CENTRIFUGAL PUMP - 1

Transcription

1 CENTRIFUGAL UM

2 CENTRIFUGAL UM Objeties. At onstant pump speed, determine the harateristi ure (pressure hane s. flow rate) of a entrifual pump.. At onstant pump speed, determine the effiieny as a funtion of flow rate for a entrifual pump. 3. At onstant speed, determine the effiieny as a funtion of power output for an eletri motor. Theory A pump is a deie for supplyin enery to a fluid. The effet of supplyin enery an be studied ia the mehanial enery equation. E z α F Q h W s This equation nelets all shearin stresses. Note: all nomenlature is defined at the end of the setion. In eneral, the power oriinates from a pressure hane ( > ), a hane in kineti enery ( > ), a hane in leel (z > z ), or as frition ( ΣF ). Sine frition is always present in mehanial deies, the enery supplied oes partly into frition and the rest to one of the other effets ited. made. For the pump used in the present experiment, the followin assumptions are E 0 no internal enery hane 0 no kineti enery hane Q h 0 no heat enerated or lost z 0 no sinifiant heiht hane The enery balane simplifies to

3 3 Thus, the atual shaft work done (W s) is the total work (W T ) minus frition losses Net ositie Sution Head (NSH) NSH differene between the stati head at the sution inlet and the head at the inlet at the apor pressure. Caitation At ery lare flow rates, the pressure ( 3) deeloped at the undersides of the blades or positions lose to the eye of the impeller blades beomes ery small. F W Head Total s F W W T s α α NSH or F z NSH or NSH 3 3 OVER

4 If 3 < ap Bubbles form. The bubbles form at the low pressure areas, moe to the hih pressure areas (tip of impeller blades), and ollapse. This ollapse auses mirojets orientated toward the blade at extremely hih pressures (>0,000 atm). This impat (or aitation phenomena) auses seere erosion or erosion/orrosion (in a orrosie medium) of the impeller blades. Noise and ibration are deteted in the pump. ower (in Watts or Hp) Enery (J) Time (s) Horsepower (Hp) Fluid Hp ( f): This is the liquid Hp deliered by the pump (also known as the f Q Q x TDH x hydrauli Hp). Brake Hp ( b): This is the atual Hp deliered to the pump shaft (also alled π N b Ω Q x TDH η x pump input ). e W Eletri Motor Hp ( e): The eletrial Hp deliered by the pump. Note: Hp 550 ftlb f /s 33,000 ftlb f /min Watts 4

5 Effiieny (η) In eneral, the effiieny of a pump is defined as: η Output Input x 00 Work done by the Eletrial enery supplied pump by the pump x 00 The numerator is the head supplied by the pump. η m b e x 00 Motor (η m): η p f b x 00 ump (η p): η o f e x 00 Oerall (η o): Affinity Laws Known harateristis at one impeller diameter or speed extrapolate to some other D or N. A. Q Q N N B. H H N N C. b b N N 3 Case I: Impeller diameter (D) onstant 5

6 6 Case II: ump Speed (N) onstant Aeleration and Deeleration A entrifual pump is a deie whih supplies mehanial enery to a fluid by aeleratin it radially outward and also irumferentially. The sudden redution in radial eloity as the asin is enountered plus the equally sudden redution in irumferential eloity as the fluid enters the dishare line has the net effet of inreasin the pressure. Thus, the equation of enery an be isualized in two steps. In a entrifual pump, the fritional term arises from a ombination of fators:. Frition between the impeller and fluid. Frition between the asin and fluid 3. Shok losses assoiated with rapid aeleration and deeleration. D D C. D D H H B. D D Q Q A. 3 b b Deeleration F Aeleration F W s α α

7 Frition between the impeller and fluid tends to be reatest at low flow rates, while frition between the asin and fluid tends to be reatest at hih flow rates. Shok losses tend to be ery seere at low flow rates and less seere at hih flow rates. Dependin on the atual pump desin, shok losses will also tend to be a minimum at some intermediate flow rate, and the total effet of the three losses on the effiieny is to produe an intermediate flow rane of maximum effiieny. In seletin a pump, this flow rate rane should math the intended operatin rane. In addition to fluid losses, all pumps show mehanial losses whih enerally ary with pump speed. Sine the shaft work inludes these losses, the effiieny is also affeted by these mehanial losses. Nomenlature D Impeller diameter (ft) E Internal enery (lb f ft/lb m) ΣF Sum of all fritional fores (lb f ft/lb m) Graitational fore onstant (3. ft/s ) Graitational onersion fator (3. lb mft/lb f s ) H Total head (ft) m Mass flow rate (lb m /s) N ump speed (rpm) NSH Net positie sution head (ft) N s Speifi speed (omplex units) ressure from D ell (inhes of H) b Brake horsepower (Hp) e Eletri horsepower (Hp) f Fluid horsepower (Hp) ower ower (ftlb f /s) ap Vapor pressure (lb f /in ) Q Volumetri flow rate or apaity (al/min or al/s) Q h Heat enerated (lb f ft/lb m) TDH Total Dynami Head (ft) Fluid eloity (ft/s) W Wattae (Watts) W s Shaft work (ftlb f /lb m) W T Total work (ftlb f /lb m) z Heiht (ft) α Kineti enery orretion fator γ Speifi raity Density (lb m /al) Ω Torque (lb f ft/rad) η Effiieny η m Motor effiieny η o Oerall effiieny η p ump effiieny 7

