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Npsha calcs

  • 1.  Npsha calcs

    Posted 05-28-2020 21:28
    Guys,

    When pumping crude oil from a floating roof tank, should the npsha calculation still include the vapor pressure as there is not much of a vapor space?

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    Shantal Ramdeo PE
    Senior Process Engineer
    Heritage Petroleum Company Limited
    Princes Town
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  • 2.  RE: Npsha calcs

    Posted 05-30-2020 01:58
    Shantel
    The vapour pressure should be included irrespective of whether a vapour phase exists. The NPSHR is intended to ensure that vapour doesn't form (cavitation) in the pump.

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  • 3.  RE: Npsha calcs

    Posted 27 days ago
    Simon is correct, of course.

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    Emmett Miller
    Consultant
    Lafayette, CA
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  • 4.  RE: Npsha calcs

    Posted 28 days ago
    I agree with Simon.

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    Tracie Tibiletti
    Process Safety Engineer
    Seadrift Coke LP
    Port Lavaca TX
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  • 5.  RE: Npsha calcs

    Posted 26 days ago
    The other responses are correct in that vapor pressure must still figure in the calculation.

    I think a lot of the confusion is over the meaning of the term "vapor pressure".   The term is misleading in that it seems to suggest that it is a system proper like pressure or temperature.   It is not the pressure in the vapor space.    It is a fluid property similar to the boiling point of a liquid.    Where boiling point is the temperature of boiling for a given pressure,  the vapor pressure is the pressure of boiling for a given temperature.

    Or put another way, the vapor pressure is a value from a reference source -- not something read off a gauge.

    Bruce

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    Robert Coulter PE
    Associate Professor
    Chattanooga State Community College
    Chattanooga TN
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  • 6.  RE: Npsha calcs

    Posted 26 days ago
    Following up on my previous post, maybe you meant to refer to the head space pressure ?    In atmospheric tank, at seal level, this would be 0 psig (14.7 psia).   I don't have any experience with the operation of floating roof tanks but I would presume that the roof must move very easily to avoid a vacuum buildup under the roof.   In that case, the pressure under the roof is the same as atmospheric pressure.   Also, the roof would have some weight.   I don't know if this is significant enough to add pressure.  Maybe when the level is low it is significant?    In any case, the weight of the roof would improve the NPSHA.

    Bruce

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    Robert Coulter PE
    Associate Professor
    Chattanooga State Community College
    Chattanooga TN
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