Nitrogen Purging of Natural Gas Headers

<...how much Nitrogen in m3 is required to purge the whole system....>

As much as is needed to purge the gas below the lower flammable limit and the oxygen below acceptable limits for welding. Only a test can establish this.

Methane or natural gas is LIGHTER than air.... suck only from the top.

Or stick a big fan on one end and blow it all out the other....

There is no importance of pressures. first you have to depressurise the pipelines before purging. then calculate the volume of natural gas exists in the pipe lines. we require at least four to five times times nitrogen to purge out the natural gas. we have to take analysis after purging. pressurisation and depressurization shall help to purge properly that lines.

Kanakaiah G

Airliquide India

In case the de-pressurisation re-pressurisation process is something you aren't familiar with

Depressure your natural gas piping to flare to atmospheric pressure.

Close it up and pressure Nitrogen into it - ideally up to the pressure of nitrogen supply or the operating pressure of the natural gas system. Allow a little time.

Then depressure to flare to atmospheric pressure.

repeat typically 4 times BUT you MUST test before using sparking / flame equipment

I do have an LEL meter..

Contact a company (Haliburton is one) that specializes in delivering nitrogen. They can fit a rental unit with operator to fit your needs. Then, purchase a portable Gas Detector. ITX, by Industrial Scientific, is one manufactered by an international company. They provide all the equipment accessories for caring for and calibrating the portable detector.

After venting the gas, does filling the gas header with water sound feasible?

Then we could drain the water and purge it with Nitrogen.

I would have two issues with water

Are the supports designed for the weight of water? Depending on how big the piping is having the pipe full of water might overload the pipe supports (although presumably the pipe was hydrotested before use so perhaps it should be ok)

Second its generally a bad idea to introduce water to a system that is normally dry. What pressure testing are you going to use to retest the piping after all this cutting welding and grinding. If you are going to hydrotest then why not use water at this stage. I guess its just the issue of getting the water in and disposing of it.

Not all natural gas supplies are 'fully scrubbed'.

They may contain a variety of hydrocarbons that may condense and coat the system, or be held in moisture in the system.

So in addition to what Bud recommends, (which assumes a high grade of gas purity), I would recommend you;

1. nitrogen purge, to a safe reading,

2. then apply vacuum to the system for several hours (preferably 10),

3. then re-purge whilst testing for any rise compared to the first purge final reading.

Should you find a rise, it means it is advisable to keep a flow of nitrogen in the system from both sides of a cut*, as cutting can allow oxygen to enter.

(*which, I think is advisable anyway)

We did it successfully with the following procedure,

First we vented all the pressurized Natural gas on both our headers.

Then we pressurized the whole system with Nitrogen, upto 2.0 bar and then we vented it.

The vent was closed again, we pressurized the header again.

Checking LEL continuously on all vents and drains, The process was repeated 4 times. LEL came down to 0. Then we started blowing air.

Overall, the whole shutdown was successful!

Thanks for the update. Bud

USA's Chemical Safety Board released a video, Deadly Practices, on the hazards of venting or purging gas piping with natural gas (a practice known as "gas blows"), based on recent explosion and fire catastrophes in the US. The video streams on-line or downloads in .wmv format or Quicktime formats) 15:08 minutes in length.

http://www.csb.gov/videoroom/detail.aspx?vid=49&F=0&CID=1&pg=1&F_All=y

A lesson for all of us.

It's true, blowing with Natural gas, since it is readily available in large quantities is a very common practice and has been a source of major accidents in power plants throughout the world.

I've done Natural Gas purging quite a few times during commissioning phases, common practice is to use the highest venting point available to purge the debris, air or residual water after hydrotest to the atmosphere.

Gas detectors, LEL meters should be installed indoors.

Very scary. Thanks for bringing this to my attention.

I'll make sure I'm not on site when the gas lines are scheduled for a gas blow.