Wet-end materials science

Heads, diaphragms, valves and elastomers for metering pumps. A selection window — not an ASTM chart, not a warranty, not an offer.

Ratings assume typical water-treatment concentrations at ambient temperature, one chemical at a time. Mixtures, heat, slurry and impurities change the answer. Engineering confirms on the quote.

Liquid end

What actually wets

Pump head

The chamber that sees the chemical. PVC, PP, PVDF or a metal alloy. The rest of the pump can be painted steel — this cannot.

Diaphragm

PTFE is chemically almost inert but creeps under load. A FKM or EPDM backing takes the mechanical stress. The process sees only the face. Hydraulic oil on HD stays behind that sandwich.

Balls and seats

Check valves set accuracy as much as the stroke. Ceramic and PTFE balls survive oxidizers. SS balls pit in chlorides. Viscous polymer can stick a ceramic ball — that is a rheology problem.

O-rings

The weakest link is often a two-dollar elastomer. FKM for bleach and oils. EPDM for caustic, steam and polymer. PTFE envelope gaskets when neither rubber is honest.

Plunger & packing

Packed plungers are not sealless. Packing weeps by design. Ceramic or Hastelloy rod, compatible packing, lantern ring. Choose HD when leak-free matters more than 350 bar.

Peristaltic tube

On PR the tube is the entire wet-end. No valves to clog on sludge or polymer. Fatigue life is the spare-parts plan. Norprene for chemicals, silicone for sanitary short runs.

The science

Why the polymer or alloy matters

Pump head, valves, day tanks

PVC

Rigid vinyl. Polar, resists dilute mineral acids, hypochlorite and coagulants. Aromatics, ketones and many solvents craze it.

Service window: About 0–50 °C. Softens above that.

Typical failure: UV and NaOCl embrittlement over 12–24 months. Not a long-life bleach head — PVDF is preferred.

Heads, tanks, frames

PP

Polyolefin. Strong in alkalis and many acids. Weak against oxidizers at concentration and against chlorinated solvents.

Service window: About 0–80 °C.

Typical failure: Stress cracking, UV, concentrated bleach.

Preferred plastic head for oxidizers

PVDF

Fluorinated vinylidene polymer. Resists halogens, HCl, ferric chloride and industrial NaOCl. Hot concentrated NaOH attacks it.

Service window: About −20–120 °C. Caustic service much lower.

Typical failure: 50% NaOH near 25 °C is already a limit. Confirm concentration and temperature.

Diaphragm face, liner, seats, tape

PTFE

Near-inert fluoropolymer. Process chemicals see PTFE; the metal or elastomer behind it carries load. Not for elemental fluorine or molten alkali metals.

Service window: Process face often −20–150 °C. Limited by the backing, not the skin.

Typical failure: Cold flow (creep) without a backing diaphragm. ClO₂ can permeate a thin PTFE face.

Heads, plungers, clean-water hardware

SS316

Austenitic stainless with Mo for pitting resistance in mild chlorides. Passivated by nitric and aerated water. Reducing acids and free chlorine break the film.

Service window: Wide mechanically. Chloride SCC risk rises with heat.

Typical failure: Pitting, crevice corrosion and stress-corrosion cracking in NaOCl, HCl, FeCl₃. Do not specify 316 for bleach.

High-chloride metal head and plunger

Hastelloy C-276

Ni–Cr–Mo alloy. Holds mixed acids and chlorides that destroy 316. Default metal for seawater RO antiscalant and high-temp acid.

Service window: Process often −20–200 °C. Confirm the mix.

Typical failure: Few exceptions in water treatment. Still confirm oxidizing + reducing cocktails.

Valve balls, plunger coating

Ceramic (Al₂O₃)

Sintered alumina. Hard, chemically quiet in acids, bleach and coagulants. Hydrofluoric acid and strong thermal shock are the enemies.

Service window: Well above polymer heads. Limited by seats and body.

Typical failure: Chipping on impact. Viscous polymer can wet-seat and stick the ball.

O-rings, seats, diaphragm backing

FKM

Fluoroelastomer. Oils, many acids and NaOCl. Swells in ketones, amines and strong caustic.

Service window: About −20–200 °C in compatible media.

Typical failure: NaOH above ~5–15% is a common misspec. Use EPDM on caustic.

O-rings, backing, polymer service

EPDM

Ethylene-propylene rubber. Water, steam, dilute acid and caustic. Oils and oxidizers attack the unsaturation.

Service window: About −40–150 °C.

Typical failure: NaOCl is a life question, not a yes. Field failures are common — FKM or PTFE on bleach.

Peristaltic tube

Norprene

Thermoplastic elastomer tube. The tube is the wet-end: no seals, no valves. Acids, bases, food-grade options. Not a solvent hose.

Service window: About 0–80 °C. Fatigue, not melt, sets the interval.

Typical failure: Wall fatigue and abrasion. Plan a 6-month tube change on continuous duty.

Sanitary peristaltic tube

Silicone

Platinum-cured silicone. Clear, food and pharma. Poor against oxidizers, many acids and solvents.

Service window: Wide, but pump life is short at speed.

Typical failure: Swelling and tearing in bleach and strong acid. Use Norprene or PTFE tube instead.

Duty chemicals

Compatibility window

5–13% available chlorine, ambient, off-gassing

Sodium hypochlorite

Off-gasses O₂ and Cl₂. Heat and metals accelerate decay. Auto-degassing heads (EM / DX) prevent vapour lock. Keep the tank cool and vented.

Typical liquid end

PVDF or PTFE-lined head · PTFE-faced diaphragm · ceramic or PTFE balls · FKM or PTFE seals. Not SS316.

Pump types for this duty

Electromagnetic Smart digital Motor diaphragm

Heads & metals

PVCLimits

PPLimits

PVDFPreferred

PTFEPreferred

SS316Avoid

Hastelloy C-276Preferred

Seals, balls, tubes

PTFEPreferred

FKMPreferred

EPDMLimits

Ceramic (Al₂O₃)Preferred

NorpreneLimits

PVC	PP	PVDF	PTFE	SS316	Hastelloy C-276
Limits	Limits	Preferred	Preferred	Avoid	Preferred
Preferred	Preferred	Limits	Preferred	Preferred	Preferred
Limits	Limits	Preferred	Preferred	Avoid	Preferred
Preferred	Preferred	Preferred	Preferred	Avoid	Limits
Preferred	Preferred	Preferred	Preferred	Avoid	Limits
Preferred	Preferred	Preferred	Preferred	Limits	Limits
Preferred	Preferred	Preferred	Preferred	Preferred	Preferred
Limits	Limits	Preferred	Preferred	Limits	Preferred
Limits	Limits	Preferred	Preferred	Limits	Preferred
Limits	Avoid	Preferred	Limits	Avoid	Preferred

Seals, balls, tubes

PTFEPreferredFKMPreferredEPDMLimitsCeramic (Al₂O₃)PreferredNorpreneLimits

Related models

Metering Pump

DFD

Electromagnetic Diaphragm Metering Pump

Flow range: 1–55 L/h
Max pressure: 20 bar
Pump head: PP / PTFE / PVDF / SS316
Supply: 220 V 50 Hz