Water In Oil Measurement Concepts And Selection Methods

Abstract: There are a variety of methods to measure the water in oil and the appropriate method need to be selected according to the specific requirements. Based on the clarification of the concept and comparison from principle and application level, a relatively complete analysis is provided.

0 Introduction

       With the development of the economy, the deepening of technological innovation and the refinement of production, the measurement of water in oil has become a requirement. However, before deciding which measurement method to use, it is necessary to have a clear concept of water in oil measurement, and to compare the advantages and disadvantages of various measurement methods in order to use the most suitable method to obtain the best results.

1 Basic concept and status of water in oil measurement

1.1 Basic concepts

       First of all, we need to find out what is water in oil and the form in which water exists in oil.

       Water has three forms: gaseous, liquid and solid. In the gaseous state, it exists in the form of water molecules, while in the liquid state and solid state, it exists in the form of chemically combined water. According to the definition of ISO3171, the existence of water in oil in the form of dissolved water, suspended water, free water. The definitions are as follows.

1) Dissolved water   The water contained in the oil at the current temperature forms a solution, i.e. is dissolved in the oil. At this point the water exists in the form of water molecules.

2）Suspended water   Water in oil is suspended as droplets in the oil. At this time, the water exists in the form of chemically combined water. With the change of temperature and pressure conditions, the suspended water may become dissolved water or free water.

3）Free water   The water forms a stratification with the oil and usually sinks at the bottom of the oil. The water at this point exists in the hydrated water form.

4）Total water   Includes the sum of all three forms of water mentioned above.

       According to the definition can be seen, the first thing to understand the measured exactly the kind of water, in order to choose the right product.

1.2 Current status of water-in-oil measurement

       The commonly referred to crude oil water content measures the total water content in the oil, and the measurement range is from 0% to 100%, i.e. from the oil continuous phase to the water continuous phase. It is mainly used for metering handovers, material calculations, contractual agreements, etc. There are numerous online methods of measurement in addition to laboratory methods. As for the measurement of dissolved water, it is usually used in lubricating oil systems, turbines, transformers, etc. Because of its low water content (ppm level), none of the online measurement methods for water content can effectively measure it. Therefore it is necessary to clarify the concept of measurement of water in these two types of oil and to compare the commonly used measurement methods.

2 Measurement of dissolved water in oil

2.1 Basic concepts

       Any liquid can dissolve a portion of water. The maximum amount that can be dissolved is related to its saturation point. Once the saturation point is reached, the water will precipitate out and become free water. Since the density of most oils is less than that of water, the free water will stratify with the oil and settle below it.

       The saturation point of oil is related to many factors, such as the basic component of the oil (mineral oil or synthetic oil), additives, emulsifiers, type of oxidizer, etc. In addition to this, the saturation point of oil as a working fluid can change during its service life. The change of temperature is one of the main factors affecting the saturation point. Another major factor is the change in the chemical composition of the dynamic oil system.

       Traditionally, water in oil is measured in ppm (parts per million), which expresses the absolute value (volume or mass) of the ratio of trace moisture to oil in the oil. By measuring the ppm value, the absolute value of trace moisture can be determined.

       However, ppm measurement has its limitations in that it does not reflect the saturation point of water in the oil. That is, in a dynamic oil system where the saturation point of the oil is fluctuating, the ppm measurement will not provide information on whether the trace moisture in the oil is approaching its saturation point. When the moisture content approaches the saturation point, free water will most likely precipitate out and for most oil systems, free water will cause damaging contamination.

       To avoid these problems, a method of measuring water activity can be used. Water activity represents the ratio of dissolved water in the medium at the current temperature to the maximum water that can be dissolved in that medium at the same temperature.

aw =  p  ∕  p0

where

p------ indicates the partial pressure of moisture in the medium at the current temperature

p₀------ denotes the saturation vapor pressure of pure water at the current temperature

       It can be seen that a_w is a function of both the saturation point and the actual moisture in the oil. That is, a_w will always provide the saturation margin of the water in the oil. A correspondence between a_w and ppm can be obtained for any oil, and this correspondence will change over time. Changes in components due to chemical reactions will affect the saturation point and also its correspondence with a_w.

