Predicting water transit times for fire sprinkler systems controlled by conventional differential dry pipe valves has long been the nemesis of the design technician. Section 4-2.3.1 of the 1999 edition of NFPA 13 states that not more than 750 gallons (2,839 L) of system capacity shall be controlled by one dry pipe valve. The "exception" to this limitation allows the system piping volume to exceed 750 gallons (2,839 L) provided that the system design is such that water is discharged from the inspector's test connection in not more than 60 seconds. This exception gives contractors the opportunity to test the limits of their design creativity and nerve.

Table 1 shows test data for the LDX system.

How Big Is Too Big?

How big is "too" big for the dry pipe sprinkler system? This is the real question. Traditionally the contractor has had to build the dry system and open the inspector's test valve in order to get a measure of the system performance. Should the dry system fail to meet the 60-second criteria, the traditional response has been to lower the system operating air pressure to a minimum in order to improve the water transit time.

A new dry pipe system called the LDX system from The Reliable Automatic Sprinkler Company was designed to lower system air pressures. LDX stands for Lo-Pressure Dry system with an external reset. Lower system operating air pressures provide the contractor with the opportunity to install larger dry pipe systems with faster water transit times.

Traditional differential dry pipe valves have a 5 to 1 or 6 to 1 air to water area ratio of the clapper assembly. The LDX system has a 17 to 1 ratio. The high ratio of the LDX valve allows high static water pressures to be held back with very low air pressures. Lower system air pressures also mean that smaller air compressors can be used to fill the system. The air fill time of the system is also reduced, even when a smaller air compressor is utilized. Shorter fill times translate into labor hours saved on test day.

Even with the high ratio LDX system and lower system air pressures, the question remains, how big can the system be? How does one calculate the water transit time? Hydraulic calculations for fire sprinkler systems are relatively simple due to the fact that the water traveling through the sprinkler system piping is in a steady state condition. Although the process can be arduous to tabulate all of the flows and friction losses in the piping network, the governing equations are relatively simple compared with the mechanics of water entering a dry pipe sprinkler system that is charged with air. When both air and water are simultaneously flowing through the same piping network, the system is experiencing a two-phase flow of fluid. The air is the gas phase fluid and the water is the liquid phase fluid. Each phase of fluid flow is subject to its own set of governing equations, and to complicate the mechanics further, the air is being compressed and hence is not in a steady state flow condition. Simply stated, the empirical calculation for the water transit time in a dry pipe sprinkler system is far too complex to be solved economically. As discouraging as this sounds at this point, there does exist a simpler approach to this complex task.

As industry professionals, we are all familiar with the Hazen-Williams formula for calculating the pressure loss of water, traveling under steady state conditions, through standard piping geometry. But perhaps we are not as familiar with the history or the mechanics behind the Hazen-Williams equation. Messrs. Hazen and Williams did not take the empirical approach to their equation, but rather a measured, statistical approach. Hazen and Williams actually built piping networks connected to a water storage vessel and measured the actual pressure drop across the piping for an array of various pipe sizes and flow rates. Thus armed with volumes of data, they returned to the laboratory and plotted the results. Their equation is a fit to the graphed curves of the measured data and not empirically derived.

Reliable has taken an analogous approach to the water transit time issue in large dry pipe systems. The company systematically measured the LDX performance in an effort to tabulate a base of data for which large dry systems meet the 60-second requirement. From this approach, they identified some of the limiting factors in dry system performance.

First, the water supply must be adequate to operate the system. The true measure of the kinetic energy of a water supply is the flow rate, in gallons per minute (gpm). The flow rate directly translates to the velocity at which the water will travel through the piping. As a general guideline or rule of thumb, the minimum required flow rate is twice the systems capacity. The second limiting factor is the overall travel distance from the LDX clapper to the inspector's test discharge orifice. The test data reveals that 450 ft. of travel distance approaches the upper limit of success. Table 1 illustrates many of LDX successes.

Travel distance and water supply flow rate are the two most important factors to consider when designing large dry pipe sprinkler systems. The following is a list of design suggestions that will improve a systems ability to meet the 60-second rule:

  • Do not loop the cross mains. Center- or end-fed systems work best. Looping the cross mains is very hydraulically efficient, but can double the water transit time since the supply needs to fill both legs of the loop simultaneously.

  • Locate the LDX system riser as close to the hazard as possible.
  • Avoid long runs of feed mains.

  • Use large orifice sprinklers (K = 8.0 or greater). A large orifice smooth bore bushing in the inspector's test will discharge the air in the system exponentially faster than standard sprinklers.

  • Keep the length of the inspector's test piping to a minimum.

  • Make the pipe size of the inspector's test line the same size as the branch line tailpiece that supplies the last sprinkler.

  • Use a quick opening, full port ball valve as the inspector's test valve.

There are many advantages and benefits of the LDX dry pipe system in addition to faster water transit times.

  • Smaller, less expensive air compressors fill the system faster, which saves labor.

  • Segmented preassembled trim is fast and easy to install.

  • Allows the economical use of nitrogen in lieu of costly regenerative air compressors for freezer applications.

  • No priming water is required to seal the clapper.

  • The system can be hydrostatically tested with the clapper assembly in the seated position.

  • The external reset feature eliminates the need to remove the cover plate in order to reset the system.

  • For static supply pressures greater than 125 psi, accelerators may be eliminated.