Removing moisture from hydraulic oil is critical for maintaining system performance and preventing rust, oxidation, cavitation, and loss of lubricity. Moisture can exist in three forms:
- Dissolved water – Invisible, fully blended in oil
- Emulsified water – Cloudy, forms tiny droplets in oil
- Free water – Settles at the bottom, visible separation
🔧 Methods to Remove Moisture from Hydraulic Oil
🌀 1. Vacuum Dehydration (Most Effective)
- How it works: Oil is heated under vacuum to vaporize and remove water.
- Removes: Free, emulsified, and dissolved water.
- Used in: Industrial hydraulic systems, turbines, power plants.
✅ Pros: Very effective, works for all moisture forms
❌ Cons: Requires specialized equipment
💨 2. Desiccant Breathers
- How it works: Installed on reservoirs, these remove moisture from incoming air before it contacts the oil.
- Removes: Prevents new moisture from entering
- Used in: Long-term moisture control
✅ Pros: Passive, cheap, easy to install
❌ Cons: Doesn’t remove existing water in oil
🧊 3. Coalescing Filtration
- How it works: Special filter media causes water droplets to merge (coalesce) and be removed mechanically.
- Removes: Free and some emulsified water
- Used in: Mobile equipment, industrial systems
✅ Pros: Good for large free water removal
❌ Cons: Not effective on dissolved water
🔁 4. Centrifugal Separation
- How it works: High-speed spinning separates water and particles by density.
- Removes: Free water and heavy particles
- Used in: Power stations, marine systems
✅ Pros: Continuous processing
❌ Cons: High cost, not effective on dissolved water
🔥 5. Thermal or Hot-Air Drying
- How it works: Oil is heated; moisture is evaporated.
- Removes: Some free and emulsified water
- Used in: Emergency situations or batch treatment
✅ Pros: Simple method
❌ Cons: Risk of oil oxidation if overheated
🧴 6. Chemical Absorbents
- How it works: Water-absorbing additives or clay packs absorb moisture from oil.
- Removes: Limited free/emulsified water
- Used in: Small systems or temporary fixes
✅ Pros: Low-cost, portable
❌ Cons: Limited capacity, needs replacement
🧪 How to Check for Moisture in Hydraulic Oil?
| Method | Accuracy | Notes |
|---|---|---|
| Crackle test | Quick check | Heat a drop of oil on a hot plate; crackles = water |
| Karl Fischer titration | Very precise | Lab test for exact water content (ppm) |
| Infrared or Capacitive Sensors | Real-time | Installed in-line for continuous monitoring |
✅ Best Practice Approach
| Step | Action |
|---|---|
| Prevent | Use desiccant breathers and sealed systems |
| Monitor | Regular oil analysis (moisture content, cloud point, etc.) |
| Remove | Use vacuum dehydration or coalescing filtration as needed |
| Replace | If oil is saturated, severely emulsified, or degraded |
📌 Rule of Thumb:
- Water > 500 ppm = Action needed
- >1000 ppm = Immediate dehydration or oil replacement recommended
Hydraulic oil is a liquid medium widely used in hydraulic systems to transfer action between two surfaces that are in contact with each other, making machine parts more flexible. However, hydraulic oil may contain moisture, which can cause various problems in the hydraulic system, so it is very necessary to remove the moisture from the hydraulic oil.
In hydraulic oil, water exists in two forms: dissolved and suspended. Dissolved water cannot be removed by simple sedimentation because it forms hydrogen bonds with the hydrocarbons in the hydraulic oil. Suspended water refers to small water droplets formed due to factors such as the viscosity and temperature of hydraulic oil, which can be removed through physical methods.
For removing moisture from hydraulic oil, the following methods are available:
Natural drying method The natural drying method is a relatively simple method. It is to place the hydraulic oil in a dry and ventilated place to allow the water in the oil to evaporate naturally. The advantage of this method is that it is simple and easy to implement, but it takes a long time to dry and is greatly affected by weather conditions.
Drying method The drying method uses heating to remove moisture from hydraulic oil. This method can remove moisture from hydraulic oil in a short period of time, but requires the use of heating equipment, so the cost is higher and may affect the performance of the hydraulic oil.
Chemical treatment method The chemical treatment method removes moisture from the hydraulic oil by adding an appropriate amount of demulsifier, desiccant or water absorbent to the hydraulic oil. This method can achieve drying effect in a shorter time and has less impact on the properties of hydraulic oil. However, chemical treatments are more expensive and require regular tank cleaning and desiccant replacement.
When choosing a method to remove moisture from hydraulic oil, there are a number of factors to consider. If the amount of hydraulic oil is small and the time required to remove moisture is short, drying or chemical treatment can be selected. If the amount of hydraulic oil is large and it is required to have a small impact on the properties of the oil, you can choose the natural drying method. Of course, there are many aspects such as equipment maintenance and cost that need to be taken into consideration when choosing a method.
Finally, it needs to be emphasized that no matter which method is chosen, it should be ensured that the quality of the hydraulic oil after treatment is completed meets the requirements for use. During handling, moisture should be avoided from reentering the hydraulic oil. At the same time, if the hydraulic oil contains a large amount of impurities, metal particles and other harmful substances, it is also necessary to consider using filtration and other methods for pretreatment to ensure the normal operation of the hydraulic system.