Objective
Achieve and verify the cleanliness target required by the OEM — typically ISO 4406 16/14/11 or cleaner — by circulating filtered lube oil at a flow and temperature that produces turbulent flow (Reynolds number Re ≥ 4,000 minimum; Re ≥ 10,000 recommended) in every pipe segment. The objective is to dislodge construction debris, scale, weld slag, and varnish from internal surfaces prior to commissioning.
High-velocity flushing protocol per API 614 / ASTM D6439 for turbomachinery lube oil systems.
Prerequisites & Preparation
| # | Item | Detail |
|---|---|---|
| 1 | Documentation | OEM flushing specification, system P&ID, cleanliness acceptance criteria, ITP. |
| 2 | Bypass jumpers | Install bypass spools/jumpers across bearings, seals, control valves, and coolers — protect tight clearances. |
| 3 | Filter rating | Install temporary flushing filters; start at 25 µm absolute, step down to 10 µm then 6 µm or finer as cleanliness improves. |
| 4 | Flushing pumps | Use temporary pumps sized for 2–3× normal system flow; primary + redundant or a dedicated flushing skid. |
| 5 | Heater | Provide a heat source (immersion or in-line) capable of raising bulk oil to 65–80 °C (150–175 °F). |
| 6 | Reservoir | Clean reservoir mechanically; remove preservative oil; lint-free wipe; verify drains are blocked or directed to a waste drum. |
| 7 | Sample points | Designate upstream and downstream sample points; use clean ISO 3722 sample bottles; label per ISO 4021. |
| 8 | Safety | Hot-oil PPE, spill containment, fire watch where required, LOTO of production equipment, hot-work permit if heating. |
Execution — Step-by-Step
| Step | Action | Setpoint / Criterion | Duration |
|---|---|---|---|
| 1 | Initial fill — Fill reservoir with fresh flushing oil (compatible with service oil) up to normal operating level. Vent high points. | Oil grade per OEM (typically ISO VG 32 / 46 / 68). | — |
| 2 | Static heat — Heat the bulk oil to flushing temperature with main pump OFF. Circulate via kidney loop if available. | 65–80 °C (150–175 °F) | 2–4 h |
| 3 | Low-velocity warm-up — Start the flushing pump at low flow; warm piping; vent air pockets; check for leaks. | 0.5–1.0× normal flow; ΔP < 1 bar on temp filters. | 1 h |
| 4 | Coarse flush — Ramp to maximum flushing flow (2–3× normal). Confirm Re ≥ 4,000 (10,000 target) in the largest pipe segment using the Reynolds Calculator. | Q sized for Re ≥ 10,000 in worst-case pipe. | 24 h min (48–72 h on large systems) |
| 5 | Sparging / vibration — Apply pneumatic vibrators or rap pipes with rubber mallets; introduce nitrogen sparging at the front end to dislodge sticky debris. | Rotate locations every 30–60 min. Inert atmosphere. | Throughout coarse flush |
| 6 | Thermal cycling — Cycle bulk oil between 40 °C and 80 °C (or per OEM) to induce expansion/contraction stresses on debris bonded to pipe walls. | Hold ≥ 30 min at each extreme; complete ≥ 4 cycles. | 8–12 h |
| 7 | Filter stepdown — As ΔP stabilizes, step filter rating down (25 → 10 → 6 → 3 µm absolute). Replace elements when ΔP reaches vendor change-out value. | Filter ΔP < 50 % of element rated ΔP. | Concurrent |
| 8 | Sample & inspect — Pull upstream + downstream samples; run particle count per ISO 11171; record ISO 4406 codes after every filter element change. | Trend ISO 4406 vs. time; stop only when two consecutive samples meet target. | Every 4–8 h |
| 9 | Inspection screens — Install 200-mesh + 100-mesh acceptance screens (or per OEM) at the farthest point from the filter; inspect after the required run hours. | No metallics, no fibers > pass criterion (typ. NAS class 6). | Run 1 h after install, then inspect |
| 10 | Two clean screens — After a screen passes clean, restart pumps and run another screen. Both must pass independently. | Both consecutive screens pass acceptance. | Per screen cycle |
| 11 | Restore configuration — Drain flushing oil if not service-compatible; remove bypass jumpers; reinstate bearings, seals, coolers, control valves; install service-grade filters. | Verify torque, gaskets, alignment marks per WO. | — |
| 12 | Final fill & circulate — Charge with service oil; circulate at normal flow + temperature for 4 hours; pull a final ISO 4406 sample. | ISO 4406 ≤ OEM target (typ. 16/14/11). | 4 h + sample turnaround |
| 13 | Acceptance & sign-off — Issue flushing completion report: ISO 4406 trend, filter records, screen photos, hold-point sign-offs by user engineering + OEM/vendor. | All ITP hold points signed. | — |
Acceptance Criteria
- Cleanliness: ISO 4406 ≤ OEM target (typical API 614 general-purpose 16/14/11; special-purpose may require 15/13/10 or cleaner).
