5 replies to this topic

[maldini]

Dear all,

In order to design a complete relief system for gas plant we have to perform sizing of the safety valves for different cases, K.O. Drum sizing, flare headers, blow down valves and choose the flare max rate based on following scenarios:
1- The maximum relief load from safety valves (i.e blockage case)
2- Total plant depressurizing (most of the equipment within 15 min. except slug catcher finger type)
3- Safety valves installed on the same fire zone as per API 521 (the fire zone diameter is about 18-24 meters)

First question: the client requirements is to determine the required time before vessel mechanical fail when it is exposed to pool fire and depressurizing initiated , is it will be less than 15 min. or more with different thicknesses less than 1" or more

Second question: it is the same of the above but the cause of the fire is not pool but jet fire due to pipe rupture or flange leakage, how to calculate the required time that the vessel can withstand the fire before mechanical fail
(The vessel may fail after 8 min. of starting the depressurizing of the vessel will be about 15 min.)

thanks for any advice

[JoeWong]

It is difficult to understand your questions i.e. "... is it will be less than 15 min. or more with different thicknesses less than 1" or more...". Can you please rephrase ?

Different material with different strength and wall thickness will have different allowable stress level at elevated temperature when it is under fire exposure. The depressuring time may be within or exceeded 15 minutes. However, experience shown that steel vessel with more than 1" tends to "stay" longer than 15 minutes when it is under fire exposure and proper depressuring device in place.

Pool fire and jet fire will have different heat flux, momentum, coverage, etc. (Jet fire will have higher heat flux and momentum however less coverage). This will lead to different in vessel wall temperature increase rate and hence different in vessel allowable stress level.

Regardless pool or jet fire, you have to ensure the vessel will not rupture within an "acceptable" time. The vessel rupture criteria could be vessel allowable stress at elevated temperature less than internal pressure induced stress.

[maldini]

if i assume that i have a fire zone for example and it consists of 4 vessels (i.e H.P. separators) and the design pressure is about 90 barg so the i have to reach half the design pressure by depressurizing in 15 min. as their thickness is more than 1” as per API 521 This is what we have done, if the thickness of the vessels is more than 1" we reach by depressurizing to the half of the design pressure and if the thickness is less than 1" we will reach 7 barg within 15 min.
(As i know that this case is for pool fire only and API didn't say anything about jet fire)
then the client requirement now is to consider the jet fire so if there is a flange leakage in one of the 4 vessels it will affect the other vessels and as i know that the jet fire have a higher heat flux
So my question now how to know the required time (may be 8 min,) to depressurize the other 3 vessels before they reach max. allowable pressure (mechanical fail) in case of the vessels thickness is more than one inch and in case of less than one inch
Thanks for your reply

[fallah]

if the thickness of the vessels is more than 1" we reach by depressurizing to the half of the design pressure and if the thickness is less than 1" we will reach 7 barg within 15 min.

As per API 521 FIFTH Ed. sec. 5.20.1 for vessel with thickness thinner than 1" the destination pressure of depressuring would be 50% of vessel design pressure (same as vessel with 1" thickness or more) but with faster depressuring rate.

Depressuring to around 7barg is commonly considered when the
depressuring system is designed to reduce the consequences from a vessel leak (based on API 521).

[JoeWong]

Below are some steps may be considered to approach depressuring involve fire...

(i) Establish credible fire scenario and associated heat flux
(ii) Establish depressuring model with heat flux as decided in (i) together with vessel wall temperature rise This could involve heat being transferred to vessel (temperature rise) and heating/vaporizing content in vessel (internal pressure reduction with mass removed from system).
(iii) Base on vessel wall temperature as established in (ii), establish the allowable stress (Sa)
(iv) Base on content internal pressure as established in (ii), establish the pressure induced stress (Sp)
(v) Ensure vessel not rupture (SP <= Sa) throughout the depressruing path.
(vi) Re-adjust depressuring model to achieve (v)

[maldini]

Below are some steps may be considered to approach depressuring involve fire...

(i) Establish credible fire scenario and associated heat flux
(ii) Establish depressuring model with heat flux as decided in (i) together with vessel wall temperature rise This could involve heat being transferred to vessel (temperature rise) and heating/vaporizing content in vessel (internal pressure reduction with mass removed from system).
(iii) Base on vessel wall temperature as established in (ii), establish the allowable stress (Sa)
(iv) Base on content internal pressure as established in (ii), establish the pressure induced stress (Sp)
(v) Ensure vessel not rupture (SP <= Sa) throughout the depressruing path.
(vi) Re-adjust depressuring model to achieve (v)

please can give me more explanation or send me an example