Qty. of Nitrogen for Expanded Pre-Action System?

12/21/2021

I'm currently designing for an addition to an existing pre-action system. This existing pre-action system (with three existing pre-action valves) and is connected to a 130 cubic-foot nitrogen cylinder.

How can I calculate if this much nitrogen would be enough if I added in another preaction valve?

Do I need to calculate the volume of all the pipes downstream of the preaction valve?

What other information should I gather?

How much percentage of the piping downstream of the valve needs to be filled with nitrogen?

Thanks in advance.

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5 Comments

Dan Wilder

12/21/2021 07:56:00 am

NFPA 13 says that the single largest system shall be refilled within 30 minutes but depleting the air/N2 supply isn't a good practice unless additional cylinders are readily available.

Yes, volume calculations of the system are needed to confirm if the system triggers additional requirements for quick-opening devices and/or delivery calculations.

NFPA 13 has several sections of requirements for preaction systems, too many to adequately cover here.

Nitrogen works from a purity standpoint so simply calculating the volume of the new system isn't enough unless you've only assumed a single time fill and aren't trying to keep the system pressure maintained. Both ECS & SouthTek (there are a couple others) utilize a purging method that cycles N2 into a system, replacing the standard mix of "air" to a level of 98% purity over a period of time (1-2 weeks and checked via purity sensors) then there is the maintenance side that replaces the N2 as it bleeds off/changes pressure due to temp changes (the reason for the AMD).

Alex

12/21/2021 08:45:42 am

To add off of Dan’s response, like Anthony mentioned below, it may be worth while to look into a generator if you are going to be modifying the system further.

Jesse

12/21/2021 08:27:07 am

Dan beat me to it with his great answer.

Anthony

12/21/2021 08:35:26 am

130 cubic-foot nitrogen cylinder isn't enough information to determine if its enough. You need to know the pressure and do some Boyles law math.

You need to know the total volume of all the systems supplied by your tank and what pressure it needs to be at and if the tank has the volume and pressure to supply that.

You'll also need to know if the system can be refilled from the single tank with in 30 min, which is a question for the type of flow/ pressure regulator used.

You'll need to account for any losses due to bleed off as well. when you fill the system you need to purge the non purified air. There are products that do this automatically and maintain the pressure in the system.

The whole system should be filled with N2, the % will be determined by the effective ness of your purge and maintenance system.

If you're already at 3 valves controlling a large volume its time to think about an on site generator.

sean

12/31/2021 06:25:20 pm

a straight nitrogen system isnt super common either, typically a nitrogen purging system is more common, helps you get around the 30 min issue.

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Qty. of Nitrogen for Expanded Pre-Action System?

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