Hydraulically Calculate Length of Dry Sprinkler?

5/10/2022

When performing hydraulic calculations, do I need to account for the length of the barrel of a dry sprinkler?

The sprinkler itself starts at the threads, but the length can sometimes be quite long before water hits the orifice of the sprinkler.

Under NFPA 13, under friction loss, the closest I can find says "friction loss shall be excluded for the fitting directly connected to the sprinkler."

Thanks in advance.

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18 Comments

Alex Riley

5/10/2022 07:18:21 am

Hi,

The code section you referenced, NFPA 13 (2013) 23.4.4.7.1(9), I believe is referring to the reducer. For example, a 1" to 1/2" reducer you would typically find on an upright/pendant.

I have always added the length of the barrel for my calculations.

Thanks,
Alex

Jesse

5/10/2022 08:07:26 am

If you're using 1" dry pendents and also using 1" drops for other pendents the calculations is the same. If I was using a 3/4" dry pendent I would certainly put the dry barrel length in, but in most case I'm just doing it for fab purposes.

M. Newell

5/10/2022 08:10:36 am

It has always been my understanding that the way the dry sprinklers are tested and listed they have already factored in the length during the testing process. So based on their listing and testing it is the same for a dry sprinkler with a 12” barrel as it would be for a standard pendent in a bell or ell. You don’t add the drop length to your calculations as friction loss.

This can be seen with the Victaulic VS-1 series dry flexible sprinklers, they calculate the same as a normal sprinkler in a fitting or outlet with no additional friction loss.l even though they come with a flexible sprinkler hose up to 48” in length.

Ultimately it would be best to check the cut sheet for the specific dry sprinkler you are using to verify during your design.

Christopher J Logan link

5/10/2022 08:33:34 am

I have always been under the understanding that the length of the sprk is factured in as well.

part of the 48" max for listed assembly.

Anthony

5/10/2022 08:34:58 am

There is a significant hydraulic difference between a 4'-0 dry barrel and a 1'-0.

If you're flowing 30 gallons through that 4' pipe you're losing 1.5 psi compared to 0.38 psi through a 1' pipe (assuming 1'' nominal diameter sch 40, c100)

So yeah, you have to account for the barrel on the dry pipe.

Dan Wilder

5/10/2022 08:36:26 am

Many of the cut sheets have this kind of language (this was taken from a Viking Cut Sheet) which means you don't need to adjust for the dry length per NFPA 13-19' Section 27.2.4.10.3 - "Flow from a sprinkler shall be calculated using the nominal K-Factor except that the manufacturer's adjusted K-Factors shall be utilized for dry-type sprinklers"

"K-Factor applies for standard lengths (“A” Dimensions indicated above)."

Nominal K-Factor: 5.6 U.S. (80.6 metric*) for all listed and approved lengths.

These sprinklers are listed and approved in lengths from 4” to 48-1/4” (102 mm to 1,226 mm)

I have seen some lengths at 36" but it needs to be reviewed on a MFG sheet specific basis.

CJ Bonczyk

5/10/2022 08:41:10 am

Yes the length of the matters significantly for hydraulics. As the length changes the K-Factor changes for many manufacturers on their data sheets. Secondly pay attention to the C-Factor. Lastly the exposure temperature charts will dictate the minimum barrel length. All of this information is typically listed on the manufacturer data sheets as it varies for each type and manufacturer.

Glenn Berger

5/10/2022 09:09:36 am

You need to document what k-factor being used. K-factor for the dry barrel assembly should be used and assumed at the tee-connection.

David Kendrick

5/10/2022 09:13:54 am

The engineering behind a UL listed product probably exceeds my skill, knowledge and intelligence.

As an example from the Viking technical bulletin.

Viking K Factor 5.6 Dry Sprinkler -- "Nominal K-Factor: 5.6 U.S. (80.6 metric*) for all listed and approved lengths."

Pulled technical bulletins from two other manufacturers. None showed any changes in the K Factor for their dry sprinkler heads based on the length of the barrel.

The K Factor for dry sprinkler heads is the number you use. Lengths are limited by their listing but the K Factor remains the same.

Brian Gerdwagen FPE

5/10/2022 09:15:03 am

Glenn is 100% correct, as are some others. The adjusted K factor from the manufacturer gives you the K factor at the fitting the sprinkler is attached. There is no need to add additional friction loss for the length of the barrel of the dry pendent.

Alex

5/10/2022 12:46:42 pm

Brian, Glenn,

I didn't pose the original question but I want to follow up on both of your responses. I would agree to use a provided discharge k factor that takes in account the barrel length/diameter/C factor/etc, but what if they aren't provided? To me, its not common they are.

For example, how would you handle Tyco's TY3155 dry upright? There is no various K factors provided on the length of the barrel. Are you saying that Tyco has engineered the sprinkler to already adjust for the friction loss through the pipe? I don't see how that is possible since its contingent based on the flow.

