PSV Sizing for Fire Case - Relief Temp above Design Temp
PSV Sizing for Fire Case - Relief Temp above Design Temp
(OP)
In reviewing API 521 (4th ed. - 1997), Section 3.15.1.1 Effect of Fire on Wetted Surface of a Vessel says that one should refer to Section 3.15.4 "if elevated temperature is likely to cause vessel rupture." The following section on Effect of Fire on the Unwetted Surface of a Vessel says one should refer to section 3.15.4 AND 3.15.5.
The referenced section 3.15.4 is titled, "Protective Measures Excluding Insulation." It suggests water sprays, depressuring, fire-proofing, earth-covered storage, and diversion walls. The section referenced only for unwetted walls (3.15.5) is for External Insulation.
Why isn't the external insulation section referenced for wetted wall vessel failures? The suggestions of fire-proofing and earth-covering are essentially equivalent. Was this an oversight or am I missing something?
The referenced section 3.15.4 is titled, "Protective Measures Excluding Insulation." It suggests water sprays, depressuring, fire-proofing, earth-covered storage, and diversion walls. The section referenced only for unwetted walls (3.15.5) is for External Insulation.
Why isn't the external insulation section referenced for wetted wall vessel failures? The suggestions of fire-proofing and earth-covering are essentially equivalent. Was this an oversight or am I missing something?





RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
I think. I just recently am trying to get my head around API 521, so any other thoughts or steering are welcome.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
5.15.5.1 (assume its the same as 3.15.5.1 but im not sure): Credit for thermal insulation is typically not takenbecause it usually not meet the fire protection insulation requirements given in 5.15.5.2 through 5.15.5.4. If these requirements are met, a reduction in fire heat input can be obtained by using the environmental factor F...
So (as jistre also says) the destiction is because often insulation does not meet the requirements for fire protection. But if it do meet these requirements then you are allowed to take credit.
Best regards
Morten
PS: Get the new copy not much use learning an old code.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
Whenever I apply fire scenario for wetted and unwetted vessel case, unwetted case always gives the highest temperature sometimes even higher than the carbon steel maximum allowable temperature. Reason is simple. In wetted vessel, majority of heat input is used to vaporize liquid. so temperature increase is lower than that of the unwetted vessel where all the heat is used to increase the vapor temperature. Chances of failure of wetted vessel are small. That's why API has not mentioned wetted vessels.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
What ever liquid you are dealing with , you should first know the boiling point of the liquid at relief pressure. If you have a heavy oil, boiling point could be very high. Depending upon the type of oil,i have seen heavy oil boiling points of 700-800 degC exceeding carbon steel maximum temperature (593 degC).In such cases, there will be no vaporization and the vessel will fail before the PSV lifts.
Since there is no vaporization, we can dismiss this case and consider only the unwetted case.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
My first time posting as well.
For PSV sizing, taking credit for insulation (assuming the API requirements are met), would only reduce the heat input rate (Q) from fire (lowering the F factor in the equation), reducing the relief flow rate if vaporization occurs. For a vessel containing a heavy oil with a bubble point temperature (at relieving condition) that exceeds the max vessel wall temperature (593 oC for CS as Propacket stated earlier), taking credit for insulation would not prevent vessel failure due to high temp, and the case would be dismissed for PSV sizing purposes.
There is a theory that heavy hydrocarbons (heavy gas oils, resids, etc.) undergo thermal cracking reaction under prolonged exposure to fire (similar to a Visbreaker); producing light hydrocarbon vapors which could be used for conservative PSV sizing.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
The way I read API 521 (the 1997 version) Section 3.15.5 is that credit for insulation reducing heat input from fire can be taken for both dry vessels, and vessels with a wetted surface areas. When taking credit for insulation on a vessel (refer to Table 5, and Equations 4, 6 and 9) the Environmental factor (F) is lowered; thus reducing the heat input rate, as well as vessel wall temperature rise. Ultimately, if you look at the equations, the the impact is a reduced relief rate.
API 521 ("Pressure Relieving and De-Pressuring Systems") is a guideline specifically for examining the causes of overpressure as well as to determine relief rates. It evaluates fire in terms of overpressure protection and relief system design. If a fire is likely to cause a vessel to fail from excessive temperatures, even if the PSV is sized adequately, other means of protection should be evaluated. This applies to both vessels in "dry" service and vessels containing high BP liquids. Section 3.15.4 mentions the alternative methods, but outside of depressuring, guidelines for applying these methods are be beyond API 521's scope.
There are other codes and standards that address overall plant Fire Protection, not just for overpressure protection. These documents should cover determining Fire-proofing needs, evaluating emergency response times, and fire-proofing material requirements and applications (including the use of insulation). I would recommend referring to API Publication 2218 - Fireproofing Practices in Petroleum and Petrochemical Processing Plants. This document provides a more comprehensive guideline for fire-proofing (particularly for passive methods). If I'm wrong, please let me know.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
You write "..When taking credit for insulation on a vessel (refer to Table 5, and Equations 4, 6 and 9) the Environmental factor (F) is lowered; thus reducing the heat input rate, as well as vessel wall temperature rise." But for the wetted case the wall temp will not come down since the wall temp=boiling point - that wont change due to insulation. The question was re. with relevance of API 521 when the boiling point was above the temp where the metal has lost so much strength that it will burst anyway.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
For the situation being discussed, API 521 suggest other means of protection since PSV cannot protect against the vessel from failing due to excessive temperature. "Fire-proofed" insulation, and other means of protection can be used as a safeguard to ensure adequate time for emergency response, however guidelines for its application is outside of API 521's scope.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp
You can either reset the relief valve to say 100 psia, added deluge water or add a blast wall or just decide the failure of that vessel will not add enough risk to worry about.
If the gases inside are deadly you might then take a track to mitigate the risk of failure, but if they are that deadly, the relief had better be piped to a safe point and you'd better get real particular with the seals on valves and connects because they will be close to failing.
RE: PSV Sizing for Fire Case - Relief Temp above Design Temp