ME 4880 Experimental Design Lab Centrifugal Pump Performance Experiment Instructors: Dr. Cyders, 294A Stocker, cyderst@ohio.edu Dr. Ghasvari, 249B Stocker, ghasvari@ohio.edu Spring 2014 1
Part I. General topics on Pumps Categories of Pumps Pump curve Cavitation NSPH
Pumps Basic definitions to describe pumps and pumping pipe circuits Positive displacement pumps and centrifugal pumps The Pump Curve Net Positive Suction Head
Pump analysis: energy equation 2 2 P1 V1 P2 V2 z1 z2 hfriction h g 2g g 2g pump 1 2 Q Shaft work delivered by pump is translated into a pressure rise across the pump: P 2 > P 1 How does h pump vary with Q? Typically data is gathered from experiments by manufacturer and is presented in dimensional form (pump curve)
Definitions in a typical pump system: Liquid flows from the suction side to the discharge side Suction head is head available just before pump, h s : Discharge head is head at the exit from pump, h d : Pump head, h p : hp hd hs = head required from pump Flow rates affect terms h fd & h fs 2 2 P1 V1 P2 V2 z1 z2 hfriction h g 2g g 2g h z P h g s s s fs h z P h g d d d fd pump
Positive Displacement Pumps Properties of a PD pump: Pumps fluid by varying the dimension of an inner chamber. Volumetric flow rate determined size of chamber + RPM of pump. Nearly independent of back pressure. Application for metering fluids (example, chemicals into a process, etc.) Develops the required head to meet the specified flow rate Head limit is due to mechanical limitations (design/metallurgy). Catastrophic failure at limit. High pressure applications Able to handle high viscosity fluids. Often produces a pulsed flow
Types of Positive Displacement Pumps A. Reciprocating piston (steam pumps) B. External gear pump C. Double-screw pump D. Sliding vane E. Three lobe pump F. Double circumferential piston G. Flexible tube squeegee H. Internal gear
Positive Displacement Pumps
Centrifugal pumps Characteristics Typically higher flow rates than PDs. Comparatively steady discharge. Moderate to low pressure rise. Large range of flow rate operation. Sensitive to fluid viscosity.
Efficiency of centrifugal pumps: From the energy equation, pumps increase the pressure head The power delivered to the water (water horse power) is given by The power delivered by the motor to the shaft (breaking horse power) is given by Therefore, efficiency is: 2 2 P1 V1 P2 V2 z1 z2 hfriction h g 2g g 2g H Pw Pbhp P g QP T P P w BHP Pw gqh T gqh pump Note: 1HP = 746W
Centrifugal pumps pump curves Real pumps are never ideal and the performance of the pumps are determined experimentally by the manufacturer and typically given in terms of graphs or pump curves. Typically performance is given by curves of: Head versus capacity Power versus capacity NPSH versus capacity As Q increases the head developed by the screen decreases. Maximum head is at zero capacity The maximum capacity of the pump is at the point where no head is developed.
Centrifugal pumps Sample Pump P P L Q 2 1 H pump z2 z1 f hm 2 2 g D D 2g 4 Curve 3500 is the RPM Impeller size 6¼ to 8¾ in. are shown Maximum efficiency is ~50%. Note that pumps can operate at 80-90% eff. Maximum normal capacity line Should not operate in the region to the right of the line because pump can be unstable. Semi-open impeller Max sphere 1¼ This pump is designed for slurries / suspensions and can pass particles up to 1¼. This is why efficiency is relatively low. Motor horse power. Remember to correct for density using previous equation Operating line (system curve) This is dependent on the system you are putting the pump into. It is a plot from the energy equation. That is, analyze the system to determine the pump head required as a function of flow rate through the pump This will form the 2 system line.
