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Use Nitrogen Cylinders Without Pressure Regulator

10/20/2020 1:40 PM

We have a natural gas piping volume of 50 m3 (piping design pressure 70 bar) at atmospheric pressure that needs purging with N2. We have 12 nitrogen cylinders (each cylinder volume 50 litre) at 200 bar, with total normal volume 120 m3. The hose that connects the cylinders with the piping is rated at 150 bar, so there will be no closed valve between the hose and the piping. The purging should be done in limited time, and I am thinking if it is safe to do it without pressure regulator, since the final pressure will be a little more than 2 bar. Then we will vent to atmosphere, and we will repeat this procedure 5 times to get a low gas concentration.

Has anybody done this job or a simillar one, without pressure regulator? Controlling by the shut off valve of the cylinder bundle?

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#1

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/20/2020 3:06 PM

No personal N2 purge experience, but I would advise you use a pressure regulator anyway to reduce potential risks. That and ensure you are safely venting the natural gas into an open, safe area to mix and dissipate into the atmosphere safely away from people and ignition sources.

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#2

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/20/2020 3:31 PM

So:

  1. What is the outcome of the HazOp Study over this intended operation and what does it say?
  2. How does one read the pressure in the <...gas piping...> to establish what it is?
  3. Where is the hydraulic test record document for this <...natural gas piping...>, and what does it say?
  4. Does the <...gas piping...> have any pressure relief systems on it?
  5. What is wrong with applying the <...nitrogen...> at one end and letting it flow through once instead of <...5 times...>?
  6. What does <...low...concentration...> actually mean? What is the Lower Explosive Limit of this material in air?
  7. How is the <...low...concentration...> of <...natural gas...> to be established?
  8. Why is there no <...pressure regulator...> to hand?
  9. Why is it being done in <...limited time...> instead of safely instead?
  10. What is going to happen to all the frost that is going to develop on the equipment as a result of the expansion?
  11. What are the reaction forces on the pipework and its support systems and can they withstand the inrush of all this <...nitrogen...> in a <...limited time...>?
  12. What is going to happen to the hose as a result of the low temperatures it is going to experience in the <...limited time...>? If it fractures, what happens to the <...natural gas...> in the rest of the system?
  13. "Cock on the block" - <...We...>: whose signature is going on the procedure to authorise its use and how close is this individual going to be to the installation when the purge begins by way of warranty with his/her life that it is safe to do it that way?

These are the sorts of questions that need to be asked. Answering them in this forum would be superfluous.

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#3

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/20/2020 4:43 PM

Seems like a pressure regulator set to something less than 70 bar or a relief valve on the piping set to something less than 70 bar would be a prudent feature in your configuration.

The math supports having none of these features but the math won't matter when you are trying to defend yourself in court should some sort of disaster occurs.

(Someone hooks up too many bottles, the piping wasn't properly vented on the previous purge, the hose rated for 150 bar 'sees' 200 bar because some valve was left closed, and so on. Trust me, nothing is 'soldier-proof'. )

Disasters usually happen because someone didn't think of EVERY POSSIBLE THING that could go wrong or be done wrong and the delusion that the configuration is 'inherently safe'.

Nothing is inherently safe about compressed gases.

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#4

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/21/2020 3:15 AM

Don't be bullied by managers, accountants etc into doing an unsafe procedures in a hurry... they are not the ones who will be dead or in court if it goes wrong.
THEY always walk away free while the poor sucker further down the food chain takes the hit.
Del

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#5

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/21/2020 9:45 AM

Typically nitrogen cylinders are attached to a manifold with a regulator to control the purge medium pressure. The size and length of the hose between the manifold and the piping system to be purged also has an impact on the velocity of the purge medium. I suggest you take a look at a document from the American Gas Association called Purging Principles and Practices. The piping configuration to be purged also plays a role in which of the purge methods described in the document is best suited for the work you are trying to perform. Also, the use of an air mover may be appropriate to evacuate the piping configuration. All of this is explained in the document. Purging apparatus diagrams are included. Hope this helps.

