The seawater reverse osmosis (SRO) industry has been steadily gathering momentum. For the valve industry, that growth opens up a number of opportunities for companies that understand the unique issues the industry presents.
Global Water Intelligence reported that in 2011, new construction within the desalination industry was about five million cubic meters per hour (m3/hr), forecast to increase by 140% to 12 million m3/hr by 2016. To meet the forecasted increase in demand, individuals charged with valve applications in this industry know that, more than ever, paying attention to materials selection, equipment configuration and energy efficiency are critical for improving financial returns on the sizeable investments required.
This article briefly reviews the seawater reverse osmosis (SWRO) process and broadly addresses those valve applications. It discusses materials of construction and the all-important pitting resistance equivalent number (PREN), along with several new desalination valve configurations. It also touches upon energy recovery devices (ERDs) and the critical role they play in SWRO efficiency.
It’s important to note that valves used in the SWRO process are applied to low-pressure and high-pressure services, which affect both valve configurations and the materials of construction. The ultimate goals are to maximize process efficiency, minimize maintenance and downtime, and contain or reduce energy expenses. Proper selection can improve the chances of extending the life of valves and increasing system uptime, while improving energy efficiency.
The SWRO plant is typically divided into four units: intake, pretreatment, desalination and final treatment (Figure 1).
The goal of the intake process is to remove materials that could foul or damage the efficiency of the expensive, sensitive membranes. In the intake process, raw seawater, with its high content of dissolved chlorides, enters the plant and all solids, such as silt, sand and organic materials, are removed. This is accomplished through standard sand and cartridge filters and, more recently, by ultra-filtration techniques—along with chemical pretreatment. For the process, water intake valves are typically specified with rubber-lined cast iron or ductile cast iron bodies.
This low-pressure process also requires a large number of butterfly valves ranging in size from 3 inches (80 millimeters) to 110 inches (2,500 millimeters). These valves may be specified in a wide range of configurations and in a variety of seat and disc materials, including Viton, Halar, Hypalon and PTFE, as well as other plastics and elastomers. Valve bodies are typically offered in seawater-resistant materials such as aluminum bronze, duplex stainless steel and super-duplex stainless steel. Valve sizes to 24 inches (600 millimeters) will have lug-style bodies, while larger sizes are normally flanged.