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CENTRIFUGAL PUMP SYSTEMS TIPS
This is a list of ideas or DOS AND DON'TS for pump systems. You
may not of thought of some of
these and they will help you design and trouble-shoot pump
systems and select the proper pump. Also
there is information here that is hard to find elsewhere. You
can think of this list as GUIDELINES for
the pump system designer.
1. Flow and pressure relationship of a pump
When the flow increases, the discharge pressure of the pump
decreases, and when the flow
decreases the discharge pressure increases
2. Do not let a pump run at zero flow
Do not let a centrifugal pump operate for long periods of time
at zero flow. In residential
systems, the pressure switch shuts the pump down when the
pressure is high which means there
is low or no flow.
3. Use pressure gauges
Make sure your pump has a pressure gauge on the discharge side
close to the outlet of the pump
this will help you diagnose pump system problems. It is also
useful to have a pressure gauge on
the suction side, the difference in pressure is proportional to
the total head. The pressure gauge
reading will have to be corrected for elevation since the
reference plane for total head
calculation is the suction flange of the pump.
4. Do not let a pump run dry, use a check valve
Most centrifugal pumps cannot run dry, ensure that the pump is
always full of liquid. In residential
systems, to ensure that the pump stays full of the liquid use a
check valve (also called a foot valve) at
the water source end of the suction line. Certain types of
centrifugal pumps do not require a check
valve as they can generate suction at the pump inlet to lift the
fluid into the pump, see
http://www.watertanks.com/category/43/. These pumps are called
jet pumps and are fabricated by
many manufacturers Goulds being one of them.
Make use of check valves to isolate pumps installed in
parallel.
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5. Suction valves
Gate valves at the pump suction and discharge should be used as
these offer no resistance to flow and
can provide a tight shut-off. Butterfly valves are often used
but they do provide some resistance and
their presence in the flow stream can potentially be a source of
hang-ups which would be critical at the
suction. They do close faster than gate valves but are not as
leak proof.
6. Eccentric reducer
Always use an eccentric reducer at the pump suction when a pipe
size transition is required. Put the flat
on top when the fluid is coming from below or straight (see next
Figure) and the flat on the bottom
when the fluid is coming from the top. This will avoid an air
pocket at the pump suction and allow air to
be evacuated.
7. Use a multi-stage turbine pump for deep wells
For deep wells (200-300 feet) a submersible multi-stage pump is
required. They come in different sizes
(4" and 6") and fit inside your bore hole pipe. Pumps with
different ratings are available, see
http://www.webtrol.com/waterwell%20homepage.html
8. Flow control
If you need to control the flow, use a valve on the discharge
side of the pump, never use a valve on the
suction side for this purpose.
This is an excellent treatment of the types of control systems
for a centrifugal pump. Thanks to Walter
Driedger of Colt Engineering a consulting engineering firm for
the petro-chemical industry in Alberta,
Canada.
9. Plan ahead for flow meters
For new systems that do not have a flow meter, install flanges
that are designed for an orifice plate in a
straight part of the pipe (see next Figure) and do not install
the orifice plate. In the future, whoever
trouble-shoots the pump will have a way to measure flow without
the owner having to incur major
downtime or expense. Note: orifice plates are not suitable for
slurries.
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10. Avoid pockets and high points
Avoid pockets or high point where air can accumulate in the
discharge piping. An ideal pipe run is one
where the piping gradually slopes up from the pump to the
outlet. This will ensure that any air in the
discharge side of the pump can be evacuated to the outlet.
11. Location of control valves
Position control valves closer to the pump discharge outlet than
the system outlet. This will ensure
positive pressure at the valve inlet and therefore reduce the
risk of cavitation.
When the valve must be located at the outlet such as the feed to
a tank, bring the end of the pipe to
the bottom of the tank and put the valve close to that point to
provide some pressure on the discharge
side of the valve making it easier to size the valve, extending
it's life and reducing the possibility of
cavitation.
12. Water hammer
Be aware of potential water hammer problems. This is
particularly serious for large piping systems such
as are installed in municipal water supply distribution systems.
These systems are characterized by long
gradually upward sloping and then downward sloping pipes.
