Why is system pressure high even when the purge valve is open?

Technical Troubleshooting Guide for Pump-Side Flow Restrictions

Overview

In HPLC and LC–MS systems, opening the purge (or prime) valve is intended to create a low-resistance flow path directly from the pump outlet to waste. Under normal conditions, this should reduce system pressure to near atmospheric levels. When pressure remains elevated after opening the purge valve, the system is compressing against a restriction or falsely reporting pressure.

This condition is not subtle. If the purge valve is functioning correctly and the flow path is unobstructed, pressure should collapse almost immediately. Persistent pressure indicates a mechanical blockage, a purge valve malfunction, or a pressure measurement artifact.

Rule of thumb:
If opening the purge valve does not reduce pressure close to zero, the restriction lies between the pump head outlet and the purge outlet, or the purge valve is not actually venting.

How Pressure Is Generated, Measured, and Released

Most analytical LC pumps measure pressure at or immediately downstream of the pump head outlet. This measurement reflects resistance to flow anywhere downstream of that point.

The purge valve is designed to bypass the analytical flow path by providing a short, wide-bore route to waste. With the purge valve open and a modest flow rate applied (typically 0.5–1.0 mL/min), expected pressure is:

• 0–5 bar (0–70 psi) on most systems

• Slightly higher values may occur due to sensor offset or minimal tubing resistance

If pressure remains significantly elevated (for example, >20–30 bar), the pump is compressing solvent against a restriction or the pressure signal is invalid.

Primary Root Causes

1. Restriction or Malfunction Upstream of the Purge Path

Stuck or Clogged Check Valves

• Salt precipitation, particulate matter, microbial growth, or polymerized residues can prevent check valves from opening.

• A stuck outlet check valve is especially common after buffer use.

• Typical symptoms: Rapid pressure rise at very low flow rates and little to no liquid exiting the purge outlet.

Pump Head Contamination

• Crystallized buffers (phosphate, acetate, carbonate) or viscous residues can restrict internal flow channels.

• In severe cases, the pump behaves as if dead-headed.

Damaged or Swollen Pump Seals

• Solvent-incompatible seals may swell or deform.

• Debris trapped beneath seals increases friction and causes pressure buildup without proportional flow.

Active Inlet or Proportioning Valve Faults (Quaternary Pumps)

• Debris or misalignment can limit solvent delivery and cause unstable or unexpectedly high pressure during purge.

• These faults often present as erratic pressure behavior rather than a clean pressure drop.

Note: Degasser issues alone do not typically cause sustained high pressure during purge, though severe cavitation can distort pressure readings.

2. Purge Valve or Purge Drain Path Obstruction

Purge Valve Not Fully Opening

• Solenoid or mechanical needle failure may leave the valve partially or fully closed.

• A functioning valve usually produces an audible or tactile click when toggled.

Blocked Purge Port or Drain Tubing

• Salt crystals, particulates, or collapsed tubing can block the purge line.

• Kinks or overly long narrow-bore tubing increase resistance.

Waste Bottle Backpressure

• A sealed or poorly vented waste container can generate significant backpressure.

• The waste bottle must always be vented when connected to purge lines.

Downstream Restrictions Installed on the Purge Line

• Fine-bore tubing, restrictors, or manifolds installed on the purge outlet defeat the purpose of purging.

• The purge line should be wide-bore and unrestricted.

3. Pressure Measurement Artifacts

Pressure Transducer Drift or Failure

• Calibration drift, trapped air, or solvent ingress into the sensor cavity can result in falsely high readings.

• Flow may appear normal despite high reported pressure.

Incorrect Pump Operating Mode

• Analytical run settings applied during purge into a blocked path can drive pressure spikes.

• Confirm that the system is in prime/purge mode, not gradient run mode.

Solvent Temperature and Viscosity

• Cold or highly viscous solvents increase pressure, but purge pressure should still be near atmospheric unless a restriction exists.

4. Instrument-Specific Plumbing Effects

• Some systems position the pressure sensor upstream of a partial restriction that is not bypassed by the purge valve.

• LC–MS systems may include capillaries, restrictors, or inline filters that must be excluded from the purge path.

