- Email: [email protected]
Pulsation problems in rotary lobe pumps
S PECIAL
FEATURE
P u l s a t i o n problems in rotary lobe pumps One disadvantage with the use o f rotary lobe p u m p s is the unsteady dis-
proc
wb c l oduceslz -
sure ~ speaflcally in long and rigid pipelines a n d non-compressible liquids. The result is vibration of p u m p and piping, which im'mases the risk of cavitation on the suafon side. This article explains the mechanical causes o f the
unsteady dUplacemem process and
a new lobe design is d e r i v e d a n d described as a consequence. Slurries and high viscosity fluids with an apparent viscosity of more than 0.5 Pa s can no longer be p u m p e d economically by centrifugal pumps. Especially for inhomogeneous suspensions it is than better to use self-sealing, valveless, rotary positive displacement pumps with large cross-sections. These are eccentric screw pumps or rotary lobe p u m p s operating according to the Roots principle.The constant chamber volumes and low s p e e d s of these p u m p designs mean sensitive materials can be p u m p e d gently and metering tasks easily fulfilled. The rotary lobes of the two-shaft pump design are constructed with two or three lobes and preferably provided with rubber lining for use in highly solid contaminated suspensions. Compared with eccentric screw pumps, rotary lobe pumps with large capacities have been successfully established due to their compact design. Preferred areas of application are:
By H Vogelsang, B VerhUlsdonk, M T@rk, and G H@rnig
Principles of rotary lobe geometry Theory of the displacement process Rotary displacement pumps with a periodically fluctuating volume flow Q generate pressure pulsations in the c o n n e c t e d piping due to acceleration and deceleration. The internal displacement mechanism of rotary lobe pumps of traditional design according to the Roots principle bring about flow fluctuations with peak values determined by the basic design characteristics shaft distance A and lobe tip diameter D (Figure 1). In rotary displacement pumps the drive torque is directly proportional to the displacement volume and that in any angular position. Friction in bearings, gears and lobes (friction of lobes against each other and the pump casing) are disregarded at first.The course of the volume flow fluctuation versus the angular rotation corresponds to the course of the torque.The torque at a specific angular position can be calculated according to Figure 1 when the line of action is known. For this purpose the pressure forces imposed by the pumped medium on the lobes are resolved in a force component in the direction of rotation and a force component in the direction opposite to the direction of rotation. The sum of the two A
pumping systems for agricultural suspensions (liquid manure and biogas slurries) with long pipelines (up to 500 m and in exceptional cases even up to 5 km)
pl < p2
pumping systems for sewage slurries and high viscosity industrial fluids mobile pumping systems on road tank cars for extracting liquid organic waste and faecal matter. Figure 1. Pressure loads on the lobes.
0262 1762/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
WORLDPUMPS FEBRUARY1999
S PECIAL m
FEATURE
lobe torques gives the total torque.The maximum and minimum torques and displacement volumes can also be determined that way. The p u m p delivers least w h e n the lobes contact at the points S of the line of action (Figure 1), i. e. the addendum of one lobe contacts the dedendum of the other lobe. Conversely, maximum delivery is achieved w h e n the lobes contact at pitch point W. All rotary lobe pumps with a lobe design to prevent backFigure 3. Helically toothed lobes (HiFIo®). flow through the pump centre have a figure-eight shaped lobe line of action. the number of teeth because the time for load Irrespective of the lobe shape and number of reversal decreases according to the number of teeth, the extreme values of the volume flow fluc- teeth.The average pumping capacity is not affecttuations are determined from shaft distance A and ed by the number of teeth, the shape of the lobe tip diameter D.The average displacement volume flank has only a minor effect. at a constant shaft distance A varies roughly according to the square of the tip diameter D.The Solutions for reducing pressure pulsation maximum displacement volume is limited by the gearability of the lobes.The largest tip diameter for So the most urgent problem is the reduction of flow fluctuations. The easiest way to achieve this two-lobe piston is approximately Dmax = 1.6 *A. The pumping capacity of a three-lobe rotary is by phase shifting two