How to Decide on Type of Pre-Action System?

12/30/2019

I would like to open up a topic to this amazing forum.

Preaction systems...types, cost, challenges, panel set-ups...I continue to see the vast lack of knowledge amongst the design and management community.

Which type is best? Which is more economical? Which is best for your company to install or to maintain? How does a double interlock system affect design/cost? Can a single interlock system be set up to operate the way you need and at a lower cost? Does the AHJ typically have any special requests?

Knowing these answers could only make everyone's lives easier. Thanks in advance!

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7 Comments

Mike L

12/30/2019 02:40:37 pm

Yes, there seems to be a mystery about Pre-Action systems. Even within our office, electrical engineers who design the activation system don't fully understand the proper sequence of operation.

Here in NYC, single interlocks are the only type of systems permitted (unless it's protecting an area subject to freezing, which we would use a conventional dry system). The FDNY requires an immediate availability of water in the suppression system.

Outside NYC, in the tri-state area, double interlocks are common. I can't imagine that there is much of a cost difference between single and double interlock, as all the equipment is the same. The difference is in the programming.

Assuming the AHJ doesn't have any requirements (like the FDNY), then the decision of which type of pre-action comes down to a discussion with the stakeholders. Typically, the answer I hear is "how can we avoid having water near our servers?". On the hand, we want to meet our client's wants/needs but on the other hand, as fire protection engineers, we need to provide a system that will also provide property protection and not let a potential fire get out of control.

Key to remember, single interlock systems can be considered wet systems while double interlock systems are more akin to dry systems and thus the 30% area enlargement needs to accounted for.

MIGUEL ANGEL D ADARIO

12/30/2019 03:49:49 pm

To the comments made by MIKEL, I would add when we have a double action system in frozen areas, great care must be taken with the shapes and slopes of the pipe, so that after acting the system can be completely emptied.

Kelly

12/31/2019 10:39:46 am

This post does a good job of summarizing the differences:
https://community.nfpa.org/community/open-xchange/fireprotection/blog/2015/06/16/single-vs-double-interlock-preaction-systems-explained

In the DC area we tend to get a lot of requests for double-interlock. Mainly due to IT managers not wanting a sprinkler to "accidentally activate" and get water on their systems. I think that single interlock systems would be perfectly adequate to meet this need, however they also present more of a unique risk - for example if a smoke detector has a false alarm due to dust or someone vaping (yes I've seen it) - then the system could get charged for no reason. This isn't a problem most of the time, but if your system has leaks, which aren't uncommon for dry pipe systems that are of a certain age, then you'll have leaky pipe where with a double-interlock you would not be in that situation.

Usually I try to talk down clients from any type of preaction system as the maintenance requirements and corrosion in them tends to be worse for the facility's lifetime cost than the risk of a wet pipe sprinkler system spontaneously springing a leak (the roof is more likely to leak, or an HVAC condensate drain). Some clients are just set on what they want and won't listen to expert advice.

James

12/31/2019 10:42:40 am

Very well said Kelly!

Michael Millman

1/15/2020 02:17:01 pm

I typically push for spot heat detectors or even linear heat detection to be used for pre-action activation for exactly the issue you mention (false alarms due to dust etc, vape, etc.). VESDA/ASD is an option as well especially for data centers that tends to avoid false alarms but provides a very accurate level of detection.

Also theoretically you'd want to monitor your air (and maybe compressor run) so that you'd get a supervisory signal if there was a leak. You'd theoretically know about the leak due to low air alarms or a compressor running over and over again. You'd theoretically detect and fix the leak quickly and then you don't have to worry about a false activation actually discharging water. Sure, you'll charge the pipe.. but you theoretically won't have leaks since you'll have fixed them as soon as they crop up due to continually monitoring the supervisory medium in the pipe.

Michael Millman

1/15/2020 02:03:48 pm

I try to push my clients to use Single Interlock for numerous reasons.
1) Treated like a wet system hydraulically (C=120)
2) No pesky delivery time issues to worry about
3) Still provides protection against discharge due to mechanical damage

I think the big problem is that many people don't understand the difference between single-interlock and non-interlocked. Most people that push for Double Interlock think that in a single interlock system, water is discharged into the pipe upon either detection event or breaking of a sprinkler link/bulb.... they basically see it as a non-interlocked system. I try to explain to them that in a single-interlock electric detection system, that no water is introduced until electric detection event occurs. Worst case scenario you'll get a low air alarm but accidental discharge is very unlikely.

I personally don't like the thought of double-interlock systems if it can be avoided. If it's a must, I usually push for a fail safe (rather than "fail secure") system... i.e. one that acts as a standard dry system in the event of a power (and batter) failure. Surefire by Viking/Fireflex is a good system that meets this requirement.

Ed Poulin

7/2/2025 12:27:53 pm

In what section of NFPA 13 do you find a 120 Cfactor for a single interlock preaction?
Also I assume the piping needs to be pitched as it would as a dry system?
-Ed

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How to Decide on Type of Pre-Action System?

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