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Anonymous Poster

Nitrogen Purging of Natural Gas Headers

12/13/2010 10:06 PM

We are in the process of modifying our Natural gas headers in our power plant.

Before we start grinding, cutting and welding things...

We need to purge the whole system with Nitrogen.

The volume of the suction header towards our gas compressors is 10 m3

The volume of the discharge header to the gas turbines in 4.5 m3

Typical operating gas pressures at the suction header is 4.0 bar and the discharge header is 47 bar.

I want to know how much Nitrogen in m3 is required to purge the whole system.

Is there a rule of thumb? or a standard?

What should be the methodology?

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Guru
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#1

Re: Nitrogen Purging of Natural Gas Headers

12/14/2010 3:27 AM

<...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.

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#2

Re: Nitrogen Purging of Natural Gas Headers

12/14/2010 2:47 PM

There are multiple standards for purging residual fuel from combustion chambers and piping for combustion and flame safeguard control, which might give you an idea for calculating purge times and purge rates for whatever volume is involved.

NFPA 85 (2001) Boilers (pgs 85-27)
[2.5.2.4.1] (single burner)
(8) b. Purge airflow shall reach no less than 70 percent of the airflow required at maximum continuous capacity of the unit.
(9) The purge shall be for at least eight air changes.

[3.4.3.3.4] (multiple burners) (pg 85-38)
Purge rate shall not be less than 25 percent of full load mass airflow. Purge rate shall not be greater then 40 percent of full load mass airflow for coal-fired units.

[3.4.3.3.5]
Components (e.g. precipitators, fired reheaters) containing sources of ignition energy shall be purged for either
(1) a period of not less than 5 minutes or (2) five volume changes of that component, prior to being placed in operation, whichever is longer.

NFPA 86 (date?) (furnaces, ovens and kilns)
[8.4.1.2] requires a minimum of four (4) system volume changes.
"The system volume shall include the heating chambers and all other passages that handle the recirculation and exhaust of products of combustion." (Ref: 8.4.1.2(B))

North American's Combustion Handbook Vol 2 1997 says 5 air changes (pg 79)

National Board of Boiler Inspectors
http://nationalboard.org/index.aspx?pageID=164&ID=204
"at least four air changes of the combustion chamber and gas passes"

Given that hot work is the purpose, I'd be inclined to check the LEL with a sniffer, like PWSlack wisely suggests.

Be aware that the greenfield Kleen power plant explosion in Connecticut (USA) earlier this year involved purging the gas supply lines, and it's apparent that LEL detectors were not used; workers relied on their noses and mercaptan. 5 People died.

http://spectrum.ieee.org/tech-talk/energy/policy/electric-power-plant-explosion-reveals-historys-biggest-lesson

The Chemical Safety Board has several videos on the hazards of hot work

http://www.csb.gov/videoroom/videos.aspx?cid=1&F_All=y

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Anonymous Poster
#3

Re: Nitrogen Purging of Natural Gas Headers

12/15/2010 12:47 AM

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....

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Participant

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#4

Re: Nitrogen Purging of Natural Gas Headers

12/15/2010 1:58 AM

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

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#5

Re: Nitrogen Purging of Natural Gas Headers

12/15/2010 8:44 AM

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

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Anonymous Poster
#6

Re: Nitrogen Purging of Natural Gas Headers

12/15/2010 9:04 AM

I do have an LEL meter..

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Member

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#7

Re: Nitrogen Purging of Natural Gas Headers

12/15/2010 1:07 PM

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.

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Anonymous Poster
#8

Re: Nitrogen Purging of Natural Gas Headers

12/16/2010 5:17 AM

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

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

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#9
In reply to #8

Re: Nitrogen Purging of Natural Gas Headers

12/16/2010 9:07 AM

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.

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Guru

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#10

Re: Nitrogen Purging of Natural Gas Headers

12/16/2010 4:56 PM

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)

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Associate

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#11

Re: Nitrogen Purging of Natural Gas Headers

12/27/2010 4:30 AM

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!

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Power-User

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#12

Re: Nitrogen Purging of Natural Gas Headers

12/27/2010 1:02 PM

Thanks for the update. Bud

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#13

Re: Nitrogen Purging of Natural Gas Headers

09/10/2011 8:52 PM

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

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Associate

Join Date: Jan 2008
Posts: 33
#14
In reply to #13

Re: Nitrogen Purging of Natural Gas Headers

09/11/2011 5:24 AM

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.

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Guru

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#15

Re: Nitrogen Purging of Natural Gas Headers

09/12/2011 10:20 AM

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.

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