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How to Avoid Expensive Pump Failures and
Repairs from Cavitation by Determining the
“Suction Energy” of a Pump
Written by: Allan R. Budris, P.E.
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How to Avoid Expensive Pump Failures and
Repairs from Cavitation by Determining the
“Suction Energy” of a Pump
Educational Objectives
On completion of this course, students will:
1
Learn how and when cavitation exists in and
causes damage to centrifugal pumps.
4
Learn about pump “Low Flow Suction Recirculation”,
and when it will cause damage to a pump.
2
Learn a method that can predict when
cavitation will and will not cause damage to
and shorten the life of centrifugal pumps.
5
Learn when pump suction piping can
cause pump damage, and recommended
minimum straight suction pipe lengths.
3
Learn how the NPSH Margin and Suction
Energy of a pump impacts pump reliability.
Cavitation damage can drastically shorten the life of the
pump impeller, mechanical seals, bearings and possibly
other pump components, plus cavitation typically starts in
pumps with NPSH Margins (NPSHA/NPSHR) of around
4.0, and sometime even higher. This means that cavitation
exists in a high percentage of installed pumps. But not all
pumps that experience cavitation will be damaged. So the
big questions becomes, not will my pump cavitate, but will
the cavitation that likely exists cause damage to my pump.
Allan Budris has developed a relatively simple means of
answering this question in the form of “Suction Energy”.
This paper defines Suction Energy, Suction Energy Levels,
Suction Energy Gating values for various pump types, and
how Suction Energy Ratios and NPSH Margin Ratios can
predict relative pump life. Also explained are how Suction
Energy can also predict suction recirculation damage, and
when suction piping is critical to dependable pump performance. Suction pipe lengths are suggested for low and high
pump suction energy levels.
This new method for predicting cavitation damage in
centrifugal pumps, which has been adopted by the Hydraulic Institute and major pump companies, can save thousands
of dollars in unnecessary maintenance costs.
Introduction:
One way that the Best-of-Class users reduce the Life Cycle
Costs of their pump installations is by selecting pumps
and designing pump systems that will avoid cavitation
damage within their pumps. Cavitation damage can dras2
tically shorten the life of the pump impeller, mechanical
seals, bearings and possibly other pump components. In
other words it will reduce the mean-time-between-failure
(MTBF), possibly to as low as three months, which will
increase maintenance costs and pump down time.
There has been a lot written on the dangers of pump
cavitation damage when adequate suction pressure (Net
Positive Suction Head Available - NPSHA) is not provided;
when pumps are operated at low flow rates (below the start
of suction recirculation); and/or when sufficient straight
runs of piping are not provided upstream of the pump
suction. It can require NPSHA values of four times the
published Net Positive Suction Head Required (NPSHR),
or more, to avoid all cavitation in a pump. The start of Suction Recirculation, which occurs in all centrifugal pumps at
some reduced flow rate, can be as high as 85% or more of the
pump best efficiency flow rate (bep). The required minimum straight length of pipe, up stream of the pump suction,
may be as high as 15 times the pipe diameter or higher.
However, experienced pump users also know that most
smaller, and/or slower speed, pumps do not experience cavitation damage, even when operated under these unfavorable
conditions. So the question is, specifically when must pump
users take special precautions to avoid cavitation damage,
and the associated increase in maintenance costs?
Suction Energy:
Well, based on the writer’s extensive experience with several major pump manufacturers, he was able to developed a
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Suction Energy (S.E.) = De x N x S x S.G.
Where:
De = Impeller eye diameter (inches)
N = Pump speed (rpm)
S = Suction specific speed (rpm x (gpm)0.5) /
(NPSHR)0.75
S.G. = Specific gravity of liquid pumped
Based on the experience of hundreds of centrifugal
pumps, the writer was able to establish the following specific gating values for the start of “High Suction Energy”
and “Very High Suction Energy”, for various pump types.
Pump Suction Energy Ratio’s can be determined by dividing the actual pump suction energy value by the “High Suction Energy” gating values from this table.
Table 1.
