Control the cost of pumping
Ian Weybourne, water industry specialist with Dowding & Mills, considers the major factors which need to be taken into account to ensure cost-efficient operation of pumping systems
Despite being relatively simple and mature products, pump reliability and performance
continues to have a significant impact on operating costs. Pumps can be in service
for 15 to 25 years and over that period consume many times their initial value
in maintenance and energy costs.
In this context failure to meet performance and efficiency requirements is
just as important as a mechanical or electrical failure. It is not possible
to tell if a pump is operating inefficiently just by looking at it. The pump
may deliver the flow or pressure required and, unless it is in a very poor state,
it will not be making a noise or be overheating. Unfortunately, the instrumentation
that can provide the information required to evaluate performance and efficiency
is often not fitted, not working or out of calibration.
The efficiency of pumps varies significantly depending on their design and
the operating conditions. The peak efficiency of new centrifugal pumps can vary
between 60% to 90% and their efficiency characteristics are strongly flow dependent.
A pump may be operating inefficiently, even though it is in good condition,
simply because it is being asked to work outside its normal performance envelope.
Processes change and whilst the original pump design and selection may have
been perfectly acceptable it may no longer be suitable for the current requirements.
Under these circumstances it may be more cost effective to replace the pump
with a more appropriate unit than persevere with consistently poor reliability
Given the time an experienced engineer can reduce or eliminate most in-service
pump problems. The real challenge is to be able to identify and quantify the
scale of the problem. On a small plant it may be obvious where resources should
be allocated. On a more complex plant it becomes increasingly important to have
some basic technical and financial information available on the current performance
and efficiency of the pumping assets.
To improve the life of pumping equipment and reduce maintenance expenditure
the plant engineer must have access to accurate data on operating conditions
and running hours, the cause of failures and the frequency and total cost of
any repairs. This information can be incorporated into most commercially available
computerised maintenance management systems and with relatively simple analysis
it can be used to:
- identify the most expensive pumps – the illusive 20% of pumps that generate
80% of the maintenance costs,
- reduce maintenance expenditure by designing-out problems, replacing unsatisfactory
equipment or improving maintenance and operating regimes,
- monitor the effectiveness of any modifications to the pump or system and
improve future purchase specifications,
- accurately predict operating expenditure at the estimating stage of new
projects and more effectively assess products at the time of purchase.
For pump repairs data can be collected on the reported symptoms the observed
defects and, in some cases, the probable cause of the failure. The level of
detail available depends on the experience and skills of those recording the
data and the information required.
Symptoms generally relate to the pump performance (eg no flow, excessive power
consumption) or its condition (eg overheating, noisy, leaking). Defects are
normally identified on strip-down and relate to components, bearing failure
or mechanical seal failure for instance. The probable cause can be harder to
determine. For example, a bearing may fail repeatedly due to overload resulting
from operating the pump at too low a flow rate. Unless a thorough investigation
is carried out, and a connection made between the mechanical effect and the
hydraulic operating conditions, the root cause of the problem will not be identified.
With access to this data, companies can benchmark their own performance and
develop a detailed understanding of the links between pump and system design
features and reliability in different operating environments.
The function of most pumping systems is to transfer liquid from one location
to another at minimum cost. The input to this process is electrical energy that
is converted into mechanical energy by the motor and ultimately hydraulic energy
by the pump. For a well designed system the hydraulic losses within the pump
and the system pipework account for about 30% of the total energy consumed and
in many cases the system losses can be much higher.
Where a large number of pumps are involved a process needs to be put in place
to compare the performance and efficiency of different pumping systems to identify
those that should be considered in detail. The effort and costs involved in
this process need to be balanced against the potential returns that will be
generated by improved pumping system efficiency. A step-wise approach is required
to progressively focus down onto the areas that will produce the greatest savings.
Initial screening is most often based on energy consumption. Typically, 80%
of the energy costs will be associated with 20% of the plant. For each of the
pumping systems being considered data is collected on the motor rating and the
estimated annual operating hours. The product of the motor rating and the operating
hours then gives an estimate of the annual energy consumption (kW/h). This is
not a very accurate estimate as the motor rating refers to the shaft output
power not the electrical input power and it also assumes that the motor is operating
at its rated load condition.
Prioritising pumping systems on the basis of energy consumption alone makes
the assumption that the greatest potential savings are always associated with
the high-energy use plant, which may not be the case. If possible some measure
of the overall efficiency of the different pumping systems should be introduced.
This is usually done on the basis of specific power consumption (ie the power
consumed per unit volume pumped, kW/h/Ml). Effort can then be focused on systems
that have a highenergy bill and high specific energy consumption.
Where it is possible to obtain data on the actual flow and head required by
the system this can be used to further refine the screening process. Pumping
systems can be identified where there is an appreciable imbalance between the
current system requirements and the pump output and where pumps are being operated
significantly away from their specified design condition. The degree of imbalance
can be assessed by comparing the product of the actual flow and head generated
by the pump with that required on-site.
Pump reliability and efficiency are closely linked and should form part of
a company’s overall asset management strategy. The traditional approach is to
treat reliability and efficiency completely separately but this may mean the
duplication of effort and a missed opportunity.
The ultimate objective is to provide sufficient information, at minimum cost,
to be able to identify poor performance and implement the work required to reduce
operating costs. The type of information required will depend on the range of
pumping applications being considered. One thing is certain: if you can not
measure it then you can not control it
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