8 replies to this topic

[Sherif Morsi]

In API 521 6th "section 4.2.1 Hierarchy of Protective Measures (page 12)", a hierachy should be used to avoid overpressure of equipment:

 

1- Avoid (reduce) risks: set MAWP of the vessel above the max pressure of all possible sources

2- Provide engineering controls: pressure relief on the vessel

3- Provide administrative controls: include provision of block valves of the locked-open design.

 

What does "locked-open design" mean?

 

Thanks,

Sherif

[fallah]

Sherif,

 

"Locked-open design" means the inline valve to be locked in open position such that could be considered as a part of the line with no possibility of blocking...

[Sherif Morsi]

Thanks Fallah. Appreciate your help

 

Sherif

[Manan_Joshi]

I believe my understanding is correct.  It means the isolation valve or the downstream control valve of the equipment to be CAS or fail open as applicable.  Is this correct ?

[Bobby Strain]

But I would never use such for overpressure protection.

 

Bobby

[Sherif Morsi]

Bobby,

 

I wouldn't use it as well for overpressure protection, however block valves upstream and downstream relief valves are installed for maintenance purposes in case of redundant PSVs. Do you have specific reasons that would add to my knowledge?

 

Appreciate your input

 

Sherif

[fallah]

Sherif,

 

Yes, these isolation valves have to be used for maintenance and change over of the redundant PSV but those of the working PSV should be locked-open for safety issue and to avoid overpressure due to inadvertent closing.

Therefore these locked-open valves aren't basically used as overpressure protection facility, but when have to be used then have to be in locked-open position...

[Manan_Joshi]

Sherif is correct and the isolation valve are CSO and CSC type.  I want to ask does anyone will extend this to control valve bypass ? especially when the scenario becomes governing.

[Sherif Morsi]

For control valves, it is subject to the application and the pressure rating of the system. Sometimes, bypass isolation is Normally Closed "NC" and sometimes Locked Closed "LC" (CSC). From my limited experience, with high pressure systems and especially with gases, they are LC.