8 EXERIMENTAL EQUIMENT Caution: Do not operate the pump dry. Do not operate the pump with ale V3 losed. The main elements of the system are a entrifual pump, a rotameter to measure flow, a ale to ontrol flow, a differential pressure (D) ell to measure pressure, and a reseroir, as shown in the fiure below. The pump is drien by an eletri motor, and the shaft onnetin the motor to the pump is proided with a torque indiator. The motor is proided with a speed ontroller and tahometer. 8

9 In operatin the equipment, it is neessary that the intake from the reseroir should be at least 6 inhes aboe the intake. The liquid should also be lean and free of sediment. At startup and at other times (as indiated by the next pararaph), air may be in the lines. This will be obsered as air bubbles flowin throuh the rotameter. Do not attempt to make any measurements as lon as air bubbles are obsered. If the problem does not lear up after a few minutes of operation, the system has a leak and is in need of repair. In normal operation, the speed is to be ontrolled usin the speed ontroller and tahometer, flow is to be ontrolled usin ale V3, pressures are to be determined from the D ell readins, torque is to be measured from the torque meter employin a strobe liht, and flow is to be determined from the rotameter readin. Speed Control The speed ontrol reulates the motor rpm s. To start the motor, set the ontrol to zero, and flip the onoff swith to on. Flip the brake swith to forward, and adjust the ontrol until the desired speed is indiated on the tahometer readout. Flow Control The ale V3 ontrols the flow. With the pump in operation, adjust the ale to produe the flow rate desired, as indiated by the rotameter float. D Cell The D ell works by detetin a pressure differene between two points before and after the pump. The readin is in inhes of merury (note: an offset may need to be subtrated from the measurement). Maximum full sale readin: 30 inhes H. Torque Meter The torque meter is proided with a ernier sale and indiates torque in lb f in. A strobosopi liht is neessary for readin the meter and this strobosope should also be used to hek the indiated motor speed in rpm s. Rotameter The rotameter reads flow rate diretly in allons per minute for a fluid hain a speifi raity of.0. If water is used, flow an be read diretly from the rotameter. The rotameter readin is obtained from the position of the widest part of the float. 9

10 Wattmeter The wattmeter allows measurement of eletrial power input to the motor. Determine the appropriate k fator based on the wirin of the wattmeter. The k fators an be loated in the lid of the wattmeter. Do not make any hanes in the existin onnetions. 0

11 EXERIMENTAL ROCEDURE. Chek the D ell, tahometer, and torque meter to see if they are zeroed properly. If not, orretion or serie are needed.. Fill the reseroir to at least 6 inhes aboe the intake to the pump. Submerene of the return line is desirable. 3. Make sure ale V3 is slihtly open. 4. Set the speed ontrol to zero. 5. Flip the speed ontrol onoff swith to on. 6. Flip the brake swith to forward. 7. Adjust the speed ontrol to produe the desired pump speed (rpm), as indiated on the tahometer. 8. Adjust the ale V3 to obtain the desired flow rate, as indiated by the rotameter. 9. Read the D ell. 0. Read the torque on the torque meter usin the strobe liht. Read the wattmeter. Determine if a orretion fator k is needed for wattmeter onersions.. Usin the strobe liht, read the pump speed as a hek on the tahometer.. Repeat steps 8 so as to obtain at least 0 data points oerin the maximum rane attainable. 3. Repeat steps 7 so as to obtain at least 5 data sets oerin the rane rpm. 4. Repeat holdin ale wide open, and ary RM to et at least 0 more data points.

12 SAMLE DATA SHEET Run Number Flow Rate (al/min) D Cell Readin Wattmeter (Watts) Torque (lb f in) Speed (rpm)

13 MINIMUM REORT REQUIREMENTS. For eah speed hosen, determine the eletri hp, brake hp, fluid hp, motor effiieny, pump effiieny, oerall effiieny, and speifi speed at eah flow rate.. For eah speed, plot head s. flow rate (plot all speed plots on one raph). 3. For eah speed, plot pump effiieny s. flow rate (plot all speed plots on one raph). 4. For eah speed, plot lo pump effiieny s. lo brake hp at one flow rate (hoose one flow rate that is aailable at all speeds). 5. From the literature, find out the relationship between head as a funtion of speed (rpm) as well as between brake hp and speed. Use linear reression on lo plots for points onstruted from all possible ombinations (45 points from part 4) to ompare these relationships with your data. Use 95% onfidene interals to ompare your data to the affinity law oeffiients 6. Disuss briefly the onditions under whih any of the aboe miht be used to establish the optimum performane leel. 7. Disuss whether it is always most adantaeous to use the hihest rotational speed for (a) entrifual pumps in eneral and (b) the pump you used in the lab. 3