Make the saving of a lifetime
Alan Bell, marketing manager with KSB assesses the possibility of making reductions in energy consumption by looking at life cycle costing when choosing and operating pumps
It is estimated that pump usage accounts for one fifth of the world’s generated electricity therefore the correct selection of pump products and its mode of operation can make for vast energy saving potential. A number of factors need consideration when selecting pumps, plant and/or systems, in order to minimise energy use and the overall cost. All cost elements need to be considered to provide a reliable basis for purchasing decisions.
Users should assess the profitability of pumping systems including the characteristics of different pumps, drives and the various monitoring systems and control options available and their particular benefits. LCC should not be treated as an exact mathematical equation, more importantly the aim should be to estimate the relevant overall costs on the basis agreed upon between customer and supplier. By agreeing a base format for the measurement of LCC customers can make direct comparisons providing the assumptions on various costs, running times, frequency of maintenance are applied equally.
Of course these elements will vary depending on the application or industry concerned. For example, in a manufacturing situation energy costs may be less important than downtime (or loss of production) while the aspects governing submersible motor pump applications in the water industry are different.
Therefore, the application and the client’s needs are as important in the calculation as the numbers produced if they are to get the right product for their particular needs. For example, a product with a longer frequency between service intervals but with lower efficiency may provide greater savings overall to the client who has high downtime costs. The correct selection of the product and its method of control should be the starting point for energy saving. Simply trimming impellers to duty can provide significant savings.
Also, rather than accepting a like-for-like replacement we need to consider more efficient alternatives. For example, in industry there is a trend towards systems with lower nominal flow rates. Often this need is met by single-stage centrifugal pumps, which run at low-flow conditions resulting reduced efficiency.
Multistage pumps which feature lower nominal flow rates are considered an alternative due to their higher efficiency. Varying head requirements can be achieved by adjusting the number of stages. Another method of achieving these savings is through the use of variable-speed controlled pumps. In recent years much of the development with regard to pumping systems has centred more on control of the pump speed and the energy saving this can provide. No wonder, when you consider that depending on the type of pump and the application, the energy cost can account for up to 80% of the life cycle cost.
Pump systems are often designed for the maximum demand, however this is often not required. Process geared adjustments can be achieved with conventional control methods using throttle valves or by-pass lines. However, this can result in high energy loss. But if the pump is optionally matched to the plant requirements by electronic speed adjustment, the motor input power can be significantly reduced. A decrease in wear and noise levels are additional positive side effects.
Different speeds are often necessary depending on the process conditions, for example in a heating or air conditioning system or in the water supply installations if there are changes in demand.
Due to its robust construction, no-maintenance and low product cost the asynchronous motor is widely recognised as the ‘workhorse’ of electric drives, even though continuous-speed control cannot be easily be implemented.
The most simple technical solution of speed control is the pole changing asynchronous motor. It allows no more than four pre-set speeds which is why continuous process control is not possible.
The asynchronous motor with frequency inverter has become a standard solution for creating continuous control for industrial and commercial drives and is widely used today. The use of frequency inverters for pump control offers energy savings as well as a number of other additional advantages.
These advantages include infinitely variable speed adjustment, availability of higher speeds than normal direct mains operation with reactive power compensation not required. The motor can be started with limited-rated current, no star delta stages, reduction in noise and wear and high start-up/stopping frequency are also possible, a PI controller is already integrated, throttle controls are not required, easy connection to bus systems and a power range from 100W to several MW.
The majority of frequency inverters work according to the same basic principle by converting the AC components to the mains DC components and ‘chops’ them into a three-phase system of variable frequency and amplitude by means of electronic power semi-conductors. The power section of a frequency inverter is made up of a rectifier, a link and the inverter. It is the rectifier in the mains that rectifies the alternating voltage. The link decouples the rectifier and the inverter and serves as the energy store.
The inverter within the motor which converts the DC components of the link via electronic switches to a three- phase voltage system of variable frequency and voltage. The power section is monitored and controlled by an electronic control system. Traditionally the functional units have been physically separated. But increasingly, smaller electronic assemblies have provided scope for integrating the components into a single unit. The advantages, other than reduced cabling costs, which can be significant, are that all relevant function motor protection controls are also integrated reducing the possibility of interference and space that is saved.
Co-operation between process and mechanical engineers and drive specialists will lead to new ideas and technologies providing even greater efficiencies which in turn will save costs. It is these tools which will make up energy saving programmes and life cycle costing initiatives in the future