Transcript pumps
Pumps CBE 150A – Transport Spring Semester 2014 Goals • Describe how centrifugal and positive-displacement pumps operate and common applications. • Calculate system head requirements. • Determine head, pump efficiency, and pump. horsepower from a typical centrifugal pump curve. • Define net positive suction head (NPSH) and understand how it relates to cavitation. • Compute NPSH required by a pump. • Determine an appropriate pump (impeller diameter, efficiency, etc.) for a given required head. • Describe how to modify system to operate on the appropriate pump curve. CBE 150A – Transport Spring Semester 2014 Background Fluid Moving Equipment Fluids are moved through flow systems using pumps, fans, blowers, and compressors. Such devices increase the mechanical energy of the fluid. The additional energy can be used to increase • Velocity (flow rate) • Pressure • Elevation CBE 150A – Transport Spring Semester 2014 Background Pump, fan, blower, and compressor are terms that do not have precise meaning. Generally pumps move liquids while fans, blowers and compressors add energy to gasses. Pumps and fans do not appreciably affect the density of the fluids that they move and thus incompressible flow theory is applicable. CBE 150A – Transport Spring Semester 2014 Centrifugal Pumps Most common type of pumping machinery. There are many types, sizes, and designs from various manufacturers who also publish operating characteristics of each pump in the form of performance (pump) curves. The device pictured on the cover page is a centrifugal pump. Pump curves describe head delivered, pump efficiency, and net positive suction head (NPSH) for a properly operating specific model pump. Centrifugal pumps are generally used where high flow rates and moderate head increases are required. CBE 150A – Transport Spring Semester 2014 Centrifugal Pumps CBE 150A – Transport Spring Semester 2014 Impeller CBE 150A – Transport Spring Semester 2014 Positive Displacement Pump CBE 150A – Transport Spring Semester 2014 Positive Displacement Pumps To move fluids positive displacement pumps admit a fixed volume of liquid from the inlet into a chamber and eject it into the discharge. Positive displacement pumps are used when higher head increases are required. Generally they do not increase velocity. CBE 150A – Transport Spring Semester 2014 LVAD Application CBE 150A – Transport Spring Semester 2014 Pump Head What happens if the MEB is multiplied through by g (gc/g)? 2 Wˆ 1 V 2 p L V gz 4 f K i g g 2 D 2 Pump Head What are the units (SI)? CBE 150A – Transport System Head N m s2 kg m3 s 2 2 kg m kg s 2 m2 m Spring Semester 2014 Pump Cavitation Net (above the vapor pressure) Positive Suction Head (NPSH) P vap Head Required NPSH Pump Curve Pb Vb2 Head Available 2 gc Pb hfs Za Pa CBE 150A – Transport Spring Semester 2014 Pump Curve Example Pump Curves (Resources Section) What pump horsepower is required to pump 150 GPM of water at a total head of 225 ft using a Goulds Model 3196 operating at 3540 RPM ? What is the required NPSH ? What is the pump efficiency ? CBE 150A – Transport Spring Semester 2014 Pump Curves CBE 150A – Transport Spring Semester 2014 System Response 2 3 1 Tank B Tank A What happens when flow control valve is closed? • Resistance (f) increases • Flow rate decreases • Need more head to recover flow rate CBE 150A – Transport Spring Semester 2014 System Response Constant Flow Response Valve Closed Valve Open Constant Head Response CBE 150A – Transport Spring Semester 2014 10 Minute Problem A centrifugal pump (Goulds Model: 3196 - 1770 RPM - CDS 3832) with an 11 inch impeller is used to transport water at 350 GPM through the system show below. The system head with the control valve completely open is 90 ft lbf / lbm. For the desired flow rate, it is necessary to partially close the control valve. What pressure drop across the valve is required ? Tank B Tank A CBE 150A – Transport Spring Semester 2014