Lubricant’s Saturation Point & Water Retention – Myth Busted!

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My previous write-up on “Water Ingress Mechanism in Lubricants” attracted a lot of comments, many sent personal messages asking more questions which are critical/confidential. To answer those questions (in general), here is a write up on “Lube oil temperature (saturation point) – related to water retention”.

As discussed earlier, water ingress in lubricants occurs in three steps. First – water in dissolved state, followed by emulsification, and the last is its presence in free form. As water gets dissolved in lubricants, it crosses the saturation point, only after which it starts to emulsify. Saturation point – in simple terms, is nothing but the maximum amount of water that the lubricant can hold in dissolved state for a given volume.

Is the saturation point constant for a lubricant?

Definitely not! It is highly dependent on the lubricant’s temperature. How much water a lubricant can hold at saturation strongly depends on the fluid base stock type, the additive package, temperature, and pressure. By water, for the sake of this write-up, I refer to dissolved water unless otherwise specified.

It is not uncommon for many to think that a lubricant’s saturation point is directly proportional to the oil’s operating temperature – i.e., the warmer the oil, the more dissolved water it can retain.

On the converse, the correlation between the lubricant’s saturation point and its operating temperature is inversely proportional – i.e., as the temperature increases, the saturation point of any given oil will decrease. This means, as the lubricant temperature increases, the amount of dissolved water that the lubricant can hold decreases.

Total Water % = Free Water + Dissolved Water

(Emulsified water is eliminated for easy understanding)

The below graph from machinery lubrication – clearly explains this phenomenon.

From the above graph, it is evident that as you increase the temperature of the lubricant, the saturation % drops almost linearly. Though the total water % is same, as the temperature increases, the saturation point decreases, and thus, by this – some dissolved water could either become free or emulsified water (superficially speaking).

As marine engineers (and of course an engineer from any industry) would draw an oil sample when the machinery is running or when the oil/system is warm. Does this supplement the theory that when the oil is warm, the % saturation decreases and thus more dissolved water would be available as an emulsion or as free water?

When such sample is analyzed in the lab, would reflect almost near to actual water % than when an oil sample collected at lower temperatures.

To answer the question – how can this help in a practical shipboard scenario?

  1. Collecting lubricant samples when the machinery is running or at least when the system is warm
  2. Settling & draining at elevated temperatures could help to remove more water from the system (talk to lubricant experts if you have a specific problem)
  3. The above-detailed process may not hold good if the temperature exceeds 100 deg Celsius (water would evaporate – Lubricant might oxidize due to local heating – other complications)

Over to you – Please share your thoughts!

Note: The theory/thoughts expressed here are author’s own (referring to various sources – STLE, Machinery Lubrication) and shipboard experience. For specific lubricant related issues, please do not hesitate to consult lubricant experts (or) you can always get in touch with me.

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Source: Sriram-Linkedin, Machinery Lubrication

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