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Latent Heat of Vaporization of Crude Oil

  • 1.  Latent Heat of Vaporization of Crude Oil

    Posted 04-29-2020 20:20
    Good day,

    I am presently sizing a vent valve for a crude oil tank, and i require the Latent heat of vaporization of molo crude....I can use a light crude as a benchmark. Thanks in advance.

    Shantal Ramdeo PE
    Senior Process Engineer
    Heritage Petroleum Company Limited
    Princes Town

  • 2.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 04-30-2020 03:33
    Crude oil is normally degassed and stabilised prior to storage, thus there is no need for a vent valve. It can be stored in floating roof tanks (with no vapour space) or fixed roof tanks fitted with an inert gas blanket.
    If the crude oil is not fully degassed the vent ought to be connected to a flare with a purge gas stream to prevent oxygen ingress to the tank. There is no need for a vent valve. Whether vented or blanketed care is required to ensure the tank pressure stays within the definition of an "atmospheric" tank, i.e. very low pressure as these are not designed as pressure vessels.

    Simon Richards CEng,EurIng
    United Kingdom

  • 3.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 04-30-2020 06:34
    I referenced api 2000 which indicated that a pressure vacuum.vent valve can be installed. 

  • 4.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 04-30-2020 06:39
    Hi Shantal,
    a pressure vacuum/ vent valve is more to do with thermal expansion contraction in my view. This would still require a nitrogen/ fuel gas  blanket to prevent air ingress to the tank when under vacuum.

    Simon Richards CEng,EurIng
    United Kingdom

  • 5.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-01-2020 01:03
    ​I agree with Simon, crude oil storage tank normally built with floating roof so vent valve is not required


    Hiteshkumar Patel PE,CSP
    Toyo Engineering India Ltd
    Surat City

  • 6.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-01-2020 09:41
    If she has an old fixed roof tank (API-650) there still may be a need for a vent valve.  I also agree that an inert gas must be used to prevent oxygen intrusion into the tank when pumping out or normal changes in atmospheric conditions.   There are gas blanketing valves that must be sized for the pump out rate plus a safety factor.  There is a range in sophistication of these valves.   But you will need a source of inert gas or nitrogen.
    I don't think the latent heat will change much between varying crude oils.  But that is just an opinion.

    Dennis Callan
    Royal Oak MI

  • 7.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-01-2020 10:57
    Guys, the inetent is to temporarily use a fixed roof tank to store crude oil. The latent heat of evaporation of crude oil (or lightest components that are liberated during exposure to solar radiation) is what I require to determine the capacity of the vent valve. Most of the light ends would be liberated from upstream, yes, but there will be some out breathing still from the tank during pumping out or when the tank is standing and being heated via sunlight exposure (I live in the caribbean). That's the intent for the data requested. We do  it have  the flexibility of retrofitting with a floating roof at this time. Also, according to api 650 some floating roofs still have vent valves.

  • 8.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-01-2020 11:41
    API 521 suggests using 50 BTU/lb for unknown hydroacarbons

    Alfredo Saettone
    Engineering Principal - Process

  • 9.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-02-2020 06:21
    I would expect a cone roof tank to be fitted with a pressure/vacuum relief valve such as the one shown on the this product data sheet from Motherwell Tank Protection.  http://www.motherwelltankprotection.com/downloads/pressurevacuumreliefvalvecnc380-103556.pdf?

    The greatest outflow of vapor is likely to be during tank filling and if that is the case, then the question of how much vapor is generated by solar heating is not an issue since it will less than the vapor load created by tank filling. 

    I encourage you to look through API 2000 which should offer full guidance. 

    That said, the lighter molecules will be the ones to vaporize from the tank and the latent heat of vaporization of butane is 165.6 btu/lb.


  • 10.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-05-2020 01:08

    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. 

    Robert Clay, PhD, MBA, PE
    Sr. Associate Engineer
    ECI, Inc.
    Lenexa, Kansas

  • 11.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-02-2020 18:06
    Hi Shantal,
    Have you considered any potential gas blow by from upstream equipment into this atmospheric tank? This could be an important scenario to consider.

    Edmond Baruque Msc
    HSE Specialist
    Audubon Field Solutions
    Katy TX

  • 12.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-02-2020 21:11
    Check that you won't be exceeding explosive limits in the headspace of that fixed roof tank.  That's not okay even on a temporary basis.  A vent valve will increase the concentration of vapors in the headspace.


    Kirsten Rosselot
    Process Profiles
    Calabasas, CA United States

  • 13.  RE: Latent Heat of Vaporization of Crude Oil

    Posted 05-02-2020 22:29

    Suppliers of vents:
    Talk to them.
    Standard:     API-2000

    You 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. 

    Dennis Callan
    Royal Oak MI