by James Golinveaux, P.E.
For automatic dry-pipe fire-sprinkler systems, NFPA 13, Installation of Sprinkler Systems, requires a maximum water-delivery time of 60 seconds for all systems with a capacity larger than 750 gallons (2,840 liters). This time is measured from the moment the inspector's valve is opened at the location farthest from the water supply until water flows from the valve, theoretically providing a worst-case condition.
Until recent advancements were made in computer-calculation methods, however, the numerous variables affecting the water-delivery time of dry-pipe fire-sprinkler systems couldn't be anticipated because it was difficult to show how they caused other factors, such as water supply, volume, and air pressure, to change, as well. Only after the system had been installed could the required water-delivery time actually be verified.
Fortunately, computer software is now available to calculate dry-pipe system performance, and the 2002 edition of NFPA 13 allows designers to submit these calculations as acceptance testing, rather than perform a traditional field-validation test.
The advantage of dry-pipe fire-sprinkler calculation software becomes evident when we look at the complicated interaction of the four basic variables that affect water-delivery time in a dry-pipe fire-sprinkler system: air and water pressure; system capacity and piping configuration; the size of the test orifice; and the dry-pipe valve-trip ratio.
Air and water pressure includes system air pressure; static water pressure; and residual water pressure and flow. System air pressure is the pressure in dry-system piping that keeps the dry valve closed to prevent water from entering the system. This pressure is dictated by the static water pressure and the dry-pipe valve design.
Static water pressure is the pressure at the base of the sprinkler riser when it isn't flowing into the fire-sprinkler system.
Residual water pressure and flow are the pressure of the water at a given flow rate. Each water supply follows a unique curve in which the pressure drops as the flow rate increases. This pressure drop significantly affects the time required to establish a steady discharge of water from the test connection after the dry-valve trips.
The total volume of all piping on the system side of the dry-pipe valve is the system capacity. It affects the volume of air that must be discharged from the system before a steady water discharge is established at the test connection.
Piping configurations include the tree, loop, and grid. The tree, which produces the fastest water-delivery times, consists of dead-end branch lines, cross-mains, and a feed main. Water must displace air only in the feed main, cross-mains, and the branch line with the test connection for water to reach the test connection. The air in the remainder of the system can be compressed into dead-end piping.
The loop configuration is so called because its cross-mains are looped, which increases the amount of air the water must displace to reach the test connection.
Finally, there is the grid, which is no longer allowed in dry-pipe systems because water has to displace the air in all the system's branch lines and cross-mains before it can reach the test connection, resulting in very long water-delivery times.
The third variable, the size of the test orifice, controls the rate at which air is discharged from the system piping. The rate of discharge and the volume of air being discharged control the water-delivery time after the dry-pipe valve has been actuated. The test orifice is equivalent to the smallest orifice size on any sprinkler in the system.
The final variable, the dry-pipe valve-trip ratio, is the water-pressure to air-pressure ratio at which the valve opens. Dry-pipe valves are traditionally designed as differential-pressure valves in which a clapper holds water out of the dry system. The surface area on the air side of the clapper is greater than the area on the water side, allowing a lower air pressure to hold back a higher water pressure. This reduces the volume of air in the dry system that must be discharged so that water can reach the test connection.
Another dry-pipe valve design is the low-pressure latch type, which depends on actuators and a quick-opening device working together to allow the valve to function properly.
System operation and performance
To understand dry-pipe system performance, we must look at the sequence of events that occurs and the impact of valve-trip time, water-transition time, and compression.
When dry-pipe fire-sprinkler systems are "trip tested" for acceptance, certain events occur after the inspector's test valve is opened. The air pressure in the system begins to drop, causing the dry-pipe valve to trip either at its designed air/water ratio or when an optional accelerator trips the valve on loss of air pressure. When the valve trips, water begins to fill the system by compressing trapped air and forcing it from the test valve. Finally, the water reaches the test connection, and a steady water discharge is established.
Let's review these steps in a little more detail to understand the effects of variables.
For the dry-pipe valve to trip, the air pressure must drop to the point that water pressure forces the valve to open. The time this takes to happen is determined by the rate at which air can be discharged from the system and the amount of air that must be released to reach the trip pressure. This is controlled by air pressure, the size of the test orifice, the dry-pipe valve's trip pressure, and an accelerator. A higher air pressure in the system will initially cause the air to discharge faster at the test valve. A larger test orifice allows air to discharge more rapidly from a system. Of course, the test orifice can only be as large as the smallest fire sprinkler orifice in the system.
As for the dry-pipe valve-trip pressure, different models trip at different water-pressure to air-pressure ratios. The typical ratio is 7:1 to 5:1. Assuming a valve's trip ratio is 5.5:1 and the system has a static water supply of 75 pounds per square inch (psi [5,2 bar]), the dry-pipe valve will trip at 13.6 psi (0,9 bar).
The air pressure recommended by the manufacturer typically includes a safety factor, as well as the compressor on-off differential settings. These are added to the trip pressure of 13.6 psi (0,9 bar) to obtain the maximum set air pressure. In this case, the valve manufacturer would recommend a maximum set air pressure of 39 psi (2,7 bar).
Because a large system can take as long as 98 seconds to trip, accelerators that react to the rate of pressure drop in lieu of fixed pressure are commonly used to shorten the time required to trip the valve.
Accelerators, which are very sensitive to pressure changes, significantly reduce the effect of system volume. Different models of accelerators perform differently, and manufacturers' published times range from 4.5 seconds to 25 seconds. The activation time depends on the system's volume and configuration, and the test-orifice size.
