2 replies to this topic

[vinod]

Sirs

I am having a separator(handling hydrocarbon liquid) with inlet line of 50m length, inbetween I don’t have any block valves and liquid outlet line is 14 m .Pls say me whether we have to consider the inlet and outlet line to calculate my wetted surface area along with my separator for fire case.

My inlet line is 20” and outlet line is 12” .

Also is there any code requirement or is it just an engg judgement regarding the risk involved.

Vinod

[sheiko]

As far as i know, a margin of 10% is added to the calculated surface area to account for the associated piping.

[JoeWong]

When we consider the extend of fire, three main points shall be taken into account :
i) Type of fire (pool, jet or other)
Type of fire subject to inventory type, fluid properties, etc. In general, pool fire is the most likely event. Nevertheless, high pressure vapor would lead to jet fire. These two fire types will have different heat flux and has been discussed in API Std 521.

ii) Extend of fire zone
For pool fire, all facilities / equipment (drum, column, pump, pipes, etc) within a fire zone will expose to similar pool fire simulteneously.
For jet fire, number of facilities / equipment is subject to jet fire location and arrangement. Detail studies is required to check extend of jet fire.

Base on API std 521, the pool fire extended to 232 m3 (or any physical barrier that can separate fire zone) as discussed in "Extend of fire..."

iii) Trap inventory
In the event of fire, it is assumed that the plant shutdown system is activated to isolate the system. All inventory within a trapped system will expose to fire. The trapped system is generally from SDV to SDV (or stop at fire zone, or stop at physical barrier, etc). The trapped system should include piping as well.


Some "expert" may argue that piping code does not call for Pressure relief. Hence, it may not be included. Personally, i would take more caution about this statement.

Above statement may stand :
- if the INVENTORY capture in the system is LOW and associated RISK is LOW.
- if the system is TOTALLY FREE from overpressure scenario i.e. external fire, solar heating, etc. For example buried pipeline.

Requirement of PSV shall NOT be 100% judged from the design code itself. The RISK and CONSEQUENCE e.g. INVENTORY associates risk & consequences shall come into consideration to define if a overpressure protection i.e PSV is required.

Let take a simple example: A filter in DN100 line designed according to vessel code. Thus, PSV is provided. Another pipe (DN500) with same fluid in it, no PSV is provided as it is designed for piping code. The risk and consequences associated to DN500 pipe is much higher than a filter in DN100 line. The logic is somehow does not stand. Read more in "Requirement of Overpressure protection system for PIPING"

Thus, shall always take note when calculate fire relief load without considering PIPING.


10% as proprosed Sheiko could be considered. I would recommend you consider 30% if you do not have clear defintion of piping layout.