Circulating Oil Systems: Specifications, Selection and Cleanliness
A circulating oil is selected by specification, not by brand. DIN 51517 sets the ladder - C (plain), CL (inhibited R&O), CLP (anti-wear) - ISO 6743-6 with ISO 12925-1 adds the CKB / CKC / CKD classes where the circuit feeds enclosed gears, and DIN 51515-1 turbine oils TD sit at the premium R&O end. Paper machines add the OEM norms (Voith VN 108, Valmet RAU4L00659) and rolling-mill oil-film bearings add the Morgoil lubricant specification. Match the class first, then the ISO VG grade, then confirm the datasheet values that actually govern the duty: demulsibility, oxidation life, air release and - where gears are in the circuit - the FZG stage.
A circulating system is not a gearbox with extra pipes
In splash lubrication the oil lives where the work happens. In a circulating system it does not: a pump draws from a reservoir, feeds bearings, gears or rolls through coolers and filters, and the oil returns to the tank to rest before going around again. That residence time in the tank is the design point. It is where entrained air is released, where water is meant to settle out to the drain pot, and where heat is shed - which is why the properties that decide a circulating oil are exactly the ones a gearbox oil can be casual about: air release, foam behaviour and above all demulsibility.
It also changes the economics. A circulating charge is large - hundreds to thousands of litres - topped up rather than dumped, and managed on condition like a turbine oil charge. Selecting the right class up front is what makes that long service life reachable.
The specification map - which standard governs which duty
“Circulating oil” has no single standard of its own. It is covered by a small map of families, and reading the machine manual against this map answers most selection questions before a datasheet is opened.
| Class | Standard | What it adds | The duty it governs |
|---|---|---|---|
| C | DIN 51517-1 | Plain mineral oil, no inhibitor package | Simple circulation and splash duty with short oil residence |
| CL | DIN 51517-2 | Ageing (oxidation) and corrosion inhibitors | The classic R&O circulating oil - bearing circulation at elevated temperature or with water about |
| CLP | DIN 51517-3 | Anti-wear / EP on top of CL; FZG (ISO 14635-1, A/8.3/90) load stage 12 or better; FE8 bearing wear test | Circulation that also feeds gears or any mixed-friction contact |
| CKB | ISO 6743-6 / ISO 12925-1 | R&O class of the gear family | Enclosed gears plus circulation, light to moderate load |
| CKC / CKD | ISO 6743-6 / ISO 12925-1 | EP performance; CKD adds thermal and oxidative stability | Loaded gear circulation; CKD where the stabilised temperature runs high |
| FC / FD | ISO 6743-2 (family F) | Spindle and bearing oils; FD carries anti-wear | Bearing-only circulation at low viscosity - spindles and high-speed bearings |
| TD | DIN 51515-1 / ISO 8068 | Turbine-oil requirements - long oxidation life, air release, demulsibility | Turbine lube circuits and the most demanding general R&O circulation |
Two boundaries worth respecting. DIN 51524 (HL / HLP / HVLP) is the hydraulic fluid standard, not this family - see the FAQ below. And the open-gear classes (CKH / CKJ / CKM, ISO 12925-2) are not circulating duty at all.
