If you are putting crude in a fixed roof tank, you will need a conservation vent just like any other fixed roof tank. Because the vapors are flammable, you will also need a nitrogen blanket. Then size the vent per API 2000. You must consider thermal breathing as well as fill and empty rates and flow of nitrogen if the blanketing valve fails. None of these requires the knowledge of the heat of vaporization.
You will need that information for the emergency vent valve, however. It frequently takes the form of a relieving manway. If you have access to a process simulation software, you can estimate it. You will need a good characterization of the crude or at least a representative crude. Software I have used for this purpose in the past are HYSYS and UniSim. Set up a flash calculation. Start at the relieving pressure (the lower of emergency vent set pressure doubled or the design pressure of the tank) and the bubble point at that pressure. The flash is at the same pressure but now with 2 or 3% vapor. (Select a vapor pet cent that seems reasonable to you.). Then for the vapor phase the software can give you the heat of vaporization and the average molecular weight of the vapors. (You will need to set up your flash to get those properties. Someone experienced with the software should be able to help you get the properties you need if you do not know how already. . Next estimate the energy input from a fire per API 2000 or API 521. Emergency vent valves are sized for SCFH air equivalent. Again, API 2000 will direct you in how to make that calculation. A good tank vent valve vendor can help you with the rest.
if you do not have a good process simulation software available for your use, you can use an estimate. One response to your query was a recommendation of 50 Btu/lb if the actual value is unknown. My experience is that that figure is not close to reality. It is conservative, however. You will be closer to being correct if you use the value of 150 Btu/lb. but you will still need to estimate an average molecular weight for the relieving vapors.
There are more sophisticated approaches to this problem. It still involves a good process simulation software. But now you model the tank with crude at the maximum level in the tank. If you have a dynamic version, start heating the tank ( at normal operating conditions) at a fire input rate, and determine when the emergency vent valve opens. Note that the greatest demand on that vent will be when it first opens.
This has been a long a long answer, and if I have repeated what you already know, I apologize. Perhaps it will be new material for some new or would be engineers. Good luck with you project.
Suppliers of vents:ProtectosealVarecEmersonTalk to them.Standard: API-2000You might consider a flame Arrestor to prevent reverse flame propagation into the tank. Some pressure reliefs use pipe aways to send gas to flare system. There you definitely will need a flame arrestor.There might be other equipment suppliers.
Your light ends might be butane or lower. I'm not sure. I was in gas processing and flammable fluid storage. Get some analysis of your oil and make some calculations based on partial pressures at temperatures. I think close will be good enough.