Pump Type
Start of
Start of
“High Suction Energy”
“Very High
Suction Energy”
2-Vane Sewage Pumps*
100 x 106
150 x 106
Double Suction Pumps
120 x 106
180 x 106
End Suction Pumps
160 x 106
240 x 106
Vertical Turbine Pumps
200 x 106
300 x 106
Inducers
320 x 106
480 x 106
* - Applies to all pumps with less than 15° of impeller vane overlap
Low Suction Energy (S.E. Ratio’s less than 1.0):
Pumps with levels of suction energy below these
values are considered to have low suction energy. Generally speaking, low suction energy pumps are not prone to
noise, vibration or damage from cavitation or recirculation.
However, there could be detrimental effects on mechanical
seals from the air or vapors which may be liberated from
the liquid during the formation of the cavitation bubbles,
under low NPSH margin conditions (below 1.1 – 1.3 NPSH
margin ratio). One exception to this damage free zone
does, however, occur with pumps handling abrasives and/or
corrosive liquids which amplify the implosion impact of the
cavitation bubbles.
www.WaterWorldCE.com High Suction Energy (S.E. Ratio’s between 1.0 and less then 1.5):
Pumps with high suction energy and low NPSH margins, especially when operated in the suction recirculation
flow range, may experience noise, vibration and/or minor
cavitation erosion damage with impeller materials that have
low cavitation resistance, such as cast iron. High suction
energy starts at about 3,560 rpm in end suction pumps with
6” and larger suction nozzles sizes, and split case pumps
with 8” and larger suction nozzles. At 1,780 rpm, high suction energy is likely to start with 10” suction nozzle size for
end suction pumps, and 12” suction size for split case pumps.
Very High Suction Energy (S.E. Ratio’s of 1.5 and above):
Pumps with very high suction energy and low NPSH
margins, especially when operated in the suction recirculation flow range, may experience erosion damage, even with
cavitation resistant materials, such as stainless steel.
This impact of Suction Energy on pump reliability can
be seen in figure 1. It reflects a pump reliability “trend line”
that was developed from actual field reliability data at two
process plants (one with split case pumps, and the other
with ANSI end suction chemical pumps). As can be see,
pump reliability definitely decreases with increasing values
of suction energy.
Figure 1: Suction Energy Reliability Factor
Reliability factor
method that predicts when pumps are susceptible to cavitation noise, vibration, and/or damage; and when they are free
of these damaging affects. The method is called “Suction
Energy”, with specific gating values of “Suction Energy”
identified for different pump types and/or impeller vane
overlap. The amount of energy in a pumped fluid which
flashes into vapor and then collapses back into a liquid
in the high pressure areas of the impeller determines the
amount of noise and/or damage from cavitation.
Suction energy is another term for the liquid momentum
in the suction eye of a pump impeller, which means that it is
a function of the mass and velocity of the liquid in the inlet.
Suction energy is defined as:
NPSH Margin
The NPSHR of a pump does not represent the start of
1: Suction Energy Reliability Factor
cavitation. ItFigure
is actually
the NPSHA that will cause the
total head to be reduced by 3%, due to flow blockage from
cavitation vapor in the entry of the impeller vanes. It can
take from 1.05 to 2.5 times the NPSHR just to achieve the
100 percent head point, and typically 4 to 5 times the 3%
NPSHR of the pump to totally eliminate cavitation. The
question then becomes, how much NPSH Margin is really
required to minimize cavitation damage, and achieve extended MTBF rates.
Figure 2 presents a trend line that was obtained from
the same actual field test data mentioned above for Suction
Energy, and it shows a definite trend of improving reliability
with higher NPSH Margin values.
3
Reliability factor
Figure 2: NPSH Margin Reliability
lines. This is due to the fact that the maximum cavitation
damage does not actually occur at the lowest NPSH Margin,
but instead at a NPSH Margin Ratio value around 1.3 to 1.5,
above the start of suction recirculation, as shown in figure
5. This is due to the cushioning impact of the dissolved air
which is liberated (to entrained air) as the cavitation vapor
bubbles are formed, in increasing numbers, as the NPSH
Margin is reduced (as shown in figure 6).
