# Cavitation in Single-Vane Sewage Pumps

Authors: P. U. Thamsen; T. Bubelach; T. Pensler; P. Springer
Journal: International Journal of Rotating Machinery
 (2008)

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## Abstract

Chair fluidsystemdynamics at TU Berlin investigated the cavitation
behavior of a full-size single-vane sewage pump. Single-vane pumps
are used for raw sewage with high content of dirt and sediments in
larger sewage pumping stations. Cavitation measurement was done by
using standardNPSH3% and the more sensitive
incipient cavitation NPSHIC. Also, vibration
and noise where observed. Contrary to very low
NPSH3% values, very high
NPSHIC values were measured. In a second
step, the impeller was modified with special cavitation bores to
reduce the cavitation effects. The NPSH3% values
increased with cavitation bores, which underlines that this value
is not a sufficient criteria describing cavitation. Using the much
more sensitive NPSHIC, a significant
reduction was obtained. Moreover, the cavitation formation was
changed from a relative concentrated cloud to a distributed bubble
form, which is much less aggressive in view of noise and erosion.
Operational behavior improved with cavitation bores,
noise and vibration levels especially came down to acceptable level. Practical
experience demonstrates also avoidance of cavitation erosion.


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## Body

## 1. Introduction

Sewage  pumping  stations  for
larger  cities typically use different  sizes and types of sewage pumps in dry
installation as shown in Figure1. For example, the Berlin Water Works, Germany,
manages about 140 pumping stations with an overall flow of 630,000m3/day (167,000,000 USGPD).Figure 1
Sewage station with
sewage pumps.Depending on quality of sewage,
pumps are equipped with different design of impeller [1–3]. Single-vane
impeller pumps with nearly nonclogging operation are the preferred choice if
high content of dirt in the sewage exists.In some installations,
sewage pumps operate in so-called lift operation, whereas the sewage water
level is below the pump level. Thus, the availableNPSHA defined as
absolute energy at suction side is around 7 m, which may be in some cases below
the required NPSHR of the pump. Beside unacceptable noise during
operation, in several cases also cavitation erosion of such impellers was observed.
Figure 2 shows a sample of cavitation erosion at the leading edge of a single-vane
impeller after approximately 5,000-hour operation.Figure 2
Cavitation erosion at single-vane
impeller.The
motivation of this project belongs to the better understanding of the
cavitation phenomena within the special design of the single-vane impeller of a
sewage pump and how to control the operation behavior with cavitation.


## 2. Test Facilities and Instrumentation

The Department of Fluidsystemdynamics
at the Technical University Berlin operates
a closed test loop with DN 350 (12”) pipe, shown in Figure3. The test loop is
designed according to guidelines EUROPUMP [4] and is equipped to test all
relevant pump characteristics. The maximum capacity reach 2.000m3/h (8.800 USGPM)
and a pump performance of about 200 kW (270 HP) allows full size test of centrifugal pumps. Figure
3 shows the closed test loop with a single-vane sewage pump and about 150 kW
(200 HP) power consumption.Figure 3
Closed test
loop for centrifugal pumps.Instrumentation and
principle setup of test loop are
shown in Figure4.Figure 4
Instrumentation of
the test loop.Experiments are executed
with normal potable water at approximately20C° (68 F). Steady-state operation
reached after about ten minutes and readings are taken during stable operation
point. Pretest shows
no significant influence of water (aerated, deaerated) on the measurements of
NPSH.The NPSH is adjusted by the
pressure in the tank, which is formed by the geodetic head as well as the air pressure
above the liquid. This gives the opportunity to run the pump under adjustable cavitation
grades, whereas all relevant data are collected depending on the NPSH, for example, capacity,
head, power, and vibration.Additionally, the test loop
is equipped with some visual instrumentation to observe the cavitation
formations within the impeller. First, an endoscope is placed in front of the
inlet of the impeller to show the onset of cavitation bubbles on the impeller
blade suction side. Additionally, the standard service flange at the pump
casing allows a view into the impeller area from the outlet side of the blade.
A stroboscopic light source ensures sufficient power of light. CCD-camera and
computer are connected for further documentation of phenomena. Visualization
equipment is shown in Figure5.Figure 5
Visualization equipment.Simple white and black
color marks on the impeller blade at the leading edge allow the identification
of the bubble length during the tests (see Figure6). The distance between the
lines is 20 mm (0.79 inch), whereas additional lines separate the inner, middle,
and outer areas
between hub and shroud.Figure 6
Impeller marked for
cavitation observation.


## 3. Cavitation Bores

The principle of the
cavitation bores is shown in Figure7. Shroud and hub of impeller get a number
of relative large-size bores close to the leading edge of the blade. These bores are quite
different to the known axial thrust balancing bores of standard impellers.
First, developed by field experience, the cavitation phenomena, such as noise, vibration, and
erosion, were significantly reduced.Figure 7
Principle of
cavitation bores.The idea behind the
cavitation bores is to connect the low-pressure areas at impeller inlet edge to the high-pressure
areas at shroud clearances. Thus, the cavitation zone will be filled up from
the flow through the bores with the effect of reducing the cavitation grade as
well as the bubble size.


