The two materials
Polypropylene (PP) housings are the standard for municipal and general light-industrial water: injection-moulded head and sump in 2.5" standard and 4.5" Big Blue formats. A clear sump is usually not polypropylene at all — it is normally styrene-acrylonitrile (SAN), moulded clear, and it carries its own chemical and ultraviolet limits: check the housing's own data sheet rather than assuming the bowl shares the head's compatibility. PP has excellent chemical compatibility with chlorinated water, hypochlorite and most water treatment chemicals, and is the economical choice for its pressure class — typically rated to 6 – 8 bar, and only up to a stated maximum temperature — commonly 38 °C, some grades 52 °C.
Stainless steel housings take over where PP runs out: higher pressures, hot water, steam-adjacent duty, mechanical abuse, hygiene-critical process lines, and multi-round formats for industrial flows. Single-round stainless housings mirror the standard cartridge formats; multi-round housings carry 3 to 20+ cartridges behind one set of pipework connections.
Where PP stops and stainless starts
| Condition | Choice |
|---|---|
| Municipal/borehole water, ≤ 6 bar, ambient temperature | Polypropylene |
| Pressure above ~8 bar (pump dead-head included) | Stainless |
| Water above ~40 °C (or the housing's stated maximum, commonly 38 °C) | Stainless |
| Outdoor, sun-exposed installation | Stainless preferred — UV embrittles PP over years |
| Impact/mechanical abuse likely (plant rooms, forklifts) | Stainless |
| Flows above ~5 m³/h | Stainless multi-round |
| Food, beverage, pharmaceutical contact | Stainless (usually 316) |
Two PP-specific reminders: a PP housing carries one pressure rating, valid only up to its stated maximum temperature — commonly 38 °C, some 52 °C — and above that temperature the sump creeps and can burst at pressures it holds easily when cold, so do not use it there at all; and PP fatigues under water hammer, so fit arrestors or slow-closing valves on systems with solenoid valves cycling.
304 versus 316: what the grades mean
Both are austenitic stainless steels; the difference is that 316 adds 2 – 3% molybdenum, which specifically resists chloride attack — the pitting and crevice corrosion that chlorides inflict on stainless.
- 304 is the general-purpose grade: fine for treated municipal water at modest chlorides, indoor plant rooms, and general industrial duty. It is the economical grade where chlorides are low.
- 316 is the correct grade wherever chlorides are elevated or persistent: coastal installations (airborne salt attacks from the outside), brackish and borehole waters with high chloride, and hot water — chloride pitting accelerates sharply with temperature. It is also the default in food and pharmaceutical service. Note where 316 stops: it is a raised chloride tolerance, not a salt-water grade. Saturated softener brine and seawater are both far beyond it, and so is any warm high-chloride stream. Those duties are specified in a super-austenitic or duplex grade, or in a non-metallic material, on the manufacturer's own corrosion data for the actual chloride, temperature and oxygen — not by moving up from 304.
The failure mode to respect is not gradual rusting but pitting: chloride attack drills small deep holes while the surface still looks bright. That is why the grade decision is about the chloride number on the water analysis (and the salt in the air), not about how corrosive the water looks. As a working trigger, treat sustained chlorides above roughly 200 mg/L — or any coastal outdoor installation — as 316 territory.
The details that cause leaks
- Size on cartridge flow, not connection size. A 20" housing on a 25 mm line still limits you to what the cartridge can pass; the housing connection tells you about the pipework, not the duty.
- O-rings are the service item. EPDM is standard for water and hypochlorite; Viton (FKM) for hydrocarbons and some acids — never Viton on hypochlorite. Keep a spare O-ring with every housing; it is the part that fails at 17:00 on a Friday.
- Multi-round beats parallel single rounds above ~5 m³/h: one vessel, one gasket path, one differential gauge, and cheaper per cartridge slot than a bank of singles with its manifold of potential leaks.
- Fit pressure gauges before and after. Differential pressure is how you know when to change cartridges; without gauges you are changing on guesswork.
Rule of thumb: polypropylene until pressure, temperature or abuse says otherwise; 304 stainless for clean inland duty; 316 the moment chlorides, coast, heat or hygiene enter the conversation. And whatever the shell, put a gauge either side of it.