Pumps are often sold as a package with an accumulator.
Domain of application of the affinity laws for an axial flow pump.
The
affinity laws are expressed by the three following relationships where
Q is the flow rate, n the pump rpm, H the total head and P the power.
You can predict the operating condition for point 2 based on the
knowledge of the conditions at point 1 and vice versa.
The
process of arriving at the affinity laws assumes that the two operating
points that are being compared are at the same efficiency. The
relationship between two operating points, say 1 and 2, depends on the
shape of the system curve (see next Figure). The points that lie on
system curve A will all be approximately at the same efficiency.
Whereas the points that lie on system curve B are not. The affinity
laws do not apply to points that belong to system curve B. System curve
B describes a system with a relatively high static head vs. system
curve A which has a low static head.
Which apply only to a given impeller with altered D and constant efficiency but
not a geometrically similar series of impellers.If that is the case then the affinity
laws can be used to predict the performance of the pump at different diameters for the
same speed or different speed for the same diameter. Since in practice impellers of
different diameters are not geometrically identical, the author's of the section called
Performance Parameters in the Pump Handbook recommend to limit the use of this technique
to a change of impeller diameter no greater than 10 to 20%. In order to avoid over
cutting the impeller, it is recommended that the trimming be done in steps with careful
measurement of the results. At each step compare your predicted performance with the
measured one and adjust as necessary.
There are many causes of air entrainment, the air may be coming in at the suction tank due to improper piping
or
due to leakage iin the pump suction line (assuming that conditions are
such that low pressure is produced in the suction line).
Leakage in a suction pipe under low pressure will cause air to enter the pump.
This next image shows the dimensions that have been standardized (source: the Pump Handbook by McGraw-Hill)
This
next image shows a cross-section of an end-suction pump built to the
B73.1standard (source: the Pump Handbook by McGraw-Hill).
The variation of atmospheric pressure with elevation.
They
are used extensively in the state of Florida to control the water level
in the canals of low lying farming areas. The water is pumped over low
earthen walls called burms into the South Florida Water Management Disctrict main collecting canals.
Figure 1 Important points of the pump characteristic curve.
Radial force on the impeller vs. the flow rate (source: the Pump Handbook by McGrawhill).
When
selecting a centrifugal pump it is important that the design operating
point lie within the desirable selection area shown in the next figure.
Figure 2 Pressure profile inside a centrifugal pump.
as
the liquid travels through the pump the pressure drops, if it is
sufficiently low the liquid will vaporize and produce small bubbles.
These bubbles will be rapidly compressed by the pressure created by the
fast moving impeller vane. The compression creates the characteristic
noise of cavitation .
Along with the noise, the shock of the imploding bubbles on the surface
of the vane produces a gradual erosion and pitting which damages the
impeller.
Cavitation damage on an impeller of a Robot BW5000 pump (image provided by my pump friend Bart Duijvelaar).
Try this
experiment, find a plastic cup or other container that you can poke a
small pinhole in the bottom. Fill it with water and attach a string to
it, and now you guessed it, start spinning it.
Figure 3 An experiment with centrifugal force.
This animation shows my interpretation of what happens to fluid particles
(represented by gray balls) once they enter the eye of the impeller and after they turn 90 degrees.
At this point they are at the entrance of the volume formed by two adjacent impeller vanes.
The rapid rotation of the vanes (impeller blades) displaces the fluid particles by moving them in a
radial direction where they come into contact with the pump
volute and are decelerated and pressurized.
Check out the direction of rotation, not what one would expect at first glance.
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Characteristic curve : same as performance curve .
Check valve :
a device for preventing flow in the reverse direction. The pump should
not be allowed to turn in the reverse direction as damage and spillage
may occur. Check valves are not used in certain applications where the
fluid contains solids such as pulp suspensions or slurries as the check
valve tends to jam. A check valve with a rapid closing feature is also
used as a preventative for water hammer. see also check valve CV coefficient.
Various check valves (source: The Crane Technical Paper no 410)
do your own calculation of Fitting friction loss with this java applet
Colebrook equation : an equation for calculating the friction factor f of fluid flow in a pipe for Newtonian fluids of any viscosity. see also the Moody diagram figure 9.
This factor is then used to calculate the friction loss for a straight length of pipe.
Do your own calculation of pipe friction loss with this java applet
To understand how to solve the Colebrook equation for the friction factor f using the Newton-Raphson iteration technique, dowload this pdf file
Chopper pump : a pump with a serrated impeller edge which can cut large solids and prevent clogging.
Chopper pump
see specialty_pumps.pdf for more information
Closed impeller :
the impeller vanes are sandwiched within a shroud which keeps the fluid
in contact with the impeller vanes at all times. This type of impeller
is more efficient than an open type impeller. The disadvantage is that
the fluid passages are narrower and could get plugged if the fluid
contains impurities or solids.
Open and closed impellers
for more information on an open impeller
CV coefficient :
a coefficient developed by control valve manufacturers that provides an
indication of how much flow the valve can handle for a 1 psi pressure
drop. For example, a control valve that has a CV of 500 will be able to
pass 500 gpm with a pressure drop of 1 psi. CV coefficients are
sometimes used for other devices such as check valves.
CV coefficients for a wafer style check valve.
Cutwater: the narrow space between the impeller and the casing in the discharge area of the casing.
this
is the area where pressure pulsations are created, each vane that
crosses the cutwater produces a pulse. To reduce pulsations in critical
process', more vanes are added.
Darcy-Weisbach equation : an equation used for calculating the friction head loss for fluids in pipes, the friction factor f must be known and can be calculated by the Colebrook, the Swamee-Jain equations or the Moody diagram.
Dead head : a situation that occurs when the pump's discharge is closed either due
to a blocage in the line or an inadvertently closed valve. At this point, the pump will go to it's maximum
shut-off head, the fluid will be recirculated within the pump resulting in overheating and possible damage.
