Home / Featured Articles / Obstruction Rules for CMSA Sprinklers

Obstruction Rules for CMSA Sprinklers

HebiPics / Pixabay

Some Parts are Missing

The 2010 edition of NFPA 13, Standard for the Installation of Sprinkler Systems, introduced a new definition “Control Mode Specific Application (CMSA) Sprinklers.” This was a composite of the earlier definitions that included Large Drop Sprinklers, Special Sprinklers, and Specific Application Control Mode Sprinklers. The Section 8.11 titled “Large Drop Sprinklers” in the 2007 edition of NFPA 13 became Section 8.11 CMSA sprinklers in the 2010 edition of NFPA 13. As you apply this style of sprinkler, you may find out that the obstruction rules in NFPA 13 may not address your application.

When we start looking for the obstruction rules that will affect CMSA sprinklers, we have to start with Section 8.11.1. Here we find that unless modified by Section 8.11, all of the requirements of Section 8.5 will apply to CMSA sprinklers unless further modified by the installation instructions of the manufacturer. Although not an obstruction problem, Section 8.5.4.1.2 addressing corrugated metal deck roofs can create an installation problem for CMSA sprinklers. This section states that for decks deeper than 3 in., the distance from the sprinkler deflector has to be measured to the highest point of the deck. Metal deck that is greater than 3 in. is unobstructed construction per a rejected first draft Public Input No. 569 to Section 3.3.5.4 in the 2013 NFPA 13. The reason that this can be a problem for CMSA sprinklers is that Section 8.11.4.1.1 sets the sprinkler deflector distance under unobstructed construction for a CMSA sprinkler at a minimum of 6 in. and a maximum of 8 in. That means for a 5½-in. deep metal deck the maximum deflector distance would be 2½ in. below the bottom of the deck for a CMSA sprinkler.

So back to obstructions, NFPA 13 certainly addresses some issues but there are gaping holes in commonly occurring situations where there is no guidance. There is also a significant shift in philosophy on how we address the location of the obstructions. It still uses the titles “Obstructions to Sprinkler Pattern Development” and “Obstructions that Prevent Sprinkler Discharge from Reaching Hazard,” but it doesn’t mean there is a defined distance below the sprinklers creating separate zones as done with spray sprinklers. The beam rule and pattern development sections (which applies up to 36-in. below the deflector) follow the standard philosophy that location in relationship to the deflector has meaning. The section on the discharge reaching the hazard is where the wheels fall off the wagon. Most of the criteria in this section apply to items less than 36-in. below the deflector. As such, you have to combine both sections when determining whether a situation is addressed.

The defined criteria for the beam rule are pretty typical, so this needs little attention. It is interesting that some of the requirements are more demanding while others are less demanding than those for spray sprinklers. For example, at 1 ft of separation, the distance above the bottom for standard spray pendent (SSP) is 2½ in. and with CMSA it is a more demanding 1½ in. but at 5 ft it is only 18 in. for SSP but a less demanding 22 in. for CMSA. It’s the same dimensions as required for ESFR. There is also the expected requirement for obstructions above the 36-in. plane (Section 8.11.5.2.1.3), but only covers three items. That is the “Three Times Rule,” which only addresses obstructions 8 in. or less in width. For this reason there is not a maximum up to distance like the 24 in. for SSP sprinklers. This obstruction rule will require that a CMSA sprinkler be located at least a distance of three times greater than the maximum dimension of an obstruction such as structural members, pipe, columns, and fixtures. There is also the requirement that solid continuous obstructions shall meet the beam rule but does not include the clarification added to the SSP rule in the 2013 edition that it applies only when the top of the obstruction is level with or above the plane of the deflector. It is reasonable to apply the same understanding since, otherwise, it conflicts with the other existing criteria. A change was made in the 2013 addition to Section 8.11.5.2.2 that allows an upright sprinkler to be attached directly to a 4-in. nominal pipe diameter. This was previously a 2-in. limitation. This is an exception to the three times rule. The sprinkler can be directly attached to the branch line up to 4 in. but if larger than that, a 12-in. riser nipple will be needed. This is still less demanding than the three times rule. It’s not that much of an issue since how often does one deal with 6-in. branch lines?

