PSV requirement for fire case caused by deflagration

[Sawsan311]

Dear All,

For vessels connected to atmosphere via an open vent, vessel design pressure has been selected to be inline with API 14 J recommended minimum design pressure for mitigating flashback and deflagaration associated increase in pressure caused by the flame front travelling behind the unburnt combustion mixture at a speed less than sonic velocity.
However, I came across that some Engineers tend to install a PSV on these pressure vessels for protecting the vessel against fire caused by the potential flashback initiated by an external ignition source at or near the open atmospheric vent. However, as per ASME SECVIII UG-125 through UG-140, I haven't seen a similar addressed potential scenario since the ''fire'' definition is conventionally a pool fire around the vessel (or jet fire) and not n internal fire inside the vessel initiated by flame propagation...
Accordingly, do you agree with this concept of overpressure relief protection against deflagation, in my view, deflagation mitigation is strongly associated with the inherent mechanical integrity of the vessel and by selecting its design pressure in addition to proper purging for reducing the flammability and explosive range with the increase of the value of the minimum ignition energy...

Thanks

 

[don1980]

Deflagration protection is outside of the scope of UG 125 - UG 140. Refer to NFPA 68 and NFPA 69.

Designing the system with sufficient strength to withstand a deflagration is one option, but that's not always practical, possible, or economical. Other options are fast acting suppression systems and explosion vent panels.

 

[Sawsan311]

Thank you Don,

I agree with you.. Appreciate if you can kindly advise on the relevant sections in NFPA..

PSV protection against a fire initiated by deflagration is not actually a design scenario addressed by any standard.

Regards,

 

[TiCl4]

"PSV protection against a fire initiated by deflagration is not actually a design scenario addressed by any standard. "

That's likely because PSVs cannot be relied upon to open quickly enough to handle the sharp pressure rise of a deflagration event. If the flashback makes it into the vessel with only a PSV for protection, you are looking at a severe vessel rupture event.

A better option would be to prevent flashback into the vessel to start with - explosion isolation valves (as made by Fike or other companies) serve this purpose admirably, especially if you do not want to have a loss of containment as with blowout panels.

 

[georgeverghese]

Even if there were to be a fire in the PSV tailpipe, it is generally acknowledged that , when the PSV is open, the forward stream velocity is too high to permit flashback into the vessel. Vent line purge is the primary means of flashback prevention. Vessel design pressure should be at least 700kpag to withstand a detonation if purge were to fail.

 

[TiCl4]

Georgeverghese, it is my understanding from the OP that the vessel has a free vent to atmosphere that has the potential for flashback, and that a previous engineer placed a PSV on a separate nozzle on the vessel to relieve an overpressure wave from the atmospheric vent piping. The concern was not flashback through the PSV, but rather the suitability of a PSV of relieving a flashback event from another inlet to the vessel.

Sawsan,

To answer you question more firmly, Sawasn, PSVs are NOT acceptable for handling deflagration relief due to what I previously stated - they are too slow to act. See API 521 excerpt below:

4.4.10.1Internal Explosion (Excluding Detonation)

If overpressure protection is to be provided against internal explosions caused by ignition of vapor-air mixtures where the flame speed is subsonic (i.e. deflagration but not detonation), rupture disks or explosion vent panels, not relief valves, should be used. These devices respond in milliseconds. In contrast, relief valves react too slowly to protect the vessel against the extremely rapid pressure buildup caused by internal flame propagation. The vent area required is a function of a number of factors including the following:

a) initial conditions (pressure, temperature, composition),
b) flame propagation properties of the specific vapors or gases,
c) volume of the vessel,
d) pressure at which the vent device activates,e) maximum pressure that can be tolerated during a vented explosion incident.

