MCHE 3450 Mechanical Engineering Laboratory
Centrifugal Pump Experiment Performed 01/28/15 Report Submitted 02/11/15 Group 2 By: Damon Dunwody And team
I certify that all the writing here is our own and not acquired from external sources. sources. We have cited sources sources appropriately and paraphrased correctly. correctly. We have not shared our writing writing with students outside our group, nor have we acquired any written portion of this document from past or present students.
Abstract:
Working Working with the centrifugal pump in this lab allows the students to have a better visual of total dynamic head (TDH) and cavitation. For the lab the pump used was setup with clear casings and piping flow rate sensors sensors a rotometer and and pressure sensors at the the inlet and outlet of the the pump. !art " of of the procedure called for the impeller to be spinning at #$%% &!'. The computer recorded data after every one full turn of the discharge valve until the valve was fully closed. During the eperiment the impeller spun at #*+ &!'. The information collected consisted of the flow rate motor speed motor tor,ue motor power outlet pressure and inlet pressure. -n the eperiment the flow rate ranged from .*# /!' to %.%# /!' the motor speed essentially stayed constant at #*+ &!' motor tor,ue ranged from #.%# ft0lbf to %.1 ft0lbf motor power ranged from %.*H! to %.#23H! the outlet pressure ranged from 2.2 !4-/ to #2.## !4-/ and the inlet pressure ranged from 0#.*%2 !4-/ to 0%.+* !4-/. With this data the total dynamic discharge head (TDDH) the total dynamic suction head (TD4H) the TDH hydraulic power and the pump5s efficiency were calculated. The results of the calculations was that the TDDH ranged from 2.%3ft to +.+1ft the TD4H ranged from 3.21%ft to 3.3ft the TDH ranged from 3.2#3ft to .%1*ft the hydraulic power ranged from %.1+++H! to %.%%%#3H! %.%%%#3H! and the pump efficiency ranged from %.+# to .%%%$#. !ump performance curves were then made with the calculated TDH motor power and pump efficiency values. !art 6 of the procedure helped visuali7e cavitation. The pump was run with the suction valve at first wide open but then slowly closed by full rotations. "t each full rotation of the suction valve the same information was recorded that was measured in part ". 6ubbles began to form once cavitation began and pictures were taken.
Introduction:
This lab uses a centrifugal pump powered by a variable0 speed electric motor which pumps water through a closed0loop system to demonstrate concepts of total dynamic head (TDH) li,uid flow rate pump efficiency and cavitation. cavitation. The total dynamic head data will be gathered at one pump speed by using a specific pump i mpeller. mpeller. The cavitation concept will only be visually observed by the students as to understand its harmful characteristics. For this pump and lab the !ump8ab system will be used for collecting the data from the centrifugal pump in real time. The system is made up of a closed circuit with a reservoir and all see0through piping and valves along with flow rate sensors a highly visual flow rotometer centrifugal pump with interchangeable impellers a controller connected to the computer and a software interface that allows the user to control the pump speed. With all this information that is available total dynamic head of the pump can be calculated along along with finding the efficiency efficiency of the pump pump with each closing turn of of the discharge valve. From there the students will be able to plot the performance curves of their impeller including their findings all on one plot.
Technical Background:
The principle total dynamic head the basic centrifugal pump theory and cavitation can be eplained through fluid fluid mechanics. 9et head illustrates illustrates the pumps performance. performance. The net head is the difference in the 6ernoulli head between the inlet and outlet of the pump. The general e,uation for 6ernoulli head can be epressed as:
6ernoulli head ; pressure head < velocity head < static head 2
( P + V + z ) ρg 2 g
The net head used to portray the pumps performance can also be called the total dynamic head (TDH). The total dynamic discharge discharge head and the total dynamic suction head can each be epressed as a 6ernoulli head. The total dynamic head e,uation can be epressed as:
Total Total Dynamic Head ; total dynamic discharge head = total dynamic suction head
(
2
P V TDH = + + z ρg 2 g
) (
2
P V − + + z ρg 2 g outlet
)
inlet
-n a centrifugal pump the fluid enters the pump at the eye and forced to the outer edge of an impeller by rotation. To make sure that there is enough head to make the fluid flow into the impeller eye the net positive suction head (9!4H) can be calculated. calculated. The head due to absolute pressure ( H a ¿ the head due to elevation (
H z ¿ the head due to friction loss ( H f ¿ the
velocity head ( H v ¿ and the head due to vapor pressure (
H vp ¿ are added together to
obtain the net positive suction head as seen in the e,uation below:
+ ¿ H z − H f + H v − H vp NPSH = H a ¿
For a centrifugal pump to work properly the available 9!4H should be higher than the re,uired 9!4H. The re,uired 9!4H is the the minimum 9!4H that is needed needed for the pump to operate operate in a desired manner. -f the available 9!4H is not higher that the re,uired 9!4H bubbles will form in the pipe due to lack of pressure. The bubbles will travel into the impeller. >nce in the impeller the bubbles will be pushed pushed to the outer edge where where the pressure is higher. higher. The The bubble will then implode releasing energy. energy. This process is called cavitation. This released energy is transmitted to the impeller which can cause damage over time and loss in efficiency. efficiency.