- Verification: Two consecutive inspection screens pass with no fibers, no metallics greater than the acceptance criterion.
- Sampling: Per ISO 4021 (sampling) and ISO 3722 (bottles); particle count per ISO 11171.
- Documentation: Time-stamped trend of ISO 4406 codes, filter ΔP, oil temperature, and flow rate over the entire flush.
Common Pitfalls — Avoid
- Targeting pressure instead of flow — flushing is a flow-driven activity; pressure is incidental.
- Treating Re = 4,000 as sufficient — it is the minimum; aim for Re ≥ 10,000 for effective debris transport.
- Leaving coolers in the flush path — they act as debris traps. Flush them separately or bypass them.
- Skipping bearing and seal bypasses — debris can lodge in tight clearances and cause immediate damage on startup.
- Inserting acceptance screens before the coarse flush is complete — the screens load up with bulk debris and tell you nothing about final cleanliness.
- Stopping after one clean screen — debris re-mobilizes on pump start/stop; always run a second confirmation screen.
- Ignoring OEM warranty cleanliness specification — the vendor cleanliness override always wins.
References
- API 614 — Lubrication, Shaft-Sealing and Control-Oil Systems and Auxiliaries.
- ASTM D6439 — Standard Guide for Cleaning, Flushing, and Purification of Steam, Gas, and Hydroelectric Turbine Lubrication Systems.
- ISO 4406:1999/2017 — Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles.
- ISO 4021 / ISO 3722 / ISO 11171 — Sampling, sample bottles, and automatic particle counter calibration.
- ASTM D341 — Viscosity-temperature charts (Walther equation) for petroleum liquids.
Reynolds Number & Turbulent-Flush Sizing
Compute Re for the largest pipe segment in the flush loop, with viscosity interpolated from
ISO VG @ 40 °C and 100 °C using the Walther / ASTM D341 equation.
Liters / minute (L/min)
Millimeters (mm) — pick a preset or enter measured ID
Mineral lube oil — ASTM D2422 / ISO 3448
Degrees Celsius (°C) — flushing temperature
Awaiting input
Reynolds Number (Re)
—
Enter flow, ID, ISO VG, and temperature
Kinematic Viscosity
—
cSt at oil temperature
Mean Velocity
—
m / s
Pipe Cross-Section
—
cm²
Q for Re = 10,000
—
L / min — recommended target
Enter the flushing parameters above. Turbulent flow (Re > 4,000) is the minimum;
target Re ≥ 10,000 for effective debris transport during the coarse flush.
Equations & Method
Kinematic viscosity at temperature is interpolated between the ISO VG published values at
40 °C and 100 °C using the Walther / ASTM D341 equation:
log₃₀(log₃₀(ν + 0.7)) = a + b · log₃₀(T)
where T is absolute temperature (K).
