6 replies to this topic

[splendid.ye]

Dear Forum members,

 

I have recently found that initial pressure of the vessel for fire case depressuring is different from standard to standard.

For example, design pressure, PSV set pressure, high-high pressure of the vessel are recommended.

But most of the case, it seems design pressure to be used.

While operating temperature or maximum operating temperature is used as initial temperature which seems to be appropriate, why is design pressure used as the initial pressure for depressuring?

Is it just because higher pressure is more conservative and more peak flow?

For me, it looks more reasonable to use maximum operating pressure.

Please let me know the reason why we use design pressure.

 

Thanks in advance.

 

[Bobby Strain]

I am not aware of any standard. But you can consult API recommended practices. If you don't have any company requirement, then you can pick your own. What tends to happen with these issues is that a group votes in some fashion. This is probably not the best method. Look at whatever guides and standards are prevalent in the country/province/city with jurisdiction for guidance. 

 

Bobby

[fallah]

Hi,

In most cases using maximum operating pressure as initial pressure of depressuring is reasonable.

In some rare cases in which there is no considerable difference between MOP and DP it is better and more reasonable using design pressure.

[Fakhri]

Hi,

This also depends on how the depressurization systems is initiated. In onshore applications, depressurization is initiated by manual push buttons. In this case, you have to take the PSV set pressure as the initial pressure as this is the maximum pressure that can happen during fire.. In offshore, this is imitated automatically by the Fire and Gas System (FGS). In this case, you have to select the PSHH set point which represent the possible maximum operating pressure of the system.

 

Best regards,

 

Fakhri

[fallah]

 

 In onshore applications, depressurization is initiated by manual push buttons. In this case, you have to take the PSV set pressure as the initial pressure as this is the maximum pressure that can happen during fire..

 

Fakhri,

 

Although above mentioned criteria looks like reasonable, in most cases; especially those in which the design pressure differs more than 15% from maximum operating pressure; selection of design pressure as initial pressure of de pressuring is much conservative. In fact, in real world it's rarely supposed a vessel engulfed in a pool fire will withstand till relevant PSV activation and it's supposed the operator will push the button after hearing the F&G alarm.

Indeed, in no case one will assume the PSV and BDV on a vessel will simultaneously be in operation... the result of taking design pressure as initial pressure of de pressuring...
 

Edited by fallah, 19 November 2016 - 07:17 AM.

[splendid.ye]

Fakhri,

 

I think that even onshore application, taking PSV set pressure as the initial pressure is much conservative.

Let say a vessel is filled with vapor no liquid in it. In this case, this vessel can not be protected by the PSV when there is fire because even before reaching the set pressure of the PSV, the metal stress of the vessel will exeed the allowable stress caused by high temperaure.

When there is no chance to reach PSV set pressure at fire case depressuring, is it still reasonable to have PSV set pressure as initial pressure?

[Fakhri]

Fallah

Sorry for a late response.

If the design pressure differs much from the operating pressure, some company takes credit that operators is able to push the ESD button within 15 minutes. Thus the initial pressure is based on the pressure rise in 15 minutes.

 

Splendid Ye,

I think if we have problem with the metal temperature, we have to find another way to prevent the metal temperature exceeds the maximum allowable metal temperature by providing PFP for instance.

 

Best regards,

 

 

Fakhri