Who’s in control?
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
During a recent review of a fire sprinkler design (contract docs, not shop drawings), it was déjà vu all over again. After decades of trying to help engineers get it right, I still find myself making the same comments. My frustration concerns the design of proper controls for fire suppression systems. The specification section I was reviewing involved a preaction sprinkler system. As you may know, a preaction sprinkler system requires a detection and control system to activate the preaction valve. This control system can be hydraulic or pneumatic, but in most instances it is electrical. Preaction sprinkler systems that use air to supervise the integrity of the system piping also have controls. The problem with this design was that the controls were covered by just a page of specifications of ancient fire detection technology, with a few sketches of smoke detectors on the drawings. To compound the error, no specification for the required low air pressure switch was included; in fact, that switch as well as the alarm pressure switch and valve tamper switches were not on the plans.
Well, hope springs eternal, so let’s give it one more try. Following is a brief review of different types of fire extinguishing/suppression systems and the controls that may be needed.
Let’s do the easy one first: wet pipe sprinkler systems. Controls include sprinkler system alarm and supervisory devices, as well as the initiating device that indicates sprinkler water flow. What’s an initiating device, you say? (Ugh!) An initiating device is usually an alarm pressure switch or a water flow alarm switch. The supervisory device monitors the status of the system, and for a wet pipe sprinkler system this normally is a valve monitor or tamper switch. Occasionally a low temperature supervisory switch or a high water tank level supervisory switch also is needed.
Thus, the plumbing engineer only needs to let the electrical engineer know where these switches are located so they can be identified on the fire alarm drawings. The specification needs are simple: These devices should be provided under the wet pipe sprinkler specification with a reference to the fire alarm specification section for the connection of these devices. Oh wait; that’s not all. Many users and AHJs now prefer the use of an electric alarm bell rather than the more traditional water motor gong. This sounds simple enough; just provide a 120VAC bell on the exterior wall near the sprinkler riser, run some power through it and the water flow switch, and you’re done. However, keep in mind that many sprinkler installers are accustomed to water flow alarm switches wired to low voltage circuits from fire alarm systems. Now you have one set of contacts with 24VDC and one with 120VAC. What a shockingly bad idea. Of course, this presumes that a switch with dual contacts was specified. Take the time to coordinate with the electrical engineer to provide a separate NAC circuit for a single sprinkler alarm bell that is connected to the fire alarm system and operates only upon activation of the water flow alarm switch. You ask, “What is an NAC circuit?” (Ugh, again.)
In terms of coordination, dry pipe sprinkler systems will be similar, except for the addition of a low air pressure supervisory switch. Wet chemical fire suppression systems will require connection of the microswitch, also a device with two sets of dry contacts, to the fire alarm control panel for alarm supervision and for shutdown of power and/or fuel to protected equipment.
Okay, maybe that was not so easy. Let’s try one that should be easy: fire pump systems. Again, the controls primarily include supervisory signals required by NFPA 20. The connection points for the supervisory signal connections are located within the fire pump controller. All fire pumps must be supervised for their running condition. Diesel engine-driven fire pumps also require a supervisory signal to indicate if the controller main switch is in the “off” or “manual” position and for controller or engine trouble. Fire pumps with electric motor drives must be supervised for loss of phase and phase reversal.
If an alternative source of power is included, a signal must be generated when the alternate source is supplying power. One control feature often overlooked is an automatic transfer switch (ATS) used with an emergency generator as the alternate source. Upon loss of power to the fire pump controller (i.e., a voltage drop below 85% of the motor’s rated voltage), the ATS must send a signal to the generator to cause it to start. The supervisory signals can be handled by mounting addressable monitor modules at each fire pump controller to cover each of the required signals. ATS/genset start circuit is usually accomplished with a direct connection between these pieces of equipment.
Here’s one other tidbit. Fire suppression systems for U.S. Department of Defense aircraft hangars often require an additional control circuit to start the fire pumps upon operation of the system. All fire pump controllers have a set of contacts for this, which is normally called “deluge start.” This redundant means of ensuring fire pump operation is in addition to starting fire pumps on water pressure drop. An annex note in NFPA 409 recommends a supplementary means of ensuring fire pump start.
Getting back to the preaction sprinkler system with electric actuation mentioned at the beginning, let’s look at how the controls can be covered in the design. The preaction system can be single-interlock or double-interlock. In single-interlock systems only one event—activation of a detector (smoke or heat)—is needed to trigger the preaction valve, but two events are needed to open the preaction valve in double-interlock systems. One can use cross-zone smoke detection or any two smoke detectors to activate the valve. In our designs we use any single detector, usually a photoelectric smoke sensor, and operation of the low air pressure switch as the two events in the double-interlock case.
The preaction specification paragraphs for the detection and control components are short and sweet, including those specifying the alarm pressure, valve tamper, and low air pressure switches, as well as a call to go to the fire alarm spec section for the connection. A short paragraph also simply indicates that detection and control will be provided by the fire alarm specification.
Coordination with the electrical engineer is required if you do it this way, but the benefit is that the fire alarm specification section has all of the good stuff you need to properly specify the controls. In my opinion, the available guide specifications for preaction sprinkler systems have poorly written control sections and are destined to get your design in trouble. The other problem is that the major fire suppression equipment manufacturers have not been very good at manufacturing control units for fire suppression systems. In the old days these were some of the clunkiest systems in the world; however, things have improved in the last 10 years as the industry has consolidated and fire suppression firms have figured out that it is easier to just OEM decent fire alarm control units.
Normally the preaction sprinkler control panel should be separate from the main or building fire alarm panel. Similarly, clean agent fire suppression or carbon dioxide extinguishing systems should always be controlled by a separate control unit panel. This is a snap for the fire alarm spec in this age of addressable/networked panels. It is important to make sure the specification requires the control panel used for preaction valve operation to be listed for releasing device service. The drawings will need to include a layout of device locations, a suppression control unit riser diagram, and a suppression control unit matrix of operations.
I believe that issues with the detection and control components of fire suppression system designs are perpetuated for one simple reason: The total system crosses electrical and mechanical disciplines, and things get lost in the coordination—unless, of course, a fire protection engineer is on the design team (the other shoe drops). It is not a cross-discipline issue for the FPE who controls the preparation of fire suppression system and fire alarm system specifications.
Regardless, pick up the phone, coordinate, attend to the details, and fix the specs, and you will be the one who’s in control.
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Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.