Fire protection in MCFs (part 2)

Lance Harry PE, director of sales, Fenwal Protection Systems -- Consulting-Specifying Engineer, 3/28/2008

Fire protection for mission critical facilities (MCFs) can be a complex and daunting topic. We’ve broken up the task into several topics so that you can create manageable assignments out of each one. This three-part series will cover each topic in depth.

* Part 1: Understand the value of the facility in question. Value can be defined in a number of ways from asset value, to operational value to historical or sentimental value.
* Part 2: Evaluate the level of risk in the facility in question. What has been done to mitigate these risks? What can be done? What should be done in order to adequately protect against a potential hazard, including fire, in the facility?
* Part 3: Investigate the options available from water-based systems to waterless systems and determine what is right for your facility and your business. Understand the unique and varying levels of protection your facility will get from each option. It is the business owner’s responsibility to fully understand what each system will provide for his or her business.


Fire protection evaluation

Fire protection system selection is a complex process that can involve a number of different entities, including internal company resources (financial, engineering, IT) and outside consultants, insurance representatives, architects/engineers, and the local authority having jurisdiction (AHJ). It is important to understand the difference between code required protection systems (most often sprinkler systems) and supplemental asset protection.

Evaluating the need for supplemental protection should start with an analysis of the facility. Several different factors must be considered:

* Analysis of the facility. The physical characteristics of the facility should be noted. In new construction, specific requirements such as tightly sealed windows/doors and venting (see below for discussion of various clean agents) can be included in the facility design. In older facilities, the space provided with gaseous protection must be specifically evaluated. If a room is overly “leaky” and will likely not achieve a level of integrity that is desired (or is too costly to do so), other strategies may need to be considered. Most local fire protection contractors can assist in a determination of room integrity for a given facility.

* Hazard analysis. Understand the fire hazards within the space. It is most important to differentiate between class A (common combustibles such as plastics and fabrics) and class B (flammable liquids). Most mission critical facilities (MCFs) contain only class A hazards, but a thorough review should be performed. This is also a good opportunity to investigate and evaluate the desire for emergency power off, or EPO. Most fire protection professionals and codes recommend power be shut off to the electronic assets in the protected space, prior to discharge of the system. In a facility where power is not shut off prior to discharge, some additional design considerations may need to be implemented. Consult your AHJ and system manufacturers for additional guidance in this area.

* Overall risk assessment. A thorough risk assessment comes down to an understanding of potential harm versus likelihood of an incident occurring. It is standard to use a risk matrix to assist in the determination of how a given facility may be categorized from a risk perspective (see below).

Fire in a MCF generally is viewed as a critical or serious impact, while the probability of occurrence is either low or medium. Even with this, the risk category would either be moderate or high. Further, risk professionals agree that moderate/high-risk categories are areas that should be addressed as a priority.

Download a PDF of probability versus impact.

If supplemental fire protection is appropriate, it is natural to evaluate the two most common strategies with respect to cost versus levels of protection.

* Pre-action sprinkler systems. A pre-action sprinkler system is water-based, and incorporates several operations to minimize the risk associated with accidental discharge and water damage. Pre-action systems come in a variety of forms including single- or double-interlock, roughly designating the level of protection against accidental discharge. For the purposes of illustration, the sequence below describes the three typical steps that occur during a double interlock pre-action sprinkler event.

1. Smoke detectors sense a combustion event. An alarm is triggered and sent to the system control panel, which interprets the alarm, and then sends a signal to a solenoid-controlled valve, opening the valve. The panel typically also provides notification of the event within the surrounding facility. The open valve allows water to fill the pipe. In a double-interlock pre-action system this may fill up to a second valve.
2. With a combustion event progressing, heat builds at the ceiling of the protected space. While most standard temperature sprinklers are rated for a temperature of 135 F to 165 F, it is not uncommon for the surrounding space to be at a higher temperature before the element fuses. Sprinklers use an element, typically a glass bulb or fusible metal link, that influences the sprinklers discharge temperature. These devices are very reliable, but the thermal mass of the link requires time to reach the same temperature as the surrounding air. This effect commonly is referred to as thermal lag.
3. With at least one sprinkler open, the piping first discharges compressed air or nitrogen present in the pipe. The loss of pressure in the piping causes a pressure switch to be triggered, and opens the second in-line valve (again in a double-interlock system). Water then is allowed to flow freely into the piping network, and discharged from any sprinkler whose element has been broken.

It is worth noting that even in a pre-action system, a significant level of heat is required to activate the system. While the level of risk associated with false discharges or damaged heads is minimized significantly in this strategy, the level of protection to the assets is still based on heat detection, and water discharge. Use of water based systems should be considered as part of the overall risk profile within the MCF. (Note that use of deluge-style sprinkler heads would eliminate the element of heat detection from the discharge of water in a pre-action system. Smoke detection alone could be used to provide initiation of the water flow into the protected space.)

* Waterless/clean agent systems. Clean agent systems typically require two actions to occur prior to an actual discharge.
1. Smoke detectors provide a signal to the suppression system control panel. Two separate alarms (often one photoelectric type and one ionization type) are required prior to a countdown to discharge of the agent. This is commonly referred to as a cross-zoned detection strategy.
2. The control panel initiates a timed countdown, typically 30 s, before signaling the agent storage container to discharge the agent into the protected space. A delay is provided while occupants can vacate the facility and other preparations (mechanical door closures, automatic dampers, etc.) can be made prior to discharge. Note that evacuating the protected space is a general recommendation for any potential fire event and is not due to the discharge of the agent. Agents discussed below are all safe to potential occupants when designed in accordance with United States and international standards.

Many strategies involve the use of both a water-based system (for structural protection) and a waterless system (for asset protection) in the same protected space. This strategy provides the most complete fire protection scenario in any given facility. It follows that this also requires the greatest financial investment by the end user.

For more information on the differences in protection levels provided by clean agents versus water-based systems, view an excellent video comparison. Click one of the video links found in the red “FM-200 Video” box.