It should also be noted that the peak pressure reached during a vented explosion is usually higher, sometimes much higher, than the pressure at which the vent device activates.Design of explosion-relief systems should follow recognized guidelines such as those contained in NFPA 68

Simplified rules-of-thumb should not be used as these can lead to inadequate designs. If the operating conditions of the vessel to be protected are outside the range over which the design procedure applies, explosion vent designs should be based on specific test data, or an alternate means of explosion protection should be used. Some alternate means of explosion protection are described in NFPA 69 [119], including explosion containment, explosion suppression, oxidant-concentration reduction, and so forth.

Explosion-relief systems, explosion containment, and explosion suppression should not be used for cases where detonation is considered a credible risk. In such cases, the explosion hazard should be mitigated by preventing the formation of mixtures that could detonate.Explosion prevention measures, such as inert gas purging, in conjunction with suitable administrative controls can be considered in place of explosion-relief systems for equipment in which internal explosions are possible only as a result of air contamination during start-up or shutdown activities. [118]

Given the difficulties around relieving a deflagration inside the vessels, a better option would be to prevent flashback from reaching the vessels in the first place. There are many options for relieving or avoiding the event. Some may not be applicable based on your process design and chemistries. I'm not sure what NFPA 69 lists, but I've listed a few methods below relieving or avoiding the event.

*Purging a common header to keep vent velocity above flame propagation speed (See the DOW publication below for Su values - flame propagation speed. Most are 2-3 fps, but some are higher. Hydrogen is 49 fps!). You can also use a blower to achieve this velocity. You can also monitor %LEL in a common header and dilute with purge or additional blower air if necessary.

*Installing flame arresters on each vessel vent. Most cost effective for vessels that have individual vents. I believe this is standard practice for many above-ground flammable storage tanks.

*Purging individual vessel with an inert gas to dilute O2 concentration to well below the limiting oxygen concentration. I believe NFPA 69 addresses this method. A publication by DOW also has a table of LOC for different materials:

http://msdssearch.dow.com/Published...ties/pdfs/noreg/779-07309.pdf&fromPage=GetDoc

*Installing an explosion isolation valve or blowout panels on the common header, if you have one.

EDIT: Added the additional guidance from API 521 and DOW publication on LOC and Su values.

 

[MJCronin]

Sawsan ....

I believe that you need a professionally designed complete vent system..

You also will need properly sized flame arrestors in the system

http://www.wermac.org/equipment/flame_arrester.html

MJCronin
Sr. Process Engineer

 

[Sawsan311]

Thank you all for your valuable inputs..

We all agree that for deflagartion mitigation and and protection, inherently safe design pressure selection is one of the essential requirements. The utilization of flame arrestors is a possible measure despite being discouraged by numerous engineering standards due to heir potential blockage of the vent lines and accordingly creating hazards of vacuum/overpressure development. API 2000 addresses the use of flame arrestors on API 650 tanks yet acknowledges their tendency of blockage.

The discussion I was pointing at is providing a PSV on the vessel not to protect against the pressure wave generated from deflagration.. but actually to relief the liquid vaporization load generated from a fire being initiated INSIDE the vessel post the flashback occurrence by the deflagartion.. again this is unconventionally odd due to the fact that we all have known that it is external pool fire which determines the relief load and NOT an internal fire starting inside the vessel.. is there any such scenario applicable..

Thanks

 

[georgeverghese]

The short answer is NO, assuming there is no flammable atmosphere in the vessel to start with, and there are no open lines connecting the vessel to the common vent/ flare system. Forward stream velocity through PSV and PCV, when operating correctly would be too high to allow reverse flow of the flame front at the vent header. Positive purge is the first line protection against a flame front appearing in the vent header.

 

[don1980]

Sawsan, recognize that overpressure due to vaporization of liquid from a sustained fire inside of an intact (and closed) vessel is exceptionally difficult to imagine. Think that through again, because it's probably not possible, and even less likely to be credible. Where is the O2 coming from to sustain this internal fire? And how is the fire vaporizing ("boiling") the liquid to create internal pressure, rather than simply combusting the vapor that's evolving from the liquid surface?