Diffuser:
located in the discharge area of the pump, the diffuser is a set of
fixed vanes often an integral part of the casing that reduces
turbulence by promoting a more gradual reduction in velocity.
The following image comes from this web site http://www.tpub.com/content/doe/h1018v1/css/h1018v1_97.htm
Diaphragm pump :
a positive displacement pump. Double Diaphragm pumps offer smooth flow,
reliable operation, and the ability to pump a wide variety of viscous,
chemically aggressive, abrasive and impure liquids. They are used in
many industries such as mining, petro-chemical, pulp and paper and
others.
An air valve directs pressurized air to one of the
chambers, this pushes the diaphragm across the chamber and fluid on the
other side of the diaphragm is forced out. The diaphragm in the
opposite chamber is pulled towards the centre by the connecting rod.
This creates suction of liquid in chamber, when the diaphragm plate
reaches the centre of the pump it pushes across the Pilot Valve rod
diverting a pulse of air to the Air Valve. This moves across and
diverts air to the opposite side of the pump reversing the operation.
It also opens the air chamber to the exhaust.
this
type of diaphragm pump is driven by pneumatic air so these can be used
where electric drives are not preferred, is self priming and can run
dry for brief periods, an handle hazardous liquids with almost any
viscosity, can pump solids up to certain sizes.
Wilden is a major manufacturer of such pumps http://www.wildenpump.com/
Dilatant : The property of a fluid whose viscosity increases with strain or displacement.
For more information see non-newtoninan fluids.pdf
Discharge Static Head :
The difference in elevation between the liquid level of the discharge
tank if the pipe end is submerged and the centerline of the pump. If
the discharge pipe end is open to atmosphere than it is the difference
between the pipe end elevation and the suction tank fluid surface
elevation. This head also includes any additional pressure head that
may be present at the discharge tank fluid surface, for example as in a
pressurized tank.
Figure 4 Discharge, suction and total static head.
See this tutorial for more information on discharge static head .
Double suction pump :
the liquid is channeled inside the pump casing to both sides of the
impeller. This provides a very stable hydraulic performance because the
hydraulic forces are balanced. The impeller sits in the middle of the
shaft which is supported on each end by a bearing. Also the N.P.S.H.R.
of this type of pump will be less than an equivalent end-suction pump.
They are used in a wide variety of industries because of their
reliabilty. Another important feature is that access to the impeller
shaft and bearings is available by removing the top cover while all the
piping can remain in place. This type of pump typically has a double volute .
The following image is provided by the Flow Serve Corporation .
This sketch will help visualize the flow inside the pump.
Double volute pump :
a pump where the immediate volute of the impeller is separated by a
partition from the main body of the casing. This design reduces the
radial load on the impeller making the pump run smoother and vibration
free.
Double volute pump (source of image the Pump Handbook by McGraw-Hill).
see specialty_pumps.pdf for more information
Drooping curve :
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. see centrifugal-pump-tips.htm
Efficiency: :
the efficiency of a pump can be determined by measuring the torque at the pump shaft with a torque
meter and then calculating the efficiency based on the speed of the pump, the pressure or total head
and flow produced by the pump. The standard equation for torque and speed provides power.
The power consumed by the pump is proportional to total head, flow, specific gravity and efficiency.
for a metric version of this formula see this page.
Flow and total head are measured and then the efficiency can be determined.
The efficiency is calculated for various flow rates and plotted on the same curve as the pump
performance or characteristic curve . When several performance curves are plotted, the equal efficiency
values are linked to provide lines of equal efficiency. This is a useful visual aide as it points out
areas of the various pump curves that are at high efficiency, which will be the preferred areas or
areas that the selected pump should operate within. The highest efficiency on a given pump curve is
known as the B.E.P. (best efficiency point), more information is available in this area of the visual
glossary .
Centrifugal pumps come in many designs and some are more suitable for low-flow high-head applications
and others for high-flow low-head and some in between. They are designed to achieve their maximum
efficiency to accommodate a particular application.
The specific speed number gives an indication of what type of pump is more suited to your application.
The effect of specific speed on pump design and how to calculate this number is
available in this area of the visual glossary .
It is possible to predict efficiency. Some years ago, a survey of typical industrial pumps was made.
The average efficiency was plotted against the specific speed and it shows what the ultimate efficiency
limits are for pumps under various operating conditions. More information is available on the
centrifugal pump tips page .
Suction specific speed is another parameter that can affect efficiency. This number is a measure of
how much flow can be put through a pump before it starts to choke (reaches it's upper flow limit)
and cavitates (the pressure at the suction becomes low enough that the fluid vaporizes). More
information is available in the visual glossary here .
End suction pump :
a typical centrifugal pump, the workhorse of industry. Also known as
volute pump, standard pump, horizontal suction pump. The back pull out
design is a standard feature and allows easy removal of the impeller
and shaft with the complete drive and bearing assembly while keeping
the piping and motor in place.
Some of its components are:
1.Casing, volute2. Impeller, vanes, vane tips, backplate, frontplate (shroud), back vanes, pressure equalising passages or balancing holes
3. Back cover parallel to Plane of the impeller intake
4. Stuffing Box - Gland/mechanical seal housing or packing/lantern ring
5. Pump shaft
6. Pump casing
7. Bearing housing
8. Bearings
9. Bearing seals
11. Back pull out
12. Bearings
13. Bearing seals
Balancing holes
Backvanes
Equivalent length :
a method used to establish the friction loss of fittings (see next
figure). The equivalent length of the fitting can be found using the
nomograph below. The equivalent length is then added to the pipe
length, and with this new pipe length the overall pipe friction loss is
calculated. This method is rarely used today. See tutotial3.htm for the current method for calculating fittings friction head loss.
Energy gradient : see Hydraulic gradient .
