5 minute read
Pump efficiency: are we heading in the right direction?
With an increasing pressure to improve the efficiency of pumps and pumping systems, Tony Keville, Managing Director of pump distributor, Tomlinson Hall, argues that more significant savings can be made by looking beyond the pumps and motors themselves.
The European Union has imposed measures designed to improve the overall efficiency of pumps and motors to save energy and carbon emissions. After all, pump systems account for 20% of the energy consumed in the UK, an estimate that is probably conservative.
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The major manufacturers have responded to the challenge, and pumps are certainly more efficient now, as are electric motors. Many of the older lowefficiency models have been eliminated. However, this has forced plant operators to adapt pipework to suit the new models, or in the case of circulators, to install new control panels and wiring, a consequence, which, along with the associated costs, was never considered at the outset.
So, when considering centrifugal pumps which represent the largest percentage of the installed base, can any more improvements be wrung out of the pump assembly and at what cost and what are the potential problems that may be encountered?
We are all familiar with the Pareto principle, the 80/20 rule. It applies here, but there is a limit, and that limit is fast approaching. It is well known and documented that as the efficiency of a centrifugal pump increases, so do attendant problems. The two issues which cause the industry the most pain are the increase in minimum continuous stable flow (MCSF) and the increased likelihood of an unstable characteristic being imposed.
If you have been in the industry for some years, you may recall the fundamental clauses which traditionally were used in enquiry documentation. It read something like: ‘the characteristic shall rise continuously from duty point to closed valve, and that rise will be a specified percentage of the duty point head’. That clause may have been written in the days before variable speed drives (VSDs), but it is as relevant now as it was then. In reality, with the use of VSDs it is more important. But why?
If we have a stable characteristic, running via a VSD that controls it at a constant pressure, then at any point if the demand increases the pressure drops and the pump then speeds up to compensate. That continues to closed valve. Now consider what happens for an unstable characteristic operating close to closed valve. As demand increases the pressure rises, so the pump slows down and ultimately stalls out. That is why VSDs are only part of the solution. As anybody who has struggled with parallel operation of unstable pumps knows only too well, such a scenario leads to a complete failure of the system.
But there is another issue to consider, and that is the MCSF. It can be seen that as efficiency rises so does the MCSF, and that means the pump must run at a closer point to the duty point, so the savings are now minimal. But why is it important? As closed valve is approached, the shaft deflection increases and thus bearing and seal life are reduced, noise increases, suction recirculation becomes significant, and then there is the likelihood of tip recirculation cavitation occurring. The nett result is a shortened pump life. The tell-tale sign is to look at the vibration analysis for an increase in amplitude around the vane pass frequency. Typically, the tips of the impeller will have a pockmarked appearance and the shrouds sometimes have a polished appearance.
There is much pressure on users to fit inverters to save money, but they should only be installed after the shape of the pump curve has been taken into consideration. The use of inverters on multistage pumps where balance discs are fitted is not recommended by many manufacturers as they require a minimum pressure to operate correctly. Always check with the pump manufacturer before proceeding. The consequence is that of the disc running against the seating with no lubrication or cooling flow with a catastrophic outcome.
If I go back to the initial statement: ‘are we going in the right direction?’ the answer is possibly no, we aren’t.
Pump and motor manufacturers are looking now at incremental changes – 2% here 4% there – but is that the end of the line?
IS IT POSSIBLE TO MAKE GREATER SAVINGS?
In many instances, much more significant energy savings can be made, perhaps up to 50%. You might think that is a bold statement to make, but it really isn’t.
In cooling or heating circulation duties, for example, the frictional resistance component of the total head can be a significant amount, and anything that can be done to reduce this will have considerable lasting financial benefits.
Put simply the friction is inversely proportional to the pipe diameter to the power of four, so going up a pipe size will reduce the friction by a large amount. In turn, this reduces the power and motor size, leading to a reduction in whole life costs, carbon footprint and environmental charges. This is not rocket science.
Whose responsibility is it to correctly specify pipe sizes? Sadly, these days specification is all down to project cost, and not running costs. So, by the time a plant is built, it is too late to install the correct energysaving pipework. Therefore, it is surely down to those in the pump industry to remind, cajole, and persuade contractors, consultants and end-users to consider the running costs at the outset, as the potential savings are enormous. The cost of the pipe frictionsoftware needed to carry out these calculations is within most companies’ budgets. Invariably it will enable the supplier to give the customer a report to back up its selection and also to sit down with the customer to show instantly how changes in pipework can change the total head and hence the power and thus what savings can be made.
The cost of the bigger pipe will fade into insignificance when compared with the whole project cost. Which project team would turn down the opportunity to save money for the long term?
