Dry-Pipe Fire Sprinkler Systems

NOTE This standard covers the dry-pipe valve, the quick-opening device, the supervisory gas supply and its pressures, water delivery time, pitch and drainage, interior corrosion mitigation, refrigerated-space provisions, and the acceptance tests specific to an automatic dry-pipe sprinkler system. (1.1)

NOTE In a dry-pipe system the piping downstream of the dry-pipe valve holds pressurized air or nitrogen instead of water. When a sprinkler opens, the supervisory gas escapes, the pressure differential that held the valve clapper closed collapses, and water enters the piping and flows to the open sprinkler after the gas ahead of it has been expelled. (1.2)

NOTE This standard is the dry-pipe layer over Wet Pipe Fire Sprinkler SystemsWet-Pipe Fire Sprinkler SystemsResolves to the current adopted revision.sync/wet-pipe-fire-sprinkler-systems. The family standard establishes how a sprinkler system is classified, sized, supplied, valved, supported, tested, spared, and warranted; this standard establishes what changes when the piping is dry: the valve that keeps it dry, the gas that supervises it, the delay before water arrives, the water that stays behind, and the tests that prove all of it. (1.3)

NOTE Freezing exposure can also be met by an antifreeze system within the limits NFPA 13 sets for it, by listed heat-tracing with insulation, or by moving the piping into heated space. A dry system delays water, increases the design area, adds an air supply and a valve that need continual attention, and corrodes from the inside faster than a wet system; the alternatives avoid those costs where their own limits are not exceeded. (1.10)

NOTE The following are outside the scope of this standard: (1.11)

NOTE Successive editions of NFPA 13 have changed the volume thresholds, the water delivery table, the nitrogen provisions, and the corrosion provisions that govern a dry system, so the edition adopted by the Authority Having Jurisdiction settles which rule applies to a given project. (2.3)

NOTE A dry system's routing is not free to change in the field the way a wet system's is. Every change moves the system volume, the delivery time, the trapped sections, and the auxiliary drain count together, so a run relocated around a duct after the plan was reviewed may have created a low point no one will drain. (3.1.3)

NOTE The auxiliary drain register is the document a future operator drains the system from. A drum drip that is not on the register is not drained before the first freeze, and the pipe above it is the pipe that splits. (3.2.2)

NOTE The device and the valve are tested and listed as a pair because the device works by sensing the rate of pressure drop and venting the intermediate chamber of one particular valve body. On a valve it was not listed with, the device can trip too early, too late, or flood itself. (4.5)

NOTE System volume is the quantity that sets every other dry-pipe decision. It fixes how much gas must escape before water reaches a sprinkler, how large the supervisory gas supply must be, whether the delivery time must be calculated and tested, and whether a quick-opening device is needed. (5.1.2)

NOTE NFPA 13 exempts a system of not more than 500 gal, and a system of not more than 750 gal that has a quick-opening device, from the water delivery time requirement, and permits a larger system where its delivery time is demonstrated by calculation and confirmed by test. A project or an insurer may hold the volume below the calculated ceiling to keep the delivery time short or to keep the exemption, and a project protecting one large unheated volume may need the ceiling. (5.1.4)

NOTE The NFPA 13 delivery table assigns a shorter time to a higher hazard because the design area is already enlarged to cover the delay, and a fire in a high-hazard occupancy outgrows even the enlarged area if water is late. A system below the exempt volume has no delivery time requirement at all, and the limit indicated here applies only where the volume exceeds the exemption. (5.2.2)

NOTE A delivery calculation run at a lower supervisory pressure than the system is set to is optimistic, because the valve trips sooner from a lower starting pressure and less gas has to leave the pipe before water arrives. (5.2.4)

NOTE An accelerator senses the rate of pressure drop when a sprinkler opens and vents the intermediate chamber of the dry-pipe valve, so the clapper releases before the system pressure has fallen to the trip point on its own. An exhauster instead vents the system gas to atmosphere through a large port ahead of the water, shortening the time to expel the gas rather than the time to trip. Accelerators are the device manufacturers now list with their valves; exhausters remain in service on existing systems and are procurable for some valve models. (5.3.2)

NOTE A quick-opening device shortens the delivery time but does not remove the delay, and the design area increase NFPA 13 applies to a dry system is not reduced because a device is installed. (5.3.5)

NOTE A differential valve holds its clapper closed with a large air-side area against a small water-side area, so a supervisory pressure of a fraction of the water pressure keeps it seated; a low-differential valve holds with a mechanical latch or a smaller area ratio and is held at a supervisory pressure closer to the water pressure. The differential valve needs less gas and a smaller supply; the low-differential valve trips from a smaller pressure loss and pairs with a nitrogen supply that is already sized to hold the higher pressure. (6.1.2)