Solutions to this can involve special
pressure/vacuum reducing valves at the high and low points or
additional tanks which provide a buffer
for pressure surges (see
http://www.ventomat.com/default.asp).
For pumps 500 gals/min or larger use semi-automatic manual
valves at the discharge that are
controlled to open gradually when starting the pump. This will
avoid water hammer during the initial
start and damage to the piping system.
13. The right pipe size
The right pipe size is a compromise between cost (bigger pipes
are more expensive) and excessive
friction loss (small pipes cause high friction loss and will
affect the pump performance). Generally
speaking, the discharge pipe size can be the same size as the
pump discharge connection, you can see if
this is reasonable by calculating the friction loss of the whole
system. For the suction side, you can also
use the same size pipe as the pump suction connection, often one
size bigger is used . A typical velocity
range used for sizing pipes on the discharge side of the pump is
9-12 ft/s and for the suction side 3-6
ft/s.
A small pipe will initially cost less but the friction loss will
be higher and the pump energy cost will be
greater. If you know the cost of energy and the purchase and
installation cost of the pipe you can select
the pipe diameter based on a comparison of the pipe cost vs
power consumption,
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14. Pressure at high point of system
Calculate the level of pressure of the high point in your
system. The pressure may be low enough for
the fluid to vaporize and create a vapor pocket which will be
detrimental to the performance of the
system. The pressure at this point can be increased by
installing a valve at some point past the high
point and by closing this valve you can adjust the pressure at
the high point. Of course, you will need to
take that into account in the total head calculations of the
pump.
15. Pump pressure rating and series operation
For series pump installations make sure that the pressure rating
of the pumps is adequate. This is
particularly critical in the case where the system could become
plugged due to an obstruction. All the
pumps will reach their shut-of head and the pressure produced
will be cumulative. The same applies for
the pressure rating of the pipes and flanges.
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16. Inadequate pump suction submersion
There is a minimum height to be respected between the free
surface of the pump suction tank and the
pump suction. If this height is not maintained a vortex will
form at the surface and cause air to be
entrained in the pump reducing the pump capacity.
17. Pump selection
Select your pump based on total head (not discharge pressure)
and flow rate. The flow rate will depend
on your maximum requirement. Total head is the amount of energy
that the pump needs to deliver to
account for the elevation difference and friction loss in your
system
Pump selection starts with acquiring detail knowledge of the
system. If you are just replacing an existing
pump then of course there is no problem. If you are replacing an
existing pump with problems or
looking for a pump for a new application then you will need to
know exactly how the systems is
intended to work. You should have the P&ID diagram and
understand the reasons for all the devices
included in your system. You should make your own sketch of the
system that includes all the
information on the P&ID plus elevations (max., min., in,
out, equipment), path of highest total head,
fluid properties, max. and min. flow rates and anything
pertinent to total head calculations.
The next figure is a typical example:
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Typical example of flow schematic used for total head
calculations.The control method is important
(on-off, control valve, re-circulating, variable speed) as it
may affect your selection. Besides the system
sketch, that you can use to record some of the data.
CENTRIFUGAL PUMP SIZING
Service: Pump number:
Fluid WATER Suction side of pump
Viscosity (cSt) 1.13 Pump suction elevation (ft)
Temperature (f) 70 Suction fluid surface elevation min. (ft)
Specific gravity 1.0 Suction fluid surface pressure (ft
fluid)
Discharge side of pump
Discharge surface elevation max. or
discharge pipe end elevation (ft)
Atmospheric pressure (ft water) 32.8 Surface pressure in
discharge tank or
pipe end pressure (ft fluid)
Vapor pressure (ft water) 22
Pipe roughness RMS (ft) .00015
1. Design flow (USgpm) 750 Comments:
2. Flow contingency %
(applied on 1.)
0
3. Flow depreciation %
(applied on 1.)
0
4. Total (USgpm) 750
5.Discharge static head (ft fluid) 40
6. Suction static head (ft fluid) 10
7. Total static head (ft fluid) 20
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Calculations
8. Pipe and fittings friction (ft
fluid)
(calc. on 1.)
Comments:
9. Equipment friction (ft fluid)
(based on 1.)
