Loss of ejector steam in vacuum tower

[fdomin]

I´m working in a review of some PSV´s and I have some doubts regarding vacuum tower protection.

When loss of ejector steam happens vacuum is lost. Should I consider that as there is no vacuum, no flash will occur? Therefore, is the any need for a PSV as tower will get pressurized slightly over atm. pressure and MAWP is bigger than that? Is this correct?

In general, Do vacuum towers in refineries have PSV´s?

Thanks in advance

 

[TD2K]

You need to look at the feed to the vacuum flasher from its charge furnce. As a first pass, check to see at normal furnace outlet temperatures if (at the MAWP of the vacuum flasher) the feed to the flasher below its bubblepoint? If so, you can argue that the light ends will build up in the flasher upon loss of ejector steam until the system pressure equals the vapor pressure of the feed leaving the furnace and then there is no more vapor released to further increase the pressure. Thus, not a PSV sizing case. If at the MAWP of the flasher you still have vapor formed, the amount of vapor formed at the MAWP (unless you can take credit for other paths out of the system) would be the basis for sizing the PSV.