tailieunhanh - THERMO_V3_6

Tham khảo tài liệu 'thermo_v3_6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Simpo PDFMerae and Split Unregistered Version - http Fluid Flow CENTRIFUGAL PUMPS Cavitation can be a very serious problem for centrifugal pumps. Some pumps can be designed to operate with limited amounts of cavitation. Most centrifugal pumps cannot withstand cavitation for significant periods of time they are damaged by erosion of the impeller vibration or some other cavitation-induced problem. Net Positive Suction Head It is possible to ensure that cavitation is avoided during pump operation by monitoring the net positive suction head of the pump. Net positive suction head NPSH for a pump is the difference between the suction pressure and the saturation pressure of the fluid being pumped. NPSH is used to measure how close a fluid is to saturated conditions. Equation 3-19 can be used to calculate the net positive suction head available for a pump. The units of NPSH are feet of water. - P NPSH Psuction saturation 3-19 where Psuction suction pressure of the pump Psaturation saturation pressure for the fluid By maintaining the available NPSH at a level greater than the NPSH required by the pump manufacturer cavitation can be avoided. Pump Laws Centrifugal pumps generally obey what are known as the pump laws. These laws state that the flow rate or capacity is directly proportional to the pump speed the discharge head is directly proportional to the square of the pump speed and the power required by the pump motor is directly proportional to the cube of the pump speed. These laws are summarized in the following equations. V n Hp n2 3 p n3 3-20 3-21 3-22 Rev. 0 Page 49 HT-03 __Simpo PDF Merge and Split Unregistered Version - http . CENTRIFUGAL PUMPS Fluid Flow where n speed of pump impeller rpm V volumetric flow rate of pump gpm or ft3 hr Hp head developed by pump psid or feet p pump power kW Using these proportionalities it is possible to develop equations relating the condition at one speed to those at a different speed. V1 A n2 1 V 1

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