Figure 4: Suction Energy/NPSH Margin Reliability
(Erosion Damage)
NPSH Margin Reliability Factor
Based on these
Suction
Energy
and NPSH Margin pump
Figure
2: NPSH
Margin Reliability
reliability trend lines, and other field experience, the author developed the graph shown in figure 3, to provide a
reasonable approximation of the overall pump reliability
(from pump cavitation and the related vibration) that can
be expected with aqueous liquids, when various NPSH
margin ratios are applied to “High Suction Energy” pumps
of increasing energy levels (Actual Pump Suction Energy
divided by the “High Suction Energy” gating value for the
pump type). These NPSH margin reliability factors are
based on the fact that, above the gating suction energy values, cavitation becomes more severe. In other words, the
greater the suction energy, the more important it is to suppress the residual cavitaiton that exists above the NPSHR,
to prevent damage. This reliability factor is only applicable
within the allowable operating flow region, above the start
of suction recirculation. Much higher NPSH margin values
are required in the region of suction recirculation for high
and very high suction energy pump applications.
Figure
5: Typical Relative Erosion Rate vs. NPSH Margin near
Figure 4: Suction Energy / NPSH Margin Reliability (Erosion
Damage)
Figure 3: NPSH Margin Reliability Factor
Figure 5: Typical Relative Erosion Rate vs. NPSH Margin near
This new methodBEP
for Flow
predicting
cavitation damage
Rate
in
centrifugal pumps, which has been adopted by the Hydraulic Institute and major pump companies, can save thousands
of dollars in unnecessary maintenance costs, as discussed
further below.
Suction Energy Impact on Low Flow Pump Internal
“Suction Recirculation”:
Figure 3: NPSH Margin Reliability Factor
Now the pump reliability curves shown in figure 3 reflect not
only the actual cavitation damage to the impeller, but reduce
life of the bearings and mechanical seals from the vibration
and air liberated due to cavitation. If we only look at the actual cavitation erosion of the impeller, pump reliability is not
only higher, as approximated in figure 4, but not a straight
4
Low flow suction recirculation can be extremely damaging in High Suction Energy pumps. Suction recirculation
caused cavitation is even more damaging than conventional
(high flow) cavitation. The writer remembers one field
problem in California, with a high suction energy pump,
equipped with a flexible rubber joint on the suction nozzle
(which did not have tie rods). The operator would not operated this pump for fear of knocking down the pier that
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it was mounted on, due to very high suction recirculation
pressure pulsations / forces acting on the unrestrained expansion joint.
pumps should not be operating for extended time periods in
the suction recirculation region.
Figure 7: Impeller Suction Recirculation
Figure 6: Cavitation impact on amount of entrained air in
pumpage
Figure 8: Start of Suction Recirculation in High Suction
Energy
Pumps
Figure
7: Impeller Suction Recirculation
6: Cavitation
impact on amount of entrained air in
WhatFigure
is Suction
Recirculation?
pumpage
Suction recirculation is a condition where, at some reduced flow rate, the eye of the impeller becomes too large
for the net through flow, and the inlet tip speed becomes
high enough to cause the flow in the inlet portion of an
impeller to separate from the vanes at the periphery and
be ejected upstream, opposite to the direction of net flow
entering the impeller, often well into the suction pipe (see
figure 7). This forms eddies and vortices within the impeller
inlet, and pre-rotation of the liquid entering the pump. Suction recirculation also causes the formation of very intense
vortices between the impeller vanes, which have high velocities at their core, and this consequently lowers the static
pressure at that location. This can in turn lead to intense
cavitation accompanied by severe pressure pulsations and
noise that can be damaging to the operation of the pump,
and to the integrity of the impeller material. It occurs in all
centrifugal pumps at some reduced pump capacity, normally
below the best efficiency rate of flow, but only causes damage in High and Very High Suction Energy pumps. In part,
it helps a pump adjust to the lower flow throughput.
