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Volume 31, Issue 4

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Monday, 29 October 2007 01:20

Choosing the Right Automatic Sprinkler

Written by  Joseph Hankins
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Automatic sprinklers are a building’s first line of defense against fire. A sprinkler system not only detects a fire, it also automatically transmits an alarm to the local fire department or central station. In the few minutes it takes for the fire department to respond, the sprinkler system already is at work controlling, or in some cases suppressing, the fire. And a sprinkler system is on duty 24 hours a day, 365 days a week.

However, not all sprinklers are the same. The fire challenges in a hotel or office building differ greatly from those in a warehouse containing large quantities of plastics. Therefore, different types of sprinklers have been designed to meet the needs of varied occupancies. (See accompanying sidebar, “The History of the Automatic Sprinkler,” for details on sprinkler development.)

The most common types of automatic sprinklers include the standard sprinkler, large drop sprinkler and Early Suppression Fast Response sprinkler.

Standard sprinklers

Standard sprinklers are, by far, the most widely used type of sprinkler. Their effectiveness is based largely on their ability to pre-wet adjacent materials that the fire has not yet reached, and to cool adjacent areas of the building.

Fire control with the standard sprinkler occurs as the original fuel burns out. Fire spread is slowed because the fire can’t ignite surrounding areas that have been pre-wetted by the sprinklers. Because the fire is confined to one area, only sprinklers above the fire operate.

Standard sprinklers continue to be the mainstay of industrial and commercial fire protection. However, new fire challenges that limit the effectiveness of standard sprinklers have been identified, which have led to design and installation modifications.

For example, one limitation of the standard sprinkler was its response time in offices, hotels, motels, hospitals and other service industry properties with smaller rooms than typically would be found in an industrial facility. A variation of the standard sprinkler, labeled the quick-response (QR) sprinkler, was developed in attempts to maintain a livable environment during a fire long enough for occupants to escape or be rescued.

Because it operates while a fire is still in the early stages of development, the QR sprinkler limits the amounts of smoke and carbon monoxide that are released by the fire. However, from a property protection standpoint, FME&R research shows that QR sprinklers are no more effective than their standard forerunner. Another, very different, fire challenge that pushed the standard sprinkler beyond its limits was rack storage fires. Warehousing practices in the post-World War II years changed significantly. As warehouses increased in size, rack storage heights grew and the combustibility of stored materials worsened. The water from ceiling sprinklers often could not find its way down to the seat of the blaze--especially if the fire started in a lower tier.

One answer was to install additional sprinklers (typically standard sprinklers) within the rack structure at one or more levels. This would allow the in-rack sprinklers to promptly deliver water much closer to the fire. However, this protection scheme has one shortcoming: lack of flexibility. In-rack piping and sprinklers are inconvenient when racks must be moved or rearranged. Higher hazard commodities and an increase in storage height may require re-piping the in-rack system. Thus, although it provides good protection, a standard ceiling and in-rack system requires substantial capital outlay in the initial installation, and potentially costly modifications to deal with storage changes.

In addition, some warehouse owners and managers worry that in-rack sprinklers are more vulnerable to being damaged as products are put into or removed from the racks. They fear that the damaged sprinklers will leak and damage the surrounding storage. However, this is mostly a problem of perception versus reality. Factory Mutual Engineering and Research studies show that there is a low frequency and low average dollar loss from in-rack sprinkler leakage. In fact, the average in-rack sprinkler leakage loss over a recent 10-year period (involving an average of six losses per year) was only $14,967. In comparison, the average loss in storage facilities where sprinkler protection was provided but more sprinklers were needed averaged $1.4 million per year.

Large drop sprinklers

To increase flexibility and maintain effective fire protection, the large drop sprinkler was introduced. The large drop sprinkler is the first sprinkler designed specifically to deal with high-challenge fires. Based on a unique design incorporating a larger orifice (opening for water discharge) and more effective discharge pattern, larger drops of water are able to penetrate the upward blast of strong fire plumes, while providing adequate cooling for the surrounding area.

Backed by a strong water supply, the large drop sprinkler is well-suited to a wide variety of commodities and storage arrangements. It also can be used to protect higher storage than standard sprinklers alone. In many situations, the need for and added cost of in-rack sprinklers are eliminated or at least reduced.

At existing warehouses, standard sprinklers sometimes can be replaced with large drop sprinklers to provide additional protection when changes in commodity or in storage arrangement have resulted in a greater fire hazard. Here, too, in-rack sprinklers either may not be needed, or fewer levels will suffice.

Early Suppression Fast Response sprinklers

To enhance the effectiveness of sprinklers on highly challenging warehouse fires, the Factory Mutual Research Corporation (FMRC) combined the quick response and large drop technologies to develop the Early Suppression Fast Response (ESFR) sprinkler.

Using a more heat-sensitive fusible element and a larger orifice than both the standard and large drop sprinklers, an ESFR installed at the ceiling allows rapid discharge of a large quantity of water in a very efficient discharge pattern to suppress, not just control, a fire in its early stages. This occurs before severe fire plume velocity develops and the heat release rate accelerates.

Fewer ESFR heads operate during a fire than standard or large drop heads, but ESFR sprinklers discharge more water. While the water demand may be higher for ESFR sprinklers, the advantage is that ESFR sprinklers extinguish the fire quickly, resulting is less water damage.

Installing an ESFR sprinkler system instead of the combination in-rack and ceiling sprinklers...

  • is easier and costs less in the long run
  • saves the repeated costs and inconvenience associated with removing and re-installing in-rack sprinklers
  • allows for easier material handling within racks
  • permits storage of varying types of products anywhere in the warehouse
  • may make a warehouse more attractive to potential buyers

A note of caution: Recent interest in faster responding automatic sprinklers has resulted in a potentially confusing variety of products that have fast-acting heat-sensitive elements. In considering which sprinkler to purchase, be sure that the product (whether it’s an ESFR, large drop, quick response or standard sprinkler) is approved by a recognized product certification laboratory.

Joseph Hankins is manager of the Protection Section of Factory Mutual Research Corporation’s Standards Division. In this capacity, he is responsible for the development of FMRC’s automatic sprinkler protection guidelines.

This article adapted from Vol. 5 #2.

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