Making the case for positive displacement pumps in SWRO

Monday, 17 October 2016
Large-flow positive displacement pumps are entering the market with potential to bring down energy consumption by 20%

In recent years, the number of desalination plants has increased dramatically along the coastlines of many continents. And the number of new plants is likely to continue to grow in the coming years as a natural response to the increasing water scarcity facing the world today. In fact, the Global Water Intelligence Association expects an 80 % increase in the number of plants from 2017 to 2020.

According to the UN, more than one billion people suffer from lack of fresh water today, and more than 300 million people in 150 countries rely wholly or partly on desalination of seawater according to the International Desalination Association.

The desalinated water resources are vital for supply of drinking water, but equally important for the growing of crops, personal hygiene and sanitation to prevent spreading of life-threatening diseases.

Reverse osmosis has become the preferred technology for desalination. Compared to other methods, Sea Water Reverse Osmosis (SWRO) is highly efficient and provides clean and contamination-free water. The technology is reliable and safe with high-pressure pumps forcing sea water through desalination membranes to create fresh water.

The global focus on CO2 emissions and climate change has led to heavy investments in development of smarter and energy efficient reverses osmosis technologies.

For decades, the centrifugal (CF) pump has been considered the best choice of high pressure pump for SWRO. In recent years, however, the increasing focus on energy savings has led to the invention of new and highly efficient pumping solutions. Large-flow Positive Displacement (PD) pumps designed specifically for SWRO are entering the market with potential to bring down the energy consumption by 20% or more compared to CF pumps.

In fact, parallel PD pumps, for instance the APP pumps from Danfoss have been tested to outperform centrifugal pumps even in large plants. The APP pumps are running with proven efficiency rates up to 90%, offering the lowest life-cycle costs in the industry.

Despite the obvious advantages of using PD pumps, some system builders still hesitate to use them for several reasons. In some cases simply because the PD pump is new in large SWRO applications. In other cases, designers are hesitant to use the PD pump because the flow control is different and requires a slightly different design of hydraulic components.

The flow capacity of a single PD pump is often too low for SWRO applications. Therefore, you need to configure multiple pumps in parallel to achieve the desired capacity. This system design is simple, since the total flow for all pumps can be calculated simply as the sum of the individual pump flow. No capacity is wasted in the parallel configuration.

At the same time, there are obvious advantages of the parallel-coupling of PD pumps. First of all, parallel coupling of PD pumps saves CAPEX as well as energy. This is due to the fact that the regulation of the flow on one pump does not affect the flow on the other pumps, which means that flow regulation needs only be done on one of the pumps. When using VFD to save energy this means that VFD needs only be applied on one of the pumps, while the rest are running directly online.

Having several pumps in the system also increases the up time of the SWRO system. If one pump is taken out for service or repair, the other pumps remain in service and can provide the required capacity in order to fulfill the demand for fresh water at all times.

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