Measuring the moisture in oil is essential to the smooth running of many processes. The presence of free moisture can cause corrosion, oxidation or acid formation, which may lead to particle contamination and reduced life of the equipment or compromised operation.
Free moisture also decreases the compressibility, insulating and cooling properties of oil affecting the performance and potentially safety of hydraulic, transformer and quenching processes.
In this article, we will provide an overview of oil, its properties, uses and applications and how to measure the moisture content in different oils to increase efficacy.
- Overview of Oil and Its Uses
- Popular Uses of Oil
- Hydraulic Oil
- Transformer Oil
- Quenching Oil
- Lubricating Oil
- How to Optimise Oil Performance
- Types of Moisture in Oil Measurement
- Moisture in Oil Transmitters
Overview of Oil and its Uses
Oil is commonly used across transportation, industry and agriculture due to its extensive range of properties that include:
- Non-compressibility
- Stable viscosity parameters
- Lubricity
- Good heat dissipation,
- Demulsibility
- Barrier layer
- High boiling and low freezing points
- Availability
- Low toxicity to humans
Popular Uses of Oil
Oil has various functions.
- The non-compressibility of oil makes it an ideal hydraulic fluid, primarily to transmit energy in hydraulic machinery and equipment.
- Transformer oils operate effectively at very high temperatures. Their main functions are cooling, insulating, and stopping corona discharges and arcing due to their effective heat dissipation, high dielectric strength, thermal conductivity and high chemical stability.
- Oil’s rapid heat transfer and wetting properties make it an ideal quencher for the fast cooling of metals.
- Its ability to reduce friction between two surfaces makes it an ideal lubricator for engines, compressors, pumps, power generators, hydraulic machinery, gearboxes and transmissions.
Hydraulic Oils
Hydraulic oil, also known as hydraulic fluid, is a synthetic (man-made) or mineral-based fluid that has several functions:
Its primary task is to provide a medium for energy transfer, so it acts as a lubricant, a sealant, a coolant for the equipment and a means of removing contaminants from the system.
Applications of Hydraulic Oil
Transportation
Aircraft – To move and actuate landing gears, thrust reversers, wing flaps, brakes and flight controls.
Nautical – For steering gears, deck machinery, valve operations, bow and stern thrusters, automation and control systems, propulsion engines and stability.
Vehicles – To operate hydraulic brakes and clutch, automotive lifts and jacks.
Agricultural machinery
Tractors – For steering and braking, heavy lifting, digging operations and scraping.
Ploughs, sprayers, weeders and harvesting equipment
Log splitters
Construction
Construction machinery – including excavators, loaders, dump trucks, graders, pavers, compactors, cranes, pile boring and driving equipment.
Shock tube – enables building product manufacturers to check if their products will withstand the shock wave from a blast event, such as an explosion.
Other Industries
Hydro-Control and electric generation – To control gates and valves that determine water flow rates for electric generation.
Mining, extraction and processing of natural resources
Manufacturing & Testing Equipment – Industrial presses, parts presses for automotives, die-casting and aerial lifts
Crushing machinery – To crush coal and other hard minerals.
Theme park rides – e.g. roller coasters
Adverse Effects of Water Ingress in a Hydraulic System
Measuring the moisture in oil is crucial because water can affect:
1. The components of a hydraulic system
Water may act as an electrolyte conducting electricity between dissimilar metals, causing galvanic corrosion to bearings and gear surfaces and through cavitation.
Water has a higher vapour pressure than oil, which can lead to vapourisation and forceful collapse of the vapour bubble against metal surfaces in the lower pressure regions of the system resulting in physical damage.
2. The oil itself
Physically
Oil’s viscosity and lubricity may be reduced. This can lead to rapid wear of moving parts in contact with one another and their load-carrying characteristics.
Also, if contaminants are present, water aids the agglomeration of smaller contaminant particles causing valves to stick or slow down and the blockage of component orifices and filters. This affects the smooth running of the system and, in extreme cases, causes system failure.
Chemically
Water’s reaction with oil additives, such as oxidation inhibitors, produces acids and precipitates, which increase wear and interference.
Thermo-oxidative stability
Water combined with heat accelerates oil oxidation, leading to higher viscosity and deposits, such as polymeric compounds or sludges.
The presence of water will increase deposition, including soot and coking.
3. The efficiency of the hydraulic system in terms of its compressibility
Compressibility can be affected by microbial growth, especially when air is present, web-like masses can rapidly form that require significant force to dissipate to unblock filter elements. This resistance adversely affects the operation of a hydraulic system.
How does water ingress occur?
