PUMPS
WORLD
Pulsation problems in rotary lobe
pumps are solved by the Hiflo'
technique
One disadaantage utitb tbe use of By H Vogelsang,B Verhülsdonk, M
rotary lobe pumps is tbe unsteady dß- Türk,and G Hörnig
pla.cementprocess wlticb produces pressure pußa.tions, specifically in long and
rigid ptpelines and non-compressible tr Principles of rotary lobe
geometry
liquids. Tbe result is uibration of pump
piping,
rßk
of
tbe
u*ticlt
inqeases
and
caaitation on tbe sudion side. Tbß afü- Theory of the displacement process
cle expl.ains tbe mecbanical causesof
tbe unsteady displacementprocess and Rotary displacement pumps with a periodically
fluctuating volume flow Q generate pressllre
a, neut lobe design is deriaed and pulsations
in the connected piping due t<l
describedas a consequence.
acceleration and deceleration.The internal disSlurries and high viscosity fluids with an apparent
viscosity of more than 0.5 Pa s can no longer be
pumped economically by centrifugal pumps.
Especially for inhomogeneous suspensittnsit is
than better to use self-sealing,valveless,rotary
positive displacementpumps with large cross-sections.These are eccentric screw pumps ()r rotary
lobe pumps operating according to the Roots
principle.The constant chamber volumes and low
speeds of these pump designs mean sensitivc
materials can be pumped gently and metering
tasks easily tulfilled.
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
Compared
highly solid contaminatedsuSpensions.
with eccentric screw pumps, rotary lobe pllmps
with large capacities have been successfully established due to their compact design.
Preferred areasof application are:
placement mechanism of rotary lobe pumps of
traditional design according to the Roots principle bring about flow fluctuations with peak values cletermined by the basic design characteristics shaft distance A and lobe tip diameter D
(Figure 1).
In rotary displacement pumps the drive
torqlle is directly proportional to the displacement volume and that in any angular position.
Friction in bearings,gears and lobes (friction of
lobes againsteach other and the pump casing)are
disregarded at first.The course of the volume flow
fluctuation versus the angular rotation corresponclsto the course of the torque.The torque at
a specific angular position can be calculated
accordin5; 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 comp()nent in the direction of rotation
and a force component in the direction opposite
to the direction of rotation. The sum of the two
pumping systems for agricultural suspensions (liquid
manure and biogas slurries)
with long pipelines (Lrp to
500 m and in exceptional
caseseven up to 5 km)
pumping systems for sewage
slurries and high viscosity
industrialfluids
mobile pumping systems on
road tank cars for extracting
liquid organic waste and
1äecalmatter.
Figure 1. Pressureloads on the lobes.
O262 7762/L)9/$ - see fiont rnatter
O 1999 ElsevierScience Ltd. AII rights reserved.
WORLD PUMPS FEBRUARY 1999
PECIAL
S
FEATURE
lobe torques gives the total
torque.The maximum and minimum torques and displacement
volumes can also be determined that way. The pump
delivers least when 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 when the
krbes contact at pitch point W.
All rotary krbe pumps with a
lobe design to prevent backFigure 3. Helicallytoothed lobes (HiFlo@).
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 decreasesaccording to the number of
teeth, the extreme values of the volume flow fluc- teeth.The averagepumping 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 averagedisplacement 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 Llrgent problem is the reduction of
flow fluctuations.The easiest way to achieve this
two-lobe piston is approximately D-", = 1.(r * A.
The pumping capacity of a threelobe 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 pumped if the identical pump chambers with a y2 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
pumped in the second lobe position when the the pump combination is also doubled by superlobe s contact each other at point I(4The liquid imposition, i. e. the acceleration values are
columns in the piping connected to the suction reduced to only < lO % of the original values.In
and discharge side must be periodically accelerat- actual pumping systems,in which for geometrical
ed and decelerated.Thatis also the main causeof 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 separatepump chambers are required.
pulsation frequency increases.Thepressurepeaks
This means additional sealing gaps,higher fricat constant speed increase almost linearly with
tion forces, no overmounted bearings of the
increasing
FlowrateQ
decreasing
acceleration
,{n
ull
?t*
deceleration
105'
Figure 2. Pulsation as the result of flow fluctuation.
WORLDPUMPS FEBRUARY 1999
124"
Rotatingangle
s
PECIAL
FEATURE
pump lobes, increased assembly effort, etc.This
version has been successfullyused in practice for
some time.
A pulsation-free helical lobe, the so-called
HiFlo lobe, was developed as a new 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
occllrs. 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 achievedat €areat
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 l2O' +
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 becomes uniform. The minimum number
of teeth is then four at a casing angle of 180'.The
manufacturing costs are, howeveq considerably
higher due to the steeper lobe flanks. But the
disadvantageof 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 abovehelical.rubbedined lobes allow the
design of single-chamber pump which do not
exhibit geometry-specificflow fluctuations.This
new lobe shape can be installed in the customary
pump casingsand exhibits all the previous design
benefits such as minimum leakage gap length and
overmounted bearing with rapid accessto the
pump space for cleaning,maintenance or repair.
Besidesthe lack of pulsation, a better suction
capability was also demonstratedin practical tests
so that higher speeds with higher capacities
become possible.
E
Practical investigations
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 perfclrmance also
changes.Exact pipe and pump characteristicsin
I
I
'l
I
Liquidfeed
I
0
I
E
I
q)
(E
L
=
o
(l)
E
=
ö
Pressure
on suctionsideof the pump,bar
Figure 4. Suction characteristiccurvesof rotary pump V 186-130Q with HiFlo i lobes.
