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PHS
- Pump Education |
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Lesson
1. |
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| a)
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The
impeller spins & throws water out. |
Like swinging
a bucket of water above your head and staying dry
or
Throwing clay
on a potter's wheel and wearing it. |
| b) |
Low
pressure is formed in the inlet.
The lower the pressure, the higher the pump can "suck."
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| c)
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Atmospheric
pressure pushes more water in.
It
is this simple - this is the major part of pump theory !
Understand
this, and NPSH is easy!
PUMPS
DON'T SUCK
In fact, nothing
does.
Can you name
something that does ? |
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Consider
items b) and c) above:
Explain
the workings of several other things in our world:
breathing
flight
wind
carburettors
vacuum cleaners |
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PHS
- Pump Education |
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Lesson
2. |
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HEAD
Centrifugal pump curves show 'pressure' as head, which is
the equivalent height of water with S.G. = 1.
This makes allowance for specific gravity variations in the
pressure to head conversion to cater for higher power requirements.
Positive Displacement pumps use pressure (ie; psi or kPa)
and then multiply power requirements by the S.G. |
| STATIC
HEAD
The vertical height difference from surface of water source
to centreline of impeller is termed as static suction head
or suction lift ('suction lift' can also mean total suction
head).
The vertical
height difference from centreline of impeller to discharge
point is termed as discharge static head.
The vertical
height difference from surface of water source to discharge
point is termed as total static head. |
TOTAL
HEAD / TOTAL DYNAMIC HEAD
Total height difference (total static head) plus friction
losses & 'demand' pressure from nozzles etc. ie: Total
Suction Head plus Total Discharge Head = Total Dynamic Head. |
NPSH
Nett positive suction head - related to how much suction lift
a pump can achieve by creating a partial vacuum. Atmospheric
pressure then pushes liquid into pump. A method of calculating
if the pump will work or not. (more) |
S.G.
Specific gravity. weight of liquid in comparison to water
at approx 20 deg c (SG = 1). |
SPECIFIC
SPEED
A number which is the function of pump flow, head, efficiency
etc. Not used in day to day pump selection, but very useful
as pumps with similar specific speed will have similar shaped
curves, similar efficiency / NPSH / solids handling characteristics. |
VAPOUR
PRESSURE
If the vapour pressure of a liquid is greater than the surrounding
air pressure, the liquid will boil. |
VISCOSITY
A measure of a liquid's resistance to flow. ie: how thick
it is. The viscosity determines the type of pump used, the
speed it can run at, and with gear pumps, the internal clearances
required. |
FRICTION
LOSS
The amount of pressure / head required to 'force' liquid through
pipe and fittings. |
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PHS
- Pump Education |
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Lesson
3. |
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Reading
Centrifugal Pump Curves |
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Centrifugal
pump performance is represented by multiple curves indicating
either:
Various impeller diameters at a constant speed.
Various speeds with a constant impeller diameter. |
| The
curve consists of a line starting at "shut head"(zero
flow on bottom scale / maximum head on left scale). The line
continues to the right, with head reducing and flow increasing
until the "end of curve" is reached, (this is often
outside the recommended
operating range of the pump). |
Flow
and head
are linked, one can not be changed without varying the other.
The relationship between them is locked until wear or blockages
change the pump characteristics. |
The
pump can not develop pressure unless the system creates backpressure
(ie: Static
(vertical height), and /or friction
loss). Therefore the performance of a
pump can not be estimated without knowing full details of
the system in which it will be operating. |
Refer
to fig.2 below for a sample curve showing (text colours relate
to colours used in fig.2):
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Three
performance curves ( various impellers or speed). |
| Curves
showing power absorbed by pump (read power at operating point,
see note 1). |
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Nett
positive suction head required by the pump (NPSH
[R] ). |
| The
circled numbers indicate the following for bottom curve (ie:
smallest diameter impeller or slowest speed curve shown): |
| Maximum
recommended head.
Minimum recommended head.
Minimum recommended flow.
Maximum recommended flow. |
| The
points referred to as "shut head: and "end of curve".
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Note
1: Power absorbed by pump is read at point where power curve
crosses pump curve at operating point. However this does not
indicate motor / engine size required. Various methods are
used to determine driver size. |
| 1.