External Gear pump :
a positive displacement pump. Two spur gears are housed in one casing
with close clearance. Liquid is trapped between the gear tooth spaces
and the casing, the rotation of the gears pumps the liquid. They are
also used for high pressure industrial transfer and metering
applications on clean, filtered, lubricating fluids.
Viking Pumps is a major supplier of these pumps http://www.vikingpump.com/.
Flat curve : head decreases very slowly as flow increases, see centrifugal-pump-tips.htm
Foot valve : a check valve that is put on the end of the pump suction pipe, often accompanied with an integrated strainer. This is an example from a supplier.
Friction :
The force produced as reaction to movement. All fluids are subject to
friction when they are in motion. The higher the fluid viscosity, the
higher the friction force for the same flow rate. Friction is produced
internally as one layer of fluid moves with respect to another and also
at the fluid wall interface. Rough pipes will also produce high
friction.
Friction head loss : the friction head loss is given by the Darcy-Weisbach equation and in many tables such as provided by the Cameron Hydraulic data book . It is normally given in feet of fluid per 100 feet of pipe.
Table of head loss factors for water from the Cameron Hydraulic data book.
Try this calculator for piping friction head loss.
For more information on friction head .
Friction factor f : the friction factor f is required for the calculation of the friction head loss. It is given by the Moody diagram , or the Colebrook equation or the Swamee-Jain equation .
The value of the friction factor will depend on whether the fluid flow
is laminar or turbulent. These flow regimes can be determined by the
value of the Reynolds number .
Front cover : see end-suction pump .
Front plate : see end-suction pump .
Gland : see stuffing box .
Glandless pumps : see sealless pumps .
Hazen-Williams equation :
this equation is now rarely used but has been much used in the past and
does yield good results although it has many limitations, one being
that it does not consider viscosity. It therefore can only be applied
to fluids with a similar viscosity to water at 60F. It has been
replaced by the Darcy-Weisbach and the Colebrook equation.
Interestingly the NFPA (National Fire Protection Association) mandates
that the Hazen-Williams equation be used to do the friction
calculations.
The C coefficients use in the above Hazen-Williams equation are given in the table below.
Hazen-Williams equation C coefficients.
Head:
the height at which a pump can displace a liquid to. Head is also a
form of energy. In pump systems there are 4 different types of head:
elevation head or static head, pressure head, velocity head and
friction head loss. For more information on head see this tutorial.
Also known as specific energy or energy per unit weight of fluid, the unit of head is expressed in feet or meters. see also tutorial2
Try this calculator to obtain head from pressure.
Hydraulic gradient:
All the energy terms of the system ( for example velocity head and
piping and fitting friction loss) are converted to head and graphed
above an elevation drawing of the installation. It helps to visualize
where all the energy terms are located and ensure that nothing is
missed.
Impeller:
The rotating element of a pump which consists of a disk with curved
vanes. The impeller imparts movement and pressure to the fluid.
See this paper on impellers by the McNally Institute .
Figure 5 Major pump parts and terminology.
The
impeller consists of a back plate, vanes and for closed impellers a
front plate or shroud. It may be equipped with wear rings, back vanes
and balancing holes.
for more on the different impeller types see impeller.htm .
Impeller eye:
that area of the centrifugal pump that channels fluid into the vane
area of the impeller. The diameter of the eye will control how much
fluid can get into the pump at a given flow rate without causing
excessive pressure drop and cavitation. The velocity within the eye
will control the NPSHR, see this chart .
see also centrifugal-pump-tips.htm
For more information on pump part terminology see this web page.
Inducer:
an inducer is a device attached to the impeller eye that is usually
shaped like a screw that helps increase the pressure at the impeller
vane entrance and make viscous or liquids with high solids pumpable. It
can also be used to reduce the NPSHR.
Inducer (source: The Pump Handbook from McGrawHill).
Internal gear pump : a positive displacement pump.
The internal gear pumping principle was invented by Jens Nielsen, one of the founders of Viking Pump .
It uses two rotating gears which un-mesh at the suction side of the
pump to create voids which allow atmospheric pressure to force fluid
into the pump. The spaces between the gear teeth transport the fluid on
either side of a crescent to the discharge side, and then the gears
re-mesh to discharge the fluid. Viking's internal gear design has an
outer drive gear (rotor- shown in orange) which turns the inner, driven
gear (idler-shown in white).
Viking Pumps is a major supplier of these pumps http://www.vikingpump.com/.
Jet pump :
a jet pump is a commonly available residential water supply pump. It
has an interesting clever design that can lift water from a well (up to
25 feet) and allow it to function without a check valve on the suction
and furthermore does not require priming. The heart of the design is a venturi
(source of water is from the discharge side of the impeller) that
creates low pressure providing a vacuum at the suction and allowing the
pump to lift fluids.
see this article for more information
visit this manufacturer (and no, I don't get a commission) for more info
Another good web site on this topic.
K factor : a factor that provides the head loss for fittings. It is used with the following equation
The
K factor for various fittings can he found in many publications. As an
example, Figure 6 depicts the relationship between the K factor of a
90° screwed elbow and the diameter (D). The type of fitting dictates
the relationship between the friction loss and the pipe size.
Note: this method assumes that the flow is fully turbulent (see the demarcation line on the Moody diagram of Figure 9 ).
Figure 6 K factor vs. diameter of fitting (source: Hydraulic Institute Engineering data book)
Another good source for fitting K factors is the Crane Technical Data Brochure.
Figure 7 Values for the K factor with respect to the friction factor for a standard tee.
The Crane technical paper gives the K value for a fitting in terms of the term fT as in this example for a standard tee.
As
is the case for the data shown in Figure 6, the friction loss for
fittings is based on the assumption that the flow is highly turbulent,
in fact that it is so turbulent that the Reynolds number is no longer a
factor and pipe roughness is the main parameter affecting friction.
This can be seen in the Moody diagram . There is a line in the diagram that locates the position where full turbulence starts.
The term fT used by Crane is the friction factor and is the same as that given by the Colebrook or the Swamee-Jain equation.
When the Reynolds number becomes large the value of fT (using the Swamee-Jain equation ) becomes:
furthermore the Crane Technical Paper No. 410
assumes that the roughness of the material will correspond to new steel
whose value is 0.00015 ft. Therefore, the previous equation for fT becomes:
Therefore the value of the K factor is easily calculated based on the diameter of the fitting, the friction factor fT and the multiplication factor for each type of fitting.
Laminar : A distinct flow regime that occurs at low Reynolds number (Re <2000). It is characterized by fluid particles in layers moving past one another without mixing.
Figure 8 Laminar flow velocity profile.
Lobe pump :
a positive displacement pump. Primarily used in food applications
because they handle solids without damaging them. Lobes are driven by
external timing gears as a result the lobes do not make contact. Liquid
travels around the interior of the casing in the pockets between the
lobes and the casing, meshing of the lobes forces liquid through the
outlet port under pressure. They also offer continuous and intermittent
reversible flows and can operate dry for brief periods of time. Typical
applications are in following industries: food, pharmaceuticals, paper
& pulp, beverages, chemical and biotechnology.
Viking Pumps is a major supplier of these pumps http://www.vikingpump.com/.
Low NPSH pump : a pump designed for application with a low N.P.S.H. available , usually has an inducer. see inducer
see specialty_pumps.pdf for more information
Mechanical seal :
a name for the joint that seals the fluid in the pump stopping it from
coming out at the joint between the casing and the pump shaft. The
following image (source: the Pump Handbook by McGraw-Hill) shows a
typical mechanical seal. A mechanical seal is a sealing device which
forms a running seal between rotating and stationary parts. They were
developed to overcome the disadvantages of compression packing. Leakage
can be reduced to a level meeting environmental standards of government
regulating agencies and maintenance costs can be lower.
For more information on mechanical seals see this Goulds web page .
Mercury (Hg) :
A metal that remains liquid at room temperature. This property makes it
useful when used in a thin vertical glass tube since small changes in
pressure can be measured as changes in the mercury column height. The
inch of mercury is often used as a unit for measuring vacuum level or
pressures below atmospheric pressure.
The relationship between inches of mercury, psi and psia units of pressure.
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. ( What is the B.E.P.? )
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.
see also the pump glossary BEP
How is the minimum flow of a centrifugal pump established (answer from the Hydraulic Institute http://www.pumps.org/public/pump_resources/faq.htm )
The factors which determine minimum allowable rate of flow include the following:
*
Temperature rise of the liquid -- This is usually established as 15°F
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".
Minimum NPSHA :
the margin of safety or minimum NPSHA that should be available depends
in part on the amount of suction energy of the pump. The suction energy
level of the pump increases with:
The casing suction diameter The pump speed The suction specific speed The specific gravity of the fluid
Anything
that increases the velocity of the pump impeller eye, the rate of flow
of the pump, or the specific gravity, increases the suction energy of
the pump.
The Hydraulic Institute has offered these guidelines for minimum NPSHA depending on the level of suction energy.
Minimum NPSH Margin Ratio Guidelines NPSHA/NPSHR
Suction energy levels
Application Low
Medium
High
Petroleum
1.1-a
1.3-a
Chemical
1.1-a
1.3-a
Electrical power
1.1-a
1.5-a
2.0-a
Nuclear power
1.5-b
2.-a
2.5-a
Cooling towers
1.3-b
1.5-a
2.0-a
Water/Waste water
1.1-a
1.3-a
2.0-a
General industry
1.1-a
1.2-a
Pulp and paper
1.1-a
1.3-a
Building services
1.1-a
1.3-a
Slurry
1.1-a
Pipeline
1.3-a
1.7-a
2.0-a
Water/Food
1.2-a
1.5-a
2.0-a
"a" - or 0.6 m (2 feet) whichever is greater
"b" - or 0.9 m (3 feet) whichever is greater
"a" - or 1.5 m (5 feet) whichever is greater
Motor frame :NEMA
(National electrical Manufacturers Association) provides standards to
which electric induction motors are built. Each frame size (for example
frame 254T) is built to specified dimensions. The amount of room
required for the pump assembly will depend on the size and construction
of the motor. It is easy to find a chart that provides the motor
dimensions vs. the frame size (see following chart).
but I looked long and hard to find a chart that provides the frame size vs. the rpm and hp, and here it is:
Moody diagram : A graphical representation of the laminar and turbulent (Colebrook) flow equations.
Figure
9 the Moody diagram, a graphical representation of the laminar flow
equation and the Colebrook equation for the friction factor f.
Net Positive Suction Head Available (N.P.S.H.A.) :
Net positive suction head available. The head or specific energy at the
pump suction flange less the vapor pressure head of the fluid. see NPSHA.PDF
See this calculator for N.PS.H.A.
Also for those who need to know about NPSHA but hate that stuffy word .
Net Positive Suction Head Required (N.P.S.H.R.) :
Net positive suction head required. The manufacturers estimate on the
NPSH required for the pump at a specific flow, total head, speed and
impeller diameter. This is determined my measurement. see also NPSHR.PDF
This
next figure provides an estimate for NPSHR for centrifugal pumps
(source: Centrifugal Pump Design & Application by Val.S.Labanoff
and Robert R Ross, contributed by a pump forum friend, Ravi Sankar.
You can join the centrifugal pump discussion forum at http://www.lightmypump.com/forum
For a larger scale image download npshr-predict.pdf
Newtonian fluid :
A fluid whose viscosity is constant and independent of the rate of
shear (strain). For Newtonian fluids, there is a linear relationship
between the rate of shear and the tangential stress between layers.
