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

and efficiency.

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

Action inspires action. Stay ahead of the curve with sustainability and energy newsletters from edie