The next obstruction rules are in Section 8.11.5.3. This is the section where the obstructions would normally be greater than 36 in. below the deflector. Section 8.11.5.3.2 deals with a narrow group of obstructions that are located entirely below the sprinkler but the bottom of the obstruction is less than 6 in. below the sprinkler deflector. They have a minimum width of 24 in. so are not covered by the three times rule. They are treated as the beam rule with less vertical allowance.

Section 8.11.5.3.4 deals with another narrow band of obstructions that are no greater than 24-in. wide and where the bottom of the obstruction is greater than 24-in. below the sprinkler deflector. The solution is that sprinklers are to be located on both sides of the obstruction but it must be located within 12 in. of the center point between the sprinklers. If this tight requirement is not met, a line of sprinklers is required beneath the obstruction. Also if the obstruction is greater than 24 in. in width (noting the bottom of the obstruction is still required to be more than 24 in. beneath the deflector) then additional sprinklers will have to be installed below the obstruction. This is much like the 4-ft rule for SSP, except there is no guidance when the bottom of the obstruction is less than 24 in. below the deflector. There is also no minimum size assigned to this section. It just says less than 24 in. This is where, if it is 8 in. or less, you would jump to the three times rule.

The final item in Section 8.11.5.3.5 is an even narrower band of obstructions that NFPA 13 even calls a special case. This is for obstructions that run parallel to the branch lines. They must be greater than 36 in. below the deflector and are limited to a maximum width of 12 in. If the obstruction is directly beneath the branch line, the edge can be no more than 6 in. beyond the centerline of the branch line. These obstructions can be ignored. What if it is installed perpendicular to the branch line? Isn’t that parallel to a row of sprinklers, though not on a single branch line, and presents the same impact on the discharged water? A reasonable interpretation is that any obstruction no more than 12-in. wide (including the directly beneath the sprinkler requirement) can be ignored. If it is greater than 12 in., you default to Section 8.11.5.3.4.

A big gap in obstruction protection criteria occurs between that of Section 8.11.5.2.1.3 (the three times rule) and Section 8.11.5.3.2 (obstructions more than 24-in. wide). As an example, what happens when you have a bank of 1-in. diameter conduits that is 20-in. wide and located 6 in. below the deflector and 2 ft off of the side of the branch line? It could also be a 12-in. wide circular light that is 15 in. below the deflector. At this point no guidance is provided on how to address this and similar type obstructions.

The CMSA sprinkler is a control mode sprinkler and is very effective in providing protection against high-challenge fire hazards. CMSA sprinklers are, however, more sensitive to obstructions than standard spray sprinklers. Sections 8.5 and 8.11 do not provide sufficient guidance to avoid the types of obstructions discussed previously, which will require that other solutions will have to be sought. An ESFR sprinkler is a suppression sprinkler and as such has a very stringent set of criteria for obstruction avoidance in Section 8.12.5. Technically, the criteria for ESFR sprinklers cannot be enforced on a CMSA sprinkler. On the conservative side, however, where no guidance is provided for obstruction avoidance for the CMSA sprinkler the use of ESFR’s obstruction criteria may be the only logical recourse. Naturally, any issue not explicitly addressed by NFPA 13 requires concurrence from the Authority Having Jurisdication (AHJ).

ABOUT THE AUTHOR: Phill Brown, a NICET Level IV, is AFSA’s director of technical program development and codes in the Technical Services Department. He has an associate degree in Applied Science/Fire Science and has over 50 years industry involvement, including 26 years of contracting experience. He is a Senior Engineering Technician (S.E.T.) and a Certified Fire Protection Specialist (C.F.P.S.). Brown represents the interest of AFSA and its members in the building codes. He assists in responding to technical questions and is the primary instructor for the AFSA design schools.

IMPORTANT NOTICE: The article and its content is not a Formal Interpretation issued pursuant to NFPA Regulations. Any opinion expressed is the personal opinion of the author and presenter and does not necessarily represent the official position of the NFPA and its Technical Committee.

Share

Check Also

Low Flow Attic System Design

A New Scope for Roofs with Slope Combustible concealed attics have always been a difficult …

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.