For more information see non-newtoninan fluids.pdf
If
you want to understand what a non-Newtonian fluid feels like and what
it means for viscosity to change with the rate of shear, try this
experiment.
In
a large shallow bowl make a solution of approximately 1 part water and
2 parts corn starch, try moving this fluid rapidly around with your
fingers. When the fingers are moved slowly, the solution behaves as
expected, offering little resistance. The faster you try to move
through the fluid, the higher the resistance. At that rate of shear,
the solution almost behaves as a solid, If you move your fingers fast
enough they will skip over the surface. This is what is meant by
viscosity being dependent on rate of shear. Compare this behavior to
that of molasses; you will find that even though molasses is viscous
its viscosity changes very little with the shear rate. Molasses flows
readily no matter how fast the movement.
See a video presentation of this experiment .
Open impeller :
the impeller vanes are open and the edges are not constrained by a
shroud. This type of impeller is less efficient than a closed type
impeller. The disadvantage is mainly the loss of efficiency as compared
to the closed type of impeller and the advantage is the increased
clearance available which will help any impurities or solids get
through the pump and prevent plugging. See an image of an open impeller.
Operating point : The point (flow rate and total head) at which the pump operates. It is located at the intersection of the system curve and the performance curve of a pump. It corresponds to the flow and head required for the process.
Figure 11 Operating point on a pump performance curve.
Packing : see stuffing box .
Partial emission pump : see radial vane pump .
Peripheral pump :
also known as regenerative or regenerative turbine pump. These are low
capacity (150 gpm or 34 m3/h) high head (5400 ft or 1645 m) pumps. The
impeller has short vanes at the periphery and these vanes pass through
an annular channel. The fluid enters between two impeller vanes and is
set into a circular motion, this adds energy to the fluid particles
which travel in a spiral like path from the inlet to the outlet. Each
set of vanes continuously adds energy to the fluid particles.
Peripheral
pumps are more efficient at these low flow high head conditions than
centrifugal pumps, they also require much less NPSHA than an equivalent
centrifugal pump. They can also handle liquids with up to 20% entrained
gases.
They are used in a wide range of domestic and industrial applications.
For a good explanation of the principal of operation see the Mepco web site .
Performance curve :
A plot of Total Head vs. flow for a specific pump model, impeller
diameter and speed (syn characteristic curve, water performance curve).
see Figure 1
For more information on performance or characteristic curve see this tutorial
Pipe roughness :
A measurement of the average height of peaks producing roughness on the
internal surface of pipes. Roughness is measured in many locations and
then averaged, it is usually defined in micro-inches RMS (root mean
square). Download or view a pipe roughness chart in pdf format
Piping pressure (maximum) :
it may be necessary in certain applications to check the maximum rating
of your pipes to avoid bursting due to excessive pressure. The ASME
pressure piping code B31.3 provides the maximum stress for pipes of
various materials. Also the pipe flange rating will have to be checked.
for more information see max_piping_oper_press.pdf
Table of allowable piping stress from the ASME pressure piping code B31.3
Pitot pump :
also know as rotating casing pump. This pump’s specialty is low to
medium flow rates at high pressures. It is frequently used for high
pressure shower supply on paper machines.
Pitot (Roto-jet ) pump
see specialty_pumps.pdf for more information
Pressure :
The application of a force to a body producing more or less compression
within the liquid. In a static fluid pressure varies with height.
Fluid
weight is the cause of hydrostatic pressure. A thin slice of fluid is
isolated so that the forces surrounding it can be visualized. If we
make the slice very thin, the pressure at the top and bottom of the
slice will be the same. The slice is compressed top and bottom by force
vectors opposing each other. The fluid in the slice also exerts
pressure in the horizontal direction against the pipe walls. These
forces are balanced by stress within the pipe wall. The pressure at the
bottom of the slice will be equal to the weight of fluid above it
divided by the area.
The weight of a fluid column of height (z) is:
The
pressure (p) is equal to the fluid weight (F) divided by the
cross-sectional area (A) at the point where the pressure is calculated :
where F : force due to fluid weight
V : volume
g : acceleration due to gravity (32.17 ft/s2 )
Pressure head : an expression of energy, specifically it is energy per unit weight of fluid displaced. More information on pressure head .
We
often need to calculate the pressure head that corresponds to the
pressure. Pressure can be converted to pressure head or fluid column
height for any fluid. However, not all fluids have the same density.
Water for example has a density of 62.34 pounds per cubic foot whereas
gasoline has a density of 46.75 pounds per cubic foot. Specific gravity
is the ratio of the fluid density to water density at standard
conditions. By definition water has a specific gravity (SG) of 1. To
convert pressure to pressure head, the specific gravity SG of the fluid
must be known. The specific gravity of a fluid is:
where
where
The quantity 3 for water at 60 °F) is:
After simplification, the relationship between the fluid column height and the pressure at the bottom of the column is:
Progressive cavity pump :
a positive displacement pump. These pumps are ideal for fluids that are
just too tough for other pumps to handle. e.g. – pastes, greases,
sludge etc. They consist of only one driven metal rotor rotating within
an elastomer lined (elastic) stator.
Liquid
enters the Suction Inlet under pressure or by gravity and as the ROTOR
1 turns within the flexible rubber STATOR 2 forming tightly sealed
cavities 3 which moves the Liquid toward the Discharge Outlet. Pumping
action starts the instant the ROTOR turns. Liquid acts as the lubricant
between the pumping elements.
Pseudoplastic : The property of a fluid whose viscosity increases slowly with rate of shear.
For more information see non-newtoninan fluids.pdf
Radial flow pump : refers to the design of a centrifugal pump for medium head and medium flow or high head and low flow. The value of the specific speed number will provide an indication whether a radial pump design is suitable for your application. see radial flow pumps.
