By Elizabeth A. Adams

Seawater is used to make drinking water in many parts of the world. Some treatment facilities are very large and supply municipal water for an entire city. Other installations serve small, regional communities, and even smaller installations are used to serve mobile, drinking water needs like those on cruise ships. While all installations require reliable, efficient and cost-effective filtration solutions, mobile installations—due to the highly self-sufficient nature of such operations—are perhaps even more reliant on these factors, as the filtration systems are their only method of supplying clean drinking water to passengers during lengthy excursions.

Before
The systems to produce potable water generally fall under WHO jurisdiction, although the actual regulatory body varies from country to country, depending on where the cruise ship is operated and registered. US-based ships fall under USPHS (US Public Health Service) guidelines.

A British water purification company has focused on the cruise ship industry to improve filtration efficiency and save on capital costs. One of its key customers recently replaced a bulky housing and numerous string-wound cartridges with a single fiberglass industrial system for prefiltration of blue seawater (open ocean water, away from shorelines and bays), prior to RO membrane treatment. Conventional RO prefiltration systems use 2.5-inch (63.5-mm) O.D. cartridges that are typically 30 to 40 inches (76.2 to 101.6 cm) long. The filters are constructed from a variety of materials, from melt-blown fibers to cotton or polypropylene string-wound cartridges. Cartridges are typically installed in metallic housings. Generally, prefilters remove algae, particulate and sea vegetation. Occasionally, the filters will need to remove large, floating debris as well. The supplier needed to provide a cost-effective, prefiltration system for the RO membranes that was as lightweight and compact, and with as few change-outs monthly as possible.

Since seawater is highly corrosive to many materials, including steel, housings used for these applications are fabricated from exotic metals, such as hastelloy (a highly corrosion-resistant super alloy, primarily made of nickel), and rubber lined or coated with special materials to tolerate long-term exposure to high-salinity seawater. The use of these materials makes housings designed for seawater applications very expensive compared to some alternative systems.

Before
In addition to high purchase cost, conventional housings have many operating disadvantages:

  • Require tools to open numerous swing or eye-bolt closure fittings
  • Housing internals can be lost or damaged, compromising seal efficiency.
  • Numerous, small cartridges require extended change-out time.
  • Large housings consume valuable production footprint and can be cumbersome for operators.
  • Varying efficiency elements provide inconsistent removal and on-stream life.
  • Line flow is interrupted for change-out cycle.
  • Numerous spare sets of cartridges need to be kept in inventory for unexpected change-outs.

The cruise ship corporation previously used fifteen 30-inch, string-wound cartridges for filtration at 150 gpm (35 m3/hr). The cartridges were housed in a coated, metallic vessel designed to tolerate highly corrosive seawater salinity. This steel housing had an installed weight of approximately 480 lbs. (217.7 kgs) and required a height clearance of approximately 56 inches (142.2 cm) for cartridge change-out. At least one spare set of 15 cartridges was kept in inventory on each ship as a precaution against unplanned cartridge change-outs. By replacing the conventional cartridges with the newer system, the corporation was able to reduce costs as well as inventory, a driving factor for this project. The industrial system featured rugged fiberglass construction for long-lasting performance— even in demanding seawater environments—and a significant cost savings over exotic metal or coated metallic housings. It included large-diameter, high-efficiency elements to provide the desired removal level and long on-stream life. Plus, the compact design delivered a small footprint and low-weight installation—a key advantage on a cruise ship. Additional features included a quick-change design, allowing operators to access and change out cartridges in a matter of minutes, without disassembling nuts and bolts or interrupting the flow. A single element could be changed while the whole system stayed online, eliminating the need for ad- ditional costly downtime. The system also featured significantly fewer elements to change, purchase and keep in inventory.

After
The upgrade from conventional cartridges to an industrial system allowed the customer to dramatically reduce cartridge change-out time and system downtime, as well as the number of cartridges in inventory from 15 to one. Filtration removal efficiency was measurably improved, which had a cumulative benefit on the performance, as well as operating and maintenance costs of their reverse osmosis system. A single industrial element housing was capable of the same flow rate as a large, multi-round metal housing, but was purchased at one-third the price of a conventional-style housing. The modular design, which could be expanded to meet any flowrate requirement, also provided flexibility for easy, cost-effective expansion in the future.

About the author
Elizabeth A. Adams runs Applications Engineering for Pentair’s Industrial Filtration Group. She holds a BS Degree in biology and chemistry as well as an MS Degree in environmental. Adams has been with Pentair for one year and has over 15 years experience in technical management and engineering in the filtration industry. Her expertise includes the application of filtration products, the design of large, custom, filtration systems, and the development of new filtration products for the industrial markets.

About the company
Pentair Industrial manufactures and markets a broad range of industrial filtration products including filter cartridges, filter bags, cartridge housings and bag housings, in addition to numerous related filtration accessories. Long known for quality filtration products and patented technologies, Pentair engineers key filtration solutions for oil and gas, food and beverage, power generation, petrochemical, paint and ink, municipal, medical, pharmaceutical and other industrial market sectors. For more information, contact Pentair Industrial, 1040 Muirfield Drive, Hanover Park, IL 60133, phone (800) 869-0325.

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