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This question is related with capacity estimation for safety relief valve for external fire case for heavy oil services. Typical VDU bottoms stream which is sent to delayed coker plant as feed stream is received in a surge drum inside coker plant. Being a heavy stream, the boiling point is high say 550 Deg.C plus. For external fire case, the heavy oil will start cracking and will release the lighter gases. Being a carbon steel vessel, the vessel will start approaching to rupture conditions after temp reaches 400 Deg.C but the material has still not started boiling or not started cracking. How do we protect such equipment from external fire and can anyone guide on arriving/estimating cracked gas quantity? How does one estimate cracking temperature? Please help on sizing PSV for such cases.
 
Answers
08/07/2008 A: Graham Bennett, DNV , graham.bennett@dnv.com
Submitted by one of my colleagues.
The heavy steam in the VDU should be a mixture of various hydrocarbons. Normally it is a bit misleading to talk about the boiling point of a mixture, since a boiling point is only true for a pure component. For a mixture, we normally talk about bubble point (the temperature point at which the first bubble of vapor is formed) and dew point (the temperature point first liquid is formed). If a vessel containing a mixture is under fire, the mixture will start to evaporate while the liquid temperature reaches the bubble point, and all the mixture will evaporate completely when the temp reaches to the dew point. During this period (from bubble point to dewpoint), the latent heat (L) is not constant, but can be decided by the difference between vapour enthalpy and liquid enthalpy at each temperature point. A series of required release rates can then be calculated by the absorbed heat (Q: defined by the vessel wetted area - API 521 section 3.15.2) divided by the various latent heats (defined above) at different temperatures. We can then use the equation at API 520 Part 1 section 3.6 Sizing for Gas or Vapor Relief to get a series of required orfice areas of a pressure relief valve. We should remain aware that while using this API520 equation, temperature and molecular weight are different at different release temperatures. The relief valve should finally be selected based on the maximum orifice size that is calculated.
The normal methods to prevent equipment from external fire are (1) passive fire protection both to the vessel and to the support structure (2) adequate drainage (eg, slope the ground) to drain the released liquid to a safe place (3) adequate fire detection and firefighting system (fire wire, flame detection, firewater etc).
The rupture condition of a vessel is normally decided by the vessel internal stress (defined by the vessel internal pressure and vessel thickness), the vessel wall temperature, and the time under this temperature. For example, API 521 figure 2 shows that for a steel plate (ASTM A 515, Grade 79), at internal stress 10,000 pounds per square inch and at temperature 480 DegC, the vessel does not rupture even after 1000 hours. Whilst if the internal rupture stress reaches 40,000 pounds per square inch, at the same temperature, it will rupture after 20 hours. At an internal rupture stress of 80,000 psi, at the same temperature, it will rupture about 1 minute.
How to estimate cracking temperature? Again, whether the vessel will rupture or not is a function of internal pressure, wall thickness, wall temperature, time under fire etc. Because Figure 2 at API 521 is based on steel ASTM A 515, Grade 79, it can only be used as a reference for calculation. However if we know the vessel wall thickness, the design pressure of the vessel, the wetted area of the vessel, all these questions can be answered.
07/07/2008 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
The inventory of vacuum residue in the feed surge drum provides a heat sink for the heat entering from the external fire in the vicinity of the vessel. Depending upon the envisioned active duration of the fire incident before being controlled and the incoming temperature of the vacuum residue and the typical minimum inventory level, this heat sink effect can greatly mitigate the incidental generation of thermal cracking vapors.
To almost eliminate incidental generation of thermal cracking vapors, the best practice is to insulate the feed surge drum [or at least bottom half of horizontal drum or bottoms 20 feet of vertical drum].
Insulation type is selected to be resistant to high temperatures during fires and the exterior of the insulation is jacketed with stainless steel sheathing [minimum TP 410 SS].
Insulation and jacketing of the feed surge drum is generally recommended since vacuum residues should not be cooled below 120C for safety reasons; and there is increasing interest in transferring "heavier / more viscous" vacuum residues from the Vacuum Distillation Unit to the Delayed Coker Unit at 230C to 285C. Remember that vacuum residues exhibit quirky / non-Newtonian / high viscosities in the real world [though not necessarily in the computer simulation world]---so specific heat energy recovery ["encon"] from vacuum residues becomes increasingly more challenging below say 250C.