The tests that decide it
Circulating-oil datasheets all quote the same short battery, and each test exists because of something that goes wrong in a tank:
| Test | Method | What it proves in a circulating system |
|---|---|---|
| Demulsibility | ASTM D1401 (54 °C up to VG 90, 82 °C above); ASTM D2711 for heavy duty | Water sheds in the tank instead of riding through bearings as an emulsion |
| Oxidation life | ASTM D943 (TOST) | The charge survives years of hot, wet, aerated service - premium R&O oils quote four-digit hours |
| Rust protection | ASTM D665 A/B | Ferrous surfaces survive the water the system will inevitably see (B uses synthetic sea water) |
| Air release | ASTM D3427 / IP 313 / DIN 51381 | Entrained air escapes within the tank residence time - the pump does not draw foam |
| Foam | ASTM D892 | Surface foam collapses instead of overflowing the reservoir |
| FZG scuffing | ISO 14635-1, A/8.3/90 | Where gears share the circuit - CLP and paper-machine oils are expected at load stage 12 or better |
| Bearing wear | FE8, DIN 51819-3 | Rolling-bearing wear protection - added to CLP and demanded by paper-machine norms |
Viscosity by duty
Once the class is right, the grade follows the duty rather than habit. Paper-machine circulation runs almost entirely on ISO VG 150 and 220, with synthetic grades extending to 320 and 460 where dryer-section temperatures demand them. Rolling-mill oil-film bearings run VG 150 to 460 depending on mill speed and load. General bearing circulation sits lower - VG 32 to 100 by shaft speed and bearing temperature - and where the circuit feeds a gearbox, the gear set’s own grade requirement governs, commonly VG 220 to 460 on heavy drives. When one circuit serves mixed duties, the most demanding contact sets both the class and the grade.
Paper machines: what Voith and Valmet actually ask for
Paper-machine wet ends are the hardest circulating duty in general industry: hot, permanently wet, continuously filtered, and expensive to stop. The machine builders therefore publish their own oil norms on top of DIN 51517-3 - Voith’s paper-machine oil norm VN 108 (current OEM-approval lists also cite VS 108 sections for wet end, dry end, coaters and shoe presses) and Valmet’s oil recommendation RAU4L00659, with a separate recommendation for zone-controlled rolls.
What they demand beyond the DIN class is consistent across both: fast water separation at the 82 °C test condition, FZG load stage 12 or better, FE8 bearing wear performance, compatibility with the fine filtration modern machines run continuously, and thermal stability that keeps hot piping and dryer-section journals free of deposits. The OEM documents themselves are not public, so in practice selection works from the approval and norm references printed on the oil’s datasheet - which is exactly how we quote this duty.
Rolling mills: the Morgoil bearing and its oil
The backup rolls of flat-product mills run in oil-film bearings - the Morgoil type, from the Morgan Construction lineage - in which the roll neck floats on a hydrodynamic oil wedge. There is no rolling element to fail; the oil film is the bearing, fed by a large circulating system that also swallows whatever roll cooling water gets past the seals.
That is why the Morgoil lubricant specification (the SN-180 lineage) is written almost entirely around water handling and cleanliness: the standard New Oil level is proven with ASTM D1401 and D2711 demulsibility, and the Advanced level adds a dynamic demulsibility endurance test that simulates sustained water ingress in service. Danieli publishes a parallel mill-oil standard including a super-demulsibility class. Grades run VG 150 to 460. The practical rule for a mill lubrication engineer is simple: this duty takes a Morgoil-class circulating oil, selected from the current approval documents - never a general-purpose gear oil of convenient viscosity.
Cleanliness and water - the operating half of the job
Specification gets the right oil into the tank; operation keeps it working. Two disciplines carry most of the weight. The first is particle cleanliness: circulating systems live or die by filtration, and the target and measurement framework is the same ISO 4406 code system used for hydraulics - our ISO 4406 cleanliness guide covers how to read the code and what targets to run. The second is trending the charge itself - water content, acid number, demulsibility retained, varnish potential - on the same condition-based logic as a turbine charge, which the turbine oil condition monitoring guide sets out in full. A circulating charge managed this way lasts years; one managed by topping up and hoping does not.
The Castrol ranges for circulating duty
Castrol covers the map above from four directions, and the honest summary is by class rather than by slogan. Castrol Alpha SP is the DIN 51517-3 CLP range for gearboxes on forced circulation or splash - the circulation-fed gear duty in the table. The Castrol Perfecto range covers turbine-grade R&O circulation, the most demanding end of the CL/TD column. Magna is Castrol’s circulating and simple gear oil family, and Hyspin VG covers plain R&O duty at hydraulic viscosities. For the OEM-normed duties - Voith or Valmet paper-machine lists and Morgoil-class mill oils - we quote against the current Castrol India range and confirm the approval line on the datasheet with Castrol before anything is supplied, because on these duties the approval is the product.
