Adding Loss for Flex in Pipe Schedule System?

7/7/2022

NFPA 13 makes provisions for the addition of a backflow preventer to a pipe schedule system (i.e., device pressure loss must be taken into account for minimum residual pressure available, per Section 19.3.2.6.2 of the 2019 Edition).

When adding flexible sprinkler drops to a pipe schedule system (1 for 1), is a recalculation necessary, or wouldn't the basic addition of that extra pressure loss to the residual pressure for the pipe schedule system be all that's needed?

The AHJ is requiring a recalculation rather than simply adding the pressure loss to the residual pressure as we would do for adding a backflow preventer.

Thanks in advance.

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10 Comments

Danefre

7/7/2022 07:30:26 am

We're in MA. Typically, recalculation is required. We use a specific flexible braided hose manufacturer and calculate it at 55 equivalent feet to be conservative. Max length & number of bends.

Not as simple as adding a flat PSI loss to your residual pressure and most AHJs would agree.

Glenn Berger

7/7/2022 08:10:16 am

There is a limit to the amount of modifications made to an old pipe schedule system as still be considered viable. You might have found this AHJ's limits.

I <3 Sprinklers

7/7/2022 08:16:10 am

We would definitely require a recalculation for adding flex drops to a pipe scheduled system in our area outside of very specific circumstances (matching another floor of an existing building that was previously calculated or something like that).

If not for this situation, where would you draw the line about where full hydraulic calculations should be conducted compared to when preliminary theoretical calculations suffice?

The Victaulic AH2 shortest hose (31") with the fewest bends (3) has a 15'-0 equivalent length. Do you think the plans reviewer and inspector would raise any concerns if you had 15'-0 arm overs for every sprinkler on an add/relocate project?

Dan Wilder

7/7/2022 08:35:26 am

So (spit-balling here, I've worked on about 5 pipe schedule systems in my career), tricky subject as this isn't specifically mentioned in the "Revamping" section and how would you justify pressure loss differences between a hard pipe and a flex hose? If using the SxL x Density to get PSI loss though 1" hard pipe/fittings then applying that to the EQ length of a flex hose, you're effectively applying hydraulic calculation procedures to justify not using hydraulic procedures....

However, the one condition I would stress to make your argument (or not allow it), is to verify that all components of the flex hose are a minimum 1" ID to match a Sch 40 pipe (1.049" ID)...some hoses are a lesser ID.

There is nothing in the current NFPA about how the 1 sprinkler fed from 1" pipe is limited by the number of feet, elbows, or tee's in the line so you may be able to argue that point (again, providing that the hose is at least 1" ID throughout).

Casey Milhorn

7/7/2022 09:00:49 am

I agree with the AHJ on the need for a calc in this situation (being an old pipe schedule system means it's possibly never had a calculation performed on the system). Water supplies can change a lot over that many years.
On an existing calculated light hazard system, I'm not in favor of asking for calcs every time a change is made, even when adding flex drops. When you factor in 2 to 4 90s, tees, X amount of pipe, etc. you can actually exceed the pressure loss vs some flex drops. Also, sometimes a little more friction loss is a good thing and can help balance systems, especially when 5.6k heads aren't maximized in their spacing. I also have an issue with the fact that we calculate down to a tenth or hundredth of a PSI yet flex heads are all over the board in real life friction loss, dependent upon how many bends there are. Long story short, I think there are bigger fish to fry than worrying about a little extra friction loss in flex heads. BUT, if there are any concerns with water supply change, occupancy change, EC heads being used, etc.... that would warrant a calc, I'm always in favor of erring on the side of caution in those cases. Great question.

schulman

7/7/2022 10:18:24 am

I would require full recalc.

Anthony

7/7/2022 03:09:17 pm

Pipe schedule systems came before flex drops. There's no prevision to using them or not using them per code. That being said I would require a calc as it is not "like for like" replacement or repair. Especially if corrugated hoses was used as those have a higher tendency to crimp and restrict flow as compared to a braded hose.

Jesse

7/8/2022 07:37:42 am

Every AHJ we deal with would require a full calc. Even if they didn't, I would require my designers to hydraulically prove adequacy before I sealed it.

Alex

7/9/2022 09:48:17 pm

Hi,

Responding a few days late here - took the week away with family for the 4th.

I agree with all the comments above requiring calculations. If you are replacing a sprinkler with the same sprinkler, different story. If you are replacing a pendant with a flex head, I would require calculations.

Alex

sean

7/11/2022 06:28:53 am

you cant mix and match calculation methods either pipe schedule or hydraulic.

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Adding Loss for Flex in Pipe Schedule System?

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