Like most "little" things, the final few feet between the branch line and the sprinkler head in a fire protection system usually gets little, if any thought. But as increasing numbers of new and retrofit commercial projects are proving, these few feet can make a noticeable difference, both initially and over the life of the property. Using flexible connections in place of traditional hard-pipe armovers pays dividends that at first blush would seem out of proportion for such a relatively minor detail.

Flexible connections install quickly, have fewer points of potential failure, have an installed cost that is at least competitive with hard pipe, and are very accommodating of building, use or code reconfigurations. This article discusses the physical attributes of flexible fire sprinkler connections, and the financial/economic aspects of specifying flexible connections rather than hard-pipe armovers.

History of Flexible Fire Sprinkler Connections

The first viable flexible fire sprinkler connections were developed by FlexHead Industries of Holliston, MA, for semiconductor manufacturing facilities. Initially designed for exhaust-duct protection, the technology was then adapted for suspended ceilings in cleanrooms. The flexibility of the connection eliminated concerns about armover joint leaks or particulate generation caused by building shifts or seismic activity, as well as providing fast-track construction and the ability to easily accommodate future space changes. The high-quality, welded stainless steel construction and a custom-designed fit-up also contributed to eliminating particulate generation, which is an important concern in the ultra-filtered cleanroom environment.

Several years ago, the technology was adapted for use in suspended ceilings commonly found in commercial projects. Currently, a handful of manufacturers offer flexible sprinkler connections that employ various designs, materials and construction approaches. Typically, the units consist of a threaded pipe nipple to connect to the main or the branch feeder line, flexible connecting hose in standard lengths, a reducing fitting that the sprinkler head is attached to, and a mounting bracket that attaches to the ceiling grid (Figure 1). The mounting brackets are designed so that the sprinkler head accurately hits center-of-tile with no adjustments or calculations required of the installer. Usually, the brackets also allow for vertical adjustments of the heads to achieve consistent, specified exposure through the ceiling tile.

Regardless of design and construction, flexible connections offer some advantages over traditional hard-pipe armovers. For instance, they involve fewer joints between the feeder line and the sprinkler head, which both eliminates most of the handwork that a hard-pipe armover requires and also substantially reduces the number of potential sources of leaks. Also, because the mounting brackets typically take the guesswork out of hitting center-of-tile, fussy adjustments are eliminated. Field reports also indicate that flexible connections enable installers to quickly and easily overcome unexpected obstacles, such as ducts or wall segments. In many cases, reconfiguration of the system to accommodate space or code changes is also much faster because the connection doesn't need to be disassembled and re-installed. Assuming the hose is long enough, it's simply a matter of depressurizing the system, repositioning the mounting bracket and reconnecting the sprinkler head assembly.

Flexible Approaches

Within the realm of flexible sprinkler connections, however, there are several comparison points to consider that affect cost (initial, installed and lifetime) and performance. These include the material used, the design, and the degree of factory vs. onsite assembly required.

The most obvious feature, and an important delta, is the corrugated hose. Both the inside diameter and the material used affect friction loss, which can vary among different brands by more than 350% over a six-foot hose length. In practice, as systems are sized from the heads back to the main--regardless of whether they are hard pipe or flexible--the friction loss values can impact the size of the mains. One-inch "true" inner diameter stainless steel hose exhibits the best friction loss, producing equivalent-length values similar to that of a hard-pipe armover, as well as providing superior resistance to corrosion over time. Flexible sprinkler connection friction loss data is expressed in equivalent lengths of one-inch Schedule 40 pipe, and should be included in the hydraulic design calculations the same as a valve or fitting

Both the strength of the hose and its resistance to reverting to its original shape under pressure are greatly enhanced by the use of a braided sheath. The fact that only braided hose can pass the stringent testing that Factory Mutual requires for its certification (see UL/FM Testing Sidebar) illustrates its importance. As with the hose itself, the sheath material used also affects the performance.

Another consideration is the way the various elements of the system are joined: hose, fittings, and sprinkler head. Some flexible systems use o-ring joints between the hose and the fittings; others use welded joints. For the reducing fitting that attaches the sprinkler head, some manufacturers swage pipe and then thread the reduced inside diameter. Other manufacturers machine their fittings from solid stock, which produces very consistent tolerances and durability.

