❗ “Never Use NAS Again”?

❗ “Never Use NAS Again”? — A Technical Reality Check for Cleanliness Standards in Turbine & Hydraulic Oils

1. What You Saw in the Refinery: A Common but Outdated Practice

Many large plants—especially refineries, power plants, and legacy oil & gas facilities—still report NAS values alongside ISO particle counts.

This usually means they are using:

• NAS 1638 (introduced in the 1960s for aerospace hydraulics)
• Alongside ISO 4406 (modern international standard)

At first glance, this looks like “extra control.”
In reality, it often creates confusion, misinterpretation, and poor decision-making.

2. Why NAS 1638 Is Considered Obsolete

❌ 2.1 It Was Designed for a Different Era

NAS 1638 was:

• Developed for aerospace hydraulic systems
• Based on older particle counting technologies
• Designed when filtration, oil chemistry, and system sensitivity were very different

Modern turbomachinery, servo valves, and high-pressure systems operate at a completely different level of cleanliness sensitivity.

❌ 2.2 It Uses a Single “Class” — Which Is Misleading

NAS compresses contamination into one number (e.g., NAS 6, NAS 8).

👉 The problem:

• It does NOT distinguish particle size distribution clearly
• It can hide critical contamination trends

Example:

• Two oils can both be NAS 7
• But one may have dangerous >14 µm particles affecting servo valves
• While the other has mostly fine particles

➡️ Same NAS class ≠ same risk profile

❌ 2.3 It Does Not Align with Modern Failure Modes

In my world (turbomachinery, journal bearings, servo valves, EHC systems):

Failure sensitivity is strongly linked to:

• Particle size thresholds (e.g., >4 µm, >6 µm, >14 µm)
• Surface interactions (varnish + particles)
• Clearances in microns

NAS does not:

• Properly reflect critical particle size bands
• Support root cause analysis
• Link directly to modern reliability strategies

3. Why ISO 4406 Is the Industry Standard Today

✔️ 3.1 It Uses Differential Particle Size Ranges

ISO 4406 reports contamination as:

ISO Code: 18 / 16 / 13

Which corresponds to:

• ≥4 µm
• ≥6 µm
• ≥14 µm

👉 This is extremely powerful, because:

• You can see which particle sizes are increasing
• You can link contamination to specific failure risks

✔️ 3.2 It Supports Modern Reliability Engineering

ISO 4406 integrates directly with:

• OEM cleanliness targets (e.g., servo valves, turbines)
• Standards like:
  • ISO 11171
  • ISO 4407

It allows:

• Trend analysis
• Alarm setting
• Root cause diagnostics

✔️ 3.3 It Matches Modern Filtration & Oil Management

Technologies like:

• High-efficiency filtration
• Electrostatic filters
• Ion-exchange systems (like your ICB)
• Varnish mitigation systems

➡️ All rely on particle size distribution, NOT a single aggregated number.

4. The Hidden Danger of Using NAS Today

⚠️ False Sense of Security

A plant may say:

“We are NAS 6, we are clean.”

But in reality:

• ISO could be 19/17/14 → dangerous for servo valves
• Varnish precursors + fine particles may be ignored

⚠️ Poor Decision-Making

Using NAS can lead to:

• Incorrect filtration strategy
• Wrong alarm limits
• Misinterpretation of contamination sources

⚠️ Incompatibility with Modern Oil Analysis Programs

Today’s programs integrate:

• MPC (varnish potential)
• RULER (antioxidant health)
• TAN (acid number)
• Particle count (ISO-based)

➡️ NAS does not integrate into this ecosystem

5. Why Plants Still Use NAS (Reality from the Field)

Let’s be fair and technical:

🔸 Legacy specifications

Old procedures, manuals, and contracts still reference NAS

🔸 Habit & familiarity

Technicians and engineers are “used to NAS numbers”

🔸 Dual reporting from labs

Some labs still provide:

• ISO 4406
• NAS 1638
  ➡️ to “keep everyone happy”

6. What Should Be Done Instead

✔️ Step 1 — Eliminate NAS from Reporting

• Remove from lab reports
• Remove from KPIs
• Remove from acceptance criteria

✔️ Step 2 — Standardize on ISO 4406

Define targets like:

• Turbine oil:
  • 16/14/11 or cleaner
• Servo valve systems:
  • 14/12/9 or cleaner

✔️ Step 3 — Link Cleanliness to Failure Modes

Instead of saying:

“NAS 7 is acceptable”

Say:

• “Particles >14 µm must be controlled to prevent servo valve sticking”
• “Particles >6 µm impact bearing wear and varnish interaction”

✔️ Step 4 — Educate the Organization

This is critical:

• Maintenance
• Reliability
• Lab personnel
• Procurement

➡️ Cleanliness must be understood as a risk parameter, not just a number

7. Final Technical Verdict

Your statement:

“Never use NAS again — it is obsolete.”

✔️ From a modern reliability and turbomachinery lubrication perspective, this is technically justified.

However, a more diplomatic and strategic version in the field would be:

“NAS 1638 served its purpose historically, but ISO 4406 provides far more meaningful and actionable data for modern equipment reliability. We should transition fully to ISO-based cleanliness control.”

🔚 Closing Thought (Khash-Level Insight)

In today’s turbomachinery:

• Clearances are in microns
• Varnish layers are in microns
• Failure mechanisms are driven by microns

And yet…

👉 NAS tries to summarize everything into one number

That alone is enough reason to retire it.