Mechanical Technology June 2016

⎪ Pump systems, pipes, valves and seals ⎪

The systems approach to pumping In this issue we welcome new columnist, Harry Rosen, from TAS Online and 2KG Training. Rosen is currently one of the two international pump experts for the United Nations Industrial Development Organisation (UNIDO). In this, his first column, he outlines the main differences between the component and systems approaches to pump efficiency analysis and optimisation.

T he world is split into two camps when it comes to improving the energy efficiency of pumps. The component-based approach is being driven in Europe through legislation and setting minimum efficiency levels for pump and motor manufacturers. The systems approach has been championed by the USA ever since the US Department of Energy piloted a successful energy savings project in the mid 90s in China. Although it started off as an electric motor optimisation project, it was very quickly discovered that the major sav- ings’ opportunities came from looking at the pumping system, rather than just concentrating on pumps and motors. The main difference between the component and systems approach comes down to how wide you intend to set your system boundary when evaluating a pumping system. Take the typical pump system of a pump taking fluid from a reservoir and pumping it to a discharge tank a suitable distance away and at a higher elevation – as shown in Figure 1.

of flow and pressure. The pressure down- stream of the PRV is what the system actually requires, and the pressure loss through the control valve must also be treated as wasted energy. By using the flow rate at F2 and the pressure after the PRV in our calcula- tions, we can determine the overall system efficiency, which could be dra- matically less than our original calcu- lation of pump efficiency. Our system level opportunity would be to remove the throttling valve, close the bypass line and find another way to meet the required system demand – by installing a VSD, trimming the impeller or changing the control methodology, for example. If we assume that 20% of the flow rate is being returned to the suction tank, and the pressure drop across the throttle valve is around 30% of the upstream pres- sure, then the overall system efficiency can be calculated to be around 42%. Suddenly there is a major energy savings opportunity. This is the benefit of look- ing at the system rather than individual components. The system: Sugar mills provide great op- portunities for reducing pumping energy costs. There are numerous pumps used in all aspects of the process, as well as for cooling of process heat. In addition, many mills have cogeneration plants with boil- ers running on bagasse, the high calorific dry pulpy residue left after the extraction of juice from sugar cane. These plants require additional pumping systems for boiler feed water and cooling pumps for condensing steam back to water. The system investigated included four hot-well pumps (three operating normally) that pump hot return water from the refinery to a set of spray pans – a low cost alternative to traditional forced- convection cooling towers. The water is cooled down through natural convection by approx 10 °C and then pumped by another set of pumps back to the plant A case study: The bypass flow at a sugar mill in the Philippines

the pump and motor. Power is measured from the MCC, flow rate from the flow meter situated just downstream of the pump, and head from the difference in pressure between the suction and discharge pressure gauges. The pump efficiency is calculated to be 75.4% and, by comparing this to that on the pump curve from the manufacturer, we find that it is close to the maximum of 79% efficiency for this pump. On a component level the pump is operating efficiently and does not warrant any further attention. The system approach Now let us expand the system bound- ary to incorporate the flow control valve (FCV). This opens to allow bypass flow back to the suction side when demand is low. It is thus not the flow rate through the pump that is important in our exam- ple, but the flow to fill the second tank, or to supply a downstream process. The energy consumed for pumping any liquid back to the suction tank is wasted energy. If we expand the system boundary once again to incorporate the pressure control valve (PRV), we get the true picture of the system demand in terms

Pumping systems 101:

The component approach Let us start with the box surrounding only

Figure 1: The main difference between the component and systems approach comes down to how wide you intend to set your system boundary when evaluating a pumping system.

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Mechanical Technology — June 2016