Where Does a Continuous Hissing Sound During RO System Operation Come From?| Insights by AQUALITEK

Introduction

During the operation of an industrial reverse osmosis (RO) system, operators may notice a continuous hissing or “air-leak–like” sound. Although the system may still appear to run normally, this sound is never something to ignore.

In practice, persistent hissing usually indicates:

•Pressure release

•Fluid leakage

•Cavitation

•Abnormal flow conditions

This article explains where the hissing sound most commonly originates, how to identify the source quickly, and what risks it may pose to the system.

Why a Continuous Hissing Sound Matters

In RO systems:

•All pressurized sections are designed to be sealed and stable

•Any continuous sound usually indicates energy loss

If left unaddressed, a small hissing sound can develop into:

•Pressure vessel damage

•Pump failure

•Membrane fouling

•Sudden system shutdown

Most Common Sources of a Continuous Hissing Sound

1. Concentrate (Brine) Control Valve or Throttle Valve

Most common and often normal—but only within limits

When the concentrate valve is:

•Partially closed

•Regulating recovery rate

High-velocity brine flow can create a:

•Continuous hissing or rushing sound

✔ Normal if:

•Sound is stable

•Pressure and flow are normal

⚠ Abnormal if:

•Sound suddenly increases

•Recovery rate has changed unintentionally

2. Pressure Relief Valve or Safety Valve Micro-Release

A pressure relief valve may:

•Partially open due to overpressure

•Leak continuously if spring tension weakens

This creates:

•Constant gas-like hissing

•Invisible pressure loss

⚠ Risks:

•Inaccurate pressure readings

•Hidden safety risk

3. Minor High-Pressure Water Leakage at Fittings

Small leaks at:

•High-pressure tubing

•Compression fittings

•Pressure gauge connections

May not produce visible dripping, but:

•High-pressure water escaping creates a sharp hissing sound

These leaks often worsen rapidly under pressure cycling.

4. O-Ring or End Plate Sealing Failure in Membrane Housing

If an O-ring:

•Is damaged

•Is misaligned

•Has lost elasticity

High-pressure feed water may escape internally or externally, producing:

•Continuous hissing

•Slight water mist

•Pressure instability

⚠ This is a high-risk fault requiring immediate shutdown.

5. Air Ingress or Cavitation at the High-Pressure Pump

If air enters the pump due to:

•Low feed pressure

•Suction line leakage

•Insufficient NPSH

The pump may produce:

•Hissing

•Crackling

•Whistling sounds

⚠ Long-term cavitation can cause:

•Impeller damage

•Bearing failure

•Rapid pump wear

6. Automatic Vent Valves or Air Release Valves

Some systems include:

•Automatic air vents

•Degassing valves

If these are:

•Stuck partially open

•Malfunctioning

They may continuously vent air or water vapor, producing:

•A steady hissing noise

7. Chemical Dosing Injection Points

At scale inhibitor or reducing agent injection points:

•High-velocity flow through injection quills

•Improper non-return valve operation

May generate:

•Localized hissing sounds

⚠ This may indicate:

•Backflow risk

•Chemical dilution instability

How to Quickly Identify the Source

✔ Walk along the system and listen closely
✔ Compare sound intensity near valves, pumps, and vessels
✔ Lightly spray suspected fittings with water (sound change indicates leak)
✔ Observe pressure gauge stability
✔ Temporarily reduce system pressure (if safe) and check sound changes

When Is Immediate Action Required?

Stop the system immediately if:

•Hissing sound suddenly appears

•Sound increases rapidly

•Pressure fluctuates abnormally

•Water mist or vibration is detected

Never assume a new hissing sound is “normal.”

Best Practice Recommendations

✔ Record sound changes as part of daily inspection
✔ Treat unusual sounds as early warning signals
✔ Train operators to distinguish normal valve noise vs. fault noise
✔ Never operate long-term with unexplained hissing

Conclusion

A continuous hissing sound during RO system operation is not merely a noise issue—it is a pressure and reliability issue. While some sounds originate from normal flow regulation, many indicate leakage, cavitation, or sealing failure.

Early identification and correction:

•Prevent major equipment damage

•Extend membrane and pump lifespan

•Improve overall system safety

In RO operation, sound is data—ignore it at your own risk.