10. Velocity head (ft fluid)
11. Sub-total (ft fluid)
(7+8+9+10+11)
12. Total head contingency (ft
fluid)
(applied on 8, 9, 10)
13. Total head depreciation (ft
fluid)
(applied on 8, 9, 10)
14. Total head (ft fluid)
Depending on the industry or plant that you work in, you will be
forced to either select a certain type of
pump or manufacturer or both. Manufacturers are normally a very
good source of information for final
pump selection and you should always consult with them, do your
own selection first and confirm it
with the manufacturer. They can help you select the right type,
model, and speed if you have all the
operating conditions and if not they will rarely be able to help
you
PUMP SELECTION DATA
Pump Manufacturer
Pump Model
Type
Suction dia. (in)
Discharge dia. (in)
Impeller speed (rpm)
Operating head (ft)
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Operating Flow (USgpm)
Pump efficiency (%)
Specific speed
Suction specific speed
Fluid type
Viscosity (cP)
Temperature (F)
Specific gravity
Vapor pressure (psia) at ______(F)
Brake horsepower (hp)
Selected horsepower (hp)
Motor frame
Motor speed
Direct drive (yes/no)
Pump shut-off head (ft)
System high point (ft) zhigh z1
NPSH required (ft abs.)
NPSH available (ft abs.)
Max. impeller size (in)
Min. impeller size (in)
Selected impeller size (in)
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Note: units for specific speed and suction specific speed are
75.0
5.0
ft
gpmrpm
Aside from the normal end suction pump, vertical turbine and
submersible pumps, there is a wide
variety of specialized pumps that you should consider for your
application if you have unusual
conditions.
SPECIALTY PUMPS Jacques Chaurette p. eng.
www.lightmypump.com October 2004
Synopsis
My intention in this article is too highlight some specialty
pumps with characteristics that
are unusual and helpful in particular situations. Some of these
pumps are mentioned in
the Hydraulic Institute classification of centrifugal pumps by
mechanical type which you
can view at
http://www.lightmypump.com/pumpdatabase/hydraulic_institute-chart.htm.
1.0 JET PUMPS
This pump is used frequently for domestic water supply. It is a
typical centrifugal pump
with the difference that the suction is augmented by a venturi
which creates a vacuum allowing water to be lifted from a deep
well. It is an ingenious use of the pump own
discharge pressure and flow to provide pressure water at the
inlet of a venturi which is located in the pump suction. The jet of
water is accelerated in the small diameter of the
venturi which creates a low pressure or vacuum, this vacuum is
used to assist in lifting the water in the well to the suction
compartment of the pump (see Figure 1b). One big
advantage is there is no need to use a foot valve (i.e. check
valve) at the end of the suction pipe, this reduces the maintenance
on this item and potential plugging.
Figure 1a A typical jet pump by Goulds, see
http://www.goulds.com/master.asp?id=3
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Figure 1b The venturi action of a jet pump.
1.1 VISCOUS DRAG PUMP
The impeller of this pump is a flat disc that accelerates the
fluid by shearing the fluid. The ability of the fluid to resist
this shear force (this is the definition of viscosity) means
that a certain quantity of fluid will follow the disc and be
accelerated towards the pump casing (see Figure 2). As in a normal
pump, the velocity energy of the fluid is converted
to pressure energy when the fluid hits the stationary casing.
The advantage of this pump is that it can handle large quantities
of air or gazes and still perform which is not the case for
traditional centrifugal pumps. Because the discs are open there are
no tight
passages as in traditional curved impeller vane passages and
therefore solids can be handled effectively. These pump are
available from the Discflo Corporation in a variety
of sizes.
Figure 2 The discflo pump by Discflo
Corp. see http://www.discflo.com/
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1.2 DOUBLE VOLUTE PUMPS
A double volute pump is one where the immediate volute of the
impeller is separated by a partition from the main body of the
casing. The result is that the impeller is subjected
to equal forces that are generated at the cutwater positions
(see Figure 3a) and
therefore is balanced hydraulically. A single volute pump always
has a net hydraulic
force that acts on the impeller causing wear and tear on the
rotating components, the
force gets larger the further one operates away from the optimum
flow at the Best
Efficiency Point (B.E.P.).
