343
1 INTRODUCTION
A cascade control system of a sewage pumping
stationiswidelyknownandapplied.Thisisprimarily
decidedbysimplicityofthesystem.Noimpactonthe
pumpingsystemispresentotherthanthefunctionof
pump switch on and off. This is enabled by drive
systems of
induction motors using frequency
converters.
A dry well sewage pumping station (pumps
operatinginadrywellseparatefromasewagetank)
is illustrated in Figure 1. A suction manifold is
immersed in a storage reservoir with capacity Q
x,
where sewage comes. The pump operates to change
the sewage level from a minimum head H
zmin to a
maximum of H
zmax. Sewage is pumped through a
discharge line at capacity Q
o to a sewage receiving
body. As sewage flows, dynamic pressure losses
ΔH=aQ
o
2
are generated. Figure 1 shows head of
pressure H
t across the pressure manifold meets the
dependence[3,5,9,14]:
2
tst ozn
HHaQH
(1)
Thefollowingequationisalsotrue:
tzn
H
HH
(2)
where:
H
zn – head of sewage at suction pipe relative to the
pump’shorizontalaxis(Fig.1).
Figure1.Anexampleofheadofpressuredistributionfora
sewagepumpingstation[8,12]
Variable Rotational Speed Control System of Sewage
Pumping Station
E.Szychta&L.Szychta
UTPUniversityofScienceandTechnologyinBydgoszcz,Bydgoszcz,Poland
ABSTRACT:Cascadeandvariablerotationalspeedcontrolsystemsofsewagepumpingstationsarecompared
inthisarticle.Therangeofpumpefficiencyvariationsandpotentialforbreakdownsareadoptedascriteria.A
pumpingstationincludingtwoorthreepumpsisanalysed.
Acontrolsystematavariablerotationalspeedand
maximumsewagelevelispresented.Propertiesofacascadecontrolsystemandasystematamaximumhead
ofsewagearecomparedusingtheexampleofanypumpperformancechart.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 13
Number 2
June 2019
DOI:10.12716/1001.13.02.10
344
Figure2. Pump flow characteristic p1 relative to the
standardcurver
1ofapumpingsystem;H[m],Q[l/s][13]
(2) implies the greater Hzn, the lower the head of
pumpH.Thisreducesdemandforelectricityfromthe
pump electric motor. Figure2 implies the pumping
station’scapacityQ
2isthesameforoperatingpoints
W
2andW2
*
.HeadH2
*
ofthepumpisgreaterforthe
point W
2
*
, which means a greater demand for
electricity.AtpointW
2
*
,thepumpoperatesatarated
rotational speed along the flow characteristic p
1.
Operationalong theflow characteristicp
2at a lower
rotationalspeedn<n
Nreducesconsumptionofelectric
power.Pumpoperationshouldbecontrolledinsuch
awayastogeneratemaximum sewage levels inthe
storagetankinordertoarriveattheforegoingeffect
[6,7,10,11].
2 COMPARATIVEANALYSISOFCONTROL
SYSTEMS
Sewage flows into a pumping station stochastically.
For
the purposes of comparative analysis, certain
conditions ensuring correct function of the entire
pumpingsystemauthorsassumed:
Sewage cannot be allowed to putrefy if it comes
slowly,
Minimum pumping station efficiency should
provide for a rate of sewage flow across the
pressure manifold sufficient for particulate
movement,
Maximum
pumping station efficiency should not
allowforanexcessiverateofsewageflow,which
wouldproducehighdynamicpressurelosses,
Rangeofpumpefficiencyvariationsshouldfollow
the manufacturer’s recommendations in order to
meetthecriterionofreliableoperation(Fig.3),
Other conditions characteristic for a medium
transported
and ensuring safe and reliable
pumping.