• Always verify plumbing against the manufacturer’s flow diagram.

Step-by-Step Diagnostic Workflow

A. Immediate Functional Checks

1. Toggle the purge valve open and closed.
   Listen and feel for mechanical actuation.

2. Remove the column and any post-pump inline filters.

3. Set flow to 0.5–1.0 mL/min with the purge valve open.

Expected result:
Near-zero pressure with visible flow to waste.

Abnormal result:
Sustained pressure >20–30 bar or no visible purge flow.

B. Purge Outlet and Waste Line Inspection

• Inspect purge tubing for kinks, salt buildup, or collapsed sections.

• Confirm the waste container is vented.

• Temporarily route the purge outlet directly into a beaker to visually confirm free flow.

C. Isolate the Pump Outlet

With the purge valve open:

• Disconnect tubing directly at the pump outlet (upstream of the purge tee if possible).

Interpretation:

• Little or no flow: Internal pump restriction (check valve, pump head, seals).

• Free flow: Purge valve, purge port, or drain line obstruction.

D. Check Valves and Pump Head

Check Valves

• Remove inlet and outlet check valves.

• Inspect for debris or crystalline deposits.

• If allowed by SOP, clean ultrasonically in filtered water followed by IPA, then dry.

• Replace if flow remains inconsistent.

Pump Seals and Plunger

• Inspect for wear, swelling, scoring, or corrosion.

• Replace seals if any damage is visible.

• Lubricate only if specified by the manufacturer.

Solvent Channel Priming

• Prime each solvent channel individually.

• Use low-viscosity solvents first (water, then methanol or acetonitrile).

• Recovery after switching solvents often indicates buffer precipitation.

E. Purge Valve Assembly

• Inspect the valve seat and needle for salt deposits or particulates.

• Verify electrical connections and actuation.

• Replace the purge valve cartridge or assembly if cleaning does not restore normal venting.

F. Pressure Transducer and System Electronics

• If purge flow is clearly normal but pressure remains high, suspect sensor error.

• Perform zeroing or calibration per service procedures.

• Replace the transducer if calibration fails or readings remain unstable.

G. LC–MS-Specific Considerations

• Confirm that the purge path bypasses the ESI capillary or MS restrictor.

• Temporarily remove post-pump inline filters or tee manifolds during diagnosis.

• Incorrect plumbing can force purge flow through the MS interface, sustaining high pressure.

Corrective Actions Summary

• Clean or replace pump check valves (most common resolution).

• Clear or replace purge valve and purge drain tubing.

• Vent waste containers properly.

• Replace worn pump seals and inspect plungers.

• Flush buffer residues thoroughly:
  Water → IPA → Water → Starting mobile phase

• Calibrate or replace the pressure transducer if readings conflict with observed flow.

• Verify purge plumbing provides a true low-resistance path to waste.

Preventive Best Practices

• Filter solvents through 0.2 µm membranes and use high-purity salts.

• Avoid non-volatile buffers in LC–MS when possible.

• Allow cold solvents to equilibrate to room temperature before priming.

• Perform routine post-buffer flushing with water and organic solvent.

• Keep purge tubing short, wide-bore, and unobstructed.

• Replace pump seals and check valves on a preventive schedule.

Rapid Diagnostic Decision Tree

Purge open, no liquid at waste

• Check valve actuation → Clean or replace purge valve → Inspect drain tubing → Vent waste bottle

Purge open, liquid flowing, pressure remains high

• Suspect pressure transducer → Calibrate or replace sensor

Purge open, no flow at pump outlet

• Clean or replace outlet check valve → Inspect pump head and seals

Issue occurs only after buffer use

• Salt precipitation → Extended water flushing → Improve buffer handling and filtration

Brief Summary

High system pressure with the purge valve open indicates a restriction within the pump or purge pathway, a non-functioning purge valve, or an inaccurate pressure measurement. Diagnosis should focus on confirming actual purge flow, isolating the pump outlet, verifying waste venting, and inspecting check valves and seals. In most cases, restoring normal operation requires cleaning or replacing check valves, servicing the purge valve and drain line, or correcting sensor drift.