Pump cavitation and NSPH Cavitation should be avoided due to erosion damage to pump parts and noise. Cavitation occurs when P < P v somewhere in the pump Since pump increases pressure, to prevent cavitation we ensure suction head is large enough compared to vapour pressure P v Net positive suction head P P g s v NPSH zs hfs Often we evaluate NPSH using energy equation and reference values don t measure P inlet
NSPH required Manufacturers determine conservatively how much NPSH is needed to avoid cavitation in the pump Systematic experimental testing NSPH required (NPSHR) is plotted on pump chart Caution: different axis scale is common read carefully Plot NPSH vs NSPH required to give safe operating range of pump Q max Q
Part II. Dimensional analysis Affinity Laws
Dimensionless pump performance Previous part: everything dimensional Terminology used in pump systems Pump performance charts NPSH and avoiding cavitation (NPSH vs NPSHR) This part : Discuss how centrifugal pumps might be scaled Best efficiency point Examples
Dimensionless Pump Performance For geometrically similar pumps we expect similar dimensionless performance curves Dimensionless groups? Capacity coefficient Head coefficient Power coefficient Efficiency NPSH? C What to use for n (units 1/time): rad/s (), rpm, rps C Q NPSH Q nd 3 C H g NPSH 2 2 nd gh nd 2 2 C P P bh 3 5 nd C C H Q C
Dimensional Analysis If two pumps are geometrically similar, and The independent s are similar, i.e., C Q,A = C Q,B Re A = Re B A /D A = B /D B Then the dependent s will be the same C H,A = C H,B C P,A = C P,B
Affinity Laws For two homologous states A and B, we can use variables to develop ratios (similarity rules, affinity laws, scaling laws). C C Q D B B B Q, A Q, B Q A A DA 3 Useful to scale from model to prototype Useful to understand parameter changes, e.g., doubling pump speed.
Dimensional Analysis: ideal situation If plotted in nondimensional form, all curves of a family of geometrically similar pumps should collapse onto one set of nondimensional pump performance curves From this we identify the best efficiency point BEP Note: Reynolds number and roughness can often be neglected
Dimensionless Pump Performance In reality we never achieve true similarity E.g. manufacturers put different impeller into same housing Following figure illustrates a typical example of 2 pumps that are close to similar Note: See that at BEP: max = 088 From which we get * * * * CQ, CH, CHS, C x From which you can calculate Q, H, NPSH, P
Part III. More on Centrifugal Pumps Pump selection
Pump selection Previous part : Other types of pumps Centrifugal and axial ducted Pump specific speed This part Non-dimensional Pi Groups for pumps C Application to optimize pump speed (BEP) Scaling between pumps NPSH g NPSH 2 2 nd C P P bh 3 5 nd C H gh nd 2 2 C Q Q nd 3
Dynamic Pumps Dynamic Pumps include centrifugal pumps: fluid enters axially, and is discharged radially. mixed--flow pumps: fluid enters axially, and leaves at an angle between radially and axially. axial pumps: fluid enters and leaves axially.
Centrifugal Pumps Snail--shaped scroll Most common type of pump: homes, autos, industry.
Centrifugal Pumps
Centrifugal Pumps: Blade Design
Centrifugal Pumps: Blade Design Vector analysis of leading and trailing edges.
Centrifugal Pumps: Blade Design Blade number affects efficiency and introduces circulatory losses (too few blades) and passage losses (too many blades)
Open vs. Ducted Axial Pumps Axial Pumps
Open Axial Pumps Blades generate thrust like wing generates lift. Propeller has radial twist to take into account for angular velocity (=r)
Ducted Axial Pumps Tube Axial Fan: Swirl downstream Counter-Rotating Axial- Flow Fan: swirl removed. Early torpedo designs Vane Axial-Flow Fan: swirl removed. Stators can be either pre-swirl or postswirl.
Pump Specific Speed Pump Specific Speed is used to characterize the operation of a pump at BEP and is useful for preliminary pump selection.
