Pipeline dew point test
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Recently, our company was working on determining the dew point of pipes for a new project. We have encountered a problem and are seeking advice from experts. Is there any standard that specifies the pressure to be used when testing the dew point after purging the pipes with nitrogen? In my opinion, for the same set of pipes, when they are pressurized with nitrogen at different pressures, the dew point values obtained should vary, as the dew point of nitrogen is relatively low. The higher the pressure, the more the nitrogen dilutes the gases already present in the pipes, resulting in a relatively lower moisture content. Also, it is often mentioned that the dew point needs to reach a certain value; does the dew point referred to here mean the dew point at atmospheric pressure?
Additional note: The reason for asking the above question is that there is a pipe containing a small amount of water inside, and the dew point measured was around minus 20 degrees. The test was conducted at a pressure of 5 bar; could it be that the high pressure makes it difficult for the water to evaporate, resulting in a lower dew point reading?
I wonder what type of dew point meter the original poster is using? The dew point value should be independent of pressure.
Definition: The lower the temperature, the lower the saturated vapor pressure. Therefore, for air containing a certain amount of water vapor, the temperature at which the pressure is kept constant and the saturated vapor pressure drops to equal the actual vapor pressure at that time is called the dew point. Put simply, the temperature at which water vapor in the air turns into dew drops is called the dew point. So it is related to stress to some extent. For the pipelines you are purging, they are likely nitrogen pipelines or instrument air pipelines, assuming nothing goes wrong. For the production of such devices, the instrument air temperature is generally required to be above -40 degrees; otherwise, liquid accumulation will occur, affecting the functionality of the instruments. What the original poster mentioned about -20 is understandable as well; this definitely won’t pass the test.
Don’t you measure the dew point at the end of the purging process? Isn’t that end used for venting? Also, isn’t the pressure of the dew point meter determined by the gas filling it? Therefore, the dew point measured should not be closely related to the pressure used for purging. It’s understandable that the dew point measured due to liquid accumulation in the pipes is only -20°C, as when you use 5 bar of nitrogen for purging, the nitrogen stays in the pipes for a short time, so there is less moisture present. If you seal the pipes for a while before measuring the dew point again, the value will definitely be higher. Additionally, the requirements regarding dew point should be related to the requirements of each system. It depends on what requirements your system has
Reasonable. The dew point measured by the poster is that of nitrogen, so it is naturally very low. The amount of water carried away by the dynamic process is limited; that is, the water content per unit volume of nitrogen gas is very small. (Note: The dew point indicates the amount of water contained in a unit volume of gas.) In a closed container, if the water evaporating from the accumulated liquid cannot be carried away, saturation will eventually be reached (if there is enough liquid); in this case, the dew point equals the temperature of the gas inside the container.