Auxiliary Drain for Firefighting sprinkler system

Auxiliary Drain for Firefighting sprinkler system

Auxiliary Drain for Firefighting sprinkler system

What is the purpose of Auxiliary Drain?

An auxiliary drain is designed to allow drainage when the main drain valve is insufficient and results in water trapped within the system.

When is the Auxiliary Drain used and why?

Auxiliary drains are used after a system flush when the main drain leaves water in the system. Any time there is a change of direction of pipe in the system, there is a danger of water lingering in that area. The auxiliary drains help finish the job of the main drain.

Where is an Auxiliary Drain located?

Auxiliary drains are located at each point where the pipe changes direction. These drains should be accessible, especially those that are in areas subject to freezing. When the pipe changes direction, there is a “T” connection instead of an elbow. One part of the change in direction flows to the remainder of the system. The other direction flows to the auxiliary drain valve.

How to size the Auxiliary Drain?

The size of the valve will depend on the amount of trapped water and if the piping is subject to freezing conditions, NFPA 13 gets very specific in their valve size prescriptions by sprinkler system type according to the followings:

1- Wet Pipe systems and Pre-Action systems in areas not subject to freezing:

• If trapped water is 50 gallons or more, the auxiliary drain shall be a valve not less than a 1”.
• If between 5 and 50 gallons, the auxiliary drain shall be a ¾” or larger valve and a plug or nipple with cap.
• If less than 5 gallons are trapped, then a nipple with a cap or plug not less than ½ inch can be used.
• If trapped piping is less than 5 gallons, then the auxiliary drain can be omitted if the trapped water can be drained by disconnecting a pendant sprinkler head or where easily separated sections (like grooved fittings) are used.

2- Dry Pipe systems and Pre-Action systems with freezing conditions:

• If trapped water is less than 5 gallons, a valve not smaller than ½ inch and a plug, or a nipple and cap should be used.
• If trapped water is more than 5 gallons, two 1” valves should be used separated by a 2” dia. x12” length  condensate nipple as per the following figure:

Auxiliary Drain for Firefighting sprinkler Dry system

References of the article:

- NFPA 13 - 2019 edition (Standard For Installation Of Sprinkler Systems)

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• Sprinkler Distribution In Small Rooms