WORLDPUMPS FEBRUARY 1999
S
PECIAL
FEATURE
combination with the flow properties of the actual pumping media are therefore important for the
design of pump systems.
In the pump test equipment at the German
Institute for Agricultural Engineering, pipe and
pump characteristicscan be measured consecutively at different adjustable pressures and capacities in one test.Thusa 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-calledthrottling
pump (also a rotary lobe pump) is arranged in line
downstream of the pump for pressureadjllstment.
Even lbr highly concentrated high viscosity
fluids infinitely variable pressure adiustment in
the range from O to l0 bar is realised by the
infinitely variable speed using frequency converters.A throttle valve fbr adiusting the suction pressure is arranged in the suction nozzle.The following measuringdata were recorded:
.
volume flow rate Q (inductive flowmeter)
.
pressure ps Lrpstreamand pressure p4 downstream of the pump (pressure transmitter)
.
torque Mo and speed no of the pump (measuring shaft)
.
differential pressure Ap in the meter tubes
(differential pressure transmitters via avetaging 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 changesin the flow properties can
be continuously observedand documented.
The static operating characteristics of the
rotary lobe pump were determined using incremental reduction of speed and pump pressure.
The capacity characteristic is determined in the
discharge and suction ranges. The vibration
behaviour in the suction range is particulady
interesting when cavitation starts.
The pumping tests were perfclrmed 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 condensity p = 1100 kg,/m3,flow limit
tentTS = 2L)%n,
Pumpn
?wuen
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Dimmsion!
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I verminderteKovitotion f höhere
Drücke I problemlos nochrüstbor
4 9 6 3 2 E s s e n / O l . H o l t h ö gle0 - 1 4 T e l .0 5 4 3 4 / 8 3 - O
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B ü r oB i e l e f e lIde l . 0 5 2 4 7 / 1 0 1 6 0 B ü r ol l l i n g e T
WORLD PUMPS FEBRUARY 1999
s1"g#,
1,0
F
:z
d
=
o_
<D
-c
4oo
E
(t'
0
f
<t)
r
o
F
(6
_o
L
0,0
u
(t)
o_
=>
o)
L
-
(t)
ct,
(D
o--1,0
40
60
Revolutions
F i g u r e 5 . S u c t i o n t h r o t t l i n g o fw a t e r f l o w . T o r q u e p u l s a t i o n o fl o t a r y p u m p V 1 3 6 - 1 4 0 Q w i t h t w o lobe oval pistons and four-lobe HiFloh'pistonsat 900 rpm.
xn = 11 .O5Pa,consistency coefficient k = O.5727
Pa s",flow exponent n = 0.8856.If this liquid feed
mix is pumped at Q - 50 m3lh through
a pipe with an inside diameter of 100 mm, for
example, an apparent viscosity of O.7O4Pa s is
effective. The pumping tests were performed
with a type V 186-130 Q rotary lobe pump.
The rated value include, among other things,
Q = 116 m3/h ^t np = 385 rpm.
The water characteristics approach a straight
line only at higher speeds.The water can flow
back through the gaps in the pump.Also at lower
speeds the capacity changes linearly with the
pressure when high viscosity liquid feed is
pumped. The gap leakage flow is greatly
reduced.
In principle the static characteristics of
the lobe designs are not essentially different.
The tightness of the pump depending on its
Pumping test results
operating time and wear are decisive in all
cases.
The flow characteristics in the discharge range
The suction behaviour (Figure 4) is also difdepend specifically on the pumping medium. ferent for water and feed pumping. The flow
characteristics play
an increasing role
1200
with
increasing
speeds
because
1000
high viscosity media
Oval lobes
800
have poorer backflow properties and
E ooo
the pump filling
z
ratio deteriorates.
o
400
HiFlo
The variation in
l
lobes
torque vibrations of
,o
-zoo
-r
the lobe designs
becomes particular0
ly clear when the
suction side is throt-200
tled and at high
-400
speeds (Figure 5).
\ü7hile the straight
oval lobe develops
quite
substantial
Figure 6. Torquepulsation of rotary pump V 186-130Q with two-lobe oval
pulsations
and
pistons and three-lobe HiFloo pistons at water flow, 500 rpm and 2.3 bar.
greatly loads the
WORLD PUMPS FEBRUARY 1999
s
PECIAL
FEATURE
pump 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 speedswithout 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 tofques also occur
which might be an indication of cavitation in the
pump.The application range of this lobe is therefore exceeded with np = 500 rpm.
Although such a distinctive difference
between the lobe designs cannot be found when
water is pumped, the differences when liquid feed
is pumped 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.
K summary
The reason for the pressure pulsations occurring
frequently during the pumping 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.TheHiFlo lobes also allow better suction behaviour even in case of cavitation. The
pumping and vibration behaviour of HiFlo and
customary oval lobes is illustrated on the basis of
practical tests with water and a highly concentratT
ed liquid feed mixture.
1) Dipl.-Ing. H. Vogelsangand Dipl.-Ing. B.
Verhülsdonk are managers in the Hugo
GmbH,
Maschinenbau
Vogelsang
Holthöge 12-14,D-49632 Essen,Germany.
Tel: +49 5434 93O;Fax: +49 5434 83lO;
E-mail:[email protected]
2) Dr.-Ing. habil. M. Türk and Prof. Dr. G.
Hörnig are researchers in the Institute of
Ägricultural Engineering Bornim, MaxPotsdam,
Eyth-Allee lOO, D-t4469
Germany.Tel: +49 331 5699 513; Fax'.+49
33L 5699 849; E-mail: [email protected]
potsdam.de
rfuoErsANG
El ne l'r ko Ib e rr p u rrrp Er r-r
WORLDPUMPS FEBRUARY 1999