Sampling Line Not Drawing Water? Common Causes

When an online suspended solids analyzer fails to draw water, the issue typically stems from one of several mechanical or hydraulic failures. Identifying the root cause quickly is essential for maintaining reliable process control.

1. Clogged Sampling Line

The most frequent cause is a physical blockage. Sediment, biofilm, or large particles can accumulate inside the flexible tubing or rigid piping, especially if the sample contains high concentrations of solids or fibrous material. A kinked or pinched tube will also restrict flow entirely.

2. Insufficient Sample Pressure or Flow

If the sampling point is located in a pipe with low pressure or if the isolation valve is partially closed, the static head may be insufficient to push water to the analyzer. In pressurized systems, a clogged strainer or filter upstream of the analyzer can reduce pressure below the required threshold.

3. Pump Malfunction

Most analyzers rely on a peristaltic pump to draw the sample. Over time, the pump tube can wear out, lose elasticity, or rupture, leading to a loss of suction. Additionally, a failed pump motor, loose roller assembly, or incorrect pump head installation will prevent the pump from creating the necessary vacuum.

4. Leaks in the Suction Path

Air leaks are difficult to see but easy to diagnose. Any loose fitting, cracked tube, or deteriorated seal on the intake side will break the vacuum. Instead of drawing water, the pump pulls in air, causing erratic or zero flow.

5. Empty or Low Sample Source

If the water level in the sump, tank, or stream drops below the intake point, the line will draw air. This can occur due to process level fluctuations or if the intake strainer is positioned incorrectly.

6. Blocked or Clogged Intake Strainer

A strainer is often installed at the end of the sampling line to prevent large debris from entering the system. If this strainer becomes encrusted with solids or debris, it can become completely sealed, blocking water entry even if the rest of the line is clear.

7. Solenoid Valve Failure

In systems that use automated cleaning or timed sampling, a faulty solenoid valve may fail to open, preventing water from reaching the analyzer. Similarly, a valve stuck in the closed position due to debris or electrical failure will halt flow entirely.

8. Signal or Control Logic Issues

Occasionally, the problem is not mechanical but electronic. A missing start signal, a tripped circuit breaker, or a software interlock condition (such as a low-level alarm) can prevent the pump from activating.