2.2 Online measurement methods

2.1.1 Direct measurement of ppm

       Water molecules in the oil are captured by a very moisture-sensitive electronic assembly consisting of two conductive layers and a moisture-absorbing sensitive layer of the humidity sensor, whose conductivity is related to the water content. The conductivity of the sensor is enhanced only when water molecules enter the sensor's electronic assembly. The sensor uses this property for calibration and storage.

       The calibration consists of a humidity sensor, a temperature sensor and a Karl Fischer titration method to form a calibration curve, which is built into the instrument. During measurement, the actual ppm value is obtained from the humidity sensor measurement, the temperature value and the internal 3-dimensional curve.

       The feature of this measurement method is that the ppm value is measured directly and calibrated according to the actual oil used. The disadvantage is that it is not possible to determine the saturation margin of water in the oil.

2.2.2 Direct measurement of water activity a_w

       The sensor is formed by the upper and lower electrodes and the insulating material between them to form a capacitance. The absorption and release of water molecules by the dielectric material in the middle leads to a change in the dielectric constant, which causes a change in capacitance. The absorption of water molecules is proportional to the water activity.

       The calibration process is directly calibrated with an a_w standard meter, supplemented by temperature compensation.

       The advantage of this method is that the saturation margin of water in oil is measured directly, independent of the actual value of the saturation point. It is advantageous to take preventive measures in advance to prevent the appearance of free water. The disadvantage is that if the ppm value needs to be measured, the accuracy it can provide is not high.

2.2.3 Indirect ppm measurement by dew point meter

       Typically dew point meters are used to measure trace moisture in gases, but they can also measure moisture in non-polar liquids. The tiny gap in the moisture sensitive probe allows only gases to enter, not liquids. According to Henry's law, the molar concentration of a gas dissolved in a liquid is proportional to the partial pressure of the vapor in equilibrium with it at a given temperature.

       For non-polar liquids containing trace moisture, the partial pressure of water vapor measured by the moisture-sensitive probe can be used to calculate the concentration of the amount of water dissolved in the liquid, i.e.:

Cw=Kpw

where

C_w------ material concentration of water in the liquid

p_w------ partial pressure of water vapor measured by the moisture sensitive element

K-------- Henry's constant in the corresponding scale

       The value of Henry's law constant can be calculated from the known water saturation concentration of the solution and the partial pressure of water saturation vapor

K=Cs/ps

where

C_s------- saturated concentration of water in the liquid at the measurement temperature

p_s------- saturated partial pressure of water vapor at the measurement temperature

       The advantage of this method is that it is directly measured by a dew point meter, which is easy to understand; there is an explosion-proof option to measure dissolved water in finished refinery products such as gasoline and naphtha or intermediate products. The disadvantage is that the saturation constant K is different for different media and varies with temperature, causing uncertainty in the measurement. Some manufacturers only provide several standard solutions of K value, if not the standard solution, the user needs to provide. Other manufacturers provide on-site sampling, measured by Karl Fischer titration to determine the saturation constant, but the price is expensive and not conducive to promotion. In addition, the dew point meter probe aging and zero point drift problem is always present, can only be solved by regular calibration.

2.3 Summary

       The methods of measuring dissolved water in oil have their advantages and disadvantages and all follow Henry's Law. If the precipitation of free water needs to be strictly controlled, it is recommended to measure the water activity. If the oil is fixed, it is recommended to calibrate directly with the oil, and the accuracy can be guaranteed. If explosion-proof is required, indirect measurement with a dew point meter can be considered.

3 Conclusion

       When choosing the water in oil measuring instrument, we must first understand the medium of measurement, requirements, and then choose the right solution according to the principle, measuring range, and influencing factors.