Oil Varnish Potential Testing Programme

Intermittent valve sticking, shorter filter life, deposits in bearing or reservoir zones, and behaviour that changes with oil temperature can support a varnish investigation. Similar symptoms can also arise from external dirt, incompatible top-ups, oxidation, water, wear debris or maintenance residue.

Start with the failure history. Record whether the issue followed an oil change, top-up, purification, outage or temperature excursion. This separates a developing oil-degradation problem from an isolated component event and determines whether screening or a deeper root-cause package is justified.

• Electrohydraulic or governor valves become slow, sticky or temperature-sensitive.
• Filters show dark, soft or recurring deposits without a matching increase in coarse particles.
• Reservoir, cooler or bearing surfaces develop amber or brown films.
• Patch colour or deposit symptoms change after purification but return during service.

ASTM D7843 membrane patch colorimetry addresses lubricant-generated insoluble colour bodies in in-service turbine oils. A controlled sample volume is passed through a membrane and the patch colour is measured. Sampling temperature, storage, preparation and timing can influence what the membrane captures, so repeatability matters as much as the result.

The patch result becomes more useful when reviewed beside fluid condition and system evidence. Particle count, water, acid number, FTIR trends and remaining oxidation-resistance indicators answer different questions. Component inspection and retained filter material may show whether deposits are organic, metallic or external.

Package depth depends on equipment criticality, oil type, previous data and whether the decision is to trend, purify, inspect or replace the charge.

Not every method is appropriate to every turbine or hydraulic fluid. Chem-Tech confirms the fluid matrix, sample quantity, method availability and current accreditation scope during quotation.

A varnish trend is only meaningful when samples are collected and handled consistently. Changing from a live-zone sample to a cool reservoir sample can change the insoluble population being measured.

• Use the same active sampling point and comparable operating condition for routine trends.
• Record oil and ambient temperature, equipment load and time since shutdown.
• Avoid sampling immediately after top-up, purification or filter change unless the intervention is the subject of the test.
• Fill clean containers, minimise unnecessary storage and tell the laboratory when the sample was drawn.
• Send previous MPC and oil-condition history with valve, filter and deposit observations.

A high or increasing varnish indicator may justify closer trending, temperature review, electrostatic or depth-media treatment, compatibility assessment or component inspection. It does not, by itself, select a purification technology or prove that an oil charge must be replaced.

Define a post-action verification plan before intervention. Repeat samples from the same points, document filter or purification changes and compare operating symptoms. A sustained improvement in both laboratory and equipment evidence is stronger than a one-time lower patch result.

• Trend a stable baseline before establishing plant alerts.
• Investigate incompatible top-ups or product changes separately from oxidation.
• Inspect retained filters and affected valves when the failure consequence is high.
• Verify any purification or oil-change action with repeatable post-action samples.