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#6

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/21/2020 10:55 AM

on EVERY gas piping system that we have has a regulator to control pressure, an in-line orifice to control maximum flowrate, and a pressure relief valve set below the burst limit of the weakest link in the system.

just some of the questions our safety team would ask before a safety permit for the procedure would be issued:

piping diameter? what material it the pipe and "hose" is made of? pressure ratings? what is the vessel that you are purging this much nitrogen into made of? what is pressure rating of the tank/room being purged? is the tank/room pressure tested on a periodic basis and when was it last tested? how large is the exhaust port (which could be considered a system orifice for this much flow)? does the tank have a rupture disc and what pressure is it rated for?

the only thing that i can think of that is similar to a system that you are talking about would be a hood fire suppression system, i haven't looked at these systems very carefully but i would assume they have an orifice set to the exhaust rate of the room/hood at the very least, but i do not know.

without anymore info, i would not get near this setup.

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#7
In reply to #6

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/21/2020 4:31 PM

Thank you all for your replies.

piping diameter: 24" , piping is a part from gas transportation system

piping material: API 5L X65M PSL2, wt 14,27mm

hose material: synthetic rubber reinforced with steel wires, hose nominal dia: 1/2"

no vessel, only piping is being purged

no periodic pressure test is required for piping, working pressure is typically 45 bar

exhaust port is 2"

My main worries is about the freezing. Could the low temperature make the shut off valve unable to operate, or the hose fail?

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#8
In reply to #7

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/21/2020 5:17 PM

It's no good asking a bunch of anonymous strangers on the internet, as none is liable if the procedure goes wrong.

The questions above need to be asked of the individuals involved in the activity instead. If unwilling to do so, then walk away from it, and FFS don't sign anything!

<unsubscribes>

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#9
In reply to #7

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/22/2020 1:06 AM

I would be concerned first about ventilation of the area...second I would be concerned about the pressure in the tank if a rupture in the purge line should occur...Yes and the valve freezing in open position, or ice accumulation in the line someplace causing a blockage...Definitely need a pressure gauge on the tank outlet, and would like one at the end of the pipe...would be good to monitor the tank outlet for frost, and throttle back if necessary...Emergency oxygen available, just in case....

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#10
In reply to #9

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/22/2020 2:35 PM

The piping and the whole job is to be done in open area.

It seems that the job will be postponed, until we will get a pressure regulator with adequate (100 Nm3/h) capacity at 3 bar outlet pressure setting (never going to reach it).

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#11
In reply to #10

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/23/2020 3:55 AM

...is the right answer.

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#12
In reply to #7

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/23/2020 7:40 AM

I don't understand why you need to purge at such a high rate?

there are many standard techniques to purge a system, i am no expert in fast purging.

Mr. Bernoulli has a principal that you may work to your advantage using less nitrogen. but you would need a port on the nitrogen side of the system for atmosphere to enter.

and for your valve, there are cryogenic valves that will work, you may want to investigate that option?

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#14
In reply to #12

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/25/2020 10:30 AM

The piping is not one simple pipe. There are pipe branches (diameter 24") , and so we cannot rely on letting gas in from one point (diameter 1/2") and getting it out from an other point (diameter 2"), because that would let all the "blind" branches almost without purging. So dilution by pressurising and depressurising (by venting to atmosphere) is more effective to remove the gas from "blind" branches and to ensure that the gas concentration will be lower than e.g. 20 %LEL (which is the target)

The available time for this operation is limited to about 8 hour, hence the need for high flow rate.

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#13

Re: Use Nitrogen Cylinders Without Pressure Regulator

10/23/2020 9:28 PM

When you disconnect each N2 cylinder to attach the next, air from the large pipe will flow back through the hose, unless there is another valve. If there is, then the hose/piping between the two valves could conceivably be exposed to 200 bar, say if someone forgets to open the downstream valve before the cylinder valve.

Even so, I doubt that the expressed fears are justified. The hose and any small piping to the large pipe will get cold, but the N2 inside won't freeze.

If the price of a regulator is a problem, consider renting one.

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