How to perform nitrogen purging for longer pipe blind ends?
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In reality, for longer pipe dead ends, purging with nitrogen does not suffice to achieve cleanliness, and working with fire presents significant risks; so how can nitrogen displacement be carried out effectively?
Generally, a backflow preventer or vent is installed at the pipe end! If no one is available, it only proves there’s a problem with the design! Or long-term replacement can also achieve the desired environment, but it takes time!
Pressurize, release, pressurize again, release once more. Repeat this several times; as long as it’s not a liquid inside, the problem can be solved. Continuous purging with nitrogen is useless
Through repeated pressurization and depressurization, the original component level can generally be reduced to below 0.2% after four cycles
Having such pipes is clearly a design flaw. It's sufficient to install a vent valve at the blind end of the pipe.
In actual work, multiple replacements are carried out; once the replacement analysis shows satisfactory results, nitrogen protection can be used during the operation for better outcomes.
Consider adding a vent pipe at the blind end, or loosen the flange at the blind end during purging, allow air to escape for a while, and then tighten the bolts again; this way, it is safe to proceed with welding.
The vacuum displacement method can be used: first evacuate to -0.09 pressure, then inject nitrogen into the pipeline, repeating this process 3-4 times to achieve satisfactory displacement.
If there is no vent valve, can a burst disc be installed to allow pressure to build up and then cause an explosion, thereby releasing pressure instantly?
What if the blind flange is welded instead of being fastened with flanges? How do you add a vent pipe? The vacuum displacement method can be used: first evacuate to -0.09 pressure, then inject nitrogen into the pipeline, repeating this process 3-4 times to achieve satisfactory displacement. lhq6388 posted on 2009-3-14 23:07; I personally agree with lhq6388’s approach.
After reading the opinions shared by other sea friends above, I’d like to share my own views as well: Pumping out the vacuum first and then filling with nitrogen is faster and saves time compared to filling directly with nitrogen However, it is quite dangerous for pipelines carrying flammable and explosive gases; from a safety perspective, it is recommended that the owner use nitrogen to directly pressurize and displace the gas! Furthermore, if the situation described by the original poster indeed exists, it is recommended to cut this pipe out during maintenance, or to add a drain at the end of the pipe, to facilitate replacement during future maintenance!
Generally, there is a vent valve in front of the pipe dead end; this valve is opened to use nitrogen to repeatedly purge and relieve pressure in the pipeline.
We generally fill the pipeline with nitrogen to a pressure of about 2 MP, then release the pressure to around 0.2 MP (from the nitrogen filling source); after doing this five times, the pipeline can be considered to be thoroughly purged.
After pressure release, nitrogen is introduced, and a small hole is drilled near the blind flange to vent the gas.
This is a safety hazard; to eliminate it completely, cut it short during maintenance (take precautions). If replacement is necessary, it can only be done by repeatedly pressing with an inert gas to release pressure 3–5 times; vacuum extraction for replacement is also an option if conditions permit.
This can be done by vacuum pumping and then filling with nitrogen for exchange; this process needs to be repeated several times. The number of repetitions depends on the lower explosive limit of the gas that needs to be replaced. Once the tests show satisfactory results, a test report is prepared and a fire permit is obtained before work can proceed.
Charge to 4 to 5 kilograms, release to slightly positive pressure, charge again, then release. Several times like this,