3 replies to this topic

[ZoRo_UAE]

Dear all,

We have a flow line of sour dry gas need to be purged, cut, and weld. The line is not pigable and it is spaded from one end and there is no provision of venting in this end. Spade cannot be removed because it required a total shut down. Flow line is about 4 km long and 8" diameter. Cutting and welding will be 2.5 km away from the purging point and about 1.5 Km from the spade.

The issue is: if we fill the line with N2 and pressurize to some extent, then bleed it from the same point, will it be sufficient purging? If not, what is the best way for purging? What is the N2 quantity required and how much we should pressurize N2?

It vey urgent job and appreciate your kind feedback on this issue.

Regards,

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[Art Montemayor]

Zoro:

I do not recommend a dead-ended pressurization and subsequent purging. My reasons are:

• You will be there till dooms day, trying to make sure the entire length is totally purged;
• There is no certain or sure way to make sure that you have totally purged the line; your purge analysis may show no sour gas, but it can still be there, at the dead-ended portion.
• Since you have no positive purge taking place, you have no way of predicting when you will be through with this job.
• There is an inherent risk involved in turning over the job for subsequent cutting and welding after this type of “purge”. You would be responsible if a trapped portion of sour gas suddenly appears later while the line is being worked on.

What I would do is to MAKE A HOT TAP at the dead-ended portion of the pipeline, just upstream of where the spade is found. I presume that the spade is between two flanges. Therefore, you should be able to easily make a 4” or 6” hot tap on the 8” line since it would be at atmospheric pressure. This should be an easy tap and once done can be used for the thorough sweep of purging nitrogen and it allows you to monitor the analysis of the exit purge gas while you are purging. You would also be able to positively pig the pipeline section because you can always pressurize either end and send the pig back and forth. This type of operation is a positive one that ensures that all sour gas and liquids are thoroughly disposed of and the results can be verified and proven.

[ZoRo_UAE]

Dear Mr. Montemayor,

Many thanks for this excellent suggestion. This is the best way in my opinion. The only thing is it requires a lot of formalities and approvals. And this might take a long time where we don't have. So, what is your opinion if I do the hot tapping at the redundant pipe where I am going to cut?

Regards,

[Art Montemayor]

Zoro:

Your compromise solution still leaves 1.5 kilometers of pipeline un-purged in a positive manner and subject to having pockets of sour gas and liquids in it.

In my opinion, the potential risk to yourself and your personnel is too great a price to pay for bureaucratic "formalities and approvals". Granted that formalities and approvals have to be done to document safety, but this is the price that has to be paid - not the lives and welfare of human beings.

The person responsible for doing the thorough purging in order to have a safe and efficient cutting and welding operation on that pipeline is the one that must also take full responsibility for there not being any toxic or hazardous chemicals in that pipeline when the modification work begins. The ONLY WAY that I could take responsibility for that decision would be if I could prove that a positive and thorough nitrogen purge THROUGH THE ENTIRE LENGTH OF THE EMPTY PIPELINE was completely inert and free of any toxic or hazardous gas or liquids. In order to reach that level of assurance, I have to purge the length of the pipeline and be able to have documented proof of the purge gas analysis.

I realize that this means a lot of time and trouble for some people. But the basic issue is HUMAN SAFETY here and I don't accept a second-rate level of safety.