Rainwater, snow and moist air can leak into a system through a reservoir if this is located outdoors.
Process water, i.e. washdown water, cooling water or steam, can also find its way into the system along with water from leaky coolers or heat exchangers.
Moisture in Oil Measurement
Continuous online moisture in oil monitoring will indicate the presence of water so that appropriate countermeasures can be taken to avoid system breakdowns and premature ageing of components and the oil.
Transformer Oils
A transformer is an electromagnetic device that allows A.C. voltage to be increased or decreased to the required level for transmission or specific uses
A transformer comprises two separate coils that wind on a coated iron core.
A.C. voltage is supplied to the primary coil and output from the secondary coil. The output voltage is determined by the number of windings on the secondary relative to the primary coil.
Transformer oil must have high dielectric strength, thermal conductivity, and chemical stability, which should be maintained for extended periods at high temperatures.
Paper insulation is used on winding wire to minimise heating or damage to the transformer. It is also used in the form of strips between winding layers as it has high dielectric strength and is free of conducting particles. The paper is immersed in mineral oil to act as an insulator and a cooling agent.
Applications of Transformer Oil
The primary function of oil-filled transformers is to make high-voltage electricity usable for end-users.
Most of these transformers are step-down types, including one or three phases. They are typically installed outside on the ground or up a pole and are used for electrical transmission and distribution lines.
Transformers are also used in:
- Trains – on-board traction transformers convert energy into speed moving passenger and freight trains.
- Ships – partitioning and isolating different parts of the electrical distribution system to provide different voltages and phases for various functions.
- Industry – steel manufacturing, electrochemical, aerospace, data processing, communications, biomedical, biochemical, military, etc
- Electrical vehicle charging stations
- Renewable energy – to collect and distribute solar and wind power. Wind turbines are each equipped with a step-up transformer which increases the generator output voltage from a few hundred volts to that required by the collector system.
- Hospitals – 30 – 230V isolation transformers with an electrostatic shield between the primary and secondary or safety transformers with 24V in the secondary circuit are used to protect patients and staff from potential electric shock and to prevent system failures.
Adverse Effects of Moisture in Transformer Oil
Water impacts the insulation’s dielectric properties, resulting in localised dielectric failure.
You may experience arcing between windings, treeing, and creeping discharges in systems containing a high moisture level where hot spots exist or universal failure if free water forms due to a leak or ageing.
How does moisture enter transformer oil?
Moist air can enter the transformer oil in various ways.
The process of “Transformer Breathing” is due to oil expansion and contraction as its temperature changes if the desiccant within the “breathing tube” is saturated. The hotter the oil, the more it will absorb moisture.
Paper degradation by-products and the natural gassing of transformer oil generates moisture in small quantities (approximately 0.5 to 1 ppm per year). Heat and electrical stress can cause paper degradation where the cellulose chain beaks, generating hydroxide molecules. Together with hydrogen released by the oil and omnipresent oxygen, there is the potential for water formation.
The ageing rate of the paper is exponentially related to its moisture content. If the water content of the paper exceeds 2-3% of the dry paper weight, dielectric breakdown occurs.
With increased moisture in the system, cycling occurs, resulting in yet more moisture and destruction.
Dynamics
Moisture can exist in one of three states in oil:
Dissolved – where the water hydrogen bonds to hydrogen atoms of the oil molecules.
Emulsified – has a supersaturated milky appearance. The water hasn’t yet totally dissociated.
Free – where water forms as separate droplets or a layer to the oil. Free moisture poses a threat to the reliable and safe functioning of a transformer.
The moisture content in oil inside a transformer varies and is temperature-dependent.
As a transformer operates, moisture will migrate from the insulation paper into the oil as it heats and return to the paper from the oil as it cools. It never reaches a steady state equilibrium due to load changes affecting the winding temperature.
The physical state of the moisture in oil is also temperature dependent; as the temperature of the oil increases, so does the solubility of the water in oil.
The critical parameter to maintain is aw – water activity or Relative Saturation(% R.S.).
These terms mean the same; the amount of water dissolved in the oil relative to the solubility level (maximum amount of dissolved water possible) at that temperature.
This correlates far better to the dielectric breakdown voltage than moisture concentration alone.
aw should be kept to less than 0.5 ( or R.S of 50%)
Measurement of moisture in oil inside a transformer
Moisture varies with temperature through two mechanisms as described above:
- Transfer of moisture from/to the insulation paper
- Solubility
Therefore we recommend the installation of a permanent moisture in oil transmitter for monitoring purposes.
