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.
Background Fluid Moving EquipmentFluids are moved through flow systems using pumps, fans,blowers, and compressors. Such devices increase themechanical energy of the fluid. The additional energy canbe used to increase• Velocity (flow rate)• Pressure• Elevation
BackgroundPump, fan, blower, and compressor are termsthat do not have precise meaning. Generallypumps move liquids while fans, blowers andcompressors add energy to gasses.Pumps and fans do not appreciably affect thedensity of the fluids that they move and thusincompressible flow theory is applicable.
Centrifugal PumpsMost common type of pumping machinery. There are manytypes, sizes, and designs from various manufacturers whoalso publish operating characteristics of each pump in theform of performance (pump) curves. The device pictured onthe cover page is a centrifugal pump.Pump curves describe head delivered, pump efficiency, andnet positive suction head (NPSH) for a properly operatingspecific model pump.Centrifugal pumps are generally used where high flow ratesand moderate head increases are required.
Positive Displacement PumpsTo move fluids positive displacement pumps admit afixed volume of liquid from the inlet into a chamberand eject it into the discharge.Positive displacement pumps are used when higherhead increases are required. Generally they do notincrease velocity.
Pump Specification Recall Mechanical Energy Balance W ( ˆ = ∆ αV 2 ) + g∆z + ∆p + 4 f L V + ∑ Ki 2 N •m 2 ρ D 2 kg Wˆ = ( + ) ∆ α V 2 g∆z ∆p + L + 4 f + ∑ K i V2 ft • lb f 2 gc gc ρ D 2 gc lbmBoth equations describe work that must be supplied to system
Pump HeadWhat happens if the MEB is multiplied through by g (gc/g)? ˆ = ( W 1 ∆ αV 2 ) + g∆z + ∆p L V + 4 f + ∑ K i 2 g g 2 ρ D 2 What are the units (SI)? N • m s2 kg • m 3 s 2 2 = =m kg m kg • s m 2 2 ^ W/g has units of length and is known as the pump head
Example 2 3 1 Tank B Tank AWhy do we choose point 2 rather than 3 for MEB?What kind of valve to uses to control flow rate?
Example 2 3 1 Tank B Tank AMechanical Energy Balance (in terms of head) ∆p L V 2 H = ∆z + + 1 + 4 f + ∑ K i ρg D 2g V 2 = H min + φ 2g
Head vs. Flow Rate V 2 H = H min + φ 2g Quadratic In V or q 2 L V 1 + 4 f D + ∑ K i 2 g g c ∆p H min = ∆z + ρg
System Response 2 3 1 Tank B Tank AWhat happens when flow control valve is closed?• Resistance (f) increases• Flow rate decreases• Need more head to recover flow rate
System Response Constant Flow ResponseValve Closed Valve Open Constant Head Response
Pump CurvesPump manufacturers supply performancecurves for each of their pumps. These arenormally referred to as ‘pump curves’. Thesecurve are generally developed using water asthe reference fluid.The following can be read directly from a pumpcurve:• Head vs. flow rate information for any fluid• Pump efficiency for any fluid• Pump horsepower for system operating with water
Power InputFor fluids other than water: Wˆ P=m η ˆ m W g ft • lb f gal 1 ft 3 lbm H lb ∗ q min ∗ 7.48 gal ∗ ρ ft 3 gc P (hp ) = m ft • lb f s η ∗ 550 ∗ 60 s • hp min
Power Input Easier Way Pfluid ρ fluid = = Sp. Gr. fluid Pwater ρ waterNote: A less dense fluid requires less horsepower
Goulds Pump CurvesManufacturers provide series of pumps to cover broad ranges ofcapacities, heads, and suction and discharge piping diameters. Mostpumps can be equipped with different diameter impellers and can beoperated at different speeds to change capacities.The curves provided are for a few variations of the Goulds model 3196process pump. Each curve corresponds to a specific pump and aspecific RPM. Pump sizes are denoted with 3 numbers. 3x4-7 Discharge Suction Casing Diameter Diameter Diameter Inches Inches Inches Note: Try to match process piping diameters with the pump discharge and suction diameters.
