Ross Mackay Forum Moderator 94 Posts
Re: Quantifying liquid density effects on NPSH1 September 2006 at 1:29pmThe reason we use a linear measurement or head in the identification of NPSH and other heads around a centrifugal pump is to avoid the confusion that is relevant between liquids of different densities. In essence, two columns of the same height, but of liquids of different densities provide different pressures at the bottom of the column. However, the head at the bottom of both columns is the same, because the height is the same.
Your pump supplier has identified that the pump requires 3.0m of NPSH at a specific capacity flow. This can be accepted as 3.0m of any liquid. As you have an NPSH available of 4.8m, you can assume you have a cushion of 1.8m of NPSH.
When calculating NPSH available for specific pump in diesel service, I have found some contradictory data in the literature concerning the fluid density influence on NPSH required for the pump. For example, if NPSH required for the pump is equal to 3m of water at 20°C (liquid density = 1kg/dm3), what is the NPSH required for subcooled diesel liquid at 200°C, with liquid mass density of 0.635kg/dm3 at process conditions, for the same pump (diesel vapor pressure is insignificant)? Is it equal to 3m, is it lower because of lower liquid density compared to water, or is it higher because of lower hydrostatic pressure at pump suction? I have NPSH available of 4.8m of subcooled diesel at pump suction centerline, and I wonder how can I recalculate it on water terms, in order I could compare my NPSHav with NPSHr which is given for subcooled water. Generally, what is the way of comparing NPSHav (process fluid) with NPSHr given for water at 20°C? There must be some formula to quantify liquid density effects on NPSH required.
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