What Could Be Causing an RO System to Shut Down Due to a “Low Inlet Pressure” Alarm?| Insights by AQUALITEK

Introduction

A sudden shutdown of a reverse osmosis (RO) system accompanied by a “low inlet pressure” alarm is a frequent and frustrating issue in water treatment operations.

This alarm is not a fault by itself—it is a protective signal indicating that the system is operating outside safe hydraulic conditions. If ignored or misjudged, it may lead to:

•High-pressure pump cavitation

•Membrane damage

•Unstable system operation

This article explains what causes low inlet pressure alarms, how to systematically diagnose them, and what corrective actions should be taken.

What Does “Low Inlet Pressure” Mean in an RO System?

“Inlet pressure” typically refers to:

•Pressure at the high-pressure pump suction, or

•Pressure at the RO feed inlet, depending on system design

When pressure falls below the preset safety threshold:

•The PLC or pressure switch triggers an alarm

•The system shuts down to protect critical components

Most Common Causes of Low Inlet Pressure Alarms

1. Insufficient Raw Water Supply Pressure

This is the most frequent cause.

Possible reasons include:

•Municipal supply pressure drop

•Well pump malfunction

•Upstream booster pump failure

•Insufficient water level in raw water tank

If the feed source cannot provide stable pressure, the RO system cannot operate safely.

2. Blocked or Fouled Pretreatment Components

Pretreatment resistance directly affects inlet pressure.

Common problem points:

•Clogged multimedia filters

•Fouled activated carbon filters

•Plugged cartridge (security) filters

•Excessive pressure loss across pretreatment units

A rising differential pressure across pretreatment is a strong warning sign.

3. Cartridge Filter Incorrect Installation

Improper cartridge filter installation can cause:

•Partial blockage

•Filter collapse

•Bypass channeling that confuses pressure readings

Also check:

•Incorrect micron rating (too fine)

•Deformed sealing O-rings

4. Feed Valve or Pipeline Restrictions

Hydraulic restrictions often go unnoticed.

Check for:

•Partially closed inlet valves

•Mispositioned automatic control valves

•Obstructed strainers

•Pipeline scaling or internal debris

Even a slightly closed valve can trigger low-pressure alarms under high flow demand.

5. Air Entrapment in the Feed Line

Air in the system can:

•Reduce effective pressure

•Cause unstable pressure signals

•Trigger false low-pressure alarms

Common air entry points:

•Leaking suction fittings

•Empty or low raw water tanks

•Inadequate venting after maintenance

6. Feed Pump Malfunction

If the RO system relies on a feed or booster pump:

•Pump wear

•Impeller damage

•Seal failure

•Motor speed reduction

may result in insufficient discharge pressure to the RO inlet.

7. Pressure Sensor or Switch Failure

Sometimes, the pressure is normal—but the signal is wrong.

Possible issues:

•Blocked pressure impulse line

•Sensor drift or aging

•Loose wiring

•Incorrect calibration

Always verify pressure using a manual gauge before assuming a hydraulic problem.

8. Incorrect Alarm Setpoint Configuration

Low-pressure alarm thresholds may be:

•Set too high

•Changed accidentally during commissioning or maintenance

This can cause nuisance shutdowns even when the system is hydraulically stable.

Systematic Troubleshooting Checklist

1.Verify actual inlet pressure with a mechanical gauge

2.Check raw water source pressure and tank level

3.Inspect pretreatment differential pressure

4.Confirm cartridge filter condition and installation

5.Fully open and verify inlet valves

6.Eliminate air from pipelines

7.Inspect feed pump performance

8.Validate pressure sensor accuracy and alarm setpoints

Why Ignoring Low Inlet Pressure Is Dangerous

Operating an RO system under low inlet pressure may lead to:

•High-pressure pump cavitation

•Excessive vibration and noise

•Rapid mechanical wear

•Membrane fouling due to unstable flow

The alarm exists to protect equipment—not to slow production.

Best Practices to Prevent Low Inlet Pressure Alarms

✔ Maintain pretreatment units regularly
✔ Monitor differential pressure trends
✔ Keep cartridge replacement records
✔ Install redundant pressure gauges
✔ Ensure stable raw water supply design
✔ Periodically calibrate pressure sensors

Conclusion

A low inlet pressure alarm is rarely a single-point failure. It is usually the result of upstream hydraulic resistance, supply instability, or instrumentation issues.

By understanding the root causes and following a structured troubleshooting approach, operators can:

•Restore stable operation quickly

•Avoid unnecessary membrane damage

•Reduce downtime and maintenance costs

In RO systems, stable inlet pressure is the foundation of reliable performance.