Pump SelectionGoal is to find a pump whose curve matches the pipingsystem head vs. flow rate curve. We can superimpose theprevious head-flow rate curve on the manufacturers pumpcurves.To select a specific pump from a product line, find the pumpwith the highest efficiency that does not require the use ofthe largest impeller diameter. This will allow for futureproduction expansions.Suppose that we have a process that requires a flow rate of300 gpm and has a head requirement of 60 ft. at that flowrate. Can a 3x4-10 model 3196 Goulds pumps be used?
Example Impeller Diameter = For Desired Q Head = How do can you force the system to operate on the pump curve?
Net Positive Suction Head (NPSH)Associated with each H-Q location on the pump curve is aquantity that can be read called NPSH.An energy balance on the suction side of the fluid system(point 1 to pump inlet) with pinlet set to the vapor pressure ofthe fluid being pumped gives a quantity called NPSHA (netpositive suction head available). g c p1 − pv L Vinlet 2NPSHA = − 4 f + ∑ Ki + ( z1 − zinlet ) g ρ D 2
Net Positive Suction HeadThe requirement is that: NPSHA > NPSHOtherwise (if NPSHA < NPSHpump), the pressure at thepump inlet will drop to that of the vapor pressure of thefluid being moved and the fluid will boil.The resulting gas bubbles will collapse inside the pump asthe pressure rises again. These implosions occur at theimpeller and can lead to pump damage and decreasedefficiency. Cavitation
NPSHDo not use NPSH to size or select a pump unless all elsefails. Pump selection is governed by H vs. Q requirementsof system. When NPSHA is too small, it might be increasedby:• Increasing source pressure (not usually feasible)• Cooling liquid to reduce vapor pressure (not usually feasible)• Raise elevation of source reservoir• Lower elevation of pump inlet• Raise level of fluid in reservoir
If NPSHA Can’t Be IncreasedIf the pump must be modified to achieve proper NPSH:• Larger slower-speed pump• Double suction impeller• Larger impeller eye• Oversized pump with an inducer
ExampleFlow = 600 gpm of benzene 60°F 2 P2 = 16.1 psiaData for benzene: 5 ft 3 PVap = 7.74 psia P3 = 16 psia ρ = 50.1 lbm /ft3 µ = 0.70 cP 150 ft P1 = 16 psia globe valve (open) 1 5 ft L = 300 ft, 5 inch Sch40 Use Goulds 3x4-10 L = 5 ft, 6 inch Sch40 @3560 RPM
Pump Selection from Many Choices of Characteristic Curves 1. Examine pump curves to see which pumps operate near peak efficiency at desired flow rate. This suggests some possible pipe diameters. 2. Compute system head requirement for a few diameters. 3. Compute V for some diameters. For water V in the range of 1 – 10 ft/s is reasonable (see ahead). 4. Re-examine pump curves with computed head and pipe diameters. This may give a couple of choices. 5. Pick pump with highest efficiency.
Selection of Pipe SizeOptimum pipe size depends mainly on the cost of thepipe and fittings and the cost of energy needed forpumping the fluids.Cost of materials increase at a rate proportional to aboutD1.5, while power costs for turbulent flow varies as D–4.8.One can find correlations giving optimum pipe diameteras a function of flow rate and fluid density, however theoptimum velocity is a better indicator as it is nearlyindependent of flow rate.
Optimum Pipe SizeFor turbulent flow of liquids in steel pipes larger than 1 in. Vopt [=] ft s 0 .1 12 m Vopt = 0.36 m[=] lbm s ρ ρ[=] lbm ft 3
Remember• Maximize pump efficiency• Power input (hp) should be minimized if possible• Selected impeller diameter should not be largest or smallest for given pump. If your needs change switching impellers is an economical solution• NPSH required by the pump must be less than NPSHA
Variable Speed Pumps Advantage: Lower operating cost Disadvantage: Higher capital cost System head requirement (no valve) RPM1 RPM2H (ft) Pump curve for Di q (gpm) q produced by pump q* (desired) with no flow control
Affinity LawsIn some instances complete sets of pump curvesare not available. In this instance the pumpaffinity laws allow the performance of a newpump to be determined from that of a similarmodel. This can be useful when modifying theoperating parameters of an existing pump.