Why System Pressure Remains High with the Purge Valve Open in HPLC and LC

Why System Pressure Remains High with the Purge Valve Open in HPLC and LC–MS
Technical Troubleshooting Guide for Pump-Side Flow Restrictions
A comprehensive diagnostic approach to resolving persistent pressure issues in analytical liquid chromatography systems.
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:

Normal Systems

0–5 bar (0–70 psi) on most systems

Acceptable Range

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
01
Toggle the purge valve open and closed
Listen and feel for mechanical actuation.
02
Remove the column and any post-pump inline filters
03
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
Solvent Filtration
Filter solvents through 0.2 µm membranes and use high-purity salts.
Buffer Management
Avoid non-volatile buffers in LC–MS when possible.
Temperature Control
Allow cold solvents to equilibrate to room temperature before priming.
Post-Buffer Flushing
Perform routine post-buffer flushing with water and organic solvent.
Tubing Maintenance
Keep purge tubing short, wide-bore, and unobstructed.
Preventive Schedule
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.