Generally, the higher the suction specific speed (and
specific speed) of a pump, the greater the flow rate at which
suction recirculation begins. This is shown in Figure 8,
which can be used to approximate the start of suction recirculation for high suction energy pumps. The three curves
show the effect of specific speed on the start of suction recirculation, which increases with increasing specific speed
and suction specific speed. This reduces the allowable
operating region. As was stated earlier, high suction energy
www.WaterWorldCE.com Figure
8: Start
Suction
in High
Suction Energy
The
exactofflow
at Recirculation
which suction
recirculation
takes
Pumps
place is dependent on the design of the impeller, and should
be obtained from the pump manufacturer. The important
fact to remember is that the larger the impeller eye diameter (which leads to a lower required NPSH), the higher will
be the capacity at which suction recirculation takes place, as
a percentage of the capacity at best efficiency.
Identifying Suction Recirculation damage:
Figure 9 shows just how much more damaging suction
recirculation can be compared to classic cavitation. Tests of
this “High Suction Energy” pump at its best efficiency and
start of suction recirculation flow rates, demonstrate not
only a much higher suction pressure pulsations peak (from
cavitation) for the suction recirculation flow rate (in the
maximum erosion flow region), but the width of the pulsation peak is much broader and occurs at a higher NPSH
margin ratio. It can take a NPSH margin ratio (NPSHA/
NPSHR) of over 3.0 to get beyond the suction recirculation
peak, and even at that the suction pressure pulsation level is
75% that of the bep flow peak.
5
Figure 9: Pressure Pulsation level vs. NPSH Margin Ration for
Recirculation and BEP flow rates
and mounted on the impeller. Such rings are commonly
referred to as “bulk-head rings”. This prevents the recirculation vortex from extending axially beyond the plane formed
by the apron. Of course, since this does increase the required
NPSH, the use of these bulk-head rings can only be resorted
to if there is sufficient margin in the available NPSH. This
was the fix used to solve the California pump recirculation
pier damaging problem mentioned above.
Figure 10: Bulk-head ring construction
It should be noted that the location of the material damage
is an9:
excellent
diagnostic
tool in
identifying
whether
the
Figure
Pressure
Pulsation
level
vs. NPSH
Margin
cause Ratio
is classic
cavitation
or
internal
suction
recirculation
for Recirculation and BEP flow rates
caused cavitation. If the damage is to the hidden (high pressure) side of the vanes, and must be seen with the help of
a small mirror, the cause is suction recirculation. Classic
cavitation damage occurs on the visible (low pressure) side
of the impeller vane, a little way back from the leading edge.
As discussed above, flow recirculation may, or may not,
cause pump noise, vibration, erosion damage, and/or large
forces on the impeller. It can also cause the pump alignment to change, all of which may tend to affect the shaft
seal and bearing life. The likelihood of damage is heavily dependent on the suction energy level (does the pump
posses high or very high suction energy), specific speed of
the pump (above 3,500), the NPSH margin in the pump,
and the nature of the flow provided to the suction piping.
Experience has shown that low suction energy pumps are
not susceptible to damage from suction recirculation. However, solids and/or corrosives can accelerate damage during
suction recirculation (as with classic cavitation), even with
low suction energy applications.
Solving Suction Recirculation Problems:
Other than avoiding the low flow suction recirculation zone all together, or selecting only low suction energy
pumps, the normal first fix for a suction recirculation problem is to install an external by-pass lines to bring the net
flow rate above the start of suction recirculation.
If the above is not practical, there is a relatively simple
pump modification which, in a number of cases, has been
used quite successfully to reduce and even sometimes eliminate the unfavorable effects of suction recirculation. It consists of retrofitting pumps with a stationary casing ring, the
apron of which extends inwardly of the impeller eye diameter
(figure 10). If preferred, such rings can instead be rotating
6
Finally, another interesting fix, a “stabilizer,” might also
be able to alleviate problem suction recirculation. A stabiFigure
10 :smaller
Bulk-head
ring
construction
lizer is
a second
diameter
concentric
pipe, 4 to 12
inches long, installed inside the suction pipe, supported by
equidistant struts welded into the suction line, and installed
directly upstream of the pump suction nozzle (figure 11). Occasionally stabilizers are allowed to partially protrude into
the pump casing. On certain pump sizes, these stabilizer
pipes deliver stable performance over an extended flow range.
