Looking for code reference (for tank capacity of 13D system)

[JerSPK]

I've heard that the tank size for a 13D system can be reduced depending on the flow of the fill valve/domestic capacity. I've always designed the tank size for the full capacity since there has always been room for a larger tank. Now I'm dealing with a smaller home with a fairly small mechanical room. I've looked through NFPA 22 but can't seem to find anything to confirm the above.

If my system demand is 32 gpm and the domestic system can maintain a constant flow of 20 gpm, (assuming no loss through the valve) can my tank be sized to a "filled" capacity of 120 gallons?

Is there a specific code reference in NFPA 22 (or any other code) that confirms this?

 

[cdafd]

do not see and exception

nfpa 13 d::

6.1 General Provisions.
6.1.1 Every automatic sprinkler system shall have at least one automatic water supply.
6.1.2 Where stored water is used as the sole source of supply, the minimum quantity shall equal the water demand rate times 10 minutes unless permitted otherwise by 6.1.3.
6.1.3 Where stored water is used as the sole source of supply, the minimum quantity shall be permitted to equal the two-sprinkler water demand rate times 7 minutes where dwelling units meet the following criteria:
(1) One story in height
(2) Less than 2000 ft2 (186 m2) in area
6.2* Water Supply Sources.
The following water supply sources shall be considered to be acceptable by this standard:
(1) A connection to a reliable waterworks system with or without an automatically operated pump
(2) An elevated tank
(3) A pressure tank designed to American Society of Mechanical Engineers (ASME) standards for a pressure vessel with a reliable pressure source
(4) A stored water source with an automatically operated pump


do not think NFPA 22 covers capacity:::

1.1 Scope.
This standard provides the minimum requirements for the design, construction, installation, and maintenance of tanks and accessory equipment that supply water for private fire protection, including the following:
(1) Gravity tanks, suction tanks, pressure tanks, and embankment-supported coated fabric suction tanks
(2) Towers
(3) Foundations
(4) Pipe connections and fittings
(5) Valve enclosures
(6) Tank filling
(7) Protection against freezing

 

[cdafd]

maybe two small tanks??

 

[JerSPK]

Two smaller tanks or one larger tank will likely take up about the same floor area which is the main problem. I could have sworn that once upon a time I read about adding fill flow to the capacity, but perhaps it was just a discussion and not in an actual code.

 

[TravisMack]

I believe what you are talking about is sometimes called a break tank. We have done this in the past with commercial and residential systems. You just have an automatic float valve so that the domestic line fills as the tank is drawn down. I have not had any AHJ reject this arrangement.

Additionally, you should have an air gap between the tank and the domestic piping. As such, you should not need a backflow preventer as there is no real chance of cross connection.

A break tank is also referenced in the 2007 edition of NFPA 20.
3.3.6 Break Tank. A tank providing suction to a fire pump whose capacity is less than the fire protection demand (flow rate times flow duration).
5.30 Break Tanks.
Where a break tank is used to provide the pump suction water supply, the installation shall comply with this section.
5.30.1 Application. Break tanks are used for one or more of the following reasons:
(1) A s a backflow prevention device between the city water supply and the fire pump suction
(2) To eliminate pressure fluxations in the city water supply and provide a steady suction pressure to the fire pump
(3) To augment the city water supply when the volume of water available from the city is inadequate for the fire protection demand

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[wafflefryer]

The 2010 edition of NFPA 13D has a new section that deals with this issue.

6.1.4 The stored water requirement of 6.1.2 or 6.1.3 shall be permitted to be a combination of the water in the well (including the refill rate) plus the water in the holding tank if such tank can supply the sprinkler system.

Pretend you have a system where the following infomation is known.

1. 10 minute supply demand is 400 gallons.
2. Volume of the well is 100 gallons
3. The well recovers 30 gallons per minute

In this scenario, you wouldn't need a storage tank. because the recovery rate of the well over a 10 minute period is 300 gallons and the water already in the well is 100 gallons. The total is 400 gallons. You would only need a properly sized pump either in the well or in the house to meet the pressure and flow requirements.

In the following scenario you need to take extra precautions to make sure you do not undersize your tank.

1. 10 minute supply demand is 400 gallons.
2. Volume of the well is 100 gallons
3. The well recovers 20 gallons per minute.
1. The primary well pump can supply 10 gallons per minute.

At first glance it would seem that the storage tank would only need to be 100 gallons to make up the shortfall. But the water delivery is limited by the pump in the well. The most the well will produce in the above scenario is 100 gallons total over a 10 minute period. Therefore the storage tank would need to be at least 300 gallons. Usually I connect the well pump to the tank with a float valve so that if the water level drops there is a mechanical device in place that will refill the tank.

I hope this was helpful.

 

[JerSPK]

Thanks for the replies! I ended up using the 13D 2010 reference since it was a 13D design. A smaller tank was approved and it will fit well in the smaller space.