How To Read A Pump Curve
How To Read A Pump Curve
How To Read A Pump Curve
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Pumps provide a differential pressure and flow according to their installation. As there are 3 main
families of pumps being Centrifugal, Rotary Positive Displacement and Reciprocating Positive
Displacement which have different characteristics dependent on the circumstances they face.
(/upload/editor/North_Ridge_Pump_Type_Overview-20081810434176412.pdf)
Pumps are a simple machine which provide a performance based on the system it works in, as most
pumps do not have a control interface unless fitted with a pressure transducer and variable frequency
drive (VFD) and must be manually commissioned onsite.
A pumps performance will be inline with the pressure losses in the system, with pumps producing a
differential flow and pressure based on the conditions at the inlet. A pump curve is a graphical
representation of what flows and differential pressures can be produced by a pump.
As 90% of problems with pumps are caused by the system they are installed in, it is important to note
that pump selection is just part of the process of selecting a pump which is right for the process.
In order for a pump to be selected for your process it is important that the following are known:
1. Fluid being pumped
2. Application
3. Flow Required
4. Pressure required
5. Viscosity of fluid & Specific Gravity
6. Temperature
7. Power available / Power medium being used to drive pump.
There are two types of pump curves dependent on the pump selected, which are Centrifugal Pumps
and Positive displacement Pump Curves.
f pumping applications, and their curves are generally shaped with a half moon shape with the
highest point on the left showing highest pressure but lowest flow, and the far right end of curve
showing highest flow but lowest amount of pressure. The duty point is is marked typically with the
efficiency indicated in percent.
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The numbers at the end of the curve is the impeller diameter, which is trimmed to acheive the
required flow and pressure. The more an impeller is trimmed the higher the impact on a pumps
efficiency as the gap between the outside of the impeller and casing is larger creating inefficiencies.
Although a pump curve shows the various duty points that a pump can achieve, operating the pump
in some of the areas of operation can lead to many problems.
As you can see in the below illustration if the pump is operated on the left it can mean low bearing
life, mechanical seal failure and heavy vibration.
If a pump is operated too far left on its curve there is no allowance for extra capacity should there be
miscalculation in system pressures. Too far to the right and there is the risk of cavitation which can
destroy the pump casing and impeller very quickly and cause the liquid to boil. Good practice is to
always have a safety margin maybe 10% towards the left of the duty point to ensure the pump can
operate as required, as a pumps performance can always be reduced, but not increased.
This is because a positive displacement pump flow is proportional to rpm and does not decrease with
pressure like a centrifugal pump. A PD pump curve usually has a separate axis detailing viscosity,
where the pump will show a flow against viscosity as per the graph below.
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Outside of the Best Efficiency Point (BEP) the pumps performance will suffer and if operated
inefficiently can damage itself, leading to its destruction within minutes.
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Curve Basis
Curves are always based on fresh water at 20°C at sea level which may not reflect the requirements
of your application which is why the fluid viscosity and specific gravity are required to create an
accurate representation of what your equipment will achieve.
Viscosity
Viscosity can vary significant with certain fluids such as oils and it is important to ensure the figure
quoted is correct. Many fluids have a viscosity quoted at 20°C or 60°C which can be far from the
actual pumping temperature especially in cooling applications where the pump is required to work
prior to the oil being heated.
Motors on centrifugal pumps revolutions are set by the number of poles in the motor. The more poles
a motor has the slower it will operate at. Increasing the number of poles in a motor can help pumps to
produce more flow at lower pressures, and gain from a reduction in the NPSH required, suffer from
less wear and tear, and utilize a smaller powered motor. If a higher pressure is required and lower flow
pumps will operate at higher RPM to generate the pressures required.
Changing the number of poles in motor is not the only way to change pump speed. Pumps can also
be set at individual rpms if used through an inverter or mechanical variator. Positive displacement
pumps will usually use a gearbox with a pump operating at full motor speed in order to ensure the
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Some applications will require a pump to operate for a short amount of time, and others for 24/7 such
as in cooling at which point a low motor speed will be chosen. A PD pump may have a 2 pole motor
rather than a higher pole due to the starting torque. Care should also be taken as motors can be listed
as having a high RPM but in actual fact the rpm may be rated as less from the motor. North Ridge
Pump curves are specified to the exact RPM of the motor rather than using a general figure.
· 1B, 1E and 1U
· 2B and 2U
· 3B
This means that depending on the class used for testing the head can vary between +- 0% to +-7%
and flow between 0% to +-9% which requires careful consideration during pump selection. This is
often why margins are added to requested performance.
1U 1E 1B
Flow Rate tQ +0% to 10% ± 5% ± 5% ± 8% ± 9%
Pressure tH +0% to 6% ± 3% ± ± 5% ± 7%
3%
Pump η 0% 0% -3% -5% -7%
Efficiency
Pump Power p 10% 4% 4% 8% 9%
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As the shaft speed or the impeller diameter is altered, the flow will change by the same amount. If the
speed of a pump is reduced by 20% the flow at the same head will also decrease by 20%.
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