Variant questions based on real exam cases
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25. A centrifugal pump is used to transfer the 75°C fluid medium from an underground open tank to a tank at a higher elevation of 10 m. It is known that the flow rate Q = 18 m3/h, the pipe specification is φ57×3.5 mm, the length of the suction pipeline is 12 m (including the equivalent lengths of various pipe fittings), and the resistance coefficient of the pipeline is λ = 0.03. The local atmospheric pressure is 97 kPa. If the safety margin for the pump’s installation height is not taken into account, which of the following values represents the calculated installation height (in meters) for this pump? It is known that the saturated vapor pressure Pv of this medium at 75°C is 49 kPa, and its density is 890 kg/m3. The net positive suction head available (NPSHa) for this pump system is 3.0 m. (A) 1.0 (B) 0.5 (C) 0 (D) -0.5 If the question is changed to one in which the safety margin for the pump’s installation height is 0.5 m, how should the equation for the installation height be solved? Should we subtract 0.5 or add 0.5 based on a calculation that does not take safety margins into account? If this safety margin is represented by Hg, then I think we should subtract it. I believe Hg = (P1-Pv)/pg – NPSHa – Hf – 0.5. Is that correct?
This post was last edited by Higee on 2016-7-20 at 15:10. A safety margin is taken into account by reducing the installation height, which is equivalent to increasing NPSHa. In this case, no safety margin is considered, so option C is chosen; if a 0.5 margin is considered, then option D is chosen