Figure 11: Suction Recirculation “Stabilizer”
Suction Energy’s Impact on how Inlet Piping can affects Pump Reliability:
The purpose of pump piping is to provide a conduit for
Figure
Suction
Recirculation
“Stabilizer”
the
flow of11:
liquid
to and from
a pump, without
adversely
affecting the performance or reliability of the pump. Howwww.WaterWorldCE.com
ever, many pump performance and reliability problems
are caused, or aggravated, by inadequate system (mainly
suction) piping, especially with high specific speed pumps
having High or Very High Suction Energy.
Suction Piping
Generally speaking, suction piping is more critical to
the performance of a pump then the discharge piping. The
function of suction piping is to supply an evenly distributed
flow of liquid to the pump suction, with sufficient pressure
to the pump to avoid cavitation and related damage to the
pump. An uneven flow distribution is characterized by
strong local currents, swirls and/or an excessive amount of
entrained air. The ideal approach is a straight pipe (of some
minimum length), coming directly to the pump, with no
turns or flow disturbing fittings close to the pump (see figure 12). Furthermore, the suction piping should be at least
as large as the pump suction nozzle and be sized to ensure
that the maximum liquid velocity at any point in the inlet
piping does not exceed 8 ft/sec.
region, is with the straight pipe, while the highest pressure
level in this flow region is with two perpendicular short radius elbows. These higher pressure pulsation levels are the
result of higher levels of cavitation in the pump, caused by
the reduced NPSH available (due to the higher local velocities) from the flow disturbing fittings. It should be noted that
these flow disturbing fittings did not increase the pressure
pulsation level for flow rates above the suction recirculation
region, since the pump tested was a small pump with a low
specific speed value (2,464).
Figure 13: Impact of suction fittings on pump pressure
pulsation level
Figure 12: Typical Pump Piping
The most disturbing flow patterns to a pump are those
that result from swirling liquid that has traversed several
of direction
in various
planes.
Whenpressure
fittings, such
on pump
fittings
of suction
13: Impact
Figurechanges
as elbows and “T”
fittings level
(especially two elbows at right
pulsation
angles), are located too close to the pump inlet, spinning
action or “swirl” is induced. This swirl could adversely affect pump performance by reducing efficiency, head and net
If the suction piping fails to deliver the liquid to the
positive suction head (NPSH) available. It also could genpump in this condition, a number of pump problems can reerate noise, vibration and damage in high-suction-energy
sult. High “Suction Energy” pumps, as discussed above, and
pumps. Straight pipe length recommendations should be
high Specific Speed (over 3,500) pumps are most susceptible
doubled if the distance between two perpendicular elbows
to poor suction piping. More often than not, the resulting
in the suction line is less than 5 pipe diameters.
Figure 12: Typical Pump Piping
problems can include one or more of the following:
Eccentric reducers should be installed with the flat surface
• Noisy operation.
at the top (to avoid trapping air or vapors) and the slopped
• Random axial load oscillations.
surface at the bottom, for horizontal pump installations (see
• Deterioration in performance.
figure 12). An exception to this is when a double suction, split
• Premature bearing and/or seal failure
case pump is installed vertically, in which case the flat surface
• Cavitation damage to the impeller and inlet portions of
should be at the side of the fitting to avoid flow disturbances
the casing.
extending into the pump, such as shown in figure 14 for a side
Figure 13 shows the results of a high suction energy, low
facing elbow. This can cause higher flow rates to one side of
specific speed pump tested with three separate inlet piping
the double suction impeller then the other, which causes the
configurations (a straight inlet pipe, a single short radius
NPSHR to be higher on one side then the other. It could also
elbow installed on the pump suction nozzle, and two short racause the lower flow side to be in suction recirculation. Also
dius suction nozzles, at right angles to one another, installed
for this reason, elbows must be vertical (perpendicular to the
on the pump suction). As can be observed, the lowest suction
plane of the shaft) when installed close to a horizontal double
pressure pulsation level, in the low flow suction recirculation
suction pump.