Although an accelerator can significantly reduce the trip time of a dry-pipe valve, the effect of higher air pressure in the system when the valve opens may increase water-transition time. This is the time the water needs after the dry-pipe valve trips to displace the air in system piping and begin flowing from the test orifice. Water-transition time is the most complex and difficult phase of dry-system performance to predict since it's affected by several factors, including the size of the inspector's test orifice, the water supply, and the air pressure in the system when the valve trips, and the volume and arrangement of the system piping.
The size of the test orifice determines the rate at which air leaves the system.
While all water supplies are unique, they share one common trait: a corresponding pressure. As water flow increases, the pressure available decreases. For any given flow, the higher the pressure and the greater the flow, the faster the water displaces air and reaches the inspector's test valve. Air pressure resists a weak water supply, resulting in lower flow rates that delay transition time.
Air pressure in the system also resists the water-supply fill rate, which is the rate at which the system fills with water during the transition period, and the distance the water has to travel to the inspector's test valve. By way of illustration, let's compare a 2,226-gallon (8,426-liter) system with 30 branch lines and 30 sprinklers; a 1,128-gallon (4,272-liter) system with 20 branch lines and 20 sprinklers; and a 410-gallon (1,555-liter) system with 10 branch lines and 10 sprinklers.
If the pipe sizes remain the same, the volume of the system is difficult to change without changing another variable, such as trapped air or distance. Keeping the same size pipe and shortening the system to change the volume adds a distance factor. With 20 fewer branch lines, the cross-main and the branch lines are 200 feet (61 meters) shorter with 10 fewer fire sprinklers.
Piping arrangement also plays an important role in water-transition time. Take, for example, a 1,128-gallon (4,272-liter) center-feed system with 20 branch lines and 20 sprinklers per line. If we cap 19 of the branch lines and half of the end line, the resulting 421-gallon (1,594-liter) system will take 8 seconds longer to deliver water than the original 1,128.6-gallon (4,272-liter) system. Why? Because all the air trapped in the system has to be vented from the fire sprinkler before water arrival, and in the smaller system, there are no pockets of air to compress or non-flowing volumes of water to push the air anywhere but out the open fire sprinkler. The single open fire sprinkler can't exhaust air as fast as the 6-inch (150-millimeter) riser can fill the system, causing the air to push back on the water and slow the fill rate.
In a similar system with an additional cross-main and four branch lines beyond the open test line, these additional lines and main bring the system volume from 421 gallons (1,594 liters) to 633 gallons (2,396 liters). With the added pipe, the air trapped in the cross-main with the plugged branch lines has more volume beyond the flowing line so that the water compresses the air as it enters the system.
Although the test sprinkler is the same distance from the source and the pipe size is the same, the water-transition time drops from 36.5 seconds to 18.7 seconds. The water filling the system pushes air in the cross-main into the trapped volume beyond the test sprinkler faster than it could push the air out of the open sprinkler in the single path. This brings the water to the open fire sprinkler in less time, despite the fact that the volume has been increased.
Now let's look at the original system with 20 branch lines. This system contains a large amount of non-flowing volume that allows the water to displace air by compressing it into the trapped branch lines. This original 1,128-gallon (4,272-liter) system has a water-transition time of 28.5 seconds, much faster than the 36.5 seconds of the first system in our example but slower than the 18.7 seconds of the second system.
A dry-pipe valve with a properly installed and maintained accelerator will trip quicker and faster, resulting in a much higher residual air pressure than is experienced if the air pressure is allowed to reach the normal trip ratio.
When a quick-opening device is used on a dry valve, the incoming water is thus subjected to more air pressure in the system at the instant the valve trips, and this additional air pressure will delay the water-transition time.
Obviously, piping arrangements play a significant role in water transition. Volume alone isn't always the essential indicator for transition times. Volume and piping configuration must both be considered.
Compression is the time between the moment water reaches the test outlet until the water pressure can be kept above the minimum required. This is often referred to as "full flow" or "when the outlet stops spurting air."
The best description of full compression is the point at which the volume of water entering the riser equals the volume of water discharging from the fire sprinkler. This means that all of the trapped air has been compressed and is no longer fluctuating. This value, though not known in the field, can be identified by computer validation.
The more critical measure of adequate sprinkler protection is the point at which the discharging sprinkler meets or exceeds its minimum flow or density requirement. A field test or delivery-time calculation should end when the fire sprinkler is functioning at the minimum designed flow. Additional air escaping from the fire sprinkler is not significant as long as the discharge density is not disrupted.
In the software, the dry-pipe model is characterized by a system of straight pipes connected by nodes, which can represent a transition point from one pipe size to another, elbows or bends, tees and laterals for dividing or mixing streams and valves, and exit nozzles, such as an open fire sprinkler.
The water supply can be modeled as either static or variable, such as a pump-driven water supply, and the equations for the flow properties of air and water are based on the unsteady equations for fluid flow.
These equations are used to solve for flow properties in the regions of fluid flow and gas flow in the system at any point, with the appropriate boundary and continuity conditions coupling the equations for water and gas.
The input screens are as simple as tree generators, as in most computer hydraulic programs or node-by-node input for more complicated systems. The output information is similar to hydraulic calculations, but the summary data are based on trip and transition times rather than water flow and friction loss. The software model will allow more than one fire sprinkler to activate.
Dry-system performance, which is affected by water-delivery time, will be greatly improved with the knowledge of actual water-delivery times.
Future performance-based designs must take advantage of actual water delivery by modeling the process rather than simply using the prescriptive volume-time rules found in older editions of NFPA 13.
James Golinveaux, P.E., is senior vice-president of Research and Development for Tyco Fire Products, which represents Central Sprinkler, Gem Sprinkler, and Star Sprinkler.