What the vessel does
A membrane pressure vessel (membrane housing) is a filament-wound FRP tube holding 1 – 6 spiral-wound elements in series, with end closures engineered to seal at working pressure and open for element changes. Feed enters one end, concentrate leaves the other, and permeate collects from the interconnected element cores through a permeate port. Elements inside connect core-to-core with interconnectors; a thrust cone at the downstream end carries the hydraulic load of the whole element stack so the last element does not.
End-port versus side-port
End-port vessels take feed and concentrate through the end closures. Simple and economical — the right answer for single-vessel systems and small skids. The cost appears at element change: pipework connects to the very closures that must come off, so servicing means breaking and remaking pipe connections.
Side-port vessels move feed and concentrate to ports on the shell near each end. Manifolds on multi-vessel skids run straight and short, closures come off without disturbing pipework, and multi-element vessels service dramatically faster. The premium buys maintainability — which is why side-port is the standard above a handful of vessels, and worth considering sooner if element changes will be frequent.
Rating: pressure has a direction of travel
Vessels are rated by class — 300 psi (21 bar) and 450 psi (31 bar) for brackish duty, 600 psi (41 bar) for high-TDS brackish water where BW elements run near their 41 bar limit, and 1 000 psi (69 bar) or 1 200 psi (83 bar) for seawater, chosen against the train's maximum design pressure (SW30 elements are rated to 83 bar, so a seawater plant designed near the element's limit needs the 1 200 psi class). Rate the vessel above the maximum pressure the system can produce, not the nominal operating point. With a centrifugal pump that means its shut-off (dead-head) pressure, which a closed concentrate valve will find quickly. A positive-displacement (plunger) pump has no dead-head — its pressure rises until something yields — so it must have a pressure relief valve and a high-pressure trip, and the vessel is rated above the relief setting; never rely on the vessel rating alone. And note the asymmetry that catches people later: these vessels are designed for internal positive pressure — a vessel (like any FRP shell) has effectively no vacuum rating, so drain-down arrangements need air admittance thought through.
The details that actually cause problems
- Reused closure seals. The leading cause of permeate-side leaks. Every time a closure opens, its seals are replaced — the kits are cheap relative to one salty-permeate investigation. Lubricate with glycerine or approved silicone lube only; petroleum grease attacks both seals and membranes.
- Brine-seal direction. Each element carries a lip seal that forces feed through the element rather than around it. Fitted backwards or damaged, feed bypasses along the vessel wall — flow looks normal, rejection looks poor, and the "membrane fault" is a seal worth a fraction of the callout.
- Element loading order and orientation. Elements load in the feed-flow direction with interconnectors seated straight; forcing a misaligned stack cracks permeate tubes. Push from the feed end, never hammer.
- Format compatibility. A 40" industrial vessel does not accept residential 21" elements without engineered spacers, and 2.5"/4"/8" diameters are their own vessel classes. Confirm element format before ordering the vessel, not after.
- Permeate port pressure. Permeate lines that can be valved closed while the plant runs will over-pressurise the permeate side of the membrane — backpressure damage that looks like sudden rejection loss. Interlock or relief-protect the permeate line.
Rule of thumb: side-port once there is a manifold, rating above the pump's real ceiling (shut-off head, or the relief setting on a plunger pump) not duty point, and new closure seals every single opening. When permeate quality drops right after a service visit, suspect a seal before you suspect the membrane.