The different design and manufacturing approaches also produce different onsite experiences and can affect the economics of the installation and lifetime costs. Some systems are delivered fully assembled and pre-tested at the factory. Others are delivered as component parts that must be assembled onsite. Such systems require more labor for installation and more materials management and handling onsite, though they still deliver installed economic and performance profiles that are superior to traditional hard-pipe armovers. Factory pre-testing is not possible when the components are assembled onsite, so it's necessary for the contractor to verify joint integrity.

The final piece of a flexible sprinkler connection assembly is the mounting bracket that attaches to the suspended ceiling grid. All mounting brackets share the advantage of accurate center-of-tile placement, which eliminates the need for the installer to calculate where the drop should go, as in hard-pipe installations. However, some mounting brackets are single piece units, and others comprise sub-components that require onsite assembly. As with other field-assembled components, this approach to the mounting bracket requires more installation labor and can be more susceptible to potential building shifts and seismic activity.

The Economics of Flexibility

The cost of materials often leaps out in an evaluation of a flexible sprinkler system, and for obvious reasons. All else being equal, stainless steel technology can't compete with black pipe if the price of material is the only criterion. But in the case of sprinkler systems, all else is not equal, and a quick look at the overall picture shows that, just like performance, the economics of flexible systems are much better than traditional hard pipe.

First, the installed cost of flexible systems is at least cost-competitive with hard pipe, but the material/labor ratio is different (Figure 2). Using a conservative benchmark, a typical installer can complete at least four flexible installations in the time it normally takes to do one hard-pipe armover. Field assessment reveals that the actual ratio is even better than this. In the case of "difficult" installations, it can be easier as well. Why is this?

Factory-assembled flexible installations require only three steps:

• Connect to the feed line;
• Install the mounting bracket; and
• Install and adjust the height of the sprinkler connector in the mounting bracket.

These three steps do not require high skill, so contractors can concentrate their better-skilled (and usually higher-paid) people on other parts of the job where the skill requirements are higher. The installation consistently and by design hits center-of-tile, including projects where that aesthetic has not necessarily been specified. Conversely, even in standard installations, hard-pipe armovers require multiple, labor-intensive joints and trips up and down ladders to position the drop. They also usually require additional labor-intensive adjustments to hit center of tile. Even if the flexible connectors are field-assembled, the labor savings can be significant enough to offset the additional materials cost.

A non-obvious cost factor is where the sprinkler installation fits in the construction schedule. Almost always, sprinkler installation comes in the final stages, when many trades are converging to finish the job. Not surprisingly, the scheduling and deadline pressures are also greatest at this stage.

Typically, the sprinkler drops are installed prior to the ceiling grid being put in place. Then the installer must return and complete the panel cuts and final sprinkler installation and adjustment, working around the grid and in-place panels. Frequently, the scheduling of other trades is affected by the sprinkler installation, and vice versa, because of the normal sequence of events in final construction.

Flexible connections introduce greater flexibility and speed into the schedule for two reasons: drop installation goes much faster (because there's only one joint per drop, connecting to the feed line), and the installation and final adjustment of sprinkler head height can be completed faster. The reduced time required for these two phases frees time in the overall scheduling. Additionally, after the initial connection to the feed line, the flexible drops can be secured out of the way until the final installation. This means that other trades--ceiling grid installers, for example--don't have to work around hanging hard pipe, so the work environment is safer, and the possibility of workers bumping into pipe and disrupting the joints is eliminated.

Summary

The initial net effect of using flexible fire sprinkler connections is a shortened and more efficient end-stage construction schedule, which can enable faster time-to-occupancy. Over the life of the property, flexible fire sprinkler connections will also make space reconfigurations or code updates easier and faster, which also can affect occupancy time and lower cost. Service work is also considerably easier and faster, and costs less. These are among the reasons why well-known companies have specified flexible connections for all of their properties. Flexibles are also currently being specified in a number of new construction and retrofit projects, including schools, department stores, office buildings, airports, and a growing number of other commercial projects.