PSV relieving to atmosphere - fire case

[PetroBob]

I am being asked to stamp an existing design that includes PSV venting to atmosphere. Relief case is pool fire under the vessel, and hence causing boiling of flammable liquids in the vessel. Liquid Mr = 54, ie heavier than air (although maybe not if hot). I'm accustomed to having PSV outlet to connected to flare for these designs. Where no flare system is available, I've only previously dealt with systems containing light gases (eg hydrogen or natural gas) that will go up if the PSV opens. I see other Eng-tips threads that discuss fire-case venting of propane (also heavier than air, but lighter than our vapour) - it seems that there are plenty of propane systems venting to atmosphere for fire case.

What is typically acceptable, in terms of heavier vapours venting to atmosphere?

The PSV discharge is aimed horizontally, so any discharged vapours will travel across the skid building and may fall to grade on the far side. The system is not located in a hollow; ie any heavy vapours vented will eventually disperse and/or flow away from the skid. However I could envisage a fire scenario where the PSV burps and vents heavy flammable vapours onto firefighters below.

Are there any standards or codes I should be checking, other than area classification?

 

[Chance17]

I believe this standard comes from API 520
In flammable vapor service, the minimum exit velocity should be 100 ft/s (30 m/s) at 25% of the rated capacity of
the PR valve (i.e., calculated flow using 25% of the installed PR valve’s effective orifice area). For modulating type
POPRVs the minimum exit velocity should be 100 ft/s (30 m/s) at 25% of the rated capacity of the PR valve or at
the minimum anticipated relief load if lower than 25% of rated capacity. If this criterion is not satisfied, dispersion
calculations should be performed to confirm that the concentration of flammable vapor at grade, at any equipment or
at any platform is below 50% of the LFL at 25% of the rated capacity of the PR valve.

 

[georgeverghese]

Agreed, dispersion calcs should be done to confirm that flammable gas concentrations are at less than 25% of LFL for all locations inplant where gas detectors are installed and perhaps at no more than 50% of LFL for locations where gas detectors are not installed.

Further, these dispersion calcs should also look at concentrations for gas components with HSE concerns (H2S, benzene for example) for short term exposure levels at certain elevated locations that may be manned at the time or relief.

Also look at heat radiation levels at some locations of interest that may be manned (and also unmanned for max permissible surface temperature for short term exposure). API521 gives max permissible heat radiation levels for situations with varying escape times, and for cases where the operator is required to remain at the location for this short term fire relief case.

For dispersion and radiation levels at each location, consider a range of windspeeds and stability class, for stream rates ranging from 25% of PSV rated capacity to 100% of process design relief rate.

Most importantly, the exit stream from the PSV must be essentially single phase vapor, else a relief stream KOD will be required. In some cases, the exit pipe may have filled up with condensate over time from a leaking PSV before the relief event.

 

[EmmanuelTop]

Hard-piping the PSV discharge to a safe location may be the best solution, if the facility is still in design stage.

If this is not possible, for whatever reason, the designer needs to ensure that all potential consequences and associated risks are reduced to a tolerable (ALARP) level. This includes, among other things, performing consequence modelling as indicated above. Radiation guidelines from API 521 are very much straightforward, but you may also have to assess the risks of delayed ignition/explosion if cloud of released vapor does not catch fire immediately. For toxic gas dispersion your (or client) company probably has the relevant guidelines on maximum IRPA and PLL values.

Company standards and local regulations differ a lot, from country to country, so there is no universal answer on this query. If safe disposal of released vapor is not feasible, you may end up with performing FERA or QRA study to address the risks properly - again this depends on where you are and what are the legal and internal company requirements.




Dejan IVANOVIC
Process Engineer, MSChE