Quenching oil
A quenching process induces rapid, controlled cooling. It is used in processing alloy steels to achieve specific properties, including hardness, strength, or toughness.
Quenching oil serves two primary functions.
- It expediates the hardening of steel by regulating heat transfer during quenching and moistens the steel, minimising cracks and distortions.
- It is used rather than water because it has a slower rate of cooling compared to water but is faster than air. Its higher boiling temperature causes the slower convective cooling stage to start sooner, enabling the release of transformation stresses which is the major problem with rapid water cooling.
Quenching process
Cooling occurs in 3 stages:
- Film boiling – upon immersion of the hot metal into the oil, a vapour blanket forms at the surface and serves as a means of slow cooling via conduction.
- Nucleate boiling – the vapour blanket collapses at various points on the surface as the metal cools, giving rise to nucleate boiling (boiling of the oil). Furthermore, rapid cooling occurs.
The composition of the oil will determine the points at which this transition occurs and the rate of heat transfer. The duration of this phase depends on the boiling point of the oil.
- Convective heat transfer – Once the temperature of the metal has fallen below the boiling point of the quencher oil, slow cooling occurs. It is exponentially related to the oil’s viscosity as faster cooling occurs with lower viscosity oils.
Adverse Effects of Moisture in Transformer Oil
Water impacts the insulation’s dielectric properties, resulting in localised dielectric failure.
You may experience arcing between windings, treeing, and creeping discharges in systems containing a high moisture level where hot spots exist or universal failure if free water forms due to a leak or ageing.
How does moisture enter transformer oil?
Moist air can enter the transformer oil in various ways.
The process of “Transformer Breathing” is due to oil expansion and contraction as its temperature changes if the desiccant within the “breathing tube” is saturated. The hotter the oil, the more it will absorb moisture.
Paper degradation by-products and the natural gassing of transformer oil generates moisture in small quantities (approximately 0.5 to 1 ppm per year). Heat and electrical stress can cause paper degradation where the cellulose chain beaks, generating hydroxide molecules. Together with hydrogen released by the oil and omnipresent oxygen, there is the potential for water formation.
The ageing rate of the paper is exponentially related to its moisture content. If the water content of the paper exceeds 2-3% of the dry paper weight, dielectric breakdown occurs.
With increased moisture in the system, cycling occurs, resulting in yet more moisture and destruction.
Dynamics
Moisture can exist in one of three states in oil:
Dissolved – where the water hydrogen bonds to hydrogen atoms of the oil molecules.
Emulsified – has a supersaturated milky appearance. The water hasn’t yet totally dissociated.
Free – where water forms as separate droplets or a layer to the oil. Free moisture poses a threat to the reliable and safe functioning of a transformer.
The moisture content in oil inside a transformer varies and is temperature-dependent.
As a transformer operates, moisture will migrate from the insulation paper into the oil as it heats and return to the paper from the oil as it cools. It never reaches a steady state equilibrium due to load changes affecting the winding temperature.
The physical state of the moisture in oil is also temperature dependent; as the temperature of the oil increases, so does the solubility of the water in oil.
The critical parameter to maintain is aw – water activity or Relative Saturation(% R.S.).
These terms mean the same; the amount of water dissolved in the oil relative to the solubility level (maximum amount of dissolved water possible) at that temperature.
This correlates far better to the dielectric breakdown voltage than moisture concentration alone.
aw should be kept to less than 0.5 ( or R.S of 50%)
Measurement of moisture in oil inside a transformer
Moisture varies with temperature through two mechanisms as described above:
- Transfer of moisture from/to the insulation paper
- Solubility
Therefore we recommend the installation of a permanent moisture in oil transmitter for monitoring purposes.
Quenching oil
A quenching process induces rapid, controlled cooling. It is used in processing alloy steels to achieve specific properties, including hardness, strength, or toughness.
Quenching oil serves two primary functions.
- It expediates the hardening of steel by regulating heat transfer during quenching and moistens the steel, minimising cracks and distortions.
- It is used rather than water because it has a slower rate of cooling compared to water but is faster than air. Its higher boiling temperature causes the slower convective cooling stage to start sooner, enabling the release of transformation stresses which is the major problem with rapid water cooling.
Quenching process
Cooling occurs in 3 stages:
- Film boiling – upon immersion of the hot metal into the oil, a vapour blanket forms at the surface and serves as a means of slow cooling via conduction.
- Nucleate boiling – the vapour blanket collapses at various points on the surface as the metal cools, giving rise to nucleate boiling (boiling of the oil). Furthermore, rapid cooling occurs.