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Select
motor or engine to suit specific engine speed or operating range
- most cost effective method where operating conditions will
not vary greatly. |
2.
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Read
power at end of curve - most common way that ensures adequate
power at most operating conditions. |
3.
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Read
power at operating point plus 10% - usually only used in refinery
or other applications where there is no variation in system
characteristics. |
4.
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By using
system curves
all operating conditions can be considered - best method where
filling of long pipelines, large variations in static head,
or syphon effect exist. |
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fig.
2 |
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PHS
- Pump Education |
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Lesson
4. |
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Centrifugal
Pump Operating Range |
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| A. |
All
types of pumps have operational limitations.
This is a consideration
with any pump whether it is positive displacement or centrifugal.
The single
volute centrifugal pump ( the most common pump used worldwide)
has additional limitations in operating range which, if not
considered, can drastically reduce the service life of pump
components. |
| B.
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"BEP"
- Best Efficiency Point ( refer to fig.3 below )
is not only the operating point of highest efficiency but
also the point where velocity and therefore pressure is equal
around the impeller and volute
As the operating
point moves away from the Best Efficiency Point, the velocity
changes, which changes the pressure acting on one side of
the impeller. This uneven pressure on the impeller results
in radial thrust which deflects the shaft causing:
Excess load
on bearings.
Excess deflection of mechanical seal. or:
Uneven wear of gland packing or shaft / sleeve.
The resulting
damage can include shortened bearing / seal life or a damaged
shaft . The radial load is greatest at shut
head. |
| C. |
Outside the
recommended operating range damage to pump is also sustained
due to excess velocity and turbulence.
The resulting
vortexes can create cavitation damage
capable of destroying the pump casing, back plate, and impeller
in a short period of operation.
Refer to fig.3
which indicates range of operation ( between approximately
50% and 120% of Best Efficiency Point ) |
| D. |
When selecting
or specifying a pump, it is important not to add safety margins
or base selection on inaccurate information.
The actual system
curve may cross the pump curve outside the recommended
operating range. In extreme cases the operating point may
not allow sufficient cooling of pump, with serious ramifications
!
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| E. |
The best practice is to confirm
the actual operating point of the pump during operation ( using
flow
measurement and / or a pressure gauge ) to allow
adjustment ( throttling of discharge or fitting of bypass line
) to ensure correct operation and long service life. |
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fig.
3 |
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PHS
- Pump Education |
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Lesson
5. |
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To
ensure the correct pump is selected for your application the
following details are required.
If
you can not supply some of the information, just ask for help
from PUMP HIRE SERVICES, we can assist in identifying your
requirements.
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| These
details required
for all applications |
Additional
details if
liquid is not water. |
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Flowrate
required
Static
suction head
Suction
pipe inside diameter
Footvalve
or open pipe
Suction
pipe length & material
Static
discharge head
Discharge
pipe inside diameter
Discharge
pipe length & material
Temperature
Details
of solids
Height
above sea level
Details
of application ie:
- additional requirements,
- sprinklers or other pressure requirements
- future
expansion, etc
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Full
liquid description
Specific
gravity
Viscosity
pH value
Other details
or data sheet |
FOR
ALL APPLICATIONS ADVISE: |
Driver
requirements ie:
Electric? - voltage/phase/Hz
Electric? - hazardous location?
Diesel? - preferences
Petrol? - preferences
Hydraulic? - system available |
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PHS
- Pump Education |
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Lesson
6. |
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| System
curves allow correct selection of pumps and are invaluable
in troubleshooting of pump problems.
To draw a system curve, follow these steps & refer to
fig1: |
| 1. |
Find details
of duty. |
| ie,
in this example: Water, 2m suction
lift, 15m static discharge
(17m total static head),
360 metres of 150mm schedule 40 steel pipe. |
| 2. |
Draw a chart with flow
on bottom scale and head on left scale. |
| (estimate scale required based
on size of existing pump, or guess maximum flow expected - example
shows max flow as 100 L/S and max head as75m - sometimes you
just have to guess to get started) |
| 3. |
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| ie: 17m at zero flow. |
| 4. |
Mark 2 or 3 other points. |
| ie: at 20L/S friction loss is
0.73 m / 100m of pipe, therefore 0.73 x 3.6 + 17 = 19.6 metres.Put
mark at junction of 20 L/S and 19.6 m. Repeat for other points.