Figure3.Distribution of pump operation reliability
characteristicsasafunctionofefficiency:A1–cavitation,A2
–shorterlifeofbearingsandsealings,A3–eddycurrentsin
thedischargepart,A4–eddycurrentsinthesuctionpart,
A5–shorterlife ofrotor,A6‐shorterlife of bearings
and
sealings, A7 – cavitation for low flows, A8 –temperature
growth[1,2]
Figure4. Analysis of sewage pumping station’s operation
basedonanypumpperformancechart[17]
Authors proposed the comparative analysis of
pumping station properties for a cascade control
systemandbyvaryingrotationalspeedbasedonthe
pump’s performance chart (Fig.4). The pump is
assumedtoraisesewagelevel(theheadofsewage)in
thestoragetankby3[m].Foramaximumtankfilling,
properties
ofthepumpingsystemaredeterminedby
thecharacteristicr
1,whilefortheminimumlevel,by
thecurver
2.Forthepurposesofcomparativeanalysis
of pumping station properties under the above
assumptions, the following variation ranges were
adopted:
Ofpumpefficiency
,
OfpumpcapacityQrelativetoitsoptimumvalue
Q
opt; effect of capacity on the pump operation
reliabilitywasadoptedasthecriterion(Fig.3).In
practice,pumpcapacityshouldbeinthefollowing
range:
opt
0,8 1, 2QQ
(3)
Forcapacitybelow0.8Q
optthereisaprobabilityof
occurrence eddy currents in the discharge part. In
contrast, for capacity greater than 1.2Q
opt, the
probability of shorter life of bearings and sealings
increases. At the same time, for the designated
operating capacity range of the pump, the highest
pump efficiency values are expected. Adopting a
smallerrangeof pumpcapacity changes will reduce
the pump efficiency range. However, this can cause
thepump
tobeswitchedonandofftoooftendueto
the lack of equality between incoming sewage and
pumpcapacity.
345
In cascade control, the pump’s operating point is
alongthe flowcharacteristic for the rotational speed
n
N=2850[rpm].Asthesewagelevelshiftsby3[m],the
pump operation is between A and B. Figure 4
producesthefollowingvariationranges:
Ofcapacity
opt
39,11,1 QQQQ
BAAB
,
Ofpumpefficiency
%81%72
AB
.
Theresultantrangeofefficiencyvariationscanbe
regarded as insignificant and acceptable in
applications.Capacity,ontheotherhand,exceedsthe
maximumacceptablevaluebyapproximately0.2Q
opt.
TherangeofefficiencyQ
ABvariationsprovesapump
is liable to cavitation. It causes faster rotor wear,
reduces pump efficiency, and requires itspremature
overhaul.
In cascade control of two pumps, a pumping
station operates along the summary station flow
characteristicbetweenA
1andB1.Operationofasingle
pumpcanbeanalysedonshiftingA
1toCandB1toD
ontheflowcharacteristicofasinglepump.PointCis
locatedonanapparentcharacteristicr
1’thatrelatesto
thereal characteristicof the pumping system r
1 as a
resultofA
1shiftingtoC.Inparallel,pointDislocated
onanapparentcharacteristicr
2’thatrelatestothereal
characteristicofthepumpingsystemr
2.Thefollowing
variationrangesarederivedfromFigure4:
Ofcapacity
opt
16,19,0 QQ
CD
,
Ofpumpefficiency
%82%79
CD
.
Pumping station operation under cascade control
andgiventheseassumptionsisenergeticallyeffective
and fulfils the criterion of high reliability when
operatingattheratedoperationalspeed.Thisanalysis
leadstothefollowingconclusions:
Where one pump is in operation, it may be
overhauledearlyduetocavitation,
Two pumps in operation reach high values of
efficiencyandreliability,
Operationofoneortwopumpsexhibitsincreased
lossesin acontrol systemmaintaining asteady –
maximum – level of sewage that corresponds to
thepumpingsystemcharacteristicr
1.
