6 replies to this topic

[stu]

Hi all,

Please guide how to find psv release rate for gas filled pipe for jet fire.

Have come across jet fire heat flux as 95,500 BTU/ft2/hr , using this how to find release rate lb/hr for gas filled pipe. 

thanks 

stu

[latexman]

Hi stu,

 

My last three companies (43 years in May) did not protect piping from fire scenario.  There's just too many pipes in a chemical plant for this to be practical, since fires are rare events.  We spend a lot of effort and money making sure fires are minimized.

 

Is there something special about this gas-filled pipe?  Diameter?  Length?  MOC?  Pressure?

 

I'm not sure if a jet fire can be protected against.  It burns ferociously in a single spot, and failure in that one spot is highly probable no matter what one does.  Pool fires are easier to protect from.

[stu]

Thanks...

it is higher dia 40inches Carbon steel natural gas line runs for about 500m inside the plant and if any jet fire hits this line, pls suggest how to find the psv release rate. 

 

thanks

stu

Edited by stu, 12 February 2022 - 10:42 PM.

[latexman]

In a pool fire, since the pipe has no liquid in it, there is no boiling/heat of vaporization to temper the fires heat input.  One would normally consider isochoric (constant volume) process and calculate relieving temperature from the gas equation of state (p₁, T₁, z₁ --> p₂, T₂, z₂). What usually happens in pool fire case cases is that the calculated relieving temperature is far above the design temperature of a vessel, which means that the vessel will likely rupture much before the actual pressure reaches the PSV set pressure. Protecting the vessel with a PSV does not have any value under those circumstances.

 

Now, your scenario is jet fire, where all the heat is focused in one spot.  This is even worse than pool fire for exceeding the design temperature at the jet fire spot before pressure can open the PSV.

 

You should consider other ways of protection - API 521 recommends vapor depressuring (blowdown) which would be automatically triggered by Fire & Gas detection system (F&GS trip signal to the SIS). Other options are insulation and cold quenching (water deluge or sprinklers).

 

It's best to invest in systems that have a chance of working.

[stu]

thanks a lot... will follow the recommendations as suggested to avoid failure...but just like to know with  jet fire heat flux as 95,500 BTU/ft2/hr, how to estimate the release rate for gas filled system...if it is liquid(wetted case) and pool fire , based on heat flux and latent heat of vap. we can estimate the mass release rate ..but for jetfire and gas filled system with jet fire heat flux , how to estimate flow in lb/hr...

 

thanks,

stu

[latexman]

Maybe someone else will reply to that question.

 

PRVs are ineffective at protecting vapor-filled vessels from fire exposure. For safety, focus on other protective measures (fire resistant insulation, water spray, and automatic depressurization) which actually help reduce the risk, rather than wasting time sizing a device (PRV) which is ineffective at providing meaningful protection. Without some amount of liquid (some amount of actual wetted surface area) to remove heat from the vessel wall, that vessel is going to rapidly fail due to excessively high wall temperature. And the size of the PRV (whether you have a 1D2 or 8T10) won't have any significant effect on the time-to-failure. PRVs protect vessels from fire exposure by releasing high amounts of heat from the vessel. This occurs when heat is transferred from the vessel wall to the inner liquid (boiling liquid, Hvap) at a high rate. The liquid boils, and this heat is removed from the vessel through the PRV. The high heat transfer rate, from the vessel wall to the boiling liquid, helps to cool the vessel wall. And this buys you some time to get the fire extinguished before the vessel fails due to excessively high wall temperature.

 

When you have a vapor-filled vessel, the heat transfer from the vessel wall to the inside vapor is insignificantly small, because vapor has an insignificantly small heat capacity as compared to that for a liquid. This means that the wall temperature is going to rise unabated, until it reaches the yield temperature, and the vessel fails. Yes, the vapor inside the vessel will thermally expand. And depending on the starting T & P, the resulting pressure rise may or may not reach the PRV set pressure before the vessel fails. But even if it does, and the PRV opens, the amount of heat released is insignificantly small. The vapor isn't absorbing enough heat to have any real effect on the wall temperature. The PRV opens to burp out some hot vapor, and then it re-closes, while the vessel wall temperature (and the temperature of the vapor inside the vessel) continues to rise. With a vapor filled vessel, the PRV has no significant affect on the rising wall temperature.

 

When you have a vapor filled vessel, focus your attention on real protective measures that have real safety benefits. When you size a PRV for fire exposure, for a vapor filled vessel, you're just pretending to address the problem, and you're hiding a safety problem from the plant personnel who need to be considering alternate protective measures which have real benefits.

[breizh]

Hi ,

Consider this handbook to support your work. You should find pointers