(remember to add static head each time) |
| 5. |
Join these points with
a line. You have completed the System Curve. (Curve
may have to be extended to suit higher flow pumps.) |
| 6.
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The pump operating
point is where a pump curve crosses
the system curve. |
| Draw as many pump curves over
the system curve as you like, to see where different pumps will
operate, or draw system curve over pump curve. |
| 7. |
If pump curve does not
cross system curve, the pump is not suitable. If the pump curve
crosses the system curve twice, then the pump will be unstable
and is not suitable. |
| 8. |
Note: 'demand' pressure, ie:
sprinklers etc, should be added at each flowpoint, or for approximate
figures can be added to static head. |
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fig1:
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System
Curves for multiple branches |
The
above method shows the basics for drawing a system curve for
one pipe connected to the discharge of the pump.
What if there
is more than one pipe? |
If
2 or more pipes are connected to the discharge of a pump,
the flow through each pipe can be added together at a common
head.
The combined
flowrate can then be plotted as a system curve. Use the highest
static head at zero flow on curve. |
If
there is a common section of pipe before the multiple branch
lines, you must first do the above step then calculate friction
through the common section (at the combined flow of the multiple
branches) then add that to the "common head" used
above.
Use the common
section static head plus the highest static head of the multiple
lines as the head at zero flow on the curve.
You will need
to use two calculators: |
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PHS
- Pump Education |
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Lesson
7. |
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Parallel
& Series Operation |
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The
use of two or more pumps to increase flowrate is called Parallel
pumping.The use of two
or more pumps to increase head (pressure) is called Series
pumping. Operation of
pumps under these circumstances may appear simple, but there
are more complex issues to consider, ie: |
| In series applications:
consider the pressure rating of pump, shaft seal, pipework and
fittings.Placement is critical
to ensure both pumps are operating within their recommended
range and will have a constant supply of water. |
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Drawing a curve
for 2 or more pumps is simple, draw 1st pump curve then draw
2nd curve, adding the head each pump produces at the same flow.
More curves can be added
in the same way. |
In
parallel applications: confirm suitability of pumps by drawing
a system curve (often
2 pumps will only deliver slightly more than one pump due
to excessive friction loss.Also you can confirm that pump
operation will be within its recommended range.).Non return
valves are required especially if one pump operates alone
at times.Dissimilar pumps or pumps placed at different heights
requires special investigation. |
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Drawing a curve for
2 or more pumps is simple, draw 1st pump curve then draw 2nd
curve, adding the flows each pump delivers at the same head.More
curves can be added in the same way. |
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PHS
- Pump Education |
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Lesson
8. |
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Cavitation
- Two Main Causes |
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a)
NPSH (r) EXCEEDS NPSH (a)
Due to low
pressure the water vapourises (boils) and higher pressure
implodes into the vapour bubbles as they pass through the
pump causing reduced performance and potentially major damage.
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b)
Suction or discharge recirculation
The pump is
designed for a certain flow range, if there is not enough
or too much flow going through the pump, the resulting turbulence
and vortexes can reduce performance and damage the pump. |
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PHS
- Pump Education |
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Lesson
9. |
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NPSH
- Nett Positive Suction Head |
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There is enough
fear of it to suggest it is.But why?
Because some
people will not accept that pumps don't suck! |
If you accept
that a pump creates a partial vacuum and atmospheric pressure
forces water into the suction of the pump, then you will find
NPSH a simple concept. |
NPSH(a)
is the Nett Positive Suction Head Available, which is calculated
as follows:
NPSH(a)= p
+ s - v - f
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NPSH(a) must
exceed NPSH(r) to allow pump operation without cavitation.
(It is advisable to allow approximately 1 metre difference
for most installations) |
The other
important fact to remember is that water will boil at much
less than 100 deg C if the pressure acting on it is less than
it's vapour pressure, ie water at 95 deg C is just hot water
at sea level, but at 1500m above sea level it is boiling water
and vapour. |
The vapour pressure of water at 95 deg C is 84.53 kPa, there
was enough atmospheric pressure at sea level to contain the
vapour, but once the atmospheric pressure dropped at the higher
elevation, the vapour was able to escape.