Reynolds number for circular pipe:
Re = v · D / ν, with v = Q/A, A = πD²/4, ν in m²/s.
Targets per API 614 / ASTM D6439: Re > 4,000 is the minimum acceptable; Re ≥ 10,000
is the recommended target for coarse flushing. Sizing the largest pipe segment governs the
required flush pump capacity. Defer to the project specification and OEM requirements.
Oil Cleanliness Code Calculator
Enter the measured particle count at each size band. Results compute on every keystroke.
Reporting Basis:
First ISO code digit
Second ISO code digit
Third ISO code digit
Awaiting input
ISO 4406 Cleanliness Code
— / — / —
Enter all three particle counts to compute
SAE AS4059F Class
—
Cumulative particle-count class
AS4059F per-size Code
— / — / —
A / B / C bins (≥4 / ≥6 / ≥14 µm)
≈ NAS 1638 Class
—
Legacy equivalent (governs ≥14 µm)
≥ 4 µm(c) Range
—
particles / mL
≥ 6 µm(c) Range
—
particles / mL
≥ 14 µm(c) Range
—
particles / mL
Enter particle counts at ≥ 4 µm(c), ≥ 6 µm(c) and ≥ 14 µm(c).
The ISO 4406 code is computed as ⌈log₂(particles / mL × 100)⌉, clamped to the standard range 1–28.
The SAE AS4059F class is mapped from the worst of the three size bins.
Method & References
Particle counts are most commonly measured per ISO 11171 using a calibrated automatic
particle counter on a representative sample drawn per ISO 4021 into a certified-clean
ISO 3722 bottle. ISO 4406 reports particles per 1 mL; SAE AS4059F reports per 100 mL — the
same raw counts, multiplied by 100.
For turbomachinery commissioning, API 614 typically calls for ISO 4406 cleanliness
of 16/14/11 or better before final acceptance of a lube-oil flush. Always defer to the
project specification and OEM requirements.
Table 1 — Cleanliness Code Cross-Reference
| ISO 4406:1999 (≥4 / ≥6 / ≥14 µm) |
NAS 1638 | SAE AS4059F Class | AS4059F Per-Size Code | Particles / mL (≥4) | Particles / mL (≥6) | Particles / mL (≥14) |
|---|---|---|---|---|---|---|
| 23/21/18 | 12 | 12 | 13A/12B/12C | 83,886 | 20,972 | 2,621 |
| 22/20/17 | 11 | 11 | 12A/11B/11C | 41,943 | 10,486 | 1,311 |
| 21/19/16 | 10 | 10 | 11A/10B/10C | 20,972 | 5,243 | 655 |
| 20/18/15 | 9 | 9 | 10A/9B/9C | 10,486 | 2,621 | 328 |
| 19/17/14 | 8 | 8 | 9A/8B/8C | 5,243 | 1,311 | 164 |
| 18/16/13 | 7 | 7 | 8A/7B/7C | 2,621 | 655 | 82 |
| 17/15/12 | 6 | 6 | 7A/6B/6C | 1,311 | 328 | 41 |
| 16/14/11 | 5 | 5 | 6A/5B/5C | 655 | 164 | 20 |
| 15/13/10 | 4 | 4 | 5A/4B/4C | 328 | 82 | 10 |
| 14/12/9 | 3 | 3 | 4A/3B/3C | 164 | 41 | 5 |
| 13/11/8 | 2 | 2 | 3A/2B/2C | 82 | 20 | 3 |
| 12/10/7 | 1 | 1 | 2A/1B/1C | 41 | 10 | 1 |
| 11/9/6 | 0 | 0 | 1A/0B/0C | 20 | 5 | 1 |
| 10/8/5 | 00 | 00 | 0A/00B/00C | 10 | 3 | 0.3 |
| 9/7/4 | 000 | 000 | 000A/000B/000C | 5 | 1 | 0.2 |
Source: Invicta cross-reference (NAS 1638 / ISO 4406 / SAE AS4059F). Particles per mL = 2^(ISO code) / 100.
