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Moisture Dew Point vs Moisture Content, Which One is More Important?

Updated: Sep 29, 2023

Moisture Dew Point vs Moisture Content, Which One is More Important?

In the world of industry, moisture can cause all sorts of problems. It may seem invisible, but it can wreak havoc on equipment, affecting manufacturing, storage, and transportation.


In general, there are two measures of moisture in a gas:


a- Moisture Dew Point

b- Moisture Content.


In this article, we will dive into the world of moisture control and explore which measurement truly holds the key to achieving optimal performance and safeguarding our processes.


We focus our discussions on moisture in natural gas, although many of the same issues pertain to other industries as well. Moisture impacts gas processing, requiring dehydration to prevent problems. To address these effects, dew point analyzers are used, and natural gas is processed to remove moisture before transportation or use, employing methods like adsorption and glycol absorption.


What Is Moisture Dew Point?

Moisture dew point means the temperature at which gas becomes saturated with moisture. It's when the gas gets so full of moisture that it starts to turn into liquid droplets. This dew point (the temperature at which gas turns into liquid) is important because that is the temperature at which you will have free liquid water in the gas stream. Liquid water is what causes most of the problems in the natural gas industry. So, knowing the dew point temperature, helps one avoid getting liquid water.

Moisture Dew Point
What is Moisture Dew Point

You see, as the temperature goes down, the gas can't hold as much moisture. So, when it reaches the dew point, it's reached its limit and can't hold any more water vapor. This is when the extra moisture starts to condense into liquid form.


Knowing the dew point of moisture is useful because it helps us prevent many problems. When moisture condenses, it can cause issues like rust, mold, or equipment malfunctions in gas pipelines and power plants. By keeping an eye on the dew point, we can take steps to avoid these problems and make sure things run smoothly.


The natural gas industry uses moisture dew point measurements in areas like biogas, refineries, gas storage and distribution, etc. It helps them control moisture levels and prevent any unwanted side effects that can affect their products or equipment.


How Is Moisture Dew Point Measured?

The only way to measure moisture dew point is through a chilled-mirror device. The mirror is cooled down until moisture starts to appear on its surface. The temperature at which this happens is the dew point temperature. The device carefully controls and monitors the mirror's temperature to get an accurate reading.


Measuring moisture dew point using a chilled-mirror device is a "First-Principle" method. Because it related back to the fundamentals of physical properties.


Dew point analyzers are also considered one of the most accurate methods for measuring dew points. These devices provide precise and reliable readings. They are widely used in natural gas pipelines, refineries, gas storage, gas distribution, biogas, city gates, and gas power plants to manage moisture effectively and prevent problems caused by too much moisture. Common problems include pipeline flow issues, corrosion and erosion, pressure drop, loss of energy content, equipment damage, etc.


Moisture dew point has units of temperature.


Unfortunately, many vendors produce moisture content measurement devices but erroneously call them dew point analyzers.


What Is Moisture Content?

In contrast to moisture dew point, moisture content refers to how much water is present in the gas. The units used for moisture content in the natural gas industry are ppm (parts per million), mg/NM^3 (milligrams per Normal Cubic meters), or lbs/MMSCF (pounds per million standard cubic feet, mostly in the US). Notice that all these units specify how much water in how much gas. This is in contrast with units of dew point which is just temperature.


Moisture Content
What is Moisture Content

The amount of moisture is of course related to moisture dew point. The higher the content, the higher the dew point at a given pressure. However, moisture content's main significance in the natural gas industry is to be able to calculate its dew point. This is due to the fact that it is liquid water that causes most of the problems.


How Is Moisture Content Measured?

There are different ways to measure moisture content, depending on what kind of material you're dealing with. The methods are different from the ones used to measure moisture dew point. Here are some common methods:

  • Weighing Method: You take a sample of the material and weigh it. Then you dry the sample to remove the moisture and weigh it again. The difference in weight tells you how much moisture was in the sample.

  • Moisture Analyzer: This is a special machine that applies heat to the sample and measures how much weight is lost as the moisture evaporates. It gives you moisture content based on weight loss.

  • Karl Fischer Titration: This method is used for liquids. It involves a chemical reaction between the water in the liquid and a special substance. By measuring how much of the substance is used up in the reaction, you can figure out the moisture content.

  • Electrical Resistance: This method uses probes or sensors that are inserted into the material. The moisture content affects how well electricity can flow through the material. By measuring the electrical resistance, you can determine the moisture content.

  • Capacitance Measurement: Similar to the electrical resistance method, this technique uses probes with electrodes. The moisture content affects the way the material stores electrical charge. By measuring the capacitance (the ability to store charge), you can estimate the moisture content.

  • Quartz Crystal Measurement: Quartz crystals vibrate at specific frequencies when a voltage is applied to them. The frequency changes with the weight of the crystal. A quartz crystal is coated with a hygroscopic (moisture-absorbing) material. The change in the frequency is correlated to the amount of moisture in the gas stream.

  • Near Infrared Spectroscopy: This method uses a special light that's close to the infrared range. Different moisture levels in the material absorb this light differently. By analyzing the absorption pattern, you can estimate the moisture content.

Moisture Dew Point vs. Moisture Content

Moisture content and dew point are different measurements related to moisture. Here are the various aspects in which they are different:


Moisture dew point is a "First-Principle" measurement. As a matter of fact, moisture content analyzers get calibrated against dew point analyzers to ensure their accuracy.


In natural gas, almost all tariffs and agreements for the sale of natural gas are written in terms of dew point. In the past, reliable dew point analyzers were cumbersome. However, with the advent of advanced technologies such as CEIRS(TM), moisture dew points can be measured directly, accurately, and unambiguously.


Furthermore, moisture dew point can be measured directly at process pressures, whereas most content measurement analyzers work at reduced (close to atmospheric) pressures.


Conclusions


One can specify moisture in both moisture dew point and moisture content. However, in the natural gas industry, the dew point is the critical parameter that is used in operational control. When choosing a method, close attention should be paid to the advantages and disadvantages of the method selected.




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