Radial vane pump :
also known as partial emission pump or vane pump. A frame mounted, end
suction, top centerline discharge, ANSI pump designed specifically to
handle corrosive chemicals at low flows.
Vane pump
see specialty_pumps.pdf
Recessed impeller pump :
sometimes known as vortex pump. This 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.
Recessed impeller pump
see specialty_pumps.pdf for more information
Recirculation :
at low flow and high flow compared to the flow at the B.E.P. the fluid
will start to recirculate or move in a reverse direction at the suction
and at the discharge.
It
is well established that cavitation type of damage seen on the inlet
vanes and not associated with inadequate NPSH can be directly linked to
the pump operating in the suction recirculation zone. Similar damage
seen on the discharge vane tips can also be associated with pump
operation in the discharge recirculation zone.
The suction and discharge recirculation may occur at different points as shown on the characteristic curve below.
Regenerative pump : see peripheral pump , also known as regenerative turbine pump.
Reynolds number :
the Reynolds number is proportional to the ratio of velocity and
viscosity, the higher the number (higher than 4000 for turbulent flow)
the more turbulent the flow and the less viscosity has an effect. At high Reynolds numbers (see the transition line to complete turbulence in the Moody diagram ) the pipe roughness
becomes the controlling factor for friction loss. The lower the
Reynolds number (less then 2000 for laminar flow) the more the
viscosity of the fluid is relevant. Most applications are in the
turbulent flow regime mode unless the fluid is very viscous (for
example 300 cSt and up), the velocity has to be very low to produce the
laminar flow regime.
Rheopectic : The property of a fluid whose viscosity increases with time.
For more information see non-newtoninan fluids.pdf
Sealless pump : see pump type chart for more information, images and references on sealless pumps.
Self-priming pump :
a pump that does not require priming or a initial filling with liquid.
The pump casing carries a reserve of water that helps create a vacuum
that will lift the fluid from a low source.
Self-priming pump
see specialty_pumps.pdf for more information
Shroud : see end-suction pump .
Shut-off head : The Total Head corresponding to zero flow on the pump performance curve.
Figure 12 Shut-off head and other points on a centrifugal pump performance curve.
The
shut-off head is the Total Head that the pump can deliver at zero flow
(see next Figure). The shut-off head is important for 2 reasons.
1.
In certain systems (admittedly unusual), the pump discharge line may
have to run at a much higher elevation than the final discharge point.
The fluid must first reach the higher elevation in the system. If the
shut-off head is smaller than the static head corresponding to the high
point, then flow will not be established in the system.
2.
During start-up and checkout of the pump, a quick way to determine if
the pump has the potential capacity to deliver the head and flow
required, is to measure the shut-off head. This value can be compared
to the shut-off head predicted by the performance curve of the pump.
Siphon :
A system of piping or tubing where the exit point is lower than the
entry point and where some part of the piping is above the free surface
of the fluid source.
Figure 14 A siphon.
See this article for a description of how a siphon works .
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.
Sludge pump
see specialty_pumps.pdf for more information
Slurry pump :
a 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.
Slurry pump
see specialty_pumps.pdf for more information
Specific gravity (SG) :
the ratio of the density of a fluid to that of water at standard
conditions. If the SG is 1 then the density is the same as water, if it
is less than 1 then the fluid is less dense than water and heavier than
water if the SG is bigger than 1. Mercury has an SG of 14, gasoline has
an SG of 0.8. The usefulness of specific gravity is that it has no
units since it is a comparative measure of density or a ratio of
densities therefore specific gravity will have the same value no matter
what system of units we are using, Imperial or metric.
For more information see specific gravity.pdf
See this experiment on video showing that total head is independant of density or specific gravity .
the
above image is from the Cameron Hydraulic data book which contains a
great deal of information on fluid properties. To purchase go to the Flow Serve web site .
Specific speed :
a number that provides an indication what type of pump (for example
radial, mixed flow or axial) is suitable for the application.
Specific speed is calculated with this formula:
suction specific speed
for an article on this topic see specific-speed_primer.pdf
and here is a calculator for specific speed .
Strain : The ratio between the absolute displacement of a reference point within a body to a characteristic length of the body. see Figure 10 .
Stress : In this case refers to tangential stress or the force between the layers of fluid divided by the surface area between them.
Stuffing box :
the joint that seals the fluid in the pump stopping it from coming out
between the casing and the pump shaft. The following image (source: the
Pump Handbook by McGraw-Hill) shows a typical stuffing box with gland
packing. The function of packing is to control leakage and not to
eliminate it completely. The packing must be lubricated, and a flow
from 40 to 60 drops per minute out of the stuffing box must be
maintained for proper lubrication. This makes this type of seal unfit
for situations where leakage is unacceptable but they are very common
in large primary sector industries such a mining and pulp and paper.
For more information see this Goulds pump web page.
Submersion : Submersion as used here is the height between the free surface of a suction tank and the pump intake pipe.
Figure 13 Minimum submersion to avoid vortex formation.
Try this calculator for minimum submersion height.
Here's a nice picture of an axial flow pump with an suction intake submersion problem.
see this video on submersion
and for more information on this web site see this help file for the submersion calculation applet .
The Hydraulic Institute publishes a guide on Pump Intake Design that provides detail recommendations.
The Goulds pump company provides similar pump intake design recommendation s at no cost.
Suction guide : a device that helps straighten the flow ahead of a pump that has a 90 degree elbow immediately ahead of it.
There are two types of suction gudes as far as I know.
Suction guide by Armstrong, see http://www.armstrongpumps.com
The other type of suction guide is the Cheng vane system
The Cheng vane, see http://www.chengfluid.com
Another manufacturer of standard suction guide components from 2" to 14" diameter is Metraflex .
Suction vane : see suction guide .
Suction specific speed :
a number that indicates whether the suction conditions are sufficient
to prevent cavitation. According to the Hydraulic Institute the suction
specific speed should be less than 8500. Other experiments have shown
that the suction specific speed could be as high as 11000.
