Dry Pipe & High Temperature Density Adjustment

7/10/2019

I'm new to this industry and I've been really trying to pick apart NFPA 13.

I have a question related to determining the area of sprinkler operation of a dry pipe system with high temperature sprinklers. The system is providing coverage of an outdoor chemical storage canopy.

Design criteria:
Dry System due to freeze potential
Extra Hazard Group 1 (0.30 gpm/sf over 2500 sf) per NFPA 13 Fig. 11.2.3.1.1
High temperature rated sprinklers per NFPA 13, Section 8.4.7.2.4

My question is regarding adjustments to the area of sprinkler operation. Dry systems are to be increased 30 percent per 11.2.3.2.5, then it states in 11.2.3.2.6 that the area can be reduced by 25 percent when using High temperature sprinklers. Multiple adjustments can be compounded.

Increase: 2500*30% = 750 sf
Decrease: 2500*25% = 625 sf
2500 + 750 - 625 = 2625 sf

A conservative approach would be to ignore the 25% reduction, but I'm curious what is actually required per code.

Posted anonymously for discussion. Discuss This | Submit Your Question | Subscribe

4 Comments

Wayne Ammons

7/10/2019 07:52:21 am

To compound multiple adjustments to a design area the math would look like this (assuming you do not have to apply a 30% increase for sloped ceilings, section 11.2.3.2.4):

2,500 + 30% (750) = 3,250
3,250 - 25% (812.5) = 2,437.5

It does not matter whether you apply the increase or the reduction first. The annex material for section 11.2.3.2.7 gives a couple of examples.

7/10/2019 07:55:00 am

While the conservative approach always sounds nice and comforting to us as engineers/designers, the reality is that extra coverage area is unnecessary per the code. The exceptions exist for a reason, and your salesperson will probably not be thrilled with the extra cost that is incurred by your safety margin.
Your design area increase and decrease were calculated correctly which is good because that part seems to trip people up at first (what to take the percentage from).
If the exception applies to your scenario, take it. High-temp sprinklers will limit the number of activated sprinklers due to dry-system water delivery delays. The more sprinklers that are activated before water, the more likely your system will become overwhelmed.

7/10/2019 08:01:26 am

I stand corrected. Your (and my) design area adjustment is wrong and should follow what Wayne references and explains above. Can't overlook the annex material...

Frank

7/10/2019 02:56:57 pm

It is very important, you have an Ocupancy not an Storage Application.

this section only apply to storage areas.
High temperature rated sprinklers per NFPA 13, Section 8.4.7.2.4

Dry Pipe & High Temperature Density Adjustment

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES