What are the advantages and disadvantages of steam sealing and nitrogen sealing at startup?
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In hydrogenation units, some towers and tanks are sealed with steam, while others are sealed with nitrogen. What are the advantages and disadvantages of using each of these two mediums? There are certain parts that can only be sealed with nitrogen, but can nitrogen be used as a substitute in those areas where steam is used?
Nitrogen can be used as a substitute where steam is applicable.
Then why is steam still used in some places?
Low-pressure N2 generally refers to N2 with a pressure of 0.85 MPa; it is primarily used as a sealing gas in various tanks, such as crude oil buffer tanks, water injection tanks, and ammonia-poor liquid tanks. To prevent the medium from coming into contact with air, N2 pressure control is installed at the top of these tanks. Additionally, the N2 provided at the three-in-one service points in refinery facilities (nitrogen, steam, and industrial air) is also low-pressure N2； The N2 isolation pressure for the dry gas seal of the cyclohydrogen compressor is also 0.85 MP ； 2. Medium-pressure nitrogen generally refers to 2.5 MP nitrogen, which is installed at the outlets of the new hydrogen compressor and the recycled hydrogen compressor; it is used for system purging during startup and shutdown operations ； 3. High-pressure nitrogen refers to N2 at 7.5 MPa, which is compressed by a liquid nitrogen pump. Older systems generally did not have N2 at this pressure level; therefore, when the system needed to be made airtight and the pressure exceeded 2.5 MPa, it was necessary to use a new hydrogen pump to supply N2 to the system, which increased the load on the circulation pump. The pressure should not be too high. In newly built systems, a 7.5 MPa N2 line has been added at the outlet of the circulation pump, **which facilitates the replacement of gas and the process of making the system airtight during startup and shutdown.
I’m referring to airtightness during the construction phase, not the regular sealing of air
Why is steam sealing still needed? Aren’t you afraid something might go wrong?
I haven’t heard that steam is used for leak testing some of the towers and tanks in hydrogenation units
It is rare to use steam for airtightness testing, as it is generally difficult to achieve a nitrogen pressure of over 1.0 Mpa; in areas where high pressure is required, a wind turbine must be used to increase the pressure with nitrogen. Steam typically has a higher pressure, and in some cases steam is used for convenience – that’s just my personal opinion.
In the distillation systems of hydrogenation units, steam is generally used for maintaining airtightness; nitrogen can also be used, provided that the company’s public system can meet the requirements. Generally, the amount of nitrogen available in public systems is limited, which results in slower processing times. Steam is more airtight. Using steam to expel air during pressure testing is also convenient, as it allows one to observe the process of air expulsion directly, eliminating the need to test the oxygen content in the system. It is sufficient to see steam coming out during venting; for nitrogen, testing is required. Moreover, when checking system flanges, valves, manholes, etc., steam makes it easier to identify leakage points. If there is a concern regarding the airtightness of nitrogen, a soap water test must be conducted, which is time-consuming and requires the involvement of multiple personnel.
Low-pressure crude oil system. . . . .
However, nitrogen can be used as a substitute in places where steam is used; nitrogen is expensive