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Detection for In-racks on Double Interlock Preation

Detection for In-racks on Double Interlock Preation

Detection for In-racks on Double Interlock Preation

(OP)
Scenario: 38-ft. roof, 32-ft. open frame double and multi-row rack storage of Class II & III Commodities. 45 deg warehouse.
Wet sprinkler system at ceiling
Double Interlock System for Racks Sprinklers (2 levels) with Linear Heat Detection located at ceiling only - along ceiling sprinkler piping. For the purpose of this question - assume desgin (OH density and IRAS design) is adequate.

Several systems installed roughly 20 years ago, all with this similar layout. Slight difference in that in some cases there is a standard dry pipe system for the ceiling system and still otehrs there is a DI preaction for the ceiling. In all cases, detection is only at the ceiling.

For the case where there is a DI system for the ceiling AND a DI system for the racks, there are two separte DI systems (not just control valves), both tied to a sinlge Class A circuit LHD system at the roof level. So activation of the LHD will energize the coil for both the overhead deluge valve and the rack riser deluge valve.

No standard that I'm aware of supports any of these designs. I'm aware of the NFPA and FM exceptions to allow ceiling only detection for combined ceiling and IRAS where certain parameters are met - one of which is that there can only be a single deluge vavle (refrigerated area system).

My guess is that where temps are above 40 deg, the DI Preaction was installed in the racks to reduce water damage/accidental trips from heads getting damage by lift operators.

What are the concerns with having the detection at the ceiling only? Is it a delay? If so, why is it allowed as long as a single system feeds both? A fire in the bottom of a rack will still not get water until LHD at ceiling fuses.

With the wet system at the ceiling, there is a risk that the ceiling sprinklers could operate before the LHD fuses (on the overhead piping). It that happened, the LHD would probably never fuse, making the in-rack sprinkler inoperable. This shouldn't happen if the temp of the LHD is lower than that of the deiling sprinklers, but I guess it could happen. Could also happen with standard dry system at roof, but less likely due to the delay in getting water - allowing additional time to fuse LHD.

What am I missing? Any other thoughts??

Thank you for your time.

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