Benefits of a Tree System?

MeyerFire University | FX105.12

By Chris Logan, CFPS, RSE

RESOURCES

• One-Page Summary [PDF]
• Notes Page [PDF]​

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TRANSCRIPT

What are tree systems and what are the benefits and limitations of them?

INTRODUCTION 

It's time to get into more detail about Tree Systems. In the last module, we went over how to identify a tree system.

What are tree systems and what are the benefits and limitations of them?

It's time to get into more detail about Tree Systems. In the last module, we went over how to identify a tree system.

We discussed the comparison of “trunk” (Main, Cross main), “Branches” (branch lines) and so on, but let’s get into more detail about the design, the benefits, and the limitations of tree systems. 

Why do we use these systems? 

What sets a tree system apart from the others? 

Why were tree systems used so often early-on in the fire sprinkler industry?

EFFICIENCY

Well, for starters, tree systems are the most efficient use of material and labor.

Water follows along a single path in the most direct route to the activated sprinkler. There was no additional mains or branch lines for the water to flow. There’s just a supply, open sprinkler, and a direct pathway between the two.

Does this matter?

Well, for wet-pipe sprinkler systems, not really. A loop or a grid system has a path from the source to the open sprinkler, and they work better hydraulically. 

DRY TRIP TIME

But for dry-pipe systems, non-interlock pre-action, or double-interlock pre-action, the efficient flow of water becomes extremely important. In these systems, water is not readily available at the sprinkler when the sprinkler activates. There is a delay in time before the water flows from a dry or pre-action valve all the way to the open sprinkler. We call this the Water Delivery Time, or somewhat informally, a Dry Trip Time.

In these systems, after a sprinkler activates, we experience this delay. During this time, water from our valve has to displace air in the pipe network before it can reach the activated sprinkler.

During this time, the fire is allowed to continue to grow in size and heat release rate.

WATER FLOW

The use of tree systems here have a big benefit; water does not flow excessively down unwanted paths. It may apply pressure to the air in the system and force air to compact down some branch pipe, but on the whole, water will travel first through the mains and into the branch line serving the open sprinkler.

WATER DELIVERY TIME

Looped and gridded systems don’t have this benefit. Water has to displace air out of multiple pathways, which means it will take longer for water to reach the activated sprinkler.

For large dry or pre-action systems or for remote systems where the sprinklers are served by a long feed main, the water delivery time can be a major driver for the design of the system. It absolutely can be a limiting factor on how large a system can be, how many sprinklers it can serve, or even how much volume a system can have.

SIMPLER DESIGN

As was the case for most of the first century of fire sprinkler systems, tree systems could use the Pipe Schedule design method.

Pipe Schedule systems was a basic method of pipe sizing. 

For example, a Light Hazard Pipe Schedule under NFPA 13 would permit any two sprinklers to be fed from a 1-inch (25 mm) pipe, any three sprinklers from a 1-1/4 inch (32 mm) pipe, any five sprinklers by a 1-1/2 inch (40 mm) pipe, and so on up through the mains and the riser. 

There was not a lot of math involved in these systems; the pipe size directly related to how many sprinklers that pipe fed.

For over a century, we used cast iron or malleable iron threaded reducing fittings that would allow us to reduce the pipe size between each sprinkler connection. 

This was a basic method that could be applied consistently across structures of the same hazard.

​Until the 1970’s, the Pipe Schedule method was used all the time. Then we developed hydraulic calculations and a way to mathematically understand how these systems are supposed to actually flow water. Hydraulic calculations brought about a few things, but two of the most important features was a better understanding of how the systems could actually perform with a given water supply, and, with that understanding, the ability to design and install more efficient systems.

Since at least the turn of the century, however, NFPA 13 has only allowed pipe scheduled systems under the following conditions:

• Additions or modifications to existing pipe scheduled systems,
• New systems of 5000 sq ft (465m²) or less
• New systems over 5000 sq ft (465m²) with the minimum flows pressures indicated in NFPA.

EASIER CALCULATION

Another thing that sets these systems apart from the others is how easy it is to design and calculate. 

The pipe schedule method, as previously mentioned and described, was typical for sprinkler installation until the mid 70’s, when the hydraulic calculation was created and submitted for approval by NFPA. 

Despite not using the Pipe Schedule method anymore, tree systems still have the advantage of being easier to calculate.

Having a single path for water supply, a tree system will only show one direct path for water to flow in the calculations, and then each of the branch pipes that connect into that single water supply.

Even selecting a remote area for a tree system can be easier. These systems usually have a far end of the system that is logically the best place to calculate a remote area. Remote area sizing is fairly straightforward among the branch lines, and the calculation is performed.

This isn’t always as obvious in loop or certainly in gridded systems. In those cases, multiple remote areas adjacent to each other need to be established and tested in order to find out what the “peak” remote area is, or the “true” remote area.  

​SUMMARY 

So, what are the benefits in using a tree system?

1. Efficient use of materials
2. They’re simple
3. Direct path for waterflow
4. They’re easier to design and calculate

Those are the core benefits to using a tree system.

I’m Chris Logan, this is MeyerFire University.