HOW TO READ A PUMP
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A centrifugal pump performance
curve is simply a tool which
enables anyone to literally see
how a pump will perform in terms of
HEAD and FLOW.
Every pump will be capable of
developing a specific PRESSURE
(PSI or BAR measurement translated into feet or meters head)
at a specific FLOW
(normally represented in gallons per minute or liters per minute)
A note on establishing flow.
If you do not know the flow you require
it is relatively simple to determine if you
just take it one step at a time and add
all the outlets together.
For industrial applications, for washing, heating or cooling
the equipment will have a flow and pressure
on the design plate.
Flow in a system can be established by
understanding the requirements.
In our example we will use a house which
has two bathrooms, a dishwasher, a washing
machine, an electric geyser in the roof and
a guest toilet with hand basin. Also a sink in
the kitchen and an outside shower for the
So we have the following outlets :-
Bathroom hand basin taps 4
Bath taps 4
Shower taps 6
Toilet cistern 3
Guest basin tap 2
Sink in kitchen 2
Washing machine 1
If we assume each tap will be required to
deliver 2 gallons per minute (GPM) at 44 PSI
or 3 bar, then we simply need to determine
how many outlets we would expect to
operate at any one time. It is unlikely that
all toilets, all showers, all baths etc will
be on at the same time. A good method is to
take a third of the total outlets as your flow
need for any single moment in time.
In our example that represents 23 divided by 3 or 7.6 make it 8 outlets.
This means a flow requirement of 8 X 2 = 16 GPM or 60 liters per minute.
A note on the concept of
Think about the highest point of your
body, the top of your HEAD.
It is the same idea with pump language,
Head refers to the measurement in
feet or meters from the center line of
the pump (the pump shaft center line)
to the highest point to which the unit
is expected to deliver fluid)
The above definition of head is
limited to what is known as the
Static head. in other words, this is
the measurement of the vertical
height which should never change, it is static.
Lets say you are needing
to deliver water from a tank at
the bottom of your garden, to the
geyser at the top of your roof. The
garden has a steep bank from the
house to the lower area where the
tank is located. The house is a double
story. If one measures the height from
the base of the tank to the exact spot
where the geyser rests in the house roof,
we get a measurement of 50 feet (15,5 meters)
. This is static head, it does not change.
However when it comes to fluid and
determining the total head the pump will
feel in order to deliver your required flow
to that vertical height, there are some extra
variables which will effect the head.
These variables are where the calculation of
total dynamic head becomes a little more
involved. We are not going to get into
a huge technical discussion with formula
and major math here, all we need you to
recognise is that there are dynamic forces at
work which affect the performance of
the pump and which one needs to apply to
the system curve. Doing this right will enable
you to determine the right pump for your needs
The total dynamic head is a combination of
static head, friction losses in the pipe system
called friction head loss, and losses caused by
the equipment to which one is delivering the
fluid. These losses change in measurement
depending on the volume of fluid which is
being pumped at any one time. As such the
losses ar dynamic, they change in relation to flow rate.
In our example, we have the tank at the bottom of the yard
and the geyser in the roof, we call this the static head.
Then we have the friction head which will result
from the desired quantity of water needed flowing
through a specific pipe size, over the total distance
from the pump to the geyser. Added to that we will
also need to add the required working pressure for the geyser.
So lets see how we can make the concept a lot
more simple, lets look at a picture.
In FIG 1 we see the static head of the pipe system.
That measurement of 50 feet will not change and
it is the vertical height measurement only.
The actual pipe length given bends and horizontal
distances covered in the system is 70 feet and we
intend to install a 1” inside diameter pipe.
The pipe inside diameter is important because
it is the measurement which is presented to
the water flow path. One needs to have the total
pipe length, pipe inside diameter and the pipe
material before one can apply the formula which
establishes friction loss in a pipe at any given flow.
To calculate the friction loss presented by a pipe system
to the pump TDH (Total Dynamic Head) one normally
would apply a formula like the Hazen-Williams equation as
shown below, BEFORE YOU BLOW A GASKET
AND GIVE UP, get our free simple friction loss calculator
here. (FRICTION LOSS CALCULATOR)
f = 0.2083 (100/c)1.852 q1.852 / dh4.8655 (1)
f = friction head loss in feet of water per 100 feet of pipe (fth20/100 ft pipe)
q = volume flow (gal/min)
dh = inside hydraulic diameter (inches)
There is one more measurement that will
be required to determine the total dynamic head.
It is the actual back pressure of the geyser.
This will be on the plate of the equipment
which will be used. There will be two
measurements, working pressure and burst
pressure. The working pressure is the
pressure the equipment requires in order
to operate at design efficiency. In most
cases a home geyser requires between
30 and 60 PSI. (2to 4 Bar, 200 to 400 kPa)
Working pressure Working pressure will
need to be provided to the equipment and
therefore needs to be provided for by the
pump unit. The actual losses in the geyser
will be only around 3 or 4 PSI when new,
however we need to generate working pressure
in this instance.
So Total Dynamic head (which has many other
additions for more detailed pipe systems which
are not discussed here in detail) can be established
by keeping these basics in mind.
Get a handy calculator which will make this
far easier to establish. It is free, so have this
sent directly to you now.
HANDY HEAD CALCULATOR
BRINGING IT ALL TOGEATHER
When we know our total dynamic head and
our flow rate we require, the rest is simple.
We look up the head on the Y axis and follow
that line to the X axis flow value. Where these
two values intersect, we have our duty point.
We now search for a pump curve which will
allow this point to position on the “sweet spot”.
When we get that right, we have selected the correct
pump for our needs and we are able to purchase with
If you used our example above you will have the following.
Flow required 16 GPM (60.48 liters per minute)
Head Static 50 feet
Friction 11.6 feet
Geyser 130 feet
(working pressure assumed to be 58 PSI)
BY adding all these we have a TDH 191.60 feet (58.95 meters)
We now are able to specify a pump that is
capable of delivering 16 gallons per minute
at 191.6 feet. I would add in about 10 percent
on the head for safety.(pipe bends and other
losses) so we look for a 63 meter head or 210 feet capablility in our pump.
We are to use the primary metric pump curves so
our measurements are flow in liters per minute on
the lower x axis and GPM on the upper, and
head in meters on the Y axis left with feet on the right.
We have taken a pump manufacturers catalogue,
looked through the various models available,
searching for a pump curve that has both the
right flow rate and head characteristics. Knowing
what we are looking for because we have worked
it all out makes the exercise relatively simple.
We have now selected the PLURIJET PUMP
RANGE (A PEDROLLO PRODUCT) so lets
see how the curve works. (SEE FIG 4)
Here we have shown the correct pump selection with
the geyser working pressure included in the design.
We also show the most common mistake made in
pump selection, not including the working pressure
of the geyser. As can be seen, if we do not include
the geyser working pressure, we have a major problem.
We hope this all assists you in understanding the basics
of a fluid system design. Should you wish to have far
more information literature and instruction is available
at the link below.
MORE DETAILED INFORMATION
Further technical information link click here
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If you would like one of our designers to complete
this aspect for you, we do charge a small design
fee of US$10 per basic design and pump selection,
please contact us at this link (PUMP SYSTEM DESIGN SUPPORT)
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