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Lecture+5.++Sizing+Pumps - UNIVERSITY OF CALIFORNIA...

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UNIVERSITY OF CALIFORNIA Department of Viticulture and Enology VEN 135 WINE TECHNOLOGY AND WINERY SYSTEMS 5. SIZING PIPES AND PUMPS (CONTINUED) NET POSITIVE SUCTION HEAD (NPSH) Net positive suction head (NPSH) is the pressure available at the inlet or suction side of a pump. Every pump has a characteristic NPSH required for efficient pumping. This NPSH R can usually be found on a pump curve or from the pump manufacturer. If the available NPSH, NPSH A , is less than the NPSH R , then cavitation may occur. Cavitation is the process of forming vapor bubbles at the low pressure pump inlet that later implode at the high pressure discharge side of the pump head. These implosions can damage the pump head and/or seals and should be avoided. Cavitation frequently sounds like rocks or gravel bouncing around in the pump head. To be sure that the available NPSH is high enough it can be calculated in a fashion similar to the calculation of the total mechanical head, H m , but only the section of equipment upstream of the pump is used as follows: vap v fs a A H H H Z H NPSH - - - ± = (5.1) where H a is atmospheric pressure at a given altitude, Z is the height difference between the starting point and the pump (+ if pump is lower, - if pump is higher), H fs is the friction head upstream of the pump, H v is the velocity head (which will normally be negligble), and H vap is the vapor pressure of the fluid being transferred at the given temperature. If you are pumping water, cavitation is not usually an issue. However, there are several factors that could make it more of an issue. For instance, placing numerous fittings just upstream of the pump may cause cavitation by increasing the friction head component, thus decreasing NPSH.
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