superimposed parallel pump also corresponds to a harmonic vibration flows. But that requires parallel operation of two (Figure 2). The smallest flow is p u m p e d if the identical pump chambers with a ½ period phase lobes contact each other at point S of the line of shift. The pulsation of the resulting flow is action (home position). The largest flow is reduced to less than 5%. The number of teeth of p u m p e d in the second lobe position w h e n the the pump combination is also doubled by superlobes contact each other at point W. The liquid imposition, i. e. the acceleration values are columns in the piping connected to the suction reduced to only < 10 % of the original values. In and discharge side must be periodically accelerat- actual pumping systems, in which for geometrical ed and decelerated.That is also the main cause of pulsation reasons vibrations still occur due to the pressure pulsation. The lower curve in Figure alternating leakage flows and elastic distortion of 2 shows the acceleration dQ/dt versus the angular components, this solution produces the lowest pulsation load and optimal quiet running. But it rotation. The more teeth on a lobe, the faster the accel- needs a higher level of design expense because eration and deceleration phases alternate, i.e. the two separate pump chambers are required. This means additional sealing gaps, higher fricpulsation frequency increases.The pressure peaks at constant speed increase almost linearly with tion forces, no overmounted bearings of the
increasing Flow rate Q decreasing
acceleration
d0
r
0"
T
75"
Figure 2. Pulsation as the result of flow fluctuation.
WORLDPUMPS FEBRUARY1999
dt deceleration
90"
105"
120"
Rma~mQar~Qte
S PECIAL
FEATURE
pump lobes, increased assembly effort, etc. This version has been successfully used in practice for some time. A pulsation-free helical lobe, the so-called HiFlo lobe, was developed as a n e w solution (Figure 3). If we imagine an indefinite number of rotary lobe pumps in parallel as above, operating for at least a complete displacement period at all lines of action, any irregularity in the flow disappears. Pulsation due to geometry no longer occurs. Every lobe is helically toothed to ensure that the wrap angle along the lobe length is exactly one half of the pitch. This way an indefinite number of minimum partial flow with a uniform, pulsation-free overall flow is achieved within one period. The wrap angle must be an integral number of the half pitch, otherwise the pulsation will be reduced but not completely eliminated. To ensure the seal of the pump at any operating point, the casing must enclose every lobe (at straight outlet sides) at least along one pitch plus the wrap angle.This casing angle is 270 ° for twolobe models, which can only be achieved at great deal of effort and poor inlet and outlet cross-sections. Consequently three-lobe designs (Figure 3) for which the casing angle must be at least 120 ° + 60 ° = 180 ° and can therefore be installed in the existing pump casing - are more practical. Helical lobes along a complete pitch have the advantage that not only the driving torque of the pump but also the torque of each individual shaft b e c o m e s uniform. The minimum n u m b e r of teeth is then four at a casing angle of 180 °.The
manufacturing costs are, however, considerably higher due to the steeper lobe flanks. But the disadvantage of helical lobes is that axial forces are developed. These are, however, relatively low compared with the radial forces occurring and are safely absorbed by the customary fixed bearings. The above helical, rubber-lined lobes allow the design of single-chamber pump which do not exhibit geometry-specific flow fluctuations. This new lobe shape can be installed in the customary pump casings and exhibits all the previous design benefits such as minimum leakage gap length and overmounted bearing with rapid access to the pump space for cleaning, maintenance or repair. Besides the lack of pulsation, a better suction capability was also demonstrated in practical tests so that higher speeds with higher capacities become possible.
m
Practical investigations I
I
Material and methods The pump characteristics are normally determined with water. Concentrated organic suspensions in the field of agriculture (liquid manure, biogas slurry or liquid feed) often feature nonNewtonian behaviour with up to 1000 higher apparent viscosities. The material systems are inhomogeneous and have every different structural properties, so the pump performance also changes. Exact pipe and pump characteristics in
160 140
Water , r
_
:!:=~
~-_
500 '
Liquid feed
120
•
•
v
•
•
w
•
v.-.,.