This is why
vapour pressure is always considered in NPSH calculations
when temperatures exceed 30 to 40 deg C. |
NPSH(r) is the Nett Positive Suction Head Required by the
pump, which is read from the pump performance curve. (Think
of NPSH(r) as friction loss caused by the entry to the pump
suction.)
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PHS
- Pump Education |
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Lesson
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Affinity
Laws - Centrifugal Pumps |
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If
the speed or impeller diameter of a pump change, we can calculate
the resulting performance change using: |
AFFINITY
LAWS |
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| a) |
The flow changes proportionally
to speed
ie: double the speed / double the flow |
b) |
The
pressure changes by the square of the difference
ie: double the speed / multiply the pressure by 4 |
c)
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The power changes by the cube
of the difference
ie: double the speed / multiply the power by 8 |
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Notes: |
1. |
These laws apply to operating
points at the same efficiency. |
2. |
Variations in impeller diameter
greater than 10% are hard to predict due to the change in relationship
between the impeller and the casing. |
I
know you are thinking "what does this have to do with
anything"?, but if you can understand these 'laws' then
you can make rough estimates without having to find full information
(which might not be available anyway) |
ie:
Boss: "Hey Joe, put this new pulley on that pump"
Joe: "But that will speed the pump up by about 10 % which
increases the power by a third, do you reckon the motor will
handle it ?" |
For
rough calculations you can adjust a duty point or performance
curve to suit a different speed. |
NPSH
(r) is affected by speed / impeller diameter change = DANGER
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PHS
- Pump Education |
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Troubleshooting |
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| TROUBLESHOOTING |
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Only one
thing is a better troubleshooting tool than pressure &
vacuum gauges...
that is: readings
from pressure & vacuum gauges taken prior to the problem.
ie: monitoring gauge readings will help diagnose pump and
system problems quickly, by reducing the possibile causes.
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Flow measurement
would allow full diagnosis of pump performance but is sometimes
expensive and usually not possible (Cheap versions include:
V notch weir, measuring
discharge from horizontal pipe,
& timing of filling / emptying).
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Following
is troubleshooting table for a typical pump showing symptom
and possible cause. |
| 1. |
Pump
does not prime.. |
Suction
lift too great.
Insufficient water at suction inlet.
Suction inlet or strainer blocked.
Suction line not air tight.
Suction hose collapsed.
Mechanical seal / packing drawing air into pump. |
| 2. |
Not
enough liquid.. |
Incorrect
engine speed.
Discharge head too high.
Suction lift too
great.
Suction inlet or strainer blocked.
Suction line not air tight.
Suction hose collapsed.
Mechanical seal drawing air into pump.
Obstruction in pump casing/impeller.
Impeller excessively worn.
Delivery hose punctured or blocked. |
| 3. |
Pump
ceases to deliver
liquid after a time.. |
Suction
lift too great.
Insufficient water at suction inlet.
Suction inlet or strainer blocked.
Suction hose collapsed.
Excessive air leak in suction line.
Mechanical seal / packing drawing air into pump.
Obstruction in pump casing/impeller.
Delivery hose punctured or blocked. |
| 4. |
Pump
takes excessive ... power.. |
Engine
speed too high.
Obstruction between impeller and casing.
Viscosity and /
or SG of liquid being
pumped too high. |
| 5. |
Pump
vibrating or ... overheating .. |
Engine
speed too high.
Obstruction in pump casing/impeller.
Impeller damaged.
Cavitation due to
excessive suction lift / friction loss. |
| 6. |
Pump
leaking at ... seal housing.. |
Mechanical seal damaged
or worn. Due to:
Dry Running during priming or loss of liquid.
Cracking of ceramic seat can occur (thermal shock)
after pump has run dry or against shut
head
(heating the water) and then cool water enters
the pump casing.
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PHS
- Pump Education |
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Disclaimer |
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| Disclaimer |
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| This information is not intended
as a reference source for the design of pumping or associated
systems, but to promote interest and further investigation by
individuals and companies into the provision of reliable pumping
equipment. Pump
Hire Services would be pleased to receive enquiries to allow
full engineering investigation into your requirements. |
Due to the
complexities of pumping systems in various industries it is
difficult to predict the applications encountered.
Much of the
information is personal opinion / experience of individuals. |
The
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