fluids-2 - Fluid Flow - Pumps Available Net Positive...

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Fluid Flow - Pumps Available Net Positive Suction Head P s = suction pressure P v = vapor pressure of fluid NPSHA has to be positive. Otherwise, the fluid enters the pump with bubbles. g P g P NPSHA V S ρ - = ChE 4253 - Design I
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Fluid Flow - Pumps NPSHA: As pressure increases inside the pump the bubbles collapse. This phenomenon is called CAVITATION and it -Reduces capacity -Damages the pump ChE 4253 - Design I
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Fluid Flow - Pumps Net Required Positive Suction Head (NPSHR) Ideal pumps will not cavitate if NPSHA is positive. However a small pressure decrease can take place in a pump due to internal losses close to the suction. ====> if NPSHA = 0 bubbles can form and cavitation takes place. ====> NPSHR is a required value suggested by the manufacturer Specification Criteria NPSHA > NPSHR ChE 4253 - Design I
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Fluid Flow - Pumps Head Capacity Curves Rotary Reciprocating Centrifugal head (P d -P S ) Positive displacement pumps give an approximate constant flow Flowrate (gpm) ChE 4253 - Design I
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Fluid Flow - Pumps Thus, centrifugal pumps are chosen because they can operate in a wider range of flowrates (better control and process flexibility). -------------------------------------------------------------- If you are stuck with a positive displacement pump, the following diagram shows how you can regulate flow. However, this arrangement will: - use more energy - heat up the fluid ChE 4253 - Design I
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Fluid Flow - Pumps Centrifugal Pump Performance Curves Want to operate somewhere here 6 8 6 8 Characteristic Curve % 60 % 70 Efficiency NPHSR Impeller size h Q (flow rate) ChE 4253 - Design I
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Specifying a Pump Parameters you can control when selecting the pump: impeller diameter, speed (not very common), the model Things to look for: Maximum efficiency, NPSHA > NPSHR
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Specifying a Pump Need to calculate: System head H S = (P 4 -P 3 )+(P 2 -P 1 ) Pump head: Hp But: Hp = Hs 1 2 4 3 Control valve ChE 4253 - Design I
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Specifying a Pump Effect of throttling a valve. Hs Valve closes Flowrate ChE 4253 - Design I
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Specifying a Pump Since Hs = Hp , pick the Hs curve close to 80% open, at maximum flow H Flowrate System curve Max. flow in this interval Pump curve
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What to do if NPSHA is too low NPSHA = (P 2 -P
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This note was uploaded on 09/07/2009 for the course CBME Kinetics & taught by Professor Lobban during the Spring '09 term at The University of Oklahoma.

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fluids-2 - Fluid Flow - Pumps Available Net Positive...

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