Pressure Vessels

In a Reverse Osmosis (RO) unit, pressure vessels are cylindrical housings that contain and support the semi-permeable membrane elements. They are engineered to withstand the high operating pressures required to force feedwater through the membranes, separating purified water (permeate) from concentrated brine (reject). Their primary role is to provide a secure, pressurized environment for the filtration process while protecting the delicate membrane elements from physical damage and contamination.

Pressure vessels are typically constructed from materials such as Fiberglass-Reinforced Plastic (FRP), Stainless Steel (e.g., 316L), or Duplex Stainless Steel, chosen for their corrosion resistance and mechanical strength. Standard diameters include 4-inch (approx. 101.6 mm), 8-inch (approx. 203.2 mm), and larger industrial sizes, with lengths typically ranging from 1 to 6 meters to accommodate multiple membrane elements in series. These dimensions are reference ranges; the exact specifications must be confirmed for the specific model and application.

When selecting a pressure vessel, key inputs include the required operating pressure, feedwater chemistry, temperature, and the number and type of membrane elements to be housed. The vessel must be compatible with the system's piping and connections, and its pressure rating must match the system's design. Verification questions for procurement include: What is the maximum allowable working pressure? What are the end connection types? What materials are wetted parts made of? Are there any specific standards or certifications required for the application?

Maintenance signals include visible corrosion, cracking, or leakage at seals or fittings. Failure boundaries are defined by the vessel's pressure rating and material limits; exceeding these can lead to catastrophic failure. Regular inspection and pressure testing are essential to ensure integrity. Always verify model-specific values and standards with the legal manufacturer or supplier.

The pressure vessel receives pressurized feedwater from the high-pressure pump. This water is forced longitudinally through the vessel, passing over the surface of the spiral-wound membrane elements housed inside. The applied pressure (typically 50-1000 psi, depending on application) overcomes the natural osmotic pressure, driving water molecules through the membrane while rejecting dissolved salts and contaminants. The purified permeate is collected centrally, while the concentrated reject stream exits the vessel's end.