When
a pump has a high suction specific speed value, it will also mean that
the impeller inlet area has to be large to reduce the inlet velocity
which is needed to enable a low NPSHR. However, if you continue to
increase the impeller inlet area (to reduce NPSHR), you will reach a
point where the inlet area is too large resulting in suction
recirculation (hydraulically unstable causing vibration, cavitation,
erosion etc..). The recommended maximum suction specifc speed value is
to avoid reaching that point. (paragraph contributed by Mike Tan of the pump forum group ).
Keeping the suction specific speed below 8500 is also a way of determining the maximum speed of a pump and avoiding cavitation.
For a double suction pump, half the value of Q is used for calculating the suction specific speed.
Suction specific speed is calculated with this formula:
specific speed
The term NSS is also used to represent the suction specific speed.
According to the Hydraulic Institute
the efficiency of the pump is maximum when the suction specific speed
is between 2000 and 4000. When S lies outside this range the efficiency
must be derated according to the following figure.
source: Pump & Systems magazine August 2005
for an article on this topic see specific-speed_primer.pdf
and here is a calculator for suction specific speed .
Suction Static Head : The difference in elevation between the liquid level of the fluid source and the centerline of the pump (see Figure 4 ).
This head also includes any additional pressure head that may be
present at the suction tank fluid surface, for example as in the case
of a pressurized suction tank.
Suction Static Lift :
The same definition as the Suction Static head. This term is only used
when the pump centerline is above the suction tank fluid surface.
System :
as in pump system. The system includes all the piping, including the
equipment, starting at the inlet point (often the fluid surface of the
suction tank) and ending at the outlet point (often the fluid surface
of the discharge tank).
System Curve :
A graphical representation the pump Total Head vs. flow. Calculations
are done for the total head at different flow rates, these points are
linked and form a curve called the system curve. It can be used to
predict how the pump will perform at different flow rates. The Total
head includes the static head which is constant and the friction head
and velocity head difference which depends on the flow rate (see Figure 11 ). The intersection of the system curve with the pump characteristic curve defines the operating point of the pump.
Changes
to the system such as opening or closing valves or making the discharge
pipe longer or shorter will change the friction head which will change
the shape of the system curve and therefore the operating point. In the
following figure there is a system which has a static head of 100 feet
and a total system resistance of approximately 20 feet shown by curve
A. There is a valve at the pumpdischarge which is partially closed. If
the friction head is increased (i.e. valve is closed) then the
operating point will shift from A to point B and the flow will drop. If
the friction head is decreased (i.e. valve is opened) then the
operating point will shift to point C and the flow increases.
System requirements :
Those elements that determine Total Head: friction and the system inlet
and outlet conditions (for example velocity, elevation and pressure).
Swamee-Jain equation : an equation that can be used as a substitute for the Colebrook equation for calculating the friction factor f.
Thixotropic : The property of a fluid whose viscosity decreases with time.
Total Dynamic Head : Identical to Total Head. This term is no longer used and has been replaced by the shorter Total Head.
Total Head : The difference between the pressure head at the discharge and suction flange of the pump ( syn Total Dynamic Head. pump head , system head). see also tutorial3.htm
Total Static Head :
The difference between the discharge and suction static head including
the difference between the surface pressure of the discharge and
suction tanks if the tanks are pressurized (see Figure 4 ). See also tutorial3.htm
Turbulent :
The behavior of fluid articles within a flow stream characterized by
the rapid movement of particles in many directions as well as the
general direction of the overall fluid flow.
Vacuum : pressure less than atmospheric pressure.
Vane pass frequency :
when doing a vibration analysis this frequency (no. of vanes times the
shaft speed) and it's even multiples shows up as a peak which can
indicate a damaged or imbalanced impeller.
Figure 15 Noise vibration spectra showing vane pass frequency (source: The Pump Handbook publ. by McGrawHill)
Vane pump : see radial vane pump .
Vane pump (hydraulic) :
a positive displacement pump. Vane pumps are used successfully in a
wide variety of applications (see below). Because of vane strength and
the absence of metal-to-metal contact, vane pumps are ideally suited
for low-viscosity, non lubricating liquids up to 2,200 cSt / 10,000
SSU. Such liquids include LPG, ammonia, solvents, alcohol, fuel oils,
gasoline, and refrigerants.
1.
A slotted rotor or impeller is eccentrically supported in a cycloidal
cam. The rotor is located close to the wall of the cam so a
crescent-shaped cavity is formed. The rotor is sealed into the cam by
two sideplates. Vanes or blades fit within the slots of the impeller.
As the impeller rotates (yellow arrow) and fluid enters the pump,
centrifugal force, hydraulic pressure, and/or pushrods push the vanes
to the walls of the housing. The tight seal among the vanes, rotor,
cam, and sideplate is the key to the good suction characteristics
common to the Vane pumping principle.
2.
The housing and cam force fluid into the pumping chamber through holes
in the cam (small red arrow on the bottom of the pump). Fluid enters
the pockets created by the vanes, rotor, cam, and sideplate.
3.
As the impeller continues around, the vanes sweep the fluid to the
opposite side of the crescent where it is squeezed through discharge
holes of the cam as the vane approaches the point of the crescent
(small red arrow on the side of the pump). Fluid then exits the
discharge port.
Rexroth is a major manufacturer of vane pumps http://www.boschrexroth.com/
see also http://www.pumpschool.com/principles/vane.htm
Vapor pressure : The pressure at which a liquid boils for a specific temperature.
Figure
16 The boundary between liquid and vapor phase of a fluid. A fluid can
be vaporized by increasing the temperature or decreasing the pressure.
Figure 17 Vapor pressure vs. temperature for various fluids.
Values for vapor pressure of common liquids is available in the Goulds pump catalogue.
