Why does a sodium hypochlorite metering pump lose prime when the pump is not damaged?

Sodium hypochlorite decomposes on its own and releases gas. Tacmina publishes the decomposition as 2NaClO to 2NaCl + O2 and describes hypochlorite as a chemical that readily decomposes into a gas. Blue-White describes the same behavior from the pump side, noting that chemicals which offgas can create air pockets that cause vapor locking. A reciprocating metering pump moves a fixed displacement per stroke and depends on an incompressible liquid; when a gas pocket occupies the pump head, the stroke compresses and expands that pocket instead of pushing chemical through the discharge check. Tacmina calls the end state gas lock, where no liquid is discharged at all even with a reciprocating diaphragm.

Why is hypochlorite vapor lock worse at low dosing rates?

A published US wastewater plant case reports pumps that worked satisfactorily at higher chemical flow rates but repeatedly vapour locked at lower flows. At a low stroke rate, gas enters the pump head at the rate the chemistry produces it while the pump sweeps the head far less often, so gas production outpaces gas removal. Blue-White describes the same mechanism when it warns that on an oversized or too-slow pump, gas can accumulate in the pump head without being pushed through. This is why installing a larger pump usually makes the problem worse rather than better.

What field symptom separates gas binding from a failed diaphragm?

If the pump runs correctly right after you manually bleed it, then gradually loses output, and bleeding the head again produces a slug of gas followed by chemical, suspect gas binding before the diaphragm or the calibration. A failed diaphragm does not restore full output when you bleed it.

How does the storage tank outlet affect a hypochlorite feed problem?

Poly Processing names iron, copper, cobalt and nickel as accelerators of hypochlorite degradation, and its own general specification for 9 to 15 percent sodium hypochlorite states that an IMFO outlet is strongly recommended for full drainage of sodium hypochlorite to prevent sediment build up. Poly describes the Integrally Molded Flanged Outlet as a homogenous flange system molded as part of the tank, positioned at the bottom of the sidewall below the tank knuckle radius, which it says gives full tank discharge. A tank that never fully drains keeps a sediment heel where those metals concentrate, and a decomposing heel is a gas source feeding the same suction line.

Can a peristaltic metering pump vapor lock in hypochlorite service?

Blue-White states that peristaltic pumps are very effective when dosing fluids that contain trapped gases because they are not affected by air bubbles, that bubbles simply pass through the pump tube, and that there is no vapor lock and no loss of prime. On the FLEXFLO A3 specifically Blue-White states it cannot vapor lock or lose prime. Both are manufacturer claims with no published test method behind them and should be treated as such. Blue-White does not make a hypochlorite-specific vapor-lock claim on its A3 product pages; the hypochlorite link comes separately from its Flex-A-Prene tube compatibility statement, which describes the tube as chemically resistant to 25 percent sodium hypochlorite.

Free Field Blueprint — Sodium Hypochlorite

The Hypochlorite Suction-Path Blueprint

The bleach pump stops feeding, gets replaced, and the problem comes back. This is the 12-page blueprint for why that happens, what to check first, and the two spec decisions that keep the gas out of the pump head.

✓The five-step suction-path check, in order — each step cheaper than the one after it, and each one defeats the ones below it if it's wrong

✓Why it gets worse at low dose rates, not high ones — and why a bigger pump makes it worse

✓Storage side: the Poly Processing Vertical IMFO outlet — including the line from Poly's own hypochlorite spec, the SG rating, and the vent-sizing rule

✓Feed side: the Blue-White FLEXFLO A3 — published specs, and why the tube compound, not the model number, sets your pressure and temperature limits

✓A print-ready pre-PO checklist — plus a plain list of what these manufacturer sources do not actually say

Built for water and wastewater operators, plant and process engineers, and design engineers specifying chemical feed systems. Every technical statement is attributed to a named manufacturer source. No LibertyCES project, customer, lead time, or cost figure appears anywhere in it.

Free PDF — 12 Pages

Get the Blueprint

Enter your work email and the PDF opens immediately. No cost, and no sales call attached to it.

What's inside

01 · The failure is a fluid property

What hypochlorite does on its own, and the four things that accelerate it — every one of them a storage or layout decision, not a pump decision.

02 · Why the pump reads gas as dead

Gas lock explained from the displacement side, the two separate paths to it, and the field symptom that separates it from a failed diaphragm.

03 · The five-step check, in order

Flooded suction, line length and size, high points, liquid end selection, and where the purged gas actually goes.

04 · Storage: Poly Processing Vertical IMFO

The outlet position, the sediment argument in Poly's own spec language, the SG rating, vent sizing, gaskets, and why the schedule clock starts at drawing approval.

05 · Feed: Blue-White FLEXFLO A3

Published flow, turndown, pressure, temperature and tube-compound data — and where the 2,500:1 turndown does real work against the low-flow failure mode.

06 · What these sources do not say

Nine places where a claim is commonly made and the manufacturer source does not support it. Read this before quoting anything.

James Riggins

LibertyCES — Liberty Chemical Equipment & Supply

30+ years in industrial chemical systems, reviewing real process conditions before a part number ever gets recommended. Send the tank location, purchased concentration, dose range, and the suction run as it is actually piped — that's enough to tell whether you have a suction-path problem or a selection problem. No charge to look at a spec before the PO goes out.

(559) 395-5500 ·[email protected] ·[email protected] ·libertyces.com