Procedure: Part A: TDH study
#. 3. 1. . 2. +. .
4elect English ?9-T4 on the prompt screen. @lick Start DAQ button (data should be flowing on graphs). @lick Pump ab button. 4et 4et mot motor or spee speed d to to !"## &!' using on0screen slide switch (or type in value). @lick $un %otor button. @lick on %ap &ie' tab. @lick (lear Plot button then click %ap Point button. &ecord flow rate A/!'B motor speed A&!'B motor tor,ue Aft0lbfB motor power AH!B outlet pressure A!4-/B and inlet
pressure A!4-/B. $. @lose Discharge &al)e # full turn and allow map points to fully settle and then click %ap Point. &epeat the measurements in step ##. *. &epe &epeat at step step #3 unti untill Discharge &al)e is fully closed then click Stop %otor button. #%. #%. >pen >pen Discharge &al)e all the way. ##. @lic @lick k *rite Data to +ile button and save file as:
C/roupE&edE#$%%ETDHE''DD.ttG.
Part B: (a)itation study by )isual learning
#. 3. 1. . 2.
4et 4et mot motor or spee speed d to to !"## &!' using on0screen slide switch (or type in value). @lick $un %otor button. @lick on %ap &ie' tab. @lick (lear Plot button then click %ap Point button. @lose Suction &al)e # full turn and allow map points to fully settle and then click %ap
Point +. &epe &epeat at step step 3% unti untill Suction &al)e is fully closed then click Stop %otor button. Take
photos of any interesting interesting results. . >pen Suction &al)e all the way $. @lick *rite Data to +ile button and save file as: C/roupE&edE#$%%E9!4HrE''DD.ttG .
$esults and Discussion:
1800 RPM Experimental Data
Table Table 1 $al%e Turn Turn from 'pen 0 1 2 5 4 8 3 10 11 12 1 1 15 14 1 18 13 20 21 22 2
lo) rate *GPM+ 31 5 803 53 1 43 12 432 451 425 40 31 5 31 203 08 0 22 20 205 152 001
Motor peed *RPM+ 134 134 134 134 134 134 134 134 134 134 134 134 134 134 134 134 134 134 13 13 13 138 138 133
(Di!"ar#e $al%e& 'utlet Motor Motor preur tor,ue *ftpo)er e lbf+ *.P+ *PSG+ 101 03 55 101 03 585 101 03 525 101 03 5 101 03 55 101 03 58 101 03 545 101 03 5885 101 03 5385 101 03 5335 101 03 41 101 03 425 101 03 435 128 05 4545 128 05 485 128 05 2 128 05 485 125 023 85 118 00 32 1104 03 1015 0353 028 1142 0811 028 12 08 0252 115 0 0152 1511
nlet preur e *PSG+ -1 -1305 -132 -1 -184 -18 -1305 -1858 -1 -131 -1 -1833 -1 -184 -1 -18 -184 -1823 -182 -14 -123 -1431 -145 -13 -14 -128 -1 -1041 -0311 -0811 -0 -043
Table Table 2
#$%% &!' @alculated TDH Data
lo) rate TDD. TD.S TD. *GPM+ (ft& (ft& (ft& 31 502 250 2512 5 50 2524 2514 501 255 234 803 501 25 250 53 504 2523 2504 505 253 2515 1 508 252 2511 43 5040 250 250 12 5080 25 255 432 5081 250 251 451 5108 253 2543 425 512 252 2585 40 5155 25 2412 31 5185 2551 24 5 52 2558 243 31 50 254 20 203 532 252 2820 08 5521 2400 2321 0 543 241 0 22 5835 244 253 20 4115 244 52 205 413 248 42 152 4 20 2 001 44 22 03 Table Table