•
100
~ .........
_
~
~
~
60
~
3
o
0
. . . . . . . . . . . . .
200
~ 40 20
400
~~-::~==:
:"
:
~
=
~
:
. .O0 ...
"~
0
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
Pressure on suction side of the pump, bar Figure 4. Suction characteristic curves of rotary pump V 186-130 Q with HiFIo ® lobes,
WORLDPUMPS FEBRUARY1999
0,2
S PECIAL
FEATURE
combination with the flow properties of the actual pumping media are therefore important for the design of pump systems. In the p u m p test equipment at the German Institute for Agricultural Engineering, pipe and pump characteristics can be measured consecutively at different adjustable pressures and capacities in one test.Thus a direct relationship between the rheological properties of the medium and the operating data can be established. The testing consists of a weighing mixer, the pump unit and PVC measurement tubes. The pump drive is subject to infinitely variable control by frequency converters. A so-called throttling pump (also a rotary lobe pump) is arranged in line downstream of the pump for pressure adjustment. Even for highly concentrated high viscosity fluids infinitely variable pressure adjustment in the range from 0 to 10 bar is realised by the infinitely variable speed using frequency converters.A throttle valve for adjusting the suction pressure is arranged in the suction nozzle.The following measuring data were recorded: •
volume flow rate Q (inductive flowmeter)
•
pressure Ps upstream and pressure Pd downstream of the pump (pressure transmitter)
•
torque Md and speed np of the pump (measuring shaft) differential pressure Ap in the meter tubes (differential pressure transmitters via averaging units)
From the pipe characteristic a flow curve for laminar flow can be computed and approximated using known flow models (power laws according to Ostwald and de Waele or Herschel and Bulkley). The piping is used as a tube-type viscometer and changes in the flow properties can be continuously observed and documented. The static operating characteristics of the rotary lobe p u m p were determined using incremental reduction of speed and p u m p pressure. The capacity characteristic is determined in the discharge and suction ranges. The vibration behaviour in the suction range is particularly interesting when cavitation starts. The p u m p i n g tests were performed under defined conditions with water and with liquid feed from a mixture of crushed grain and water. Here the liquid feed serves as a sample medium for highly concentrated agricultural suspensions and is characterised as follows: dry matter content TS = 29 %, density p = 1100 kg/m 3, flow limit
•
•
(~)
Ol=v=te
Centrifugal Lined Process Pumps for:
Acids Alkalines Slurries
rr < >n" B
•
O Z
LU
n" LU
•
Z
• • •
a < UJ re O ,¢t
f-n" LU 03
Z
QualityIndustrialPumps& Motors Cencifugal/ EleCt / Waer/ Air DrivenPumps
Hypalon or Kynar lining bonded to rugged cast-iron body Light weight Kynar impeller reduces shaft deflection Adjustable impeller clearance Tangential or center discharge Multiple sealing choices
Tel: 7 1 7 - 4 5 5 - 2 0 5 1
E-mail: [email protected]
m
LU 03 < LU .J Q.
Call: 800-854-3218
~'"~"'=~'~
Website at www.shurflo.com E-mail at [email protected]
~ ~
® , ~coR cmp.y
DORR-OtJVER Member of Kraug~-Maffei Group
WORLDPUMPS FEBRUARY1999
Fax: 7 1 7 - 4 5 5 - 7 3 5 0
D rr < >fv O
Z w w w a < W nZ 0 co
In" uJ 03 Z uJ oO < w _J
S PECIAL
FF&'I'UI~
1200
1,5
800
1,0
¢t:t
E E
Z
0,5
400
E o I-
o,o ~
0
.-,=
(31.) -400
-0,5
¢t~
Ex._ -800
-1,0 0
20
40
60
80
1O0
120
Revolutions Figure 5. Suction throttling of water flow. Torque pulsation of rotary pump V 136-140 Q with twolobe oval pistons and four-lobe HiFIo ® pistons at 900 rpm.
z0 = 41.05 Pa, consistency coefficient k = 0.5727 Pa s n, flow exponent n = 0.8856. If this liquid feed mix is p u m p e d at Q = 50 m3/h through a pipe with an inside diameter of 100 mm, for example, an apparent viscosity of 0.704 Pa s is effective. The p u m p i n g tests w e r e p e r f o r m e d with a type V 186-130 Q rotary lobe p u m p . The rated value include, a m o n g o t h e r things, Q = 116 m3/h at np -- 385 rpm.
