A Required Direction for Hydraulic Calculations?

3/2/2023

Where does - or does - NFPA 13 and NFPA 13D (2022 Edition) say that hydraulic calculations should be run from the most remote sprinkler to the water supply?

Apparently some hydraulic calculation programs print out the results backwards, which makes them harder to follow.

Thanks in advance.

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10 Comments

Dan Wilder

3/2/2023 07:23:46 am

There is no requirement for direction of calculation that I can find in the Chapter 28.4 section. Just that the node tags need to be there along with several other characteristics/identifiers like elevations for each, distance between, fittings & eq lengths....

I don't follow the harder part...it's still connect the dots (nodes) while checking some other specifics. I do know that some people don't like to see negative numbers which occurs on some programs based on direction of node (A->B vs B->A).

There may be a toggle to reverse things if you reach out to the Tech Support of the software company.

Glenn Berger

3/2/2023 08:21:55 am

The requirement is that the most remote area be calculated. The how program displays the remote area is not a requirement.

Before you review a submittal, perform a quick calc to see if the end results of the submitted printout is reasonably correct.

Nate

3/2/2023 08:23:29 am

I am actually attending EKU currently, and one of my current class is sprinkler design, But in NFPA 13, 2022 chapter 19 basically answers your question in a code way. But a short answer in class we are currently just talking about your basic wet steel pipe system, there is a calculation to find your design area - (area of room (ft^2) / sprinkler coverage area (ft^2)) which is the furthest from the supply. Note: I am a rookie with sprinkler design and still learning

Chris

3/2/2023 08:32:36 am

Nate, your answer is correct for hand calculations! These are CRITICAL for your understanding of how to read calculations produced by programs, and then find the blockages to fix to optimize the pipe sizing. Keep learning and know that this industry is secretly very hard, very unique, and very rewarding!

James Evans

3/2/2023 09:00:35 am

An interesting thing about calculations. The program does not care about direction of the calculation, in the long run, the program looks at a system like a long straight pipe with flowing outlets at the end. there are no elbows or tees, those are input as equivalent lengths of straight pipe. so the computer sees only a very long run of straight pipe with a quantity of water flowing through it and outlets at the end in a straight line and each outlet relieving the flow until it his zero water left in the pipe.

Jesse

3/2/2023 08:25:13 am

I'm not familiar with anything requiring a directional calc. The nature of the Hazen-Williams calc would pretty much allow it to be reversed.

We typically say the most hydraulically remote sprinkler - as well call the pressure-end head sprinkler because we know what pressure we need at that pEH in order to get the density we need to achieve that density. The water supply, as the very foundation of the calc, is a known quantity.

I don't see it would make a difference. Its still following the route of the water from the pEH to the supply, using the hydraulic nodes as street signs.

Alex

3/2/2023 08:36:39 am

I am not aware of any requirement. I was taught that as long as the nodes on the drawing match the calculation sheet and that the two can be reviewed as one, the nodes can be “dog” or “cat” for that matter.

Like anything, there is an industry standard to work from your remote area to the source but no code requirement.

Anthony

3/2/2023 08:42:36 am

You have to run the calculation from the most demanding head to the supply for a demand calculation. Thats just how a DEMAND calc works. It determines the bare minimum supply condition required to operate the system. You can arrange the nodes in the calculation in any logical manner you'd like. NFPA 13- 16 does detail out the method for arranging calculation data on a detailed work sheet in chapter 23.3. and the procedure to follow is in 23.4 Also in the annex (specifically A23.3.2(c) there is a light smattering of examples.

If you start the calculation from the source that is called a SUPPLY calculation. That will tell you what will happen when all activated sprinklers discharge at a given supply condition. This is interesting but doesn't really do anything for a designer.

What software are you sighting? Often calculation programs will front list the direct path from the most demanding sprinkler to the supply and have additional branch line calculations listed after that as " rout #x" then have the flow added back to the main path at their respective tee nodes.

Chris

3/2/2023 08:43:18 am

As mentioned above, NFPA 13 22 - 28.2.4.7.5 states that "Calculations shall begin at the hydraulically most remote sprinkler". This comment is likely geared towards hand-calcs, and ideally should possibly be rephrased? In my career, only 1 of the hardest engineers in my region has ever commented on my calcs they didn't technically start at the most remote head. I then reorganized the way my calcs are entered to fix this.

All programs that I have seen can have the information input in any direction and in any order. In fact, it can jump as much as you want it to! I actually manually type my calcs because I hate the labeling and organization of automated calcs and it usually only takes me 30-45 min. I always start with the heads 1st, then branchlines/gridlines, then mains, then riser->hydrant.

Franck

3/2/2023 09:46:28 am

Chris indicated the right reference for the most remote sprinkler to start the hydraulic calculation.
This is the key point when doing calcuulations "by hand" (the good old way).
But with a software calculation, you don't really care where it really starts as long as:
- the calculation is done on the correct most remote area (not always the case)
- you have the most remote sprinkler (or sprinkler ending with the samller density in the calculation) in this calculation
- you can check all the steps of the calculation (whatever the order) with the correct length, fittings and pipe diameters
- the final result is below you water supply capability
- the final density meets the minimum design requirement

A Required Direction for Hydraulic Calculations?

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