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Figure 14: Flow disturbances at pump inlets will cause hydraulic load and force unbalance
If the minimum recommended straight pipe lengths
cannot be provided, flow-straightening devices should be
considered.
Field Examples: Suction Energy / Cavitation Issues with
Impellers having little or no vane overlap
The author was recently called upon to trouble shoot
two seemingly different problems, which turned out to
have a common cause. In both cases new replacement
pumps encountered problems not experienced with the
It is always recommended that a straight uninterrupted
original pumps. Detailed investigations revealed that,
section of pipe be installed between the pump suction nozzle
although the general hydraulic performance of the replaceand the nearest fitting. Isolation valves, strainers and other
ment pumps was very close to the initial pumps, the new
devices used on the inlet (suction) side of a pump should be
pump impellers had little or no vane overlap, compared to
sized and located to minimize any disturbance of the flow
the older pumps. What this seemingly subtle difference
into the pump. The specific straight pipe length recomdid was to increase the “Suction Energy” intensity of the
mendation depends on the type of fitting(s), the pump type,
pumps from the “Low Suction Energy” experienced by the
at pump
Figure
14: Flow
the suction
energydisturbances
level and the pump
specificinlets
speed.will
Onecause
original pumps, to “High” and “Very High” Suction Energy
and force
unbalancefor low
to eight hydraulic
straight pipeload
diameters
are recommended
for the new replacement pumps. In one case it caused a
suction energy, low specific speed (below 3,500) pumps, see
submersible sewerage pump to experience severe cavitation,
Table 1. Three to 16 pipe diameters are recommended for
and damage, from low flow suction recirculation. In the
high suction energy and high specific speed (above 3,500)
other case it caused excessive cavitation at high flow rates,
pumps, see Table 2.
in a vertical turbine water pump, which liberated sufficient
amounts of entrained air to damage the bowl bearings that
Table 2: Minimum Recommended Straight Pipe Length (L1)
before Pump Suction for Low Suction Energy / Low Specific
were lubricated by the fluid pumped.
Speed Pumps
However as previously discussed, in order to determine
the likelihood of cavitation caused damage the Suction EnFitting
End Suction Pump Double Suction Split Case Pumps
ergy level must be calculated and compared with the pump
Fitting in
Fitting in
Fitting type gating value from Table 1. This allows the calculation
Either
Shaft Plane Perpendicular
of the Suction Energy Ratio (Actual Suction Energy / Start
Orientation
to Shaft
of High Suction Energy Gating value). This ratio establishes
Long Radius Elbow 1D
3D
1D
pump Suction Energy Severity, where Low Suction Energy
Short Radius Elbow 2D
5D
2D
is defined as SER values below 1.0, High Suction Energy has
45° Tee
1D
5D
1D
SER values of 1.0 to less than 1.5, and Very High Suction
90° Tee
3D
8D
3D
Energy has SER’s of 1.5 and above.
Open Valves
2D
2D
Also as noted above, Low Suction Energy pumps norCheck Valves
5D
5D
mally
do not experience cavitation damage or noise, High
Filters / Strainers
3D
3D
Suction Energy pumps experience cavitation noise, but little
if any cavitation damage, and Very High Suction Energy
Table 3: Minimum Recommended Straight Pipe Length (L1)
pumps may experience major cavitation damage, along with
before Pump Suction for High Suction Energy / How Specific
Speed Pumps
cavitation noise, depending on the NPSH Margin.
As pointed out in the foot note to table 1, the “2-Vane
Fitting
End Suction Pump Double Suction Split Case Pumps
Sewage Pumps” pump type, really applies to all pumps
Fitting in
Fitting in
Fitting whose impellers have less than 15 degrees of vane overlap
Either
Shaft Plane Perpendicular
(as shown in figure 15), and this configuration has the low
Orientation
to Shaft
est gating value (100 x 106, start of High Suction Energy).
Long Radius Elbow 5D
5D
3D
Insufficient vane overlap allows the higher pressures at the
Short Radius Elbow 8D
8D
3D
discharge of the impeller to recirculate (between the impel45° Tee
8D
8D
3D
ler vanes) to the suction, causing higher velocities (energy)
90° Tee
15D
15D
6D
and increased cavitation.
