How to Prevent Vapor Lock in Sodium Hypochlorite Metering Pumps

Gas Formation Is Inherent to the Chemistry

Hypochlorite continuously decomposes, and that decomposition accelerates with higher temperature, higher concentration, UV exposure, and certain contaminants. So gas formation is inherent to the chemical — you're managing it, not eliminating it completely. (Source: Olin Sodium Hypochlorite Product Safety Management)

Keep the bleach cool and fresh. Don't buy six months of inventory because the unit price is attractive. Higher temperature and concentration substantially accelerate decomposition; Olin notes decomposition rates can increase roughly two- to four-fold for every 10°C temperature increase. (Source: Olin Sodium Hypochlorite PSM, 102-00553)

Protect the storage tank from heat and sunlight and minimize contamination, particularly transition metals, which can accelerate degradation. (Source: Olin Sodium Hypochlorite Product Safety Management)

Design the Suction Side Around Gas Escape, Not Just Flow

What I want to see on a good hypochlorite feed skid is:

• Flooded suction whenever possible. Put the pump below the normal tank liquid level. Pump manufacturers specifically recommend positive suction head for liquids that tend to outgas. (Source: ProMinent 2023 US Catalog)
• A very short, generously sized suction line. Avoid long runs, excessive fittings, high points, and little pockets where gas can accumulate.
• Sloped suction piping. Slope the suction piping so bubbles can migrate away rather than becoming trapped. ProMinent specifically recommends suction piping that is short and arranged to eliminate vapor pockets. (Source: ProMinent 2023 US Catalog)
• An auto-degassing or self-venting liquid end when the service warrants it. Those heads are specifically designed for chemicals such as sodium hypochlorite. LMI and ProMinent both describe liquid ends that automatically purge entrained gas and prevent air/vapor lock. (Source: LMI ROYTRONIC EXCEL AD Chemical Metering Pump Brochure)
• A correctly piped degassing/bleed return. The gas needs somewhere to go. A deaeration valve can return a small amount of chemical and expelled gas to the storage tank, and that return should be arranged so the gas isn't fed straight back into the pump suction. (Source: ProMinent Deaeration Valve Assembly)

One subtle point: don't automatically assume that more suction pressure solves every hypochlorite problem. You want enough positive head to fill the pump reliably, but the pump head and degassing arrangement have to be designed for that configuration. Some auto-degassing designs have specific installation requirements depending on whether you're using suction lift or flooded suction. (Source: ProMinent Proper Installation Guide)

Don't Blame the Diaphragm Before You've Ruled Out Gas

Here's the field symptom I pay attention to: if the pump runs fine after you manually bleed it, then gradually loses output, you bleed the head again and get a bunch of gas followed by chemical, I'd strongly suspect gas binding before I'd start blaming the diaphragm or calibration.

Two Feed Skid Layouts, Two Very Different Outcomes

The worst installation is usually something like: hot 12.5–15% bleach → outdoor tank → pump mounted above tank → long quarter-inch suction tube → high point in tubing → conventional pump head. That system is practically asking for trouble.

A much better arrangement is: cool/shaded tank → bottom outlet → short flooded suction → properly sized tubing → hypochlorite-rated auto-degassing pump head → vent/bleed return to tank. That is how I would approach it before trying to solve the problem electronically.

Go Deeper on Hypochlorite Feed System Reliability

Frequently Asked Questions