The wetted path
Every part the chemical touches — pump head, diaphragm face, seals and O-rings, valve balls and seats, tubing, injection quill — must individually tolerate the chemical, at its dosing concentration, at the installation's temperature. Compatibility is a chain, and the weakest link decides: a PVDF head with the wrong O-ring fails exactly as surely as the wrong head.
The good news is that water treatment's standard offering is already strong: PVDF heads (broadly resistant across hypochlorite, acids, alkalis and antiscalants), ceramic valve balls, PTFE-faced diaphragms — usually with a choice of seal elastomer. Which brings us to the decision that actually goes wrong.
EPDM versus FKM: the seal choice
The two standard elastomers split the chemical world between them, and they are not interchangeable:
- EPDM — the choice for oxidants and alkalis: sodium hypochlorite, caustic soda, and general aqueous duty. Weakness: oils, hydrocarbons, concentrated mineral acids.
- FKM (Viton) — the choice for acids and hydrocarbon-adjacent chemistry: sulphuric, hydrochloric, nitric acids, and oil-contaminated environments. Weakness: hypochlorite and other strong oxidising bleaches, hot alkalis, some ketones/amines.
The bolded weakness is the hypochlorite trap: FKM has the premium reputation ("Viton handles everything"), so it gets fitted by default — and hypochlorite attacks it. The seals harden, crack and weep over weeks to months; the pump develops mystery leaks and lost prime; the diagnosis blames the pump. The rule is blunt and worth memorising: never FKM on hypochlorite — hypochlorite gets EPDM. Understand what kind of rule it is. "FKM" and "EPDM" name families, not compounds, and a pump maker's resistance table rates a specific compound against a specific concentration and temperature — some FKM grades are listed as usable in dilute hypochlorite, and no EPDM is unconditional either. The blunt version is deliberately conservative because the field cannot tell the grades apart and the failure is slow, expensive and easy to misdiagnose. For anything outside ordinary 10–15% sodium hypochlorite at ambient temperature, work from the pump manufacturer's own resistance table for the compound they will actually ship, at your concentration and temperature. When a site doses both an acid and hypochlorite, that is two pumps, two suction lines and two injection points — never one pump with a seal change. Acid and hypochlorite must never share a pump, tubing, injection point, day tank or bund: mixing them releases chlorine gas. If a pump is ever repurposed between the two, flush the entire wetted path with clean water first.
Chemical-specific footnotes worth knowing: hypochlorite also off-gasses (a pump-mechanism problem — see the pump-types article) and its strength decays in heat and sunlight, so store it cool and dark and size doses for real strength. Concentrated sulphuric acid is a special case — dilution is violently exothermic (always acid into water), and above ~90% concentration its compatibility profile shifts; check the manufacturer's chart rather than assuming. Polymers and antiscalants are usually gentle on materials but punish stagnation and shear — follow the maker's dilution guidance.
PTFE, and the rest of the installation
Where genuine uncertainty remains, PTFE is the near-universal answer for seats and liners — chemically inert to practically everything dosed in water treatment — at a cost and mechanical-suppleness penalty that keeps it from being the default O-ring material. And extend the check past the pump: suction tubing, foot valve, injection quill and the injection valve's own seal live in the same chemical. PVC handles most aqueous duty; PE/PTFE lines suit aggressive service. The injection point itself sits in concentrated chemical at the moment of injection — the quill seal fails first on marginal material choices.
Finally: compatibility charts are the authority, not folklore. Every reputable pump manufacturer publishes one covering their head/seal/valve options per chemical and concentration. Two minutes with the chart at specification time is the entire cost of getting this permanently right.
Rule of thumb: PVDF head plus the correct elastomer — EPDM for hypochlorite and alkalis, FKM for acids, never FKM on hypochlorite — and when in doubt, PTFE and the manufacturer's chart. State the chemical and concentration on the order and the right materials arrive by default. Acid and hypochlorite never share a pump, a line or an injection point.