The w a t e r characteristics a p p r o a c h a straight line only at higher speeds. The w a t e r can flow back through the gaps in the pump.Also at lower speeds the capacity changes linearly w i t h the pressure w h e n high viscosity liquid feed is p u m p e d . The gap leakage f l o w is greatly reduced. In p r i n c i p l e the static characteristics of the lobe designs are n o t essentially different. The tightness of the p u m p d e p e n d i n g on its Pumping test results o p e r a t i n g t i m e and w e a r are decisive in all cases. The flow characteristics in the discharge range The suction behaviour (Figure 4) is also difd e p e n d specifically on the p u m p i n g medium. ferent for w a t e r and feed p u m p i n g . The flow characteristics play an increasing role 1200 with increasing A A A speeds because 1000 A it high viscosity media Oval lobes | tt have p o o r e r backflow properties and E 600 the p u m p filling ratio deteriorates. ~o The variation in ~ _ j f torque vibrations of 200 V J the lobe designs b e c o m e s particularal~r ly clear w h e n the suction side is throt-200 tled and at high speeds (Figure 5). -400 ! While the straight 0 90 180 270 360 oval lobe develops Revolution angle, deg quite substantial Figure 6. Torque pulsation of rotary pump V 186-130 Q with two-lobe oval pulsations and pistons and three-lobe HiFIo ® pistons at water flow, 500 rpm and 2,3 bar. greatly loads the
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;/
fl
J
oI
lye,.,
x r,%-
v"
V
WORLDPUMPS FEBRUARY1999
ECIAL S PFEATURE i
p u m p drive at increasing cavitation, the vibration amplitude of the helical HiFlo lobe changes only to a minor degree in this sample load. That produces clear advantages for this lobe design at high speeds and extreme loads with cavitation. These differences at high speeds without suction throttling are also shown in Figure 6. The pumping cycle of the oval lobe and threelobe helical HiFlo lobe can be followed quite well here.The shape of the oval lobe torque vibrations is interesting. Here negative torques also o c c u r which might be an indication of cavitation in the pump.The application range of this lobe is therefore exceeded with n e = 500 rpm. Although such a distinctive difference between the lobe designs cannot be found w h e n water is pumped, the differences w h e n liquid feed is p u m p e d are all the more distinctive. The vibration amplitudes of oval lobes are higher than those of the HiFlo lobes by a factor of approximately eight.
