Does Tank Meet Secondary Supply in IBC?

11/29/2021

Section 903.3.5.2 (IBC 2012)/Section 403.3.3 (IBC 2015) requires and automatic secondary water supply. I am an FPE and I work in a jurisdiction (Hawaii) that has never required this secondary water supply due to local code amendments to IBC. Recent code revisions now require this secondary supply, and I now have my first project subject to this requirement. I'm planning to meet this requirement as as follows;

Secondary Water Supply: Provide an underground storage tank capable of supplying the sprinkler system + Hose Stream demand for 30-minutues per IBC requirement (min. 12,000 gallons). A vertical turbine fire pump will sit on top of the tank and pull water from below. Pump will be sized to supply full sprinkler system and standpipe system pressure demand (750 gpm @ 250 psi).
Primary Water Supply: The city water main will flow directly into the water storage tank at a rate equal to the standpipe system demand. The tank will serve as a "break tank" as long as the city water supply is viable. Although the city water supply will fill the tank at an equal rate to the system demand, the minimum break tank size will be 750 gpm x 1.5 x 15 min = 16,875 gallons.

Am I on the right track here? Is this primary/secondary supply arrangement viable? Does anyone foresee any code or other issues that may be a problem with this arrangement?

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12 Comments

Jesse

11/29/2021 08:30:03 am

We see some AJs in Texas that have various iterations of this. Its common to use the break tank as this secondary source.

Curious though, is your standpipe demand 750-gpm @ 250-psi?

Colin Lusher, FPE

12/2/2021 05:22:51 pm

Standpipe system demand is per NFPA 14, which is 750 gpm with 100 psi at the most remote hose connection (roof); this translates to 750 gpm @ 250 psi for the fire pump on floor 1 to meet the requirements on the top floor (17th).

Michael Lantaigne

11/29/2021 09:07:51 am

My experience with this situation is where a secondary water supply is required for the sprinkler system in a high rise building with a seismic design category of C, D, E or F (2015 IBC 403.3.3). I don't believe the tank necessarily needs to be sized to provide the standpipe demand since the code specifically mentions the hydraulically calculated sprinkler demand including hose streams. It is worth noting that depending on the hazard in the building, the sprinkler demand plus hose allowance could exceed the standpipe demand.

The projects in which I've been involved have not used a break tank. Both the the city supply and the tank were directly connected to the pump suction. It seems like you could use a break tank arrangement, however you would not be able to reduce the tank size based on the refill rate since it would not be a full secondary supply in the event that the city supply was damaged.

12/2/2021 04:30:03 pm

Yes, this requirement is for the seismic design category of C, D, E or F (2015 IBC 403.3.3).

I don't understand how you can have a tank and a city water supply connected directly to a suction of a fire pump. If the pump is sized for the water tank suction pressure, it would over-pressure the system if taking suction from the city water supply. Likewise, if the pump were sized for the city water pressure, it would be way under pressure when taking suction from the water tank. Wouldn't you need two pumps in this situation?

COLIN LUSHER, FPE

12/2/2021 05:28:50 pm

"however you would not be able to reduce the tank size based on the refill rate since it would not be a full secondary supply in the event that the city supply was damaged."

I need to provide sizing based on the most demanding situation. In this case, the secondary storage tank size would only need to be around 11,000 gallons, while the break tank size needs to be 17,000 gallons, so I need to provide a 17,000 gallon tank, which would be more than required for a secondary supply if the city supply was damaged.

Sprinkler demand is OH2 over 945 sq.ft. (QR Area reduction) + 100 gpm inside hose. So secondary supply only needs to be 1.2gpm/sqft x 945 sq.ft. x (1.25 for over flow) + 100 gpm hose x 30 min = 10,087 gallons needed for secondary supply

Brad K

11/30/2021 03:52:01 pm

I feel like you are using one tank as both storage and break tank.
I don't think that is meeting the intent of the IBC 403.3.3. I must admit I have never been in that situation with Seismic zones but I am wondering if you can do a city suction supply straight to the fire pump and then have the storage tank 12k gallon as secondary. This only works if the city supply can exceed pump demand at 150% and pressure requirements. I can dream up a scenario that works with a suction tank but not a buried tank and vertical turban. That means two pumps if you must have a buried tank. in my mind, secondary water supply means two tanks or one tank and one city feed.

12/2/2021 04:34:58 pm

It is a storage tank when the city water supply is NOT working.

It is a break tank when the city water supply IS working.

So it meets IBC requirements for both Primary and Secondary supply.

The situation you're mentioning would require two pumps. There is no scenario I can foresee where you can have a single pump supply from a pressure main, and a suction tank. The output pressure would be vastly different, and wouldn't therefore work with the sprinkler system design.

Alex

12/1/2021 10:26:50 am

Hi,

I agree with Brad that a primary and secondary supply in my mind needs to be separate and cannot rely on the same pump. In your scenario, if the pump fails, you lose all water supply. Can you install the city supply to feed both the tank and act as a pump bypass if there is sufficient pressure?

Alex

12/2/2021 04:39:00 pm

The IBC code specifically states that a second fire pump is NOT required if you can meet the system demand with a single pump.

"An additional fire pump shall not be required for the secondary water supply unless needed to provide the minimum design intake pressure at the suction side of the fire pump supplying the
automatic sprinkler system"

sean

12/31/2021 06:40:31 pm

why are you sizing the secondary for sprinkler and standpipe, nothing in the code that i know of states you need both just the greater

1/10/2022 11:23:25 am

I'm not. The "Secondary" supply is only required to meet the demand of the fire sprinkler system, NOT the standpipe system. The section of IBC that specifies the secondary supply only applies to fire sprinkler systems, not standpipes, so it's not a case of meeting the "highest demand system" in this case. I confirmed this with NFSA via informal code interpretation. I only stated that the pump itself will be sized to supply the highest demand system (standpipe).

Jimmie

6/9/2022 06:33:35 am

I am in agreement here. Although, some interpret "including hose stream requirement" to mean the standpipe demand, I don't think this is the case. It is referring to the 1-1/2 inch hose connections as per 20.14 of NFPA 13 (2022), which short of storage commodities you would be unlikely to have in a building. Having said that, we have had AHJs still require 100 gpm for the hose streams, but still better than 250 gpm per standpipe.

Does Tank Meet Secondary Supply in IBC?

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