#$%% &!' @alculated fficiency Data
lo) rate *GPM+ 31 5 803 53
.6drau li! po)er *.P+ 044 4 042 4 0588 0 043 041 048
Motor po)er *.P+
Pump e7!ien! 6 *+
03
04123
03
0541
03
0304
03
04183
03
0542
03
0535
1 43 12 432 451 425 40 31 5 31 203 08 0 22 20 205 152 001
0400 0401 04 0412 5 0420 042 044 8 058 4 0415 045 2 0534 4 0582 054 021 1 0282 02253 4 011 5 00001 2
03
05142
03
05132
03
05880
03
0518
03
05581
03
054
03
041
05
083
05
0345
05
080231
05
03220
023
0851
00
08554
03
08525
028
08413
028
08123
0252
04803
0152
000081
Performance Curves - e! "mpeller
pump e7!ien!6 e7!ien!6 TD. TD. *f *ft+ motor po)er *.P9100+
i#ure 1
Figure 3
Figure 1
(alculations:
>nce the raw data was collected from the centrifugal pump the total dynamic head (TDH) of the pump at each turn was needed. -n order to find the TDH the following e,uation was used
TDH =TDDH −TDSH
(#)
To find the TDH the total dynamic discharge head (TDDH) and the total dynamic suction head were needed. The TDDH and the TD4H were f ound using the following e,uations
2
(3)
Pd V d TDDH = + + z ρg 2 g d
2
Ps V s TDSH = + + z ρg 2 g s
With ! being the !ressure
(1)
ρ as the density of water g as the acceleration due to
gravity gravity I as the velocity of the water and 7 being the elevation of the water at that point. 6y substituting (3) and (1) into (#) the new e,uation is obtained
(
2
)(
2
Pd V d P V TDH = + + z d − s + s + z s ρg 2 g ρg 2 g
)
()
However for the lab a simplified e,uation was given for the pressure and velocity heads. The e,uations have built in conversion factors and allow for more efficient calculation.. The static heads for both the suction and discharge end of the pump were also given as constant values.
h p =
h v=
2.31
( P ) (2)
SG 2
(GPM ) ( ID )
.002593
4
(+)
g
= h p
()
=h v
($)
2
g
=0 ft
(*)
z d=1 ft
(#%)
/!' stands for gallons per minute or the volumetric flow rate -D stands for inner diameter of the pipe in inches 4/ stands for the specific gravity of water and ! stands for the absolute pressure. 6ecause the absolute pressure was needed for the e,uations the atmospheric pressure had to be found. found. The atmospheric pressure pressure was found to be #.$3$ psi and was added to each pressure from the lab data accordingly. accordingly. The inner diameter is given as #.$2 in and #.+ in for the suction pipe and the discharge pipe respectively. 6y substituting substituting () ($) (*) and (#%) into () the new e,uation is reached
P 2.31
( ¿¿ d + 14.828) SG
+ ¿ ¿
2
(GPM )d +1 ( ID )d
.002593
(##)
2
TDH =¿ 6y using (##) the data from the eperiment can be used to find the TDH at each valve turn. For eample the TDH is calculated at a flow rate of .*# gallonsJmin.
