Limit for Length and Size for Underground Loops?

4/26/2024

Typically when I work on campus/multi-building systems, I am using a single underground main and splitting off to each building. Since you don't have to consider a multiple building fire problem, we have typically worked on the hydraulically most-remote building for underground sizing and use wall PIVs to offer individual building shutoffs, with FDCs on the individual buildings.

I'm getting challenged on "what allows this in code?" It's mostly a question of - where is the limit on how long a loop can be?

I'm looking at this from a sprinkler design perspective and whether each individual building's sprinkler system is able to perform hydraulically.

Is the limit for the length/layout/sizing of a campus underground loop simply the more demanding of Fire Flow or sprinkler system feeds?

Is there a code basis for this?

Thanks in advance.

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11 Comments

Glenn Berger

4/26/2024 08:10:06 am

A few questions requiring answers must be considered prior to a proper response to your question be given.

Is your single main dedicated to fire protection use only? Or is it for water needing systems on the campus?

Is there any expansion plans for the campus? Any potential more hazardous areas that can eventually pop-up on-site?

Are you considering any loss of performance as the underground system ages? NFPA standards do include a reduction in the Hazen-Williams C-factor.

Casey Milhorn

4/26/2024 08:39:42 am

No issue with it. Typically the issue has more to do with arrangement, placement of backflow, FDC placement strategy, fire pump placement (if required), and whether or not you have private hydrants on this line. If you have private hydrants, fire flow demand may be required to be met at these hydrants. Additionally I have ran into some local AHJs that have minimum size requirements on underground lines that feed hydrants based on number of hydrants and/or distance to hydrant.

Franck

4/26/2024 08:48:15 am

Limitation for size and length is directed by the hydraulic calculations. As long as you can supply all systems with a given pipe diameter and loop length, it is fine.
I survey many nuclear power plants with extensively long loop, but with large diameter to limit friction losses (generally above 8 in.).

As indicated above, check all parameters (fittings) and make regular loop tests to check internal integrity of your mains (every 5 years maximum).

Pete H

4/26/2024 09:00:00 am

None I can think of. Circulating street mains are typically gigantic compound loops or grids.

Casey's got a good point that you might have requirements for flow and size for feeding any hydrants.

Another concern I'd worry about is if your most remote building is your most demanding. Like if the most remote building is a light hazard office but you have a storage facility next to the source, the storage facility is still probably going to be the driver of your minimum flow.

Ricardo Gonzales Jr

4/26/2024 09:10:15 am

1st determine what the building needs for a proper system without and with a fire pump. Then direct the Civil folks to make that occur. Unless you're designing a private system, FPs usually don't get involved in water supplies.

That prompts the next set of questions from an FP point. What is the end game for our systems on a campus setting. Like the previous answers, there's a lot of questions before putting pen to paper.

Jeyson Ventura

4/26/2024 09:12:45 am

A different perspective on why the length of the loop system may be getting challenged.

NFPA 24 section 6.6 talks about sectional valves in looped systems. It states that sectional valves are required such that the number of fire protection connections between sectional valves does not exceed six, but it does not set a limit on the length of the loop. The intent is to limit impairments during a break or service of the underground pipe.

The only limit on the length of a system that I see is when a section of pipe splits off the loop or on a tree system. In my opinion, the intent to limit the number of connections to six would also apply in this case. NFPA defines fire protection connections as sprinkler system lead-ins, hydrants, or other fire protection connections.

Blaine

4/26/2024 12:36:37 pm

Might look into AWWA manual M31. Distribution System Requirements for Fire Protection.

Mark Harris

4/26/2024 02:17:50 pm

All great responses. May want to look at fire flow requirements in the appendix of the IFC IBC as well.

As Pete noted high hazard occupancy may make the sprinkler demand more than fire flow hydrant demand.

Jack G

4/26/2024 04:26:02 pm

Couple things.
Review Meyer fire 2020 hydrant distances, nfpa 24, ibc.
If installing underground for private water mains and hydrants, sometimes it efficient to loop the complex—- although the flow is to be calculated as dead end, Lead ins could be installed front, back or along a side was.
AHJ usually wants a minimum of 1500 gpm at 20 psi residual. Follow 24 for minimum size to fire hydrants.
Fire flow / sprinkler demand ( calculated hydraulicly, unless storage or high hazard , should be less than 1500 gpm.

Todd E Wyatt

4/30/2024 08:03:09 am

Scoping Codes (e.g. IBC-2024) and their referenced standards (e.g. NFPA 13-2022) include prescriptive requirements (e.g. shall) for the Design Professional to meet when designing a building and/or one of its systems. Some of these requirements include restrictions (e.g. "Sprinklers under walkways shall NOT be spaced more than 8 ft (2.4 m) apart horizontally.) but these are far less frequent than the use of "shall" without "not" following it.

Essentially, anything that meets the requirements (shall) and does not conflict with the restrictions (shall not) is acceptable. The Code and Standards cannot foresee all unique conditions so not all features are included in the requirements because they provide a baseline coverage that goes beyond the minimum requirements. For this reason, most Code & Standards permit equivalencies :

NFPA 13-2024
CHAPTER 1 Administration
1.5 Equivalency.
Nothing in this standard is intended to prevent the use of systems, methods, or devices of equivalent or superior quality, strength, fire resistance, effectiveness, durability, and safety over those prescribed by this standard.
1.5.1
Technical documentation shall be submitted to the authority having jurisdiction to demonstrate equivalency.
1.5.2
The system, method, or device shall be approved for the intended purpose by the authority having jurisdiction.

An equivalency statement is included in many NFPA documents to allow for products and system arrangements that are not specifically covered by the standard to be used. However, it must be demonstrated that the products or arrangements do not lower the level of safety provided by the standard or alter the standard’s intent. The AHJ is required to review the technical documentation and approve the equivalent system, method, or device.

I have experienced AHJs inquiring “Where does it state that this is permitted?” and my response (respectfully) is “Where does it state that this is NOT permitted?”.

The AHJ should not impose requirements beyond those specified in the adopted Code or Standard unless those requirements have been formally adopted in the jurisdiction.

J.H.

4/30/2024 02:10:58 pm

Generally, the governing Utilities Dept. will have guidelines regarding dead-end water mains, esp. those supplying hydrants and/or fire protection systems.

There is also some guidance in NFPA 1141. The 2017 edition is referenced below:

NFPA 1141:9.3.9 In residential areas, fire hydrants shall be supplied by not less than a 6 in. (150 mm) diameter main installed on a looped system or by not less than an 8 in. (200 mm) diameter
main if the system is not looped or the fire hydrant is installed
on a dead-end main exceeding 300 ft (91 m) in length.

NFPA 1141:9.3.11 Dead-end mains shall not exceed 600 ft (183 m) in length for main sizes less than 10 in. (250 mm) in diameter.

Limit for Length and Size for Underground Loops?

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