ITT Water & Wastewater UK Ltd
Colwick
Nottingham NG4 2AN
Tel: ++ (0)115 940 0111
Fax: ++ (0)115 940 0444
SUSTAINED PUMP EFFICIENCY – A TRUE MEASURE OF HYDRAULIC
EFFICIENCY
By Andrew Barry, ITT Water & Wastewater’s Market Development Manager
Calculating the real efficiency of a pump is a notoriously difficult task, particularly in
wastewater situations, where multiple variables can affect efficiency calculations.
Often manufacturer test-bed calculations can be severely impacted by real-life
conditions, in the same way that a car’s fuel efficiency can widely differ between
clear motorway running and inner-city stop-start driving.
Pump manufacturers usually offer efficiency calculations with their pumps, but these
are calculations based on perfect conditions, involving clean water, unsullied by any
of the problems which sewage water brings with it.
In addition, hydraulic efficiency in centrifugal pumps is related to the number of
Impeller vanes and channel depth. This is why clean water pumps have multiple
impeller vanes, but these are less suitable for wastewater because the leading edge
of each vane is an area where raggy material can gather thus reducing the
efficiency and finally resulting in a blockage. For this reason, historically, singlevane impellers have been preferred within the water industry as they only have one
leading edge where solids can attach themselves. But, this is still a compromise as
it still provides an edge where solids can gather which will reduce the efficiency in
running and potentially cause a blockage.
Therefore, although test-bed statistics have their place, they bear no relation to
actual pump performance ‘in the field’ in live unscreened sewage pumping stations.
When raggy materials get caught in the impeller, they impair the flow path and
create a drop in flow, whilst still using the same or higher amounts of power, thus
relating to a drop in hydraulic efficiency. Unless the drop in flow is identified pumps
can run in this condition for weeks or months in an unbalanced state, which
considerably adds to the energy bills for end users and puts added stress on
mechanical parts within the pumps, which can result in premature failure and costly
repairs.
The problem is that many companies use manufacturer test bed statistics to make
purchasing decisions based on life cycle costs which take little account of the effect
the pumped media has on the actual power consumption. Such calculations often
take the form of purchase cost + test-bed efficiencies + subjective maintenance
costs = pump whole life costs, which can be flawed in wastewater applications.
The raggy material problem has also increased in recent years, making the
discrepancy between manufacturer efficiency calculations and real-life conditions all
the more acute. Modern living with its emphasis on convenience has created
countless products, which make life easier for the individual but significantly more
difficult for our sewage network and, in particular, the pumps which power it.
Disposable cleaning cloths, nappies and make-up wipes all contain cloth-like
constituents which can stretch and wrap around vital components once disposed.
In addition, materials such as fat, oil and grease, commonly known by the acronym
FOG, help to consolidate solid matter in the system into a rag mass creating
blockage problems.
The result is that companies are not getting the energy or hydraulic efficiency that
they believe they will get when they purchase a pump.
Far better would be a situation where companies take into account the sustained
efficiency of pumps whilst in actual live applications. Crucially, these calculations
must be based on wastewater testing not clean water. The total efficiency
calculation would then be based upon the following: Non-clogging performance of
the pump, hydraulic efficiency, sustained efficiency, wear and motor efficiency.
Non-clogging
performance
Hydraulic efficiency
Hydraulic design
Total efficiency over time
Sustained efficiency
Wear
Down-time
Electric motor
Motor efficiency
The potential savings from ensuring the design of a pump is capable of passing
solids without a reduction in efficiency are most clearly illustrated by trials carried
out in Canada at a Pumping Station called ‘Lachenaie’.
Here, three types of impeller were run simultaneously over a nine-month period and
their actual specific energies were compared against each other and to the
theoretical specific energies based on clean water test curves. Specific Energy (E S)
is a measure of the energy consumed per unit of pumped volume and is the most
accurate way of measuring energy consumption in a system (kWh/M3).
The three Impellers were 1) two-vane design, 2) single-vane, 3) semi-open selfcleaning impeller (N-Pump).z