(
TDH =
(
2.31 5.775 1
+14.828 )
+
2
(
)
.002593 47.91 d
TDH =49.501−30.360 = 19.141 ft
2
( 1.6 )d
)(
+1 −
2.31
(−1.905 +14.828 ) 1
+
(
)
2
.002593 47.91 s 2
( 1.85) s
+0
-n addition to finding the TDH for each valve turn the hydraulic power was also determined. >nce again a simplified e,uation was given for the water hydraulic power with built in conversion factors. The e,uation is
WHP=
(GPM )( TDH )( SG)
(#3)
3960
The hydraulic power at .*# gallonsJmin is found using TDH from the previous calculation and (#3)
WHP=
( 47.91)( 19.141 )( 1) 3960
=.23158 HP
From the new hydraulic h ydraulic power calculated we can find the pump efficiency for the pump at each flow rate using the e,uation
Pump efficiency=
hydau hydaulic lic po!e po!e moto moto po!e po!e
(#1)
For an eample the pump efficiency at .*# gallonsJmin is
pumpefficien pump efficiency cy =
.23158 .479
=.48345
Discussion:
-n fluid dynamics one learns the concept of head involves the relation between the energy at a point in the pump to height of an e,uivalent static column of the fluid. To calculate head at a certain point in a pump 6ernoulli5s e,uation is used. The total dynamic head epresses each pump5s performance. Total Total dynamic head is the difference in the 6ernoulli head at the outlet and the inlet of a pump. This can also be referred to as the difference between the total dynamic discharge head and the total dynamic suction head. "s seen in figure # as the flow rate of the pump decreases the total dynamic head increases. The velocity head of the pump at the inlet and outlet are decreasing less than the pressure head is increasing at the inlet and outlet. This causes the total dynamic head to increase. -n addition to
total dynamic head figure # also shows that as the flow rate decreases the pump efficiency increases. However once the flow rate reaches a certain rate the pump efficiency plummets.
4ometimes when the pressure at a pump inlet decreases small air pockets are created in the fluid. >nce those air pockets travel to the area of the pump with higher pressure. 6eing eposed to the higher pressure the air pockets eplode and release the ecess energy to the surrounding pipe. This process is called @avitation. "s "s you can see in figure 3 the pump has fluid traveling with no cavitation. However in figure 1 you can clearly see air bubbles formed on the left side and then collapsed on the right side. @avitation is terrible for pumps and piping as it can cause efficiency in the pump to drop. @avitation can also be very harmful to a pipe and cause erosion overtime which makes it something to avoid at all costs. 4ome eamples that may cause cavitation are reducing the si7e of the suction valve or raising the pump too high above the fluid source.
(onclusion:
From Figure # it looks as though pump efficiency increases as the volumetric flow rate decreases. "lso as the total dynamic head of the pump increases so does the pump efficiency. 6ased on this one could create a pump operating guideline for someone who re,uires the most efficiency out of the centrifugal pump. The guideline would state to use a slower flow rate in order to generate a higher total d ynamic head and therefore a more efficient pump.
Appendi,:
?sing (2) and (+) from the calculations section the pressure head and velocity head were found for the inlet and outlet of the pump
h p =
h v=
2.31
( P ) (2)
SG 2
(GPM ) ( ID )
.002593
4
(+)
"t % turns the flow rate is .*#gpm the outlet pressure is 2.2psig and the inlet pressure is 0#.*%2psig. 0#.*%2psig. The inlet inner diameter diameter was given at #.$2 in and the outlet inner diameter diameter was given at #.+ in.
h p =
(
2.31 5.775
)
1
; *.2*3*1 ft
h p =
2.31
(−1.905 ) 1
; 3*.$23#1 ft
hv=
(
.002593 47.91
2
)
4
(1.6 )
; .*%$#$+ ft
hv=
(
.002593 47.91
2
)
4
( 1.85 )
; .2%$#33 ft The static head for the discharge was given at # ft and the suction head was given at % ft. With the static head the velocity head and the pressure head calculated at both the discharge and suction ends the total dynamic could be found using ()
TDH =( 49.5293 + .908186 + 1 ) −( 29.85213+ .508122+ 0 )=19.14086 ft
cel was used to calculate the values for all 31 turns of the valve in order to save time and efficiency. The The data from the lab was easily accessible and was easily transcribed into ecel.
-n order to find the hydraulic power of the pump at each turn (#3) was used
WHP=
(GPM )( TDH )( SG)
(#3)
3960
"t .*# gpm the TDH was calculated to be #*.#%$+ ft and the specific gravity of water is known to be #.
WHP=
( 47.91)( 19.14086 )( 1 ) 3960
; .31#2$ Hp "gain due to the 31 valve turns ecel was used to calculate the rest of the hydraulic powers. cel was also used to calculate the pump efficiency of the pump at each valve turn by imputing (#1). The hydraulic power formerly calculated is .31#2$ Hp and the motor power from the eperimental data is .* Hp at a flow rate of .*# gpm.
Pump efficiency= Pump efficiency efficiency =
hydau hydaulic lic po!e po!e moto moto po!e po!e
.23158 .479
; .$12
(#1)