropriate user to make it more useful thing. Sometimes we lose something, but at the same time, we get the other thing that better, at least a lesson.

Hopefully you never bored with my article.

Thank for reading,.