I may be over thinking this issue but very curious now. It doesn't appear there is a lot of information out there explaining it..

Thanks in advance,
Al

David Kendrick

5/10/2022 03:06:16 pm

When you see the size of the orifice entering and exiting the assembly well rough guess the barrel is three times the area of the orifice. Relatively little resistance to the flow.

If it's published that the K Factor of the head is x.x then that is the K Factor.

Viking put it into words that this is the way the heads are listed up to 48" in length.

Reliable and Tyco did not publish that statement. With them there is no restrictions or method to calculate their UL listed product.

I read what they publish. Follow their printed data.
For me to rethink their product would be folly.

schulman

5/10/2022 09:56:32 am

i see conflicts in the responses - my state construction office requires calculating the lengths of flexible connections to Vic's Flex Sprinklers so whether M.Newell or others are correct or incorrect, my situation requires I calc the lengths (including dry barrel). Check with your AHJ or State Construction Office and streamline your process to include the most stringent requirements ... The "costs" should be passed on to the owner anyhow so why not be safe and do it ?

Dan Wilder

5/10/2022 10:37:28 am

Vic already states in the literature:
K Factor • 5.6/8.1 S.I.
For system design purposes, no equivalent length calculations are required.

These aren't sprinklers attached to flex hoses, these are listed assemblies including the sprinkler and the dry flex hose. What criteria are you using to add the eq. length, just the length of the hose itself? No requirements for # or bends as well?

Jack G

5/10/2022 10:41:41 am

Back in 63, my father taught me hydraulic calculations. Head k factors adjusted to account for length are factored in. It hasnt changed.
Attended Star Sprinklers course in 68- same. Gem s in Rhode island in 76. Automatic sprinkler course in 77. University of Md in 70, taught you the same.
The head data sheet may not, or might obscurely state that.

Alex

5/12/2022 04:54:53 pm

UPDATE [5-12-22]

Hi everyone who has notifications on,

This post has been on my mind still (does that make me a nerd? My fiancé thinks so.)

Since I was still curious, I decided to ask JCI in Cranston, RI. They stated on their dry sprinklers, you do need to add the length of the barrel within the calculation. They stated, once the head opens, you then in turn have a wet pipe system.

A few above mentioned the Vic dry flex heads. I agree with you stating you do not need to add the equivalent length per the cut sheet. While JCI wouldn’t give advice on other brands, I would imagine it would be the same process UNLESS a discharge K factor is included in the cut.

In the end, adding the barrel length simply just adds to your safety factor!

Thanks,
Alex

Larry A Gallego

10/3/2022 11:29:18 am

Another aspect that comes into play is whether the elevation of the outlet on an 12" dry pendent is used for the calculation or is the elevation of the connection to the branchline/mains selected?
On a small layout I calculated one 12" dry pendent head at 0.15 gpm/fqft for 116 sqft with a K-factor of 5.6 at 9'-4" and re-calculated using the branchline elevation of 10'-4" as the head elevation with the following results:

Including 12" barrel (9'-4");
Total Flow From Water Supply Curve . . . . . . . . 17.40
Pressure From Water Supply Curve . . . . . . . . . 60.00
System Pressure Demand At Supply Curve . . . . . . 18.13
Safety Margin at Supply Curve. . . . . . . . . 41.87

Excluding 12" barrel (10'-4");
Total Flow From Water Supply Curve . . . . . . . . 17.40
Pressure From Water Supply Curve . . . . . . . . . 60.00
System Pressure Demand At Supply Curve . . . . . . 18.28
Safety Margin at Supply Curve. . . . . . . . . 41.72

The difference of 0.15 psi when calculating multiple heads may produce a significant pressure difference on the safety cushion.

When I calculate 8 heads the difference is 0.36 psi less pressure for the non-barrel calculation.

Total Flow From Water Supply Curve . . . . . . . . 174.69
Pressure From Water Supply Curve . . . . . . . . . 59.93
System Pressure Demand At Supply Curve . . . . . . 52.95
Safety Margin at Supply Curve. . . . . . . . . 6.98

Total Flow From Water Supply Curve . . . . . . . . 173.88
Pressure From Water Supply Curve . . . . . . . . . 59.93
System Pressure Demand At Supply Curve . . . . . . 52.59
Safety Margin at Supply Curve. . . . . . . . . 7.34

If the barrel is to be calculated, would the C Factor be 100? I will check with the local AHJ and see how they can enlighten me.

Thanks,
Larry

David Kendrick

10/4/2022 09:14:51 am

I checked several manufacturers data sheets for dry pendants.

One specifically stated that 48" and under all follow the K factor stated. The others did not publish any restrictions.

For my two cents the NFPA standards, Insurance Underwriters and especially the manufacturers hold the interpretive rule.

When we engineer outside the published standards we become responsible. Until then we're following the documented requirements.

Hydraulically Calculate Length of Dry Sprinkler?

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