Moisture – The Serial Killer of Compressors

For living things water brings life But it’s just the opposite for your compression assets.

 

How moisture gets into your lube oil

  1. Process Gas Streams
    When extracting process gases from the source, water is often inadvertently carried with it. In an ideal world this would be entirely inconsequential, however this is not the case. No seal is made perfect and there can be a transfer interface between the process gas stream and the lube oil. This can cause significant harm to your compression assets.
  2. The Environment
    A problem that is exacerbated but not exclusive to humid environments is your lube oils exposure to environmental moisture. Although the non-polarity of your lube oil makes it difficult for water to become dissolved, this does not mean it is immune to adoption of water vapour from its surroundings and over a long period of time even small amounts will damage machinery. An incorrectly set up lube oil tank could be the major source of moisture contamination. The process of the tank as it fills and drains (or as the oil expands and contracts) draws in air along with its associated dampness. This can quickly lead to your lube oil degrading to an unworkable state.

How it damages your compressor

There are many detrimental effects of having water present in your oil. Here are a few that will potentially deal the most damage:

  • Hydrogen Embrittlement
    Lube oil in compressors is naturally subject to extremely high pressures.  If moisture is present, then water splitting and hydrogen embrittlement can occur. These are the processes by which hydrogen separates from oxygen and then diffuses into metals. Over time the metals at the compressor surface will suffer the coalescence of hydrogen molecules creating regions of high pressure, eventually leading to cracking and potentially catastrophic failure.

A crack in metal alloy caused by hydrogen embrittlement.

  • Steam Micro-Explosions
    When dissolved water in oil undergoes rapid changes from high to low pressure, cavitation occurs. The almost instantaneous vaporisation of water releases a huge amount of energy – 100 millilitres of water can turn into 100 litres of steam! This creates miniature explosions which cause both pitting of internal surfaces and diminishing of lube oil effectiveness.
  • Impaired Lubrication Ability
    Like Goldilocks’ porridge, the properties of the lube oil must be just right in order for you to safely feed it to your compressor. Having moisture present can dramatically alter the viscosity of the oil. This leads to bumping when bearing component clearances are breached and damage as the lubricant cannot adequately separate components.
  • Accelerated Varnish Formation
    Varnish begins as small particulate adherent oxides which, when built up on surfaces, will cause the premature failure of critical components. Oxidation is the agent that beckons varnish nucleation, hence the inclusion of antioxidants in your lube oil additive packages. Water and high temperatures cause the rapid consumption of antioxidants. After the antioxidants become depleted substantial varnish-related problems are inevitable.
  • Sludge
    Electrostatic forces act between water and polar soot, resin, dirt and oxides. This can all cluster to form sludge that will impede oil flow.
  • Corrosion
    Acid is given its eroding power by water. Pitted and etched surfaces in rolling areas compromise the oils film strength leading to a further decrease in lubrication performance.

A combination of heavy damage by cavitation and corrosion on turbine blades.

What can you do about it?

  • Isolate Your Oil from Your Process Gas
    The Root Cause solution to this problem is to remove all moisture from the process gas before compression. Alternatively, completely isolate the bearings from the process gas. Unfortunately and obviously, these are likely to be very difficult options to implement.
  • Stop Moisture from Getting into Your Tank from the Environment
    Installing a breather on a lube tank should be a minimum requirement of your lubrication reservoir. A desiccant breather on your tank allows the passage of air while also capturing any inbound moisture and is an easy fit. As an added bonus breathers can simultaneously capture particulate matter to keep your oil cleaner. (Read more on breathers in a previous article Do You Need to Take a Breather?)
  • Remove Moisture from the Oil Directly
    If the presence of moisture in the lube oil is inevitable and a root cause solution is impractical (or too expensive) then the best practice solution is dehydration. For large volumes of water (free water) a centrifuge or even a knock-out tank may be needed but in most cases installing a vacuum dehydrator will cure the plague of moisture-ridden lube oil.

Vacuum dehydrators remove water through a combination of temperature and vacuum pressure which evaporates and removes the water. Vacuum dehydrators also remove entrained gas that is likely to be process gas that brought the moisture with it! This is an extremely effective way or eliminating water in all forms in your oil. By installing the dehydrator as a permanent attachment to a lube tank in a kidney loop, moisture can be made a problem of the past. This is a relatively cheap solution to ensure your lubricant is clean, dry, effective and lasts much longer.

Summary

In many situations the presence of moisture in lubrication oil was not anticipated at the OEM design and construction phase and the detection of this moisture doesn’t lend itself to a root cause solution. The fact is that many process gases are wet and lube oil contamination is possible. Designers may also not adequately assess the regional humidity implications and not build in suitable safeguards to protect the lubricant.  The consequences can be catastrophic and expensive with a round robin blame game on why the compressor asset has failed.  Any time that water is present in lube oil, the asset can be compromised from wear, corrosion, varnish and sludge.

It is better to address the problem with a relatively cheap solution regime of breathers and vacuum dehydration than allow a detected problem to jeopardise asset uptime or the repair and replacement of failed components.

James McAllister
james@biokem.com.au
www.biokem.com.au

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