Figure 3a Schematic of the double volute design (source:
Pumps & Systems magazine June 2004 The double volute pump is
therefore more robust and will require less
maintenance, however it is less efficient and more
expensive.
Double volute pumps are available in the medium to large size
pumps from most
manufacturers (see Figure 3b)
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Figure 3b Availability of double volute pumps (source: Pumps
& Systems magazine June 2004
1.3 CHOPPER PUMPS
This type of pump has a serrated impeller edge which can cut
large solids and therefore
prevent clogging (see Figure 4). It is used for municipal waste
handling and would no
doubt be very useful for handling slurries containing many
different type of solids such
as in the pulp and paper industry.
Figure 4 The Chopper pump by The Vaughan Co. Inc. see
http://www.chopperpumps.com
1.4 ROTATING CASING (PITOT) PUMPS
This pumps specialty is low to medium flow rates at high
pressures. It is frequently
used for high pressure shower supply on paper machines.
Figure 5a The rotating casing Roto-Jet pump by
Weir specialty Pump see http://www.rotojet.com/
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Roto-Jet pumps are designed with only two working parts, a
stationary pick-up tube (pitot tube) and a rotating casing (see
Figure 4b). These pump come with a built in
recirculation line with an orifice which can bleed high pressure
fluid from the discharge
to the inlet to avoid damage due to running the pump with a
closed discharge valve. An alternate choice to this pump is a
multi-stage centrifugal pump such as the Goulds
model 3355 which can be seen at
http://www.gouldspumps.com/cat_pumps.ihtml?pid=602&lastcatid=86&step=4
Figure 5b A sectional view of the rotating casing Roto- Jet
pump.
1.5 RECESSED IMPELLER
This pump is a frame-mounted, back pull-out, end suction,
recessed impeller, tangential
discharge pump designed specifically to handle certain bulky or
fibrous solids, air or
gas entrained liquids or shear sensitive liquids (see Figure 6).
For example, certain bulky or fibrous solids like some long denim
fiber or recycle stock can clog or abrade
parts of conventional process pumps. In addition, shear
sensitive liquids like latex are
degraded when pumped at high velocities through process pump
casings. Last, if air or
gas binding is a problem, the recessed impeller is the answer,
it can also handle liquids with up to 5% entrained air or gas.
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Figure 6 A sectional view of a recessed impeller pump by Goulds
see http://www.mackpump.com/CV3196.htm
1.6 SELF-PRIMING PUMPS
Reliable Self-Priming Operation - Before any centrifugal pump
will perform, it must
first be primed; that is, air or gases expelled from the suction
and impeller eye area,
and replaced with liquid. This is no problem when the pump is
submerged (submersible or vertical sump
pumps) or when liquid supply is above the pump. However, when
suction pressure is
negative, air must be evacuated to accomplish pump priming. The
self-priming pump is designed to insure that a sufficient quantity
of liquid to
reprime is always retained in the priming chamber (see Figure
7b).
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Figure 7a Outside dimensions for a self-priming pump by Goulds
see
http://www.gouldspumps.com/cat_pumps.ihtml?pi
d=226&lastcatid=76&step=4
Figure 7b Priming and pumping action of a self-
priming pump. 1.7 SLURRY PUMPS
The slurry pump is a rugged heavy duty pump intended for
aggressive or abrasive slurry solutions with particles of various
sizes. It achieves this by lining the inside of the pump
casing as well as the impeller with rubber (see Figure 8). All
though rubber does
eventually wear, the elasticity of its surface allows the hard
mineral particles to bounce off thereby reducing what would be
otherwise very aggressive erosion. These pumps
are used wherever abrasive slurries need to be pumped,
especially in the mining
industry. The NPSH requirement for these types of pumps is
typically higher than
comparative standard centrifugal pumps.
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Figure 8 Slurry pump by Warman see http://www.warman.co.za/
1.9 LOW FLOW HIGH HEAD PUMPS (RADIAL VANE OR PARTIAL
EMISSION)
This radial vane or partial emission pump (see Figure 10) is a
frame mounted, end
suction, top centerline discharge, ANSI pump designed
specifically to handle
corrosive chemicals at low flows. By low flows we mean:
Flows outside the recommended operating range of typical end
suction pumps. Low flows which require users to throttle end
suction pumps to
operating conditions well below their best efficiency point.
