When does it make good sense to replace a single large centrifugal pump with multiple PD pumps in an SWRO train? While there is no simple answer, we believe there are five relevant factors that must all be considered.
Redundancy and uptime
The main reason to establish multiple trains is to ensure uptime.
Water is a must-have resource for both consumers and industry; downtime is not an option that many operators are willing to live with. The redundancy of multiple trains enables operators to continue to produce water even if one train goes down. System builders, owners and operators need to consider how much redundancy is optimal and how best to configure the redundancy of all key components.
While a train driven by one very large centrifugal pump can deliver high energy efficiency, the tradeoff is reduced redundancy and increased risk of downtime: If the single pump fails, the entire train fails. Achieving redundancy by adding additional large centrifugal pumps is much more expensive than adding additional smaller HP pumps.
Redundancy and flexible production capacity
For some plants, demand for water can vary considerably according to the time of day or time of year.
Here, the additional production flexibility made possible by replacing trains driven by one large centrifugal pump with trains driven by several smaller PD pumps lets the plant operators vary the flow output more easily.
Cost
Individual cost drivers such as cap ex, energy and parts/maintenance all play a role in determining when it’s best to add more or fewer trains and how to configure and dimension these.
In general, of course, smaller components are less costly to purchase than larger components. This is true for everything from pumps to valves to piping. But using larger components might provide better economies of scale and other efficiencies.
How to configure the ideal train size? When are the disadvantages (higher initial cap ex, more piping and more complex control systems) of having more components to purchase and maintain outweighed by the advantages of using smaller parts that are cheaper to purchase and operate? Well, you will have to figure that out on a case-by-case basis.
If your model is build–transfer–operate, your focus might be more on initial rather than ongoing costs. If your model is build–own–operate, however, you’ll want to know how the number and size of individual trains – along with everything else – will have an impact on your total costs of operation (TCO) over the lifetime of your plant.
Several white papers published over the last few years have described how to use TCO calculations when comparing multiple APP pumps to single large centrifugal pumps. See, for example, IDAWC/TIAN13-046.
Technology
When it comes to SWRO trains, the tradeoffs between "bigger is better" and "small is beautiful" also have a lot to do with technology.
Membranes don't become more efficient when they are very large, and having to use lifting equipment to change membranes often doesn’t make practical sense. Currently, a single axial piston pump delivers the best energy efficiency at flow rates of only up to 1,800 CMD.
Research and development investments continue to change the game, however.
New membrane technology brings additional efficiencies or higher productivity, and positive displacement pumps for SWRO continue to grow in size as manufacturers overcome the technological difficulties of producing them at higher flow rates.
Procurement strategy
Standard, off-the-shelf components are not only less costly than made-to-order parts, but they also come with shorter lead times. Very few companies make very large centrifugal pumps.
Suppliers must be considered not only for their component and maintenance costs, but also for their lead times for standard and purpose-built components and their future reliability.