Centrifugal pumps-specific speed Use Dimensionless specific speed to help choose. Dimensionless speed is derived by eliminating diameters in C q and C h at the BEP. Proper Lazy N N ' s s C C Q* H* 1 2 3 4 nq gh Rpm( Gal / min) 3/ 4 H ( ft) * * 1 2 1/ 2 3/ 4 N s 17,182 N ' s
What we covered: Characteristics of positive displacement and centrifugal pumps Terminology used in pump systems Head vs flow rate: pump performance charts NPSH and avoiding cavitation (NPSH vs NPSHR) Examples
What we covered: Today we Developed dimensionless pump variables Extrapolate existing pump curve to different pump speeds, diameters, and densities Examples C C C C Q H P NPSH Q nd 3 gh 2 2 nd Pbh 3 5 nd g NPSH 2 2 nd
Today we: What we covered Examined axial, mixed, radial ducted and open pump designs Used specific speed to determine which type is optimal
Part IV. Lab procedure Venturi Measurements Summary of equations and calculation way Preparing graphs
Lab Objectives Understand operation of a dc motor Analyze fluid flow using Centrifugal pump Venturi flow meter Evaluate pump performance as a function of impeller (shaft) speed Develop pump performance curves Assess efficiencies
Lab Set-up Paddle meter Valve Dynamometer Venturi ( P) E I Pout Motor T Pump Pin Water Tank
D.C motor Armature or rotor Commutator Brushes Axle Field magnet DC power supply Figure 1. dc motor (howstuffworks.com)
Centrifugal pump operation Rotating impeller delivers energy to fluid Governing equations or Affinity Laws relate pump speed to: Flow rate, Q Pump head, H p Fluid power, P
24 22 1400 0.6 Head (m) 20 18 16 14 12 10 8 6 4 2 pump head 1709 rpm fluid power 1709 rpm pump efficiency 1709 rpm system load - head 0 0 0.000 0.002 0.004 0.006 0.008 0.010 0.012 Flow Rate (m 3 /s) operating point 1200 1000 800 600 400 200 fluid power (W) 0.5 0.4 0.3 0.2 0.1 0.0 pump efficiency,
Pump Affinity Laws N Q N 2 H p N 3 P 2 1 3 2 1 2 1 2 2 1 2 1 2 1 P P N N H H N N Q Q N N p p
Determination of Pump Head H p P out g P in V 2 2 V 2g 2 1 Z 2 Z 1 H p P out g P in
Determination of Flow Rate Use Venturi meter to determine Q Fluid is incompressible (const. ) Q = V fluid Area
Venturi Meter As V, kinetic energy T = 0 Height = 0 Pv or P
Calculate Q from Venturi data Q C d A 2 V 2 V 1 = inlet velocity V 2 = throat velocity A 1 = inlet area A 2 = throat area
Throat Velocity 2 2 2 2 1 1 2 1 2 2 Z g P g V Z g P g V 0 Z 2 2 1 2 2 1 B V A A V V 2 P 1 P P ),, ( 2 B P f V v A m m 2 1..
Discharge Coefficient C 0.907 6. 53 d R B ed B D D 2 1 R ed V 1 D 1 V A 2 2 1 V2 V2B A1
Solve for Q Use MS EXCEL (or Matlab) Calculate throat velocity Calculate discharge coefficient using Reynold s number and throat velocity Calculate throat area Solve for Q
Power and Pump Efficiency Assumptions Q 0 No change in elevation No change in pipe diameter Incompressible fluid T = 0 Consider 1 st Law (as a rate eqn.) Q m 1 2 2 h h V V g Z Z W 2 1 2 1 2 1 2
Pump Power Derivation h u Pv h m u P v u Pv W m h2 1 2 2 1 1 W mv P 2 P 1 m v AV Q W Q P 2 P 1
Efficiencies pump output input Q P 2 T P 1 T motor EI overall Q P 2 EI P 1
Summary of Lab Requirements Plots relating H p, P, and pump to Q Plot relating P to pump Regression analyses Uncertainty of overall (requires unc. of Q) Compare H p, P, Q for two N s For fully open valve position WRT affinity laws
905 rpm 1099 rpm 1303 rpm 1508 rpm 1709 rpm Pump Head (m) Flow Rate (m 3 /s)
Power Delevered to Fluid (W) 905 rpm 1099 rpm 1303 rpm 1508 rpm 1709 rpm Flow Rate (m 3 /s)
905 rpm 1099 rpm 1303 rpm 1508 rpm 1709 rpm pump efficiency Flow Rate (m 3 /s)
Pump Efficiency 905 rpm 1099 rpm 1303 rpm 1508 rpm 1709 rpm pump power delivered to fluid (W)
Start-up Procedure 1. Fill pvc tube with water (3/4 full) 2. Bleed pump 3. Switch breaker to on 4. Push main start button 5. Make sure variac is turned counterclockwise 6. Make sure throttle valve is fully open 7. Turn lever to pump 8. Push reset button 9. Push start button 10. Adjust variac to desired rpm using tach.
Pump lab raw data Shaft speed (rpm) DC voltage (volts) DC current (amps) Inlet Pressure (in Hg) Outlet Pressure (kpa) Venturi DP (kpa) Dyna (lbs)
Shut-down Procedure 1. Fully open throttle valve 2. Turn variac fully counterclockwise 3. Push pump stop button 4. Turn pump lever to off 5. Push main stop button 6. Switch breaker to off