Open Valves
3-5D
3-5D
Check Valves
10D
10D
Filters / Strainers
6D
6D
8
Field Problem Specifics:
So how did the Suction Energy calculations come out for the
initial and new (problem) pumps. Table 4 below presents
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these result. The fact that the new pump impellers had
less than 15 degrees of vane overlap resulted in significant
increases in the “Suction Energy Intensity”, increasing from
Low Suction Energy to Very High Suction Energy for the
submersible pump, and from Low Suction Energy to High
Suction Energy for the Vertical Turbine Pump.
Figure 15: Impeller Vane Overlap
Table 4: Field Pump Suction Energy Comparison
BASIC PUMP TYPE
ITEM
OLD PUMP
Suction Energy Level
93 x 10 6
Suction Energy 160 x 10 Gating Value
6
NEW PUMP
178 x 106
100 x 106
(< 15° vane
overlap)
Submersible
(end suction)
Suction Energy
Ratio
0.58
1.78
Suction Energy
Intensity
Low
Very High
Suction Energy Level
114 x 106
102 x 106
Vertical
Turbine
Pump
Suction Energy 200 x 10 Gating Value
Suction Energy
Ratio
0.57
1.02
Suction Energy
Intensity
Low
High
6
100 x 106
(< 15° vane
overlap)
Conclusions:
In the case of the problem submersible pump, the “Very
High Suction Energy”, for the new pump was further ag15: Impeller
Vane suction,
Overlap and the fact
gravated byFigure
an elbow
at the pump
that the system forced the pump to operate in the low flow
suction recirculation region (which can require NPSH Margin Ratios over 10 to eliminate all cavitation under these
conditions). This greatly increases the applicable X-axis
NPSH Margin Ratio values, from that shown in the figure
3 reliability chart (which only apply to flow rates above the
start of suction recirculation). The NPSH Margin Ratios
should probably be increased by a factor of 2 to 2.5 in the
suction recirculation region. So even with actual NPSH
Margin Ratios above 3.3, the impeller experienced cavitation damage.
The vertical turbine pump had a different problem. The
greater amount of cavitation caused by the lack of vane
overlap, and the lower NPSH Margin Ratio (1.2) for this
application caused the liberation of an excessive amount of
dissolved air to become entrained air (see figure 6). This
entrained air entered the bowl bearings, which caused them
to run dry and fail prematurely.
So what should be the take-away from all of this? I think
it is the importance of calculating/determining the Suction
Energy, Suction Energy Ratio, and NPSH Margin Ratio, for
all new pump applications (and retrofits), with special care
being taken to determine if the particular pump has at least
15 degrees of impeller vane overlap, and how it effects the
Suction Energy Intensity. Remember that the amount of
vane overlap decreases with increasing impeller trim, so the
determination must be made for the specific impeller diameter for the application. Finally, it is also very important,
in the case of high suction energy pumps, to prevent the
pumps from operating in the low flow suction recirculation
region, to avoid cavitation caused damage and shortened
pump life.
References:
1. Allan R. Budris & Heinz P. Bloch, “Pump User’s Handbook –
Life Extension”, Third Edition, 2010.
2.
“Effects of Entrained Air, NPSH Margin, and Suction Piping
on Cavitation in Centrifugal Pumps”, Allan R. Budris & Philip
A. Mayleben, Proceedings of the 15th International Pump
Users Symposium.
3. Allan R. Budris & Philip A. Mayleben, “The Effects of
NPSH Margin, Suction Energy and Air on Centrifugal Pump
Reliability”, (1998 Texas A&M Pump Users Symposium).