Flows that increase mechanical vibration, decrease bearing and
seal
life, increase maintenance costs, and decrease the pump life of
end
suction pumps. In other words, the radial vane pump is designed
to operate where standard end suction
pumps operate poorly - at throttled low flows.
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Figure 10 Low flow vane pump by Goulds
see http://www.mackpump.com/LF3196.htm 1.9 LOW N.P.S.H. PUMPS
(LOW FLOW, HIGH HEAD) This type of pump is used where the available
N.P.S.H. is low (see Figure 10). It is
specifically designed for low flow and high head requirements
and offers good efficiency
even under these conditions (see the article published by
Industrial Technology magazine
at
http://www.industrialtechnology.co.uk/1998/may/impeller.html).
Figure 10 Low N.P.S.H. pump (model CPX) by Flowserve see
http://www.fpdlit.com/cms/results_detail.asp?ModelID=10
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1.10 SLUDGE PUMP
Certain types of sludges tend to settle very quickly and are
hard to keep in suspension. The
Lawrence pump company has solved this problem by putting an
agitator in front of the pump
suction (see Figure 11). For more info see the Pumps &
systems magazine, issue March
2004 at http://www.pump-zone.com/.
Figure 11 Lawrence Series 5100 submersible
pump with sludge agitator,
In the selection process, you will be trying to match your flow
rate with the B.E.P. of the pump. It is
not always possible to match the flow rate with the B.E.P. (best
efficiency point), if this is not possible,
try to remain in the range of 80% to 110% of the B.E.P..
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Desirable selection area for impeller size for centrifugal
pumps.
Operating outside this range will lead to excessive vibration,
recirculation and cavitation, see the next
two figures. The first one from the Pump Handbook from
McGraw-Hill which shows how the axial
force increases with the distance in terms of percent flow from
the B.E.P. and the second from Goulds
essentially shows the same information but in terms of
vibration.
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Radial force vs. % flow of BEP
Vibration level vs. flow Electronic pump curves have been
created for many (over 50) manufacturers, . They have all been
developed by Engineered Software located in Lacey Washington USA
of which I am a representative.
Their pump sizing software PUMP-FLO can help find the best pump
for the application, it can select
the closest one to the B.E.P. for you and do all kinds of
searches based on NPSHR, efficiency, size, etc.
When you order your pump make sure that the motor is installed
with spacer blocks so that the next
largest motor frame can be installed.
18. Air in pump reduces capacity When air enters a pump it
sometimes gets trapped in the volute, this reduces the capacity,
creates vibration and noise. To remedy, shut the pump down and open
the vent valve to remove the air. If the pump is excessively noisy
do not automatically assume that the problem is cavitation, air in
the pump creates vibration and noise. Cavitation produces a
distinct noise similar to gravel in a cement mixer. If
you have never heard the sound of cavitation here's a recording
of it in WAV format, courtesy of my friend Normand Chabot, water
hammer specialist here in Montreal. 19. Effect of viscosity on pump
performance Viscosity is the main criteria which determines whether
the application requires a centrifugal pump or a positive
displacement pump. Centrifugal pumps can pump viscous fluids
however the performance is adversely affected. If your fluid is
over 400 cSt (centiStokes) in viscosity consider using a positive
displacement pump.
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20. Avoid running pump in reverse direction Avoid running a pump
in reverse direction, pump shafts have been broken this way
especially if the pump is started while running backwards. The
simplest solution is to install a check valve on the discharge
line. 21. Minimum flow rate Most centrifugal pumps should not be
used at a flow rate less than 50% of the B.E.P. (best efficiency
point) flow rate without a recirculation line. If your system
requires a flow rate of 50% or less then use a recirculation line
to increase the flow through the pump keeping the flow low in the
system, or install a variable speed drive.
