Why more and more mid-size SWRO plants use multiple trains

Increasingly, mid-size plants are relying on multiple trains running in parallel rather than a single train. Why? According to our customers and contacts in the industry, this is because the advantages of multiple-train plants – cost, speed and uptime – increasingly outweigh the challenges.

Multiple-train plants run two or more modular trains that are typically pre-built and pre-tested by the system builder, then shipped to the plant site. Some system builders also build and ship SWRO trains in containers that later also function as on-site buildings.

At first glance, it might seem paradoxical that multiple trains can result in significant cost savings compared to single-train plants. After all, this means having two or more of everything except intake systems, and this should result in overall costs that are higher, not lower. But what we hear from our customers is just the opposite – multiple modular trains reduce total costs of ownership. They give a number of reasons for this:

• Minimized design costs: System builders save skilled man-hours when they produce multiple trains based on pre-designed modules. Pre-designed modular SWRO trains mean that instead of the design process starting from scratch for every new train or even plant, only minor adjustments need to be made on already-proven designs.
  
  
• Economies of scale: Serial production of SWRO trains, in a purpose-built facility, is simply more efficient than one-off assembly on-site. Similarly, procurement and project management costs can be significantly reduced when system builders specify and even stock components, and reduce supplier complexity.
  
  
• Minimal freight costs: Modular trains are typically designed with shipping in mind, and make efficient use of space.
  
  
• Minimal on-site installation costs: Pre-fabricated and pre-tested, most modular trains are more or less ready to run once they are connected to the seawater intake system. In addition to saving system builders travel costs, this also removes the surprises and expenses associated with bringing a plant online in remote locations.
  
  
• Predictable maintenance costs: Of course, modular SWRO trains also require maintenance. The good news is that these are often proven systems with extensive maintenance histories – and that system builders can thus more easily foresee when, where and how maintenance is required and what it will cost.
  
  
• Reduced initial CAPEX: Whether they are build-own-operate or build-own-sell, plant owners can reduce their capital expenditure by reducing the number of trains they initially bring online, then add other trains in phases. This means that they can begin to produce and sell water – and generate cash flows – faster and with lower capital costs than they would if they built a full-capacity plant from day one. Later, as capacity needs grow and liquidity improves, additional trains can be added. In times when money is tight, this approach can mean the difference between getting a plant up, running and selling water – and no plant at all.
  
  
• Downsized components reduce costs: When a mid-size plant uses multiple smaller trains rather than one large train, they reduce the size of many mechanical and electrical components. Smaller sizes save money in more ways than one.
  
  
• Lower CAPEX: Smaller components cost less than large components. Larger components are typically made to order, and have long delivery times. Smaller components are typically off the shelf. For example, suppliers keep low-voltage electrical motors below 200 kW in stock, but only make large, high-voltage electric motors to order. The same is true for VFDs, stainless steel valves and pumps.
  
  
• Increased energy savings: For many SWRO plants, energy is the single largest cost driver over the lifetime of the facility. Thus, energy efficiency improvements quickly add up to boost overall profitability. When mid-size plants split production across multiple smaller trains rather than one large train, they enable the use of positive displacement pumps instead of centrifugal pumps. This alone can save more than 20% in energy costs.

Nobody likes to wait, and long lead times for an SWRO plant can be frustrating for both end users and plant owners. Modular SWRO trains can greatly reduce a new plant’s “time to water” for a number of reasons.

• Faster development and production: Reduced development processes, quicker procurement of major components and increased production efficiencies all add up to faster time to market for modular SWRO trains.
  
  
• Faster installation on-site: Because most modular trains are pre-fabricated and pre-tested, time spent on on-site installation and troubleshooting is significantly reduced.
  
  
• Quicker to add capacity: Increasing plant capacity is fast and easy, since it entails little more than adding a new train of the same type to existing trains.

The advantages of redundancy are built into multiple-train SWRO plants.

• Component failure and uptime: Multiple trains ensure continuous water production. Even if a component on one train goes down, the plant continues to produce water – at reduced capacity – on the remaining trains.
  
  
• Maintenance and uptime: Instead of bringing the entire plant offline during maintenance, multiple-train plants continue to produce water on all trains except the one being serviced.

Multiple-train plants also have their drawbacks and limitations. Fortunately, these are predictable, so system builders and operators can calculate their impact in project planning and total cost of ownership considerations.

• 100% customization is not possible: Unlike other industries, there are few agreed standards in the SWRO world regarding train capacities and sizes. System builders who rely on multiple trains make some tradeoffs. For example, they may have to compromise when it comes to dimensioning overall plant capacity. Total plant volume will always be some multiple of individual train volumes, so once module capacity is determined, the system builder may have to “round up” or “round down” rather than build to the exact volume originally planned.
  
  
• Component procurement comes in multiples: Multiple trains mean more of everything. System builders must order and stock more components, and system operators must keep more spare parts on hand. Total cost of ownership calculations must take this into account as well.
  
  
• Installation can be complex:
  
  
  • If multiple trains are not pre-built and pre-tested, the installation process can be more complicated than the installation of a single train. There are more parts that can fail, and additional troubleshooting can be required to get a new plant up and running.
    
    
  • The physical logistics of the installation process can also provide special challenges. For example, if a plant is using containerized trains, large cranes and forklifts are required to lift them into place. This is no problem if the plant site is designed accordingly, but moving heavy containers around a site that is not prepared for it can be difficult and expensive.

No two SWRO plants are exactly the same, and many variables must be considered before the total cost of ownership of multiple-train and single-train plants can be reliably compared.

Nonetheless, from what we hear from our customers, the advantages of multiple-train plants seem to increasingly outweigh their challenges – and the trend toward phased, multiple-train plants seems to be on track for the foreseeable future.

Palle Olsen is an application expert at Danfoss High Pressure Pumps with more than 15 years of experience within seawater desalination.

A member of the original team that developed Danfoss's first APP pumps and the iSave energy recovery device, Palle continues to provide input to R&D and sales. Palle is based in Denmark, and is a regular contributor to and participant in SWRO conferences around the world.