Table 2 — Filter Micron Rating ↔ Mesh Size
| Filter (µm) | US Mesh (≈) | Sieve (in) | Sieve (mm) | Typical Application |
|---|---|---|---|---|
| 1 | 12,000 | 0.000039 | 0.001 | Sub-micron / fine hydraulic & turbine oil polishing |
| 3 | 4,800 | 0.000118 | 0.003 | Servo-valve protection, high-pressure hydraulics |
| 5 | 2,500 | 0.000197 | 0.005 | Turbomachinery lube oil (API 614 systems) |
| 10 | 1,250 | 0.000394 | 0.010 | Hydraulic returns, gearbox lube |
| 15 | 800 | 0.000600 | 0.015 | General industrial hydraulic |
| 25 | 500 | 0.001000 | 0.025 | Low-pressure hydraulic, light contamination |
| 37 | 400 | 0.001457 | 0.037 | Coarse fluid filtration |
| 44 | 325 | 0.001732 | 0.044 | Process strainers, fuel pre-filters |
| 53 | 270 | 0.002100 | 0.053 | Coarse strainer, suction protection |
| 74 | 200 | 0.002900 | 0.074 | Bulk pre-filtration |
| 149 | 100 | 0.005900 | 0.149 | Wire strainer, debris removal |
| 297 | 50 | 0.011700 | 0.297 | Pump suction, coarse trash |
| 595 | 30 | 0.023400 | 0.595 | Trash basket strainers |
Source: Sigma-Aldrich particle size conversion table; PSI / Industrial Spec mesh-to-micron charts. Mesh values are nominal — actual filter beta-ratio rating governs performance.
Table 3 — Recommended Target Cleanliness by Equipment Type
| Equipment / Application | Target ISO 4406 | ≈ NAS 1638 | ≈ AS4059F | Recommended Filter |
|---|---|---|---|---|
| Servo valves (high-pressure hydraulic) | 15/13/10 | 4 | 4 | 3 µm β₃ ≥ 200 |
| Proportional valves | 16/14/11 | 5 | 5 | 5 µm β₅ ≥ 200 |
| Variable-volume piston pumps | 17/15/12 | 6 | 6 | 5–7 µm β₅ ≥ 100 |
| Vane / gear pumps, standard hydraulics | 18/16/13 | 7 | 7 | 10 µm β₁₀ ≥ 100 |
| Steam & gas turbine lube oil (API 614) | 16/14/11 | 5 | 5 | 5 µm β₅ ≥ 200 |
| Gearbox / industrial bearings | 17/15/12 | 6 | 6 | 10 µm β₁₀ ≥ 200 |
| Diesel engine lube oil | 19/17/14 | 8 | 8 | 10 µm β₁₀ ≥ 75 |
| Compressor lubrication | 18/16/13 | 7 | 7 | 10 µm β₁₀ ≥ 100 |
| New oil from supplier (typical) | 20/18/15 | 9 | 9 | Filter on delivery |
Targets are typical OEM/industry guidelines (Noria, Parker, Donaldson Hy-Pro). Always defer to the specific equipment manufacturer's spec. β-ratio (beta) is the filter efficiency: β₁₀ ≥ 200 means 99.5% capture at 10 µm.
Take it offline — LO Flushing & Lubrication Suite (Excel)
The unified workbook this assistant is built on: flushing procedure, Reynolds calculator, ISO 4406 calculator, and the full standards-comparison reference — ready to circulate during a flush or commissioning campaign.
- Step-by-step API 614 / ASTM D6439 procedure
- Reynolds calculator with Walther/D341 viscosity-temperature interpolation
- Auto-calculating ISO 4406 code + AS4059F / NAS 1638 cross-reference
- Micron-to-mesh chart and OEM target cleanliness by equipment type