I Summary The reason for the pressure pulsations occurring frequently during the p u m p i n g process are derived on the basis of the geometrical relation-
ships in lobe designs. A new lobe design, i.e. the helical HiFlo lobe, is described as a conclusion. This lobe clearly reduces pulsations especially at high speeds.The HiFlo lobes also allow better suction behaviour even in case of cavitation. The p u m p i n g and vibration behaviour of HiFlo and customary oval lobes is illustrated on the basis of practical tests with water and a highly concentrated liquid feed mixture. •
1) Dipl.-Ing. H. Vogelsang and Dipl.-Ing. B. Verhiilsdonk are managers in the Hugo Vogelsang Maschinenbau GmbH, HolthSge 12-14. D-49632 Essen, Germany. Tel: +49 5434 830; Fax: +49 5434 8310; E-mail: Vogelsang_. GmbH @t-online .de
2) Dr.-Ing. habil. M. Tiirk and Prof. Dr. G. H6rnig are researchers in the Institute of Agricultural Engineering Bornim, MaxEyth-Allee 100, D-14469 Potsdam, Germany. Tel: +49 331 5699 849; E. potsdam.de
Test our software
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o
@
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WORLD PUMPS FEBRUARY 1999
PLEASE INSERT 433 ON R E A D E R E N Q U I R Y C A R D
FEATURE
P u l s a t i o n problems in rotary lobe pumps One disadvantage with the use o f rotary lobe p u m p s is the unsteady dis-
proc
wb c l oduceslz -
sure ~ speaflcally in long and rigid pipelines a n d non-compressible liquids. The result is vibration of p u m p and piping, which im'mases the risk of cavitation on the suafon side. This article explains the mechanical causes o f the
unsteady dUplacemem process and
a new lobe design is d e r i v e d a n d described as a consequence. Slurries and high viscosity fluids with an apparent viscosity of more than 0.5 Pa s can no longer be p u m p e d economically by centrifugal pumps. Especially for inhomogeneous suspensions it is than better to use self-sealing, valveless, rotary positive displacement pumps with large cross-sections. These are eccentric screw pumps or rotary lobe p u m p s operating according to the Roots principle.The constant chamber volumes and low s p e e d s of these p u m p designs mean sensitive materials can be p u m p e d gently and metering tasks easily fulfilled. The rotary lobes of the two-shaft pump design are constructed with two or three lobes and preferably provided with rubber lining for use in highly solid contaminated suspensions. Compared with eccentric screw pumps, rotary lobe pumps with large capacities have been successfully established due to their compact design. Preferred areas of application are:
By H Vogelsang, B VerhUlsdonk, M T@rk, and G H@rnig
Principles of rotary lobe geometry Theory of the displacement process Rotary displacement pumps with a periodically fluctuating volume flow Q generate pressure pulsations in the c o n n e c t e d piping due to acceleration and deceleration. The internal displacement mechanism of rotary lobe pumps of traditional design according to the Roots principle bring about flow fluctuations with peak values determined by the basic design characteristics shaft distance A and lobe tip diameter D (Figure 1). In rotary displacement pumps the drive torque is directly proportional to the displacement volume and that in any angular position. Friction in bearings, gears and lobes (friction of lobes against each other and the pump casing) are disregarded at first.The course of the volume flow fluctuation versus the angular rotation corresponds to the course of the torque.The torque at a specific angular position can be calculated according to Figure 1 when the line of action is known. For this purpose the pressure forces imposed by the pumped medium on the lobes are resolved in a force component in the direction of rotation and a force component in the direction opposite to the direction of rotation. The sum of the two A
pumping systems for agricultural suspensions (liquid manure and biogas slurries) with long pipelines (up to 500 m and in exceptional cases even up to 5 km)
pl < p2
pumping systems for sewage slurries and high viscosity industrial fluids mobile pumping systems on road tank cars for extracting liquid organic waste and faecal matter. Figure 1. Pressure loads on the lobes.