A single pump operates at a variable rotational
speedandmaximumsewagelevelalongthepumping
system characteristic r
1. Point P1 is assumed to be
located on the similarity parabola of optimum
efficiency
opt.Thisoperationpointensuresmaximum
pumpefficiencyforagivenhead.Itisalsoassumedto
correspondtocapacityQP
1,namedoptimumcapacity
as a maximum efficiency out of the set of all
efficiencies in the characteristic r
1 is reached at this
point. In line with (3), the range of pump capacity
variations at a maximum sewage level may be
assumedtobeintherange:
1min 1max 1
0,8 1, 2QQP QP QP
(4)
The foregoing argument includes a simplification
that points P
1min, P1, P1max should be situated on the
same flow characteristic corresponding to one
rotationalspeed.Thisisnotofparamountimportance
from a practical point of view, since boundary
capacity values are assumed with a tolerance of
0.05Q
optforwhichahighpumpreliabilityisassured
(Fig.3).
Twopumpsoperateatavariablerotationalspeed
and maximum sewage level along an apparent
pumping system characteristic r
1’. P2 is on the
similarity parabola of optimum efficiency
opt. Like
for the operations point P
1, the range of pump
capacityisintherange:
2min 2max 2
0,8 1, 2QQP QP QP
(5)
Theaboveanalysisimpliesconclusionscomparing
impact of a control system on pump operation. For
controlbyvaryingrotationalspeed:
Ahighpumpoperationcapacityispreservedwith
an extensive range (ΔQ=0.4Q
opt) of pump
productivityvariations,
Pumpoperationisnearitsoptimumvalues,
Losses associated with an unreasonable head of
pump and the resultant losses, defined with the
factor of control quality for cascade regulation
2
2
1
kas
H
H
,areignored.
Inthecaseofcascadecontrol:
High efficiency values for the entire range of
pumpingstationcapacityarenotrequired,
Cavitationisverylikely,
Additional pump system losses
2
2
1
kas
H
H
are
connected with unreasonable head of pump
relativetothepumpsystemcharacteristicr
1
The adopted range of capacity variations for one
and two pumps operating at a variable rotational
speedshouldprovideforabalancebetweenincoming
sewage Q
x and pumping station capacity Qo at a
maximum level of sewage. Under such a system of
operation, fulfillment of the criteria of high energy
effectiveness and pump reliability should be
expected.Lackingsuchabalance,thefollowingmay
benecessary:
Switch the pumps off temporarily for a low
incomingsewage,
Atemporaryrise
ofrotationalspeedattheexpense
of pump reliability at times of high incoming
sewage(thisisrelativelyrare).
3 CONCLUSION
Cascade control is normally used as the simplest
application solution. The numbers of combined
sewage pumping stations require application of
variable rotational speed control. Minimising energy
consumption and limitation of
water hammers in
sewagesystemsaretheoverarchingobjectivesofthis
control.
Themethodsforcontrollingtheoperationofpump
setsaretocontroltheoutputparametersofthepump,
e.g.thepumphead.Theauthorsassumethatthereis
apossibilityofcontrollingthelevelofsewageheadat
the pump suction. The increase of this head should
aim to reduce the pumpʹs head. As a result, the
energyneededtopumpsewageshouldbeminimized.
Basedonthecomparativeanalysisofthetwocontrol
systems,theefficiencyforcascadecontrolvariesfrom
72%to82%.Atthe same
time there isanadditional
pumpsystemlosses.Theauthorsassumethat inthe
case of a pump with variable speed, the inflow of
346
sewageintothe tankis thesame asits outflow.The
level of sewage will be set at such a height that the
pump works with optimum efficiency. The analysis
shows that the pump operates at point P1 and the
efficiency is constant and amounts to 79%. An
additional benefit
of the proposed control system is
theextensionofthereliabilityoperationperiodofthe
pump.
The proposed solution is more expensive than
cascade control. A new or modernised pumping
station can utilise a new control system if incoming
sewage is steady, which will bring faster returns on
financial outlays.
The proposed control system
requires further studies to estimate unit energy
consumptionofthepumpingasafunctionofsewage
headvariationsinthesuctionmanifold.
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