NOTE An intermediate chamber that is not drained before reset holds the valve off its seat, and the valve either fails to reset or trips the moment the supply is reopened. The drain and its sight connection are what let the technician confirm the chamber state instead of guessing at it. (6.1.9)

NOTE The water side of the dry-pipe valve is full, and the trim carries priming water and drain water, so the valve is a wet component that has to live in heated space even though everything downstream of it is dry. A valve enclosure that shares the building's heating is only as reliable as that heating; a dedicated heater is only as reliable as its circuit; a listed heated enclosure trades both for a cabinet with its own thermostat and its own supervision. (6.2.3)

NOTE The supervisory gas is either air or nitrogen, and the source selected fixes which. Air is the gas a compressor supplies and the gas that oxidizes the pipe interior; nitrogen displaces the oxygen and is supplied by a generator that separates it from compressed air on site or by cylinders that are exchanged. (7.1.2)

NOTE A dedicated compressor runs only when the dry system needs gas, draws from the heated enclosure, and stops when its own pressure switch is satisfied. A plant air system supplies gas at plant pressure, carries whatever oil and moisture the plant's own conditioning leaves in it, and is unavailable whenever the plant compressor is down for maintenance or the plant is shut down. Where the plant air is dried and filtered to the condition the sprinkler system needs and is kept in service through building shutdowns, it removes a compressor from the valve room; where it is not, the dry system inherits the plant's oil, water, and outages. (7.1.3)

NOTE A membrane generator separates nitrogen continuously with no moving parts beyond its feed compressor and suits a system that leaks slowly and needs a steady trickle; a pressure-swing adsorption generator produces higher purity from a smaller feed and cycles between beds. Cylinders supply no purity risk and no electrical load, and run out, so they suit a small system, a temporary installation, or a site where a purity-supervised cylinder bank is exchanged on a schedule. (7.1.4)

NOTE An air maintenance device restricts the rate at which gas enters the system, so that a leak large enough to trip the valve is not masked by a supply large enough to keep up with it. A supply piped directly to the system without the restriction turns every leak into a pressure the valve never sees. (7.1.7)

NOTE The supervisory pressure is bounded on both sides. Below the trip pressure plus its margin, a supply pressure surge or a fire pump start trips the valve with no fire; above what the margin requires, the surplus gas lengthens the delivery time by the volume it occupies at that pressure, and every psi of surplus is a psi the leak has to bleed before the valve responds to an open sprinkler. (7.2.2)

NOTE A high-pressure signal reports a failed regulator or a stuck air maintenance device, which raises the delivery time without any other symptom. A run-time signal reports a leak by the compressor working to keep up with it, which is the earliest sign a system gives. (7.2.7)

NOTE NFPA 13 requires the automatic supply to restore normal pressure within 30 minutes. A shorter time returns the system to service sooner after a trip test or a trip, at the cost of a larger compressor and, through the air maintenance device, no faster fill on a system that has a leak. (7.3.3)

NOTE The moisture a compressor takes in leaves the receiver as vapor and condenses in the first cold pipe it reaches. Air drawn from a heated room carries less water per cubic foot than air drawn from a humid summer exterior, and it carries none of the winter condensation that an intake in the cold space itself produces. (7.3.6)

NOTE The corrosion credit NFPA 13 and FM Global Data Sheet 2-1 give a nitrogen-supervised system rests on the oxygen concentration being held low enough that the pipe interior does not corrode at a rate that matters, and 98 percent nitrogen is the purity those documents describe. A higher purity costs generator capacity and purge time and buys a smaller further reduction in oxygen. (7.4.3)

NOTE A system filled with nitrogen from a generator still holds the air it was tested with, and the nitrogen reaches the datasheet purity only as that air is vented at the far end while the generator makes up the loss. An automatic vent runs the purge unattended over the days it takes and closes on purity or on a trip; a manual valve does the same work on someone's schedule; the test connection does it only while a technician stands at it. (7.4.5)

NOTE A dry system corrodes from the inside faster than a wet system because its interior is wetted and dried repeatedly, with water left standing at every low point and oxygen supplied continuously by the gas above it. The wet-and-dry line at each puddle is where oxygen pitting concentrates, and the same standing water is where microbiologically influenced corrosion establishes. (8.1)

NOTE Galvanizing protects the interior by sacrificial zinc, which holds until the zinc is consumed and then leaves bare steel; where the zinc is damaged at threads or where water stands, the exposed steel pits faster than it would in an ungalvanized pipe, because the surrounding zinc drives the attack to the bare spot. Black steel supervised with nitrogen has no oxygen to feed general corrosion or pitting and is the combination FM Global Data Sheet 2-1 