Oystein Haldorsen is Technical Director Rotating Equipment at Bilfinger Nordics.

Oystein Haldorsen

Operators of larger rotating machinery are often confronted with varnish deposits on journal bearings. Varnish is an oil-derived deposit based on machine operating conditions, the formulation of the oil, and contamination. It is responsible for continuous temperature increases on journal bearings as it acts as an insulator, preventing the lubricants from performing one of their most important tasks: cooling. This can lead to unplanned machinery downtime and bearing damage.

In the past, it was common practice to replace the oil. The new oil dissolved most of the remaining varnish from internal parts. However, lube oils used nowadays have low solubility. Oil replacement only helps for a short period before problems return. 

OEMs and lubricant vendors tend to recommend a flushing oil to clean out the system. But the use of a flushing oil with high solubility and high temperature makes it necessary to continue flushing for at three or more weeks to dissolve all varnish deposits. 

If varnish is detected at an early stage, it is possible to increase the solubility of the lube oil using additives for several months before a planned oil replacement. An ESP (electrophysical separation process) filtration unit can also be used to remove the existing varnish. On heavy contaminated systems, though, this becomes inefficient as the ESP filters cannot handle high amounts of varnish since they saturate quickly.

The most effective solution is a chemical cleaning between oil changes. Most service suppliers specialized on chemical cleaning use the same approach as they do when cleaning a heat exchanger, a vessel, or a tank. This can lead to problems such as the wrong chemistry or leftover residue of chemistry in a lube oil system, which can have catastrophic consequences for the system and adjacent machinery. Poor cleaning results has given the practice a poor reputation.

Inspection of journal bearings for varnish.

Here is a high-level summary of best practices for chemical cleaning: 

Effective chemical cleaning of a lube oil or hydraulic oil system begins at the planning phase. It is important to prepare a complete mapping of the system and walk visually through it to verify that piping and instrumentation diagrams are correct. Also check flange sizes as there are often flanged connections that do not follow normal industrial standards. 

Flushing loops in the system should be planned based on Reynolds Number calculations, and on minimizing the number of bypasses and blinds needed. That makes it possible to calculate the correct pump size to guarantee turbulent flow and chemical flowrate. Choice of chemical depends on temperatures that can be achieved, time constraints, environmental constraints, construction materials of system, and other factors. 

During the execution phase, the oil system is drained before installing the flushing equipment. Close out or dismantle equipment that cannot be exposed to chemicals. Remove oil filters from casings. 

Manually cleaning of oil tank internals can be performed if the tank is big enough. Otherwise, chemical cleaning is performed.

When the system is ready, the compressor deck has to be cleared for half an hour to do a pressure test of the system. This is the time to fix potential leakages when only water is in the system. For a typical two- to three-week overhaul, flushing is done during night shifts when no one is working on the compressor deck. 

Flushing with water removes chemical residues. Thus, it is important to use chemicals that are soluble in water. Water flushing takes one day on average. Afterwards water is drained, and flushing skids are disconnected. 

The auxiliary oil pump can now be put into operation and the system filled with the minimum amount of oil needed to test run the system. A short oil flushing removes any remaining water then drain the system again and do a manual cleanout of the bottom of the oil tank. 

It is now time to introduce new oil through a 10-micron filter to remove particulate. Chemical cleaning can leave behind solid particles that were suspended in varnish. Therefore, it is recommended to install a high-volume circulation filter on the oil tank to remove them. 

Prior to start up, install new oil filter cartridges. If the system usually runs with 10-micron filters use five-micron filters for commissioning to ensure all particles are removed. Begin the oil circulation pump system and continue until particle levels are continuously on a low level. 

Chemical cleaning of oil systems is the most effective way to remove all varnish deposits and prevent contamination of new oil. The complexity of this process, however, requires high-level knowledge on chemistry, fluid dynamics, and filtration. 

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