Venturi (Bernoulli's law) : a venturi is a pipe that has a gradual restriction
that opens up into a gradual enlargement. The area of the restriction will have a lower pressure than the
enlarged area ahead of it. If the difference in diameters is large you can
In
certain locations I can't do this experiment, because hey don't have a
source of water in hotel suites, too bad because it's always a winner,
so I have to revert to a video here at Fisher Scientific it only costs $10, and no, I don't get a commission.
It
is not easy to understand why low pressure occurs in the small diameter
area of the venturi. I have come up with this explanation that seems to
help.
It is clear that all the flow must pass from the larger section to the smaller section. Or in other
words, the flow rate will remain the same in the large and small portions of the tube. The flow rate
is the same, but the velocity changes. The velocity is greater in the small portion of the tube. There
is a relationship between the pressure energy and the velocity energy, if velocity increases the pressure
energy must decrease. This is the principle of conservation of energy at work which is also Bernoulli's law.
This is similar to a bicycle rider at the top of a hill. At the top or point 1 (see Figure 18 below), the
elevation of the cyclist is high and the velocity low. At the bottom (point 2) the elevation is low and the
velocity is high, elevation (potential) energy has been converted to velocity (kinetic) energy. Pressure
and velocity energies behave in the same way. In the large part of the pipe the pressure is high and velocity is low, in the
small part, pressure is low and velocity high.
Figure 18 The venturi effect.
Bernoulli's law is a relationship between two points within a system that states that the sum of the
energies that correspond to pressure, velocity and elevation must be conserved.
The general form of the law (neglecting friction) is:
where p1 is the pressure, v1 the velocity and h1 the elevation
at point 1 and the same parameters are used at point 2. Gamma
In the case of the cyclist there is no pressure and only the velocity and elevation can vary, so that
Bernoulli's law becomes:
as the cyclist goes down the hill h2 becomes smaller than h1 and to
balance the equation then v2 must be larger than v1 .
In the case of the venturi tube there is no elevation change and only the velocity and pressure can vary,
so that Bernoulli's law becomes:
We can clearly see that if v2 is greater than v1 then p2 must be smaller than v1 to balance the equation.
for an article on this and related subjects see unusual_aspects-pumps-syst.pdf
Viscosity :
A property from which a fluid's resistance to movement can be
evaluated. The resistance is caused by friction between the fluid and
the boundary wall and internally by the fluid layers moving at
different velocities. The more viscous the fluid the higher the
friction loss in the system. Centrifugal pumps are affected by
viscosity and for fluids with a viscosity higher than 10 cSt, the
performance of the pump must be corrected. Use this calculator to determine the correction for viscosity to the water performance curve of the pump.
The
following figure which you can find in the Goulds pump catalogue in the
Technical Section shows the effect of viscosity on pump performance.
This next figure is a chart of values for viscosity for different liquids which you can find in the Cameron Hydraulic data book.
The
basic unit of viscosity is known as the Poise or centiPoise (cP) named
after the French scientist Poiseuille who discovered a practical method
of measuring viscosity. The greek letter
The relationship between the two is:
Viscosity data of common liquids can also be found in the Goulds pump catalogue.
Viscous drag pump :
a pump whose impeller has no vanes but relies on fluid contact with a
flat rotating plate turning at high speed to move the liquid.
Viscous drag pump
see specialty_pumps.pdf for more information
Volute : syn casing.
Vortex : see submersion .
Water hammer (pressure surge) :
If in systems with long discharge lines,(e.g. in industrial and
municipal water supply systems ,in refineries and power stations) the
pumped fluid is accelerated or decelerated, pressure fluctuations occur
owing to the changes in velocity. If these velocity changes occur
rapidly , they propagate a pressure surge in the piping system,
originating from the point of disturbance ; propagation takes place in
both directions (direct waves),and these waves are reflected (indirect
waves) at points of discontinuity ,e.g. changes of the cross sectional
area ,pipe branches, control or isolating valves, pumps or reservoir.
The boundary conditions decide whether these reflections cause negative
or positive surges. The summation of all direct and indirect waves at a
given point at a given time produces the conditions present at this
point.
These pressure surges, in
addition to the normal working pressure ,can lead to excessive pressure
and stresses in components of the installation . In severe cases such
pressure surges may lead to failure of pipe work, of fittings or of the
pump casings. The minimum pressure surge may, particularly at the
highest point of the installation ,reach the vapor pressure of the
pumped liquid and cause vaporization leading to separation of the
liquid column. The ensuing pressure increase and collision of the
separated liquid column can lead to considerable water hammer .The
pressure surges occurring under these conditions can also lead to the
failure or collapse of components in the installation.
For the maximum pressure fluctuation the JOUKOWSKY pressure surge formula can be used:
Δp = ρ . a . Δv
Where ρ = density of the pumped liquid
a = velocity of wave propagation
Δv = change of velocity of the flow in the pipe.
The
full pressure fluctuation corresponding to the change of velocity Δv
occurs only if the change of velocity Δv takes place during the period.
t ≤ reflection time tr = 2.l /a
where l = distance between the nearest discontinuity (point of
reflection ) and the point of disturbance .
A contribution from Moshe Shayan of the pump discussion forum.
This article titled Surge Control in Pumping Station by Val-Matic Valve appeared in the Pumps & Systems magazine of March 2007,
it's a very good description of how water hammer occurs and how it can be controlled.
You can join the centrifugal pump discussion forum at http://www.lightmypump.com/forum
Another interesting article from the Pump-Zone http://www.pump-zone.com/article.php?articleid=363
TOP
for the pressure rating of ANSI class flanges. 
with atmospheric pressure. 
.
Along with the noise, the shock of the imploding bubbles on the surface
of the vane produces a gradual erosion and pitting which damages the
impeller.
volute and are decelerated and pressurized.
Check out the direction of rotation, not what one would expect at first glance.