0262 1762/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
WORLDPUMPS FEBRUARY1999
S PECIAL m
FEATURE
lobe torques gives the total torque.The maximum and minimum torques and displacement volumes can also be determined that way. The p u m p delivers least w h e n the lobes contact at the points S of the line of action (Figure 1), i. e. the addendum of one lobe contacts the dedendum of the other lobe. Conversely, maximum delivery is achieved w h e n the lobes contact at pitch point W. All rotary lobe pumps with a lobe design to prevent backFigure 3. Helically toothed lobes (HiFIo®). flow through the pump centre have a figure-eight shaped lobe line of action. the number of teeth because the time for load Irrespective of the lobe shape and number of reversal decreases according to the number of teeth, the extreme values of the volume flow fluc- teeth.The average pumping capacity is not affecttuations are determined from shaft distance A and ed by the number of teeth, the shape of the lobe tip diameter D.The average displacement volume flank has only a minor effect. at a constant shaft distance A varies roughly according to the square of the tip diameter D.The Solutions for reducing pressure pulsation maximum displacement volume is limited by the gearability of the lobes.The largest tip diameter for So the most urgent problem is the reduction of flow fluctuations. The easiest way to achieve this two-lobe piston is approximately Dmax = 1.6 *A. The pumping capacity of a three-lobe rotary is by phase shifting two superimposed parallel pump also corresponds to a harmonic vibration flows. But that requires parallel operation of two (Figure 2). The smallest flow is p u m p e d if the identical pump chambers with a ½ period phase lobes contact each other at point S of the line of shift. The pulsation of the resulting flow is action (home position). The largest flow is reduced to less than 5%. The number of teeth of p u m p e d in the second lobe position w h e n the the pump combination is also doubled by superlobes contact each other at point W. The liquid imposition, i. e. the acceleration values are columns in the piping connected to the suction reduced to only < 10 % of the original values. In and discharge side must be periodically accelerat- actual pumping systems, in which for geometrical ed and decelerated.That is also the main cause of pulsation reasons vibrations still occur due to the pressure pulsation. The lower curve in Figure alternating leakage flows and elastic distortion of 2 shows the acceleration dQ/dt versus the angular components, this solution produces the lowest pulsation load and optimal quiet running. But it rotation. The more teeth on a lobe, the faster the accel- needs a higher level of design expense because eration and deceleration phases alternate, i.e. the two separate pump chambers are required. This means additional sealing gaps, higher fricpulsation frequency increases.The pressure peaks at constant speed increase almost linearly with tion forces, no overmounted bearings of the
increasing Flow rate Q decreasing
acceleration
d0
r
0"
T
75"
Figure 2. Pulsation as the result of flow fluctuation.
WORLDPUMPS FEBRUARY1999
dt deceleration
90"
105"
120"
Rma~mQar~Qte
S PECIAL
FEATURE
pump lobes, increased assembly effort, etc. This version has been successfully used in practice for some time. A pulsation-free helical lobe, the so-called HiFlo lobe, was developed as a n e w solution (Figure 3). If we imagine an indefinite number of rotary lobe pumps in parallel as above, operating for at least a complete displacement period at all lines of action, any irregularity in the flow disappears. Pulsation due to geometry no longer occurs. Every lobe is helically toothed to ensure that the wrap angle along the lobe length is exactly one half of the pitch. This way an indefinite number of minimum partial flow with a uniform, pulsation-free overall flow is achieved within one period. The wrap angle must be an integral number of the half pitch, otherwise the pulsation will be reduced but not completely eliminated. To ensure the seal of the pump at any operating point, the casing must enclose every lobe (at straight outlet sides) at least along one pitch plus the wrap angle.This casing angle is 270 ° for twolobe models, which can only be achieved at great deal of effort and poor inlet and outlet cross-sections. Consequently three-lobe designs (Figure 3) for which the casing angle must be at least 120 ° + 60 ° = 180 ° and can therefore be installed in the existing pump casing - are more practical. Helical lobes along a complete pitch have the advantage that not only the driving torque of the pump but also the torque of each individual shaft b e c o m e s uniform. The minimum n u m b e r of teeth is then four at a casing angle of 180 °.The
manufacturing costs are, however, considerably higher due to the steeper lobe flanks. But the disadvantage of helical lobes is that axial forces are developed. These are, however, relatively low compared with the radial forces occurring and are safely absorbed by the customary fixed bearings. The above helical, rubber-lined lobes allow the design of single-chamber pump which do not exhibit geometry-specific flow fluctuations. This new lobe shape can be installed in the customary pump casings and exhibits all the previous design benefits such as minimum leakage gap length and overmounted bearing with rapid access to the pump space for cleaning, maintenance or repair. Besides the lack of pulsation, a better suction capability was also demonstrated in practical tests so that higher speeds with higher capacities become possible.
m
Practical investigations I
I
Material and methods The pump characteristics are normally determined with water. Concentrated organic suspensions in the field of agriculture (liquid manure, biogas slurry or liquid feed) often feature nonNewtonian behaviour with up to 1000 higher apparent viscosities. The material systems are inhomogeneous and have every different structural properties, so the pump performance also changes. Exact pipe and pump characteristics in
160 140
Water , r
_
:!:=~
~-_
500 '
Liquid feed
120
•
•
v
•
•
w
•
v.-.,.
