More accurate Method to calculate the relieving temperature for Fire case PSV sizing by Shell DEP method.

OBJECTIVE: More accurate Method to calculate the relieving temperature for Fire case PSV sizing by Shell DEP method.

(This method is more accurate because in this method we consider the actual volumetric flow for relieving condition; this method gives somewhat less value of relieving temperature as compared to the Method 1 which is based on mass flow)

PFD: PFD developed in Aspen HYSYS

2.     Methodology: Following are the steps to be followed for developing the model in HYSYS.

a.     Open HYSYS and define the stream component, Select fluid package then go to simulation environment and pick a material stream from HYSYS utility panel.

b.     Define the stream conditions Temperature as Operating temperature and operating Pressure give some mass flow rate let say 100 kg/hr.

c.      Once stream (20) is conversed now put a Flash drum (V-105) downstream of material stream (20).

d.     Now define two outlet stream 21 & 22, 21 stream as vapor outlet stream and 22 as liquid outlet stream

e.     Define the vessel volume, vessel volume includes vessel volume on which PSV is mounted and piping volume which comes under fire zone, if piping volume is not known add 10% additional volume in the vessel volume.

f.       Now define the liquid level in vessel V-105 by adjusting the percentage, and note down the liquid volume (V_L) and gas volume (V_G) in the vessel.

g.     Split stream 21 into stream 23 & 24 and stream 22 into 26 & 27.

h.     Now adjust stream 24 mass flow rate into actual volume flow rate, select user supplied option in Adjuster connection option and specified target value is V_G   (m3/hr).

i.       Define some flow rate in stream number 23, HYSYS will adjust stream 24 according to the specified target value, if you are not able to adjust properly than you should see the adjuster parameter and define tolerance, step size minimum value, maximum value and maximum number of iterations than reset adjuster it will adjust.

j.       Now adjust stream 26 mass flow rate into actual volume flow rate, select user supplied option in Adjuster connection option and specified target value is V_L (m3/hr).

k.     Define some flow rate in stream number 27, HYSYS will adjust stream 26 according to the specified target value, if you are not able to adjust properly than you should see the adjuster parameter and define tolerance, step size minimum value, maximum value and maximum number of iterations than reset adjuster it will adjust.

l.       Now put a MIX-102 and mix stream 24, 26 and get the final mixed stream 28 from the mixture (MIX-102) outlet.

m.  Now define another stream 29, keeping mass balance same as stream 28 and pressure as Relieving Pressure.

n.     Put a Adjuster for stream 29 to adjust the temperature of stream 29 in such a way that mass density of stream number 29 is equal to the mass density of stream 28.

o.     After adjusting the temperature of stream 29.

p.     Now note down the temperature of stream 29 (Relieving Temperature) which is at relieving pressure.

Result:

1.      The Relieving Temperature at relieving Pressure is 191.8 deg C and on mass basis it was 194.6 deg C in method1.

Conclusion: Both the methods (1 & 2) is correct and according to SHELL philosophy acceptable to Shell but when you are going to analyze a Flare network, if you have some existing PSV size for Fire case at that time in Flare-net you only need to get the relieving temperature to get the value of rated capacity (Calculated by Flarenet) of Existing known size PSV at that time you can prefer METHOD1 and if you are going to size new PSV for Fire case at that time you can use METHOD1.