Natural gas pipeline blockage
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Our company has a methanol production plant that uses natural gas as a feedstock. After being shut down for nearly half a year, it was restarted in a cold-state operation. During the nitrogen circulation phase, it was found that there was a blockage in the natural gas pipeline prior to it reaching the conversion furnace. After tapping the pipeline with external force, it seemed that some adhering substance fell off, and the pipeline became unobstructed. What could be the cause of this? There is a set of control valves in front of this pipeline where abnormalities occur; the medium flowing through it is usually natural gas. Could it be that dust and impurities present in the natural gas, or impurities brought in due to the nitrogen filled into the pipeline before operation, have caused a blockage? Please help analyze it.
Usually, only solid or viscous semi-solid substances can clog pipes; therefore, when gas pipelines become blocked, it is necessarily because solid deposits have formed inside the pipes. In the cold winter, it is common to see many water droplets forming on the inner surface of glass windows, sometimes even flowing down, while there are no such droplets on the outside of the windows. Why is that? This is because water vapor in indoor air condenses on the surface of glass windows, whereas water vapor outside does not condense on their surface. The water vapor content in indoor air is higher than that in outdoor air, resulting in a higher dew point for indoor air compared to outdoor air. Since the surface temperature of the glass is lower than the dew point of the indoor air but higher than that of the outdoor air, dew forms only on the inner surface of the window glass, not on its outer surface. Natural gas inevitably contains water vapor, and therefore it also has a similar water dew point. On the pipe wall where the temperature is equal to or below the water dew point, water vapor will cool and condense into water. Under different conditions, these waters will form different solid deposits, reducing the equivalent pipe diameter, increasing flow frictional resistance, and even blocking the pipes. There are two types of sediments: one is ice, and the other is natural gas hydrates. When the wall temperature is below both the dew point of water and its freezing point, the condensate water will freeze, forming an ice plug. Natural gas hydrates appear as white crystalline solids similar to crushed ice or snow; structurally, they are cage-like inclusion complexes that can be dodecahedral, tetrahedral, or hexahedral in shape. Water molecules bind together to form a cage-like lattice through hydrogen bonds, while gas molecules are enclosed within this lattice due to van der Waals forces. The conditions for hydrate formation in pipelines are that the natural gas temperature is below both the water dew point and the hydrate formation temperature, and the presence of seed crystals is required. Hydrates are typically formed when water and natural gas are in full contact under conditions of a pressure of 3 to 5 megapascals and a temperature of 2 to 6 degrees Celsius.
How to prevent natural gas from clogging pipes? Generally, there are four methods: the first is the dehydration method or drying method. After natural gas is dehydrated, the water dew point can be reduced below the pipeline temperature, preventing condensation water from forming on the pipe walls; this eliminates the basis for ice blockages and the formation of hydrates. Our country stipulates that the water vapor content in one cubic meter of natural gas shall not exceed 125 milligrams, with the natural gas molecules enclosed within a dodecahedral cage-like lattice, or the pipeline temperature must be at least 5 degrees Celsius above the dew point of water. The next is the chemical inhibitor method. The addition of chemicals such as methanol and ethylene glycol alters the interactions between water molecules, lowering the temperature at which hydrates form, so that this temperature becomes lower than the temperature of the pipeline. This is a common method. Next is the heating and insulation method. Keep the temperature of the natural gas in the pipeline above the temperature at which hydrates form. This method is not suitable for main gas transmission pipelines, as heating increases the viscosity of natural gas. Finally, there is the pressure reduction method. Reduced pressure can lower the hydrate formation temperature. This method is often used to eliminate already formed hydrates. Venting the pipeline can dissolve the hydrates in it.
Is it possible that the temperature drop caused by a large pressure difference before and after the control valve leads to ice blockage in the pipeline?
Current conditions for hydrate formation and suppression technologies are already quite mature, and these issues can be addressed through methods such as heating and the addition of inhibitors.
It’s likely the presence of natural gas hydrates that has blocked the pipes; alcohols can be added during the processing process
Natural gas pipelines can also have the problem of black powder