Hydraulic Oil NAS 13-15: Debris, Not Structural Failure

Hydraulic oil reaches the panel, circulation continues, and the particle count stays around NAS 13-15 instead of the required NAS 4. The persistent 15-100 micron fraction points to an active debris source, trapped construction contamination, or ineffective removal. It does not by itself prove that the carbon-steel pipe is structurally failing.

Stop the fixes that only circulate the problem

Start here. Several common responses consume oil, filters, and time without identifying the source.

• Do not keep circulating on the same path indefinitely. Several days without a downward trend means the circuit is releasing particles as fast as the filtration system removes them, the flow is not reaching contaminated areas, or the sampling method is misleading you.
• Do not treat repeated filter changes as the repair. Filters capture released material but cannot stop corrosion, deteriorating internal coating, weld debris, or contamination trapped in dead legs.
• Do not blame the HPU without isolating it. New oil, reservoirs, hoses, valves, and the HPU can introduce particles, but the piping must be separated from the power unit before assigning the source.
• Do not replace all carbon-steel pipe based only on the NAS result. Carbon steel makes surface preparation, welding, drying, and preservation more demanding, but a cleanliness reading is not a pipe-wall integrity test.
• Do not repeat the chemical process blindly. The required correction depends on what chemical was used, what neutralizer followed it, and where the piping sat idle for several hours.

Find the mechanism producing the particles

Pickling removes scale and oxide from carbon steel. Neutralization stops the active cleaning chemistry, while rinsing, drying, and preservation prevent residues and newly exposed steel from becoming the next contamination source.

The idle period matters because its position in the sequence determines the failure mode:

• If the pipe sat with active pickling solution present, extended metal attack or loosened scale can leave a continuing metallic debris source.
• If it sat after partial draining or rinsing, retained chemical and dissolved contamination can dry or settle in low points.
• If it sat wet after neutralization, exposed carbon steel can flash-rust before drying and oil preservation.
• If internal lacquer or preservative remained on fittings, the oil can soften or detach it over time.
• If weld roots contain slag, spatter, oxide, or poorly attached scale, circulation can release debris long after commissioning begins.

Structural pipe failure remains a separate question. Check wall loss, leakage, deformation, and measured thickness when chemical overexposure is suspected. Particle count alone cannot answer it.

Diagnose the source before resuming the flush

First check the sample. Use the same sampling location, method, instrument configuration, and reporting basis used for the acceptance target. Flush stagnant fluid from the sampling connection, collect from moving oil, and rule out dirty bottles or connections.

1. Obtain the chemical cleaning record. Identify the pickling chemical, neutralizing chemical, sequence, measured process endpoints, dwell periods, rinse steps, and the exact stage at which flow stopped.
2. Inspect used filter media or collect debris for microscopic examination. Check whether it is metallic, magnetic, oxide-like, fibrous, elastomeric, or coating material.
3. Run the HPU and temporary flushing equipment on an isolated clean loop. Then sample the return from individual piping sections. A rise across one section localizes the source.
4. Inspect representative fittings, weld roots, low points, dead ends, and removed spool pieces. Look for coating, rust, loose scale, retained liquid, and construction debris.
5. If chemical attack is suspected, measure pipe condition with the inspection method selected by the responsible piping engineer. Compare the result with the piping design basis rather than inferring wall loss from particle counts.

Reclean the affected circuit and control the flush

Match the correction to the debris. More circulation is not a substitute for removing the active source.

1. Isolate the affected piping from clean HPU components so released contamination cannot spread through the entire hydraulic system.
2. Drain low points and trapped branches. Open or temporarily reconfigure dead legs that the original flush could not sweep.
3. Remove loose weld debris, scale, or internal coating mechanically where access permits. Repair unacceptable weld surfaces before cleaning again.
4. If residue, incomplete neutralization, or flash rust is confirmed, have the chemical-cleaning contractor issue a corrective procedure based on the chemicals actually used. Record the specified endpoints; do not select a generic neutralization value.
5. Complete every rinse, neutralization, drying, and preservation step without leaving newly cleaned carbon steel wet and idle.
6. Configure the oil-flush path to sweep each branch and component cavity. Cycle accessible valves and reverse or redirect flow where the circuit design permits.
7. Use filter media capable of capturing the particle range driving the result. Watch differential pressure and bypass indication so loaded elements do not defeat the flush.
8. Keep sensitive or already-clean components bypassed until the piping circuit has passed its acceptance checks.

If internal preservative or lacquer is the source, confirm that the removal method is compatible with seals, hoses, valves, and the final hydraulic fluid. Chemical cleaning can trade one contamination mechanism for another when residue remains trapped.

Verify that NAS 4 remains stable

A single acceptable sample does not prove the source is gone. Verify the system under conditions that release retained contamination.

1. Sample upstream and downstream of the field piping to show whether the circuit adds particles.
2. Record the cleanliness trend, filter differential pressure, element changes, flow path, valve positions, and operating state with every sample.
3. Cycle branches and valves, disturb low-flow areas through the approved flushing arrangement, and sample again.
4. Stop circulation for a controlled operational interval, restart, and watch for rebound. A rebound directs you back to settled debris, corrosion, or unswept cavities.
5. Accept the system only when the specified locations repeatedly meet NAS 4 under the owner's documented sampling and acceptance method.

Retain the removed filter elements and laboratory results until acceptance. They provide the clearest record of whether the debris type changed after corrective cleaning.

Prevent the same failure on the next installation

• Start with precleaned and suitably treated pipe and fittings rather than depending on the final oil flush to remove every manufacturing residue.
• Specify internal-surface condition, end protection, storage, weld cleaning, chemical-cleaning records, drying, and preservation in the installation plan.
• Inspect fittings for lacquer or preservative before assembly.
• Review the welding process and root condition. A cleaner root pass can reduce internal oxide and slag, but welding method alone does not guarantee a clean bore.
• Prevent uncontrolled holds between pickling, neutralization, rinsing, drying, and oil preservation.
• Consider stainless-steel piping and fittings when the cleanliness duty and lifecycle justify them. Stainless material reduces some corrosion concerns but still requires clean fabrication, controlled welding, and a verified flush.

FAQ

Stop when particle generation continues after the source has been localized and the required corrective chemistry or pipe condition cannot be validated. Escalate through the official support channels for the chemical supplier, HPU or filtration manufacturer, and responsible piping engineer before another chemical treatment or return to service.