Frequently asked questions
Should a circulating oil meet DIN 51517 CL or CLP?
CL if the circuit lubricates bearings only, CLP if it also feeds gears or any contact running in mixed friction. Part 2 (CL) adds ageing and corrosion inhibitors to the plain Part 1 (C) oil - the classic R&O circulating oil. Part 3 (CLP) adds anti-wear and EP performance on top, proven in the FZG scuffing test (ISO 14635-1, A/8.3/90, load stage 12 or better) and the FE8 rolling-bearing wear test. CLP is not automatically the better choice: EP chemistry the system does not need buys nothing, and the OEM manual's class wins every time.
What oil goes into a Morgoil oil-film bearing system?
A circulating oil built to the Morgoil lubricant specification (Morgan SN-180 lineage), typically in ISO VG 150 to 460. The governing property is demulsibility - roll cooling water will get into the charge, and the oil has to shed it continuously. The standard New Oil level is tested with ASTM D1401 and D2711; the Advanced level adds a dynamic demulsibility endurance test. Danieli publishes a parallel standard and super-demulsibility oil specification for its mills. A general-purpose gear oil is not a substitute, whatever its viscosity - the water-handling chemistry is the whole point.
What do Voith and Valmet require for paper-machine circulating oil?
Both publish their own oil norms - Voith's paper-machine oil norm VN 108 (current OEM-approval lists also reference VS 108 sections) and Valmet's oil recommendation RAU4L00659, with a separate document for zone-controlled rolls. In performance terms they sit on top of DIN 51517-3: fast water separation at 82 degrees C, FZG load stage 12 or better, FE8 bearing wear performance, compatibility with fine filtration, and thermal stability that keeps dryer-section piping clean. The OEM documents themselves are not public, so selection works from the approvals printed on the oil datasheet.
Can I use a DIN 51524 HLP hydraulic oil in a circulating system?
Not by default. DIN 51524 is the hydraulic-fluid standard and DIN 51517 is the lubricating-oil standard for circulation and splash duty - the chemistries are related but tuned differently, particularly on demulsibility and air release, which govern in a circulating tank and matter less in a closed hydraulic circuit. If the machine manual names a 51517 class, supply that class. Where one oil genuinely serves both duties the OEM will say so on the datasheet; assuming it is how tanks end up with stable emulsions.
How is demulsibility measured, and why does it decide circulating-oil life?
ASTM D1401 stirs 40 ml of oil with 37 ml of water and times the separation - run at 54 degrees C for grades up to VG 90 and at 82 degrees C for heavier grades; a good circulating oil separates in minutes. ASTM D2711 is the larger-scale version for heavy duty and also reports free water recovered. It decides oil life because a circulating system continuously collects water - condensation, seal leaks, roll cooling water - and an oil that emulsifies carries that water through bearings instead of dropping it in the tank where the drain pot can remove it.
Can Vasundhara Specialities supply circulating oils across South India?
Yes. Vasundhara Specialities Private Limited is the authorised Castrol industrial distributor for Andhra Pradesh, supplying Telangana directly and Karnataka and Tamil Nadu on special order, with pan-India industrial coverage through Vasundhara Group. Genuine Castrol circulating, gear and turbine oils with per-batch certificate of analysis, TDS and MSDS documentation, plus oil-sample analysis to support condition-based maintenance of the charge. For our complete Castrol industrial portfolio see our Castrol catalogue.
Selecting a circulating oil for a specific machine?
Send us the machine, the manual's oil class, the grade currently in the tank and the last oil-analysis report if you have one. We will map it against DIN 51517, the ISO 6743-6 classes and any OEM norm that applies, and put the recommendation in writing with the datasheet.
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