## 4. Experimental Investigations

Experimental investigations
should verify the idea of the cavitation bores and their effects on the
different cavitation phenomenon of the single-vane sewage pump. Therefore,
three variations of cavitation bores are investigated:B=0→ all
cavitations bores are open,B=1→ all
cavitations bores are closed,B=2→ half
number of cavitations bores are open.Looking on the head flow
curves in Figure8, the effect of additional gap flow due to fully open
cavitation bores reaches maximum about 5% gap flow at BEP Qopt. It
is noticeable that increased gap flow results in reduction of pump efficiency
by about up to 2%.Figure 8
Head flow curve
(n=725rpm).Some typical head-drop
curves are displayed in Figure9. Each head-drop curve at constant speed and
constant flow results
in standard NPSH3%, at which the head is reduced by 3% due to
cavitation compared to the head at high-level NPSH respective cavitation-free
operation. Figure 10
shows
the NPSH3% values for the sewage pump without (B=1) and with open
(B=0) cavitation bores.Head drop and vibration values (B=1, n=725rpm).
(a)(b)Figure 10
NPSH3% (n=725rpm).Generally,  the measuredNPSH3% values are on very low level from 1 to 2.5 m. The suction number reaches about Sq=0,7 (NSS=12,000), which is significantly higher than the standard centrifugal pumps and demonstrates normally a
high suction capability. On the other hand, the diagram of vibration level shows
a dramatic increase of
vibration at NPSH3% combined with heavy noise generation. It is not
recommendable to run the pump under such conditions. Obviously, the NPSH3% value does not give
adequate information on the operation behavior under cavitation conditions for single-vane
sewage pumps.Comparing theNPSH3% for the different cavitation bores (Figure 10), the fully open ones show slightly
higher values compared to the closed bores, even at the same low level. The explanation for
this effect is combined with the negative impact of the additional “gap flow” through
the cavitation bores on the main flow [5, 6].In this project, we use direct
flow visualization to better understand the cavitation phenomenon [7, 8].  Cavitation bubbles as well as formation of
cavitation in the impeller are observed through an endoscope in front of the
impeller. Figure 11 shows the positioned endoscope.Figure 11
Position of endoscope.Visual measurement first
gives the onset “incipient” cavitationNPSHIC, which here is defined
as observed: 20 mm bubble length at impeller leading edge. Looking at the NPSHIC (Figure 12), the absolute values are relatively high reaching approximately 30 m
BEP Qopt compared to similar standard pumps [8, 9]. This could be
explained by the special design of nonclogging single-vane impeller: the vane
inlet of the single-vane impeller opens progressively the space for all kind of
dirt in the sewage to avoid any clogging. On the other hand, this creates a
relatively large dead water zone just behind the leading edge due to flow
separation. Cavitation typically locates in this area.Figure 12
NPSHIC (n=725rpm).Figure12 also demonstrates
the positive influence of cavitation bores on the NPSHIC values:
with fully opened cavitation bores, the NPSHIC is reduced by about
10 m at BEP but remains
on a relative high value around 20 m.Remember that the pump operates in some
applications under lift conditions. Obviously, the practical operation of such single-vane
pump would have to accept some grade of cavitation. Typically, such pumps
operate atNPSHA=8–11m.Figure13 shows the observed
cavitation at BEP Qopt With
closed cavitation bores B=1 (upper photograph), the cavitation activity is
located mainly beginning at the leading edge with relatively large extension (approx.
180 mm length) of sheet cavitation which also forms some cloud cavitation. It is known that
cloud cavitation aggressively creates noise and erosion
[10, 11]
.Cavitation atNPSHA=8m
with/without cavitation bores at BEP (n=725rpm).
(a)(b)With open cavitation boresB=0
(lower photograph), the extension of cavitation bubbles is significantly
reduced. Additionally, the cavitation clouds seem to be destroyed by the jet
flow through the bores. As the cavitation bubbles move away from the wall, the cavitation activities
in middle area of the vane passage will reduce the number of cavitation
collapses in the near of impeller walls. This explains the reduction of noise
and erosion.Finally, we investigate the
cavitation grade subjective by evaluating the noise level with the same person
to decide the acceptable grade of cavitation. Results are shown in Figure14.Figure 14
Subjective
cavitationNPSHSG—curves with (B=0) and without (B=1) cavitation
bores n=7251/min.The acceptable level of
noise is approximately 15 m belowNPSHIC for the evaluated
arrangements. Whereas the evaluated level for closed cavitation bores is unacceptably
high (NPSHSG=17m at BEP), the values for open cavitation bores
are acceptable for normal operation of this single-vane sewage pump (NPSHSG=6m at BEP).


## 5. Conclusions and Summary

Cavitation in single-vane
impeller pumps will occur also under normal suction conditions. This is due to the
nonclogging impeller design, whereas the high incidence angle will create a
separation zone at the inlet edge of the impeller. On the other hand, this
sudden opening will not allow the dirt to clog the inlet. Some grades of
cavitation have to be accepted but controlled by cavitation bores, which reduce
the extension of cavitation itself and destroy the cloud formation of cavitation.
Cavitation bubbles are redirected away from the impeller walls. This could be
clearly demonstrated by visualizing the effect of the bores. Especially, noise changes to acceptable
level even for suction lift operation. In summary,  cavitation bores are an
adequate possibility to reduce negative effects of cavitation in single-vane
sewage pumps.



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*Source: 354020-2008-12-25.xml*