•
100
~ .........
_
~
~
~
60
~
3
o
0
. . . . . . . . . . . . .
200
~ 40 20
400
~~-::~==:
:"
:
~
=
~
:
. .O0 ...
"~
0
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
Pressure on suction side of the pump, bar Figure 4. Suction characteristic curves of rotary pump V 186-130 Q with HiFIo ® lobes,
WORLDPUMPS FEBRUARY1999
0,2
S PECIAL
FEATURE
combination with the flow properties of the actual pumping media are therefore important for the design of pump systems. In the p u m p test equipment at the German Institute for Agricultural Engineering, pipe and pump characteristics can be measured consecutively at different adjustable pressures and capacities in one test.Thus a direct relationship between the rheological properties of the medium and the operating data can be established. The testing consists of a weighing mixer, the pump unit and PVC measurement tubes. The pump drive is subject to infinitely variable control by frequency converters. A so-called throttling pump (also a rotary lobe pump) is arranged in line downstream of the pump for pressure adjustment. Even for highly concentrated high viscosity fluids infinitely variable pressure adjustment in the range from 0 to 10 bar is realised by the infinitely variable speed using frequency converters.A throttle valve for adjusting the suction pressure is arranged in the suction nozzle.The following measuring data were recorded: •
volume flow rate Q (inductive flowmeter)
•
pressure Ps upstream and pressure Pd downstream of the pump (pressure transmitter)
•
torque Md and speed np of the pump (measuring shaft) differential pressure Ap in the meter tubes (differential pressure transmitters via averaging units)
From the pipe characteristic a flow curve for laminar flow can be computed and approximated using known flow models (power laws according to Ostwald and de Waele or Herschel and Bulkley). The piping is used as a tube-type viscometer and changes in the flow properties can be continuously observed and documented. The static operating characteristics of the rotary lobe p u m p were determined using incremental reduction of speed and p u m p pressure. The capacity characteristic is determined in the discharge and suction ranges. The vibration behaviour in the suction range is particularly interesting when cavitation starts. The p u m p i n g tests were performed under defined conditions with water and with liquid feed from a mixture of crushed grain and water. Here the liquid feed serves as a sample medium for highly concentrated agricultural suspensions and is characterised as follows: dry matter content TS = 29 %, density p = 1100 kg/m 3, flow limit
•
•
(~)
Ol=v=te
Centrifugal Lined Process Pumps for:
Acids Alkalines Slurries
rr < >n" B
•
O Z
LU
n" LU
•
Z
• • •
a < UJ re O ,¢t
f-n" LU 03
Z
QualityIndustrialPumps& Motors Cencifugal/ EleCt / Waer/ Air DrivenPumps
Hypalon or Kynar lining bonded to rugged cast-iron body Light weight Kynar impeller reduces shaft deflection Adjustable impeller clearance Tangential or center discharge Multiple sealing choices
Tel: 7 1 7 - 4 5 5 - 2 0 5 1
E-mail: [email protected]
m
LU 03 < LU .J Q.
Call: 800-854-3218
~'"~"'=~'~
Website at www.shurflo.com E-mail at [email protected]
~ ~
® , ~coR cmp.y
DORR-OtJVER Member of Kraug~-Maffei Group
WORLDPUMPS FEBRUARY1999
Fax: 7 1 7 - 4 5 5 - 7 3 5 0
D rr < >fv O
Z w w w a < W nZ 0 co
In" uJ 03 Z uJ oO < w _J
S PECIAL
FF&'I'UI~
1200
1,5
800
1,0
¢t:t
E E
Z
0,5
400
E o I-
o,o ~
0
.-,=
(31.) -400
-0,5
¢t~
Ex._ -800
-1,0 0
20
40
60
80
1O0
120
Revolutions Figure 5. Suction throttling of water flow. Torque pulsation of rotary pump V 136-140 Q with twolobe oval pistons and four-lobe HiFIo ® pistons at 900 rpm.
z0 = 41.05 Pa, consistency coefficient k = 0.5727 Pa s n, flow exponent n = 0.8856. If this liquid feed mix is p u m p e d at Q = 50 m3/h through a pipe with an inside diameter of 100 mm, for example, an apparent viscosity of 0.704 Pa s is effective. The p u m p i n g tests w e r e p e r f o r m e d with a type V 186-130 Q rotary lobe p u m p . The rated value include, a m o n g o t h e r things, Q = 116 m3/h at np -- 385 rpm.