Figure 1
Impeller Design
Motor Power (kW)
Duty Flow (L/Sec)
Duty Head (M)
Overall Effy @ Duty
(clean Water)
Two-Vane Single-Vane N-Impeller
5.9
7.5
5.9
58.8
67.9
67
5.4
6.2
6.1
55.3
53.2
61.7
Theoretical Specific
Energy in clear
water (kWh/M3)
0.026
0.031
0.027
Average Monthly
Operation (h)
212
163
120
Monthly Average
Pumped vol. (9month average)[M3]
291,445
291,445
291,445
Actual Specific
Energy in Sewage
[kWh/M3]
0.04
0.043
0.027
% Change in Specific
Energy
55%
37%
0%
This shows that the energy consumed by both the two-vane and single-vane
impellers is much higher in a real installation than would have been calculated in a
typical Life Cycle Cost Model. In fact the increase in specific energy is directly
proportional to the energy cost so this would result in a 55% and 37% increase in
the energy bill, which is due to the pumps running partially blocked for much of the
time.
The Flygt N-Pump used in the Test is an innovative design of Semi-Open impeller
with a self cleaning mechanism which ensures that solids cannot foul on the leading
edge of the impeller but are fed through the pump eliminating any impact on the
impeller efficiency or specific energy. This is demonstrated clearly in the results of
the test carried out at Lachenaie shown in figure 1 where it will be seen that the N
pump ran for only 120 hours to pump the same volume as the Single-Vane (163
hours) pump, although only rated at the same flow on clean water.
Figure 2 – The diagrams below show the effects of sewage water on efficiency and energy
consumption with A) Single-Vane pump running continuously, B) Single-Vane pump running
intermittently and C) the Flygt N-Pump with its Self-Cleaning Impeller.
The concept of sustained efficiency is a relatively new one for many in the water
industry. If customers are to truly get an understanding of the overall efficiency of
their pump in wastewater situations, a measurement of sustained efficiency over
time is the only true method and this can only be achieved by using data gathered
from their own pumping station assets. This has historically been difficult due to the
data which needs to be gathered – flow, power & running time. However the cost of
equipment to measure these parameters has come down considerably in recent
times and there is a good opportunity for water companies to reduce their energy
costs by including these facilities into their specifications for new or refurbished
pumping stations. Only then will they get a true understanding of the potential
savings of using the correct pump selection.
For more details on ITT Water & Wastewater’s total capabilities call 01159 400111
or visit www.ittwww.com.
Ends
For media enquiries please contact Tom Leatherbarrow or Marie Beattie on 0121
456 3004 (toml@willoughby-pr.co.uk) (MarieB@Willoughby-pr.co.uk)
Notes to Editors:
Based at Nottingham, ITT Water & Wastewater UK Ltd prides itself in delivering sustainable, market
leading solutions across the UK - working at the forefront of pumping and treatment of water and
wastewater.
ITT Water & Wastewater incorporates four market leading brands - Flygt, Sanitaire, Wedeco and
Leopold, offering a comprehensive range of products and services that include systems
engineering, design and installation through to principal contractor.

Flygt is the world leader in submersible pumping and mixing technology. The Flygt product
range has applications for municipal water treatment, agriculture, clean water and industry
eg power generation, food & beverage, waste management. It offers customers quality,
outstanding life-long performance and added value. In the UK and Ireland ITT Flygt also
provides rental, monitoring and control products and aftermarket services as part of its total
solutions capability.

Sanitaire is the market-leader in diffused aeration products and systems. With over 30
years experience in optimising treatment solutions, Sanitaire’s project teams work across the
ITT Water & Wastewater product range offering process treatment alternatives, application
engineering, support services, project management and support contractors with installation,
civil engineering and operations management.

Wedeco products offer UV disinfection and ozone oxidisation disinfection for drinking water,
wastewater and process water. With a focus on water conservation, Wedeco’s
environmentally friendly, high-tech systems ensure that water can be recycled, is fit for
human consumption and that industrial process water does not harm the environment.

Leopold is a world leader in the provision of secure, maintenance free, performance filter
underdrain systems and solutions for both potable and wastewater applications.
For more information visit www.ittwww.com