The composition of the oil will determine the points at which this transition occurs and the rate of heat transfer. The duration of this phase depends on the boiling point of the oil.
- Convective heat transfer – Once the temperature of the metal has fallen below the boiling point of the quencher oil, slow cooling occurs. It is exponentially related to the oil’s viscosity as faster cooling occurs with lower viscosity oils.
Adverse effects of moisture in quenching oil
Low-level moisture
Distortion and cracking.
High-level moisture
Fires, explosions, and destruction of equipment.
Quench oils typically require moisture content levels below 0.1%.
Means of water ingress
Water ingress occurs from outside and within the quenching system via cooling equipment using water coolant, including heat exchangers, fans and doors and from badly reclaimed oil.
Moisture measurement
The most common form of measurement for moisture in quenching oil is the crackle test.
It is a qualitative test that indicates the presence of emulsified or free moisture at the time the sample is taken.
It provides no information regarding the level of dissolved moisture and how closely this might be approaching saturation.
This test is of no use as a predictive test for proactive maintenance.
Lubricating Oil
Lubricating oil reduces the friction, heat, and wear between mechanical components.
It has various applications, including hydraulic, in the automotive, marine and manufacturing industries.
Applications of lubricants:
- To minimise friction and wear under a variety of temperature and pressure conditions
- As a cooling fluid
- To remove contaminants
- To provide a protective “coating” preventing corrosion and oxidation
- To give long term stability
Adverse Effects of Moisture In Oil Used Primarily as a Lubricant.
Increased friction and oxidation
It will cause damage to the metal machinery as the wetting ability is reduced by increased emulsified or free moisture, especially when the layer is fragile.
Flash vapourisation
It will cause implosions, damaging the metal surface.
Formation of sludge
Causes a subsequent restriction in flow to moving mechanical parts and the potential blockage of filters and valves by emulsified water molecules absorbing soot and oxidation products.
How to Optimise Oil Performance
This article explains that free and emulsified water are the most harmful conditions. They cause the most damage to oil, machinery, hydraulic systems, transformers, quenchers, or other equipment where oil is used as a lubricant.
To optimise performance and prevent emulsification or free moisture, it is best to monitor the moisture in oil “in situ” and enable timely corrective action.
Due to the various factors that influence stability, it is crucial to monitor moisture continuously as taking a single sample will provide very little helpful analysis about the current state of the live system.
In transformers, continuous monitoring will capture the situation as the temperature of the oil changes and moisture migrates between the paper insulation.
The critical moisture level in oil differs according to the composition and temperature of the oil
Types of Moisture in Oil Measurement
In-line capacitive moisture in oil sensors are designed to operate on the same principle as humidity sensors. They are reliable, robust sensors which provide moisture outputs as:
· Aw (water activity)
An aw is a relative output which is extremely useful to provide meaningful, accurate information about the status of moisture in oil. It is the amount of water dissolved in the oil relative to the solubility level (maximum amount of dissolved water possible) at that temperature. It is independent of oil type and age. No calibration is required or knowledge of specific parameters.
Accuracy will not be lost as the oil ages, and you will get the same information regardless of what oil is used.
· Calculated ppm or dissolved water
A ppm reading supplies the total concentration of moisture, but no information regards the state of the water, i.e. whether it is dissolved, emulsified or free.
Moisture in oil transmitters
We supply excellent in-line monitoring moisture in oil transmitters that accurately detect water activity, temperature and ppm water content. They enable predictive and preventative maintenance to minimise the risk of damage to machines or transformers.
Compact EE364 measures aw, temperature, and calculates ppm over a range of -40-100 deg C up to 20 bar. Features 2 analogue and a Modbus RTU output
EE381 measures aw, temperature, and calculates ppm over a range of -40-120 deg C up to 20bar. A 100-bar option is also available. Features a display, and either voltage or mA analogue outputs
MOP 301 probe measures aw, temperature, and calculates ppm over -40 -120 deg C and up to 20bar. It features Modbus RTU comms. This probe comes in 200 or 400 mm lengths and can be inserted to the desired depth using a slide fitting.
It can be installed permanently in one location or moved between locations if required. An optional ball valve enables installation and removal without process interruption.
EE360 measures aw, temperature and calculates ppm over an extended temperature range of -40-180 deg C. up to 20 bar. It comes with up to 6 probe lengths from 10 cm to 1 m and features a display, two analogue outputs, a digital output and Integrated data logging. An optional ball valve is available to help with easy installation and removal from the process.
Oilport 30 is a handheld option. It offers a lower-cost solution for monitoring at multiple locations.