The w a t e r characteristics a p p r o a c h a straight line only at higher speeds. The w a t e r can flow back through the gaps in the pump.Also at lower speeds the capacity changes linearly w i t h the pressure w h e n high viscosity liquid feed is p u m p e d . The gap leakage f l o w is greatly reduced. In p r i n c i p l e the static characteristics of the lobe designs are n o t essentially different. The tightness of the p u m p d e p e n d i n g on its Pumping test results o p e r a t i n g t i m e and w e a r are decisive in all cases. The flow characteristics in the discharge range The suction behaviour (Figure 4) is also difd e p e n d specifically on the p u m p i n g medium. ferent for w a t e r and feed p u m p i n g . The flow characteristics play an increasing role 1200 with increasing A A A speeds because 1000 A it high viscosity media Oval lobes | tt have p o o r e r backflow properties and E 600 the p u m p filling ratio deteriorates. ~o The variation in ~ _ j f torque vibrations of 200 V J the lobe designs b e c o m e s particularal~r ly clear w h e n the suction side is throt-200 tled and at high speeds (Figure 5). -400 ! While the straight 0 90 180 270 360 oval lobe develops Revolution angle, deg quite substantial Figure 6. Torque pulsation of rotary pump V 186-130 Q with two-lobe oval pulsations and pistons and three-lobe HiFIo ® pistons at water flow, 500 rpm and 2,3 bar. greatly loads the
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WORLDPUMPS FEBRUARY1999
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p u m p drive at increasing cavitation, the vibration amplitude of the helical HiFlo lobe changes only to a minor degree in this sample load. That produces clear advantages for this lobe design at high speeds and extreme loads with cavitation. These differences at high speeds without suction throttling are also shown in Figure 6. The pumping cycle of the oval lobe and threelobe helical HiFlo lobe can be followed quite well here.The shape of the oval lobe torque vibrations is interesting. Here negative torques also o c c u r which might be an indication of cavitation in the pump.The application range of this lobe is therefore exceeded with n e = 500 rpm. Although such a distinctive difference between the lobe designs cannot be found w h e n water is pumped, the differences w h e n liquid feed is p u m p e d are all the more distinctive. The vibration amplitudes of oval lobes are higher than those of the HiFlo lobes by a factor of approximately eight.
I Summary The reason for the pressure pulsations occurring frequently during the p u m p i n g process are derived on the basis of the geometrical relation-
ships in lobe designs. A new lobe design, i.e. the helical HiFlo lobe, is described as a conclusion. This lobe clearly reduces pulsations especially at high speeds.The HiFlo lobes also allow better suction behaviour even in case of cavitation. The p u m p i n g and vibration behaviour of HiFlo and customary oval lobes is illustrated on the basis of practical tests with water and a highly concentrated liquid feed mixture. •
1) Dipl.-Ing. H. Vogelsang and Dipl.-Ing. B. Verhiilsdonk are managers in the Hugo Vogelsang Maschinenbau GmbH, HolthSge 12-14. D-49632 Essen, Germany. Tel: +49 5434 830; Fax: +49 5434 8310; E-mail: Vogelsang_. GmbH @t-online .de
2) Dr.-Ing. habil. M. Tiirk and Prof. Dr. G. H6rnig are researchers in the Institute of Agricultural Engineering Bornim, MaxEyth-Allee 100, D-14469 Potsdam, Germany. Tel: +49 331 5699 849; E. potsdam.de
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