4. Igor J. Karassik, “Centrifugal Pump Operation at off-design
conditions”, Chemical Processing, April 1987
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Notes
10
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Questions
1. Regarding the start of cavitation
in a centrifugal pump, as the
NPSH available to the pump is
reduced, at what point are the
first cavitation bubbles formed?
a. When the NPSHA equals the pump
“required NPSH” (NPSHR3%)
b. When the NPSHA equals
the NPSHR0% value
c. When the NPSHA is about 5 feet
above the pump NPSHR3%
d. When the NPSHA is about
4 times the NPSHR3%
2. Cavitation in a pump will
always cause damage?
a. Yes
b. No
3. “Suction Energy” can predict
cavitation damage from:
a. Conventional cavitation
b. Low flow suction
recirculation cavitation
c. Poor suction piping
d. All of the above
4. Which of the following
factors is not included in the
formula for suction energy?
a. Pump speed
b. Impeller eye diameter
c. Flow rate at pump best
efficiency point
d. Suction Specific Speed
5. “Very High Suction Energy” starts
at a Suction Energy Ratio (SER) of:
a.
b.
c.
d.
1.2
1.5
0.8
2.0
6. Under what conditions might
a “Low Suction Energy” pump
experience cavitation damage?
a.
b.
c.
d.
At high flow rates
With poor pump piping
With high impeller O.D. tip speeds
When pumping abrasives
7. What pump configuration has the
highest Suction Energy Gating value (Start of High Suction Energy)?
a.
b.
c.
d.
Vertical turbine pump
2-Vane (Impeller) Sewage Pump
End Suction Pump
Double Suction, (side
impeller inlet) pump
www.WaterWorldCE.com 8. Typically, pump reliability increases with:
a. Lower Suction Energy and
smaller impeller diameter
b. Lower Suction Energy Ratio and
Higher NPSH Margin Ratio
c. Lower flow rate and lower
Suction Energy
d. Higher specific Speed and
Lower Suction Specific Speed
9. Low Flow Suction Recirculation only occurs in pumps
with large impeller eyes?
a. Yes
b. No
10.Operation in the low flow suction
recirculation region should be
avoided because it will always
cause damage to the pump?
a. Yes
b. No
11. The flow rate (as a percentage of
the best efficient flow point) at
which suction recirculation starts
increases with increasing pump:
a. Specific Speed and Suction
Specific Speed
b. Specific Speed and Suction Energy
c. Pump flow rate and Suction Energy
d. NPSH Margin Ratio
and Pump Speed
12. Which pumps are most likely to
be damaged by cavitation from
suction recirculation?
a. Pumps with Low Suction Energy
b. Pumps with High Suction Energy
c. Pumps with Very High
Suction Energy
d. High flow rate pumps
13. Cavitation damage from suction
recirculation can be differentiated
from conventional cavitation by:
a. The location of the erosion damage
(on the hidden, high pressure
side of the impeller vanes)
b. The location of the erosion damage
(on the visible, low pressure
side of the impeller vanes)
c. The location of the erosion
damage (on the leading edge
of the impeller vanes
d. Erosion damage to the inside
of the casing suction nozzle
14.A good method to avoid
damage from pump suction
recirculation damage is:
a. Do not operate pump below the
start of suction recirculation
b. Add by-pass line to prevent
pump operation below start
of suction recirculation.
c. Select Low Suction Energy pump
d. All of the above
15. Which is typically more
damaging in a Very High
Suction Energy pump?
a. Suction Recirculation
caused cavitation
b. Conventional cavitation
16.Typically which is more
important to the reliable operation of a centrifugal pump?
a. Good discharge piping
b. Good suction piping
17. Which is not normally a
problem that might be caused
by poor suction piping?
a.
b.
c.
d.
Noisy operation
Premature bearing and/or seal failure
Cavitation damage to the impeller
Increased power draw
18. Which is more likely to experience
damage with poor suction piping?
a. High Suction Energy and
high speed pumps
b. High Suction Energy and high
Specific Speed pumps
c. High Specific Speed and High
Suction Specific Speed pumps
d. High Suction Energy and
high flow rate pumps
19.Elbows located near the suction of
a double suction, split case pump
should always be oriented to be:
a.
b.
c.
d.
Vertical
Horizontal
In the plane of the shaft
Perpendicular to the
plane of the shaft
20.What is the minimum impeller
vane overlap angle that is required
to avoid an increased Suction
Energy Ratio (SER)?
a.
b.
c.
d.
0°
10°
15°
20°
11
A Comprehensive Approach to Reducing Pump Energy Costs
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