How is the minimum flow of a centrifugal pump established
(answer from the Hydraulic Institute
http://www.pumps.org/content_detail.aspx?id=2138) The factors which
determine minimum allowable rate of flow include the following: *
Temperature rise of the liquid -- This is usually established as
15F and results in a very low limit. However, if a pump operates at
shut off, it could overheat badly. * Radial hydraulic thrust on
impellers -- This is most serious with single volute pumps and,
even at flow rates as high as 50% of BEP could cause reduced
bearing life, excessive shaft deflection, seal failures, impeller
rubbing and shaft breakage. * Flow re-circulation in the pump
impeller -- This can also occur below 50% of BEP causing noise,
vibration, cavitation and mechanical damage. * Total head
characteristic curve - Some pump curves droop toward shut off, and
some VTP curves show a dip in the curve. Operation in such regions
should be avoided. There is no standard which establishes precise
limits for minimum flow in pumps, but "ANSI/HI 9.6.3-1997
Centrifugal and Vertical Pumps - Allowable Operating Region"
discusses all of the factors involved and provides recommendations
for the "Preferred Operating Region".
22. Three important points on the pump characteristic curve The
performance or characteristic curve of the pump provides
information on the relationship between total head and flow rate.
There are three important points on this curve
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1. The shut-off head, this is the maximum head that the pump can
achieve and occurs at zero flow. The pump will be noisy and vibrate
excessively at this point. The pump will consume the least amount
of power at this point. See also the pump glossary. 2. The best
efficiency point B.E.P. this is the point at which the pump is the
most efficient and operates with the least vibration and noise.
This is often the point for which pumps are rated and which is
indicated on the nameplate. The pump will consume the power
corresponding to its B.E.P. rating at this point. 3. The maximum
flow point, the pump may not operate past this point. The pump will
be noisy and vibrate excessively at this point. The pump will
consume the maximum amount of power at this point. Sometimes the
characteristic curve will include a power consumption curve. This
curve is only valid for water, if the fluid has a different density
than water you cannot use this curve. However you can use the total
head vs. flow rate curve since this is independent of density.
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Typical centrifugal pump characteristic curve. If your fluid has
a different viscosity than water you cannot use the characteristic
curve without correction. Any fluid with a viscosity higher than 10
cSt will require a correction. Water at 60F has a viscosity of 1
cSt.
23. Normal, flat and drooping characteristic curves There are
three different characteristic curve profiles for radial flow
pumps. Figure 4 shows the various vane profiles that exist and the
relationship between them. This tip is related to the radial vane
profile which is the profile of the typical centrifugal pumps.
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Pump vane profiles vs. specific speed. There are three different
curve profiles shown in the next figure: 1. Normal, head decreases
rapidly as flow increases 2. Flat, head decreases very slowly as
flow increases 3. Drooping, similar to the normal profile except at
the low flow end where the head rises then drops as it gets to the
shut-off head point.
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Different types of radial pump characteristic curve profiles.
The drooping curve shape is to be avoided because it is possible
for the pump to hunt between two operating points which both
satisfy the head requirement of the system. This is known to happen
when two pumps are in parallel, when the second pump is started it
may fail to get to the operating point or hunt between two points
that are at equal head. Thankfully not to many pumps have this
characteristic, here are a few:
Drooping curve (Goulds).
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Drooping curve (Sundyne). A flat curve is sometimes desirable
since a change in flow only causes a small change in head, for
example as in a sprinkler system. As more sprinklers are turned on
the head will tend to decrease but because the curve is flat the
head will decrease only a small amount which means that the
pressure at the sprinkler will drop only a
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small amount, thereby keeping the water velocity high at the
sprinkler outlet. The National Fire Prevention Association
(N.F.P.A.) code stipulates that the characterictic curve must be
flat within a certain percentage. This code can be purchased at
ANSI. The normal curve can be more or less steep. A steep curve can
be desirable from a control point of view since a small change in
flow will result in a large pressure drop. The steepness of the
curve depends on the number of vanes and the specific speed.
24. Suction piping Many people are way to CONSERVATIVE about
suction piping design. The usual advice you get is make the piping
as straight, as big and short as possible. I have seen a suction
line 300 ft long, now that's not short. I believe the important
considerations are: - by all means make the pipe as short and
straight as possible, particularly if the fluid has suspended
solids which may cause plugging or hangups. - make sure there is
sufficient pressure at the pump suction (this means check the NPSHA
against the NPSHR); - make sure that the stream flow lines are
coming in nice and straight at the pump suction. This generally
means having 5 to 10D straight pipe ahead of the pump inlet. Avoid
the use of filters at the pump inlet if at all possible. Their
maintenance will often be neglected and the pump will suffer from
poor performance and perhaps cavitation. Use a 90 or 45 elbow at
the pumps inlet pipe end. This will allow almost complete drainage
of the tank and is especially useful in the case of fluids that can
not be readily dumped to the sewers. It also provides additional
submergence reducing the risk of vortex formation.
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Also be careful of elbows that are too close to the pump
suction. 25. The meaning of specific speed If you are having
trouble with a pump or want to check whether the new pump to be
installed is appropriate, check the specific speed and the suction
specific speed of the pump. The specific speed provides a number
which can help identify the type of pump (for example radial or
axial flow) that is best suited for your application. The specific
speed of the pump type selected (see Figure 4) should be close to
the specific speed calculated for your application. The suction
specific speed will tell you if the suction of the pump is likely
to cause problems in your application.
26. Different types of centrifugal pumps
There are many different types of pumps available other than the
standard end suction, submersible or
vertical multi-stage pump. In this article, you will see a
number of pumps that are specialized and may
suit a particular need.
27. Unusual aspects of pump systems
This article discusses unusual aspects of pump systems:
variation in pressure throughout the system
and effect of fluid properties.
Synopsis
There is a number called the specific speed of a pump whose
value tells us something about
the type of pump. Is it a radial type pump which provides high
head and low flow or an axial or
propeller type pump which provides low flow but high head or
something in between. If you are worried whether you have the right
type of pump or not this number will help you decide.
The article gives you an example of how to calculate this
number. Also if you are worried that
your pump may be cavitating there is another number related to
specific speed called suction specific speed that will help you
diagnose and avoid cavitation. There is a multitude of pump designs
that are available for any given task. Pump designers have needed a
way to compare the efficiency of their designs across a large range
of pump model and types. Pump users also would like to know what
efficiency can be expected from a particular pump design. For that
purpose pump have been tested and compared using a
number or criteria called the specific speed (NS) which helps to
do these comparisons. The
efficiency of pumps with the same specific speed can be compared
providing the user or the
-
designer a starting point for comparison or as a benchmark for
improving the design and increase the efficiency. Equation [1]
gives the value for the pump specific speed, H is the pump total
head, N the speed of the impeller and Q the flow rate.
NS
N(rpm) Q(USgpm) [1]
H( ft fluid)0.75
Figure 1 Specific speed values for the different pump
designs.
(source: the Hydraulic Institute Standards book, see
www.pumps.org) Pumps are traditionally divided into 3 types, radial
flow (see Figure 2), mixed flow (see Figure
3) and axial flow (see Figure 4). There is a continuous change
from the radial flow impeller,
which develops pressure principally from the action of
centrifugal force, to the axial flow
impeller, which develops most of its head by the propelling or
lifting action of the vanes on
the liquid.
Many pump types have been tested and their efficiency measured
and plotted in Figure 5. Notice that larger pumps are inherently
more efficient. Efficiency drops rapidly at specific
speeds of 1000 or less.
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Figure 2 Radial flow Figure 3 Mixed flow Figure 4 Axial flow
pump cross-section,
pump cross-section, pump cross-section,
(source: Hydraulic
(source: Hydraulic (source: Hydraulic
Institute
Institute Institute
www.pumps.org).
www.pumps.org). www.pumps.org).
-
Figure 5 Efficiency values for pump with different specific
speeds (source: The Pump Handbook published by McGraw Hill)
The following chart provides the efficiency data for pumps of
various types vs the
flow rate and maybe easier to read than Figure 5. However some
corrections are
required (use the chart in the upper left corner of Figure 6) to
the values predicted.
-
Figure 6 Efficiency values for pumps of different types (source:
The
Hydraulic Institute www.pumps.org).
-
28. Predict pump efficiency Save time in the initial phase of
the project and calculate power requirement prior to the final
pump selection using a chart that predicts the efficiency of
standard end suction centrifugal
pumps Alternatively, compare the efficiency of the final pump
selection with the industry
average.
You will notice that efficiency increases with specific speed,
this means that a pump with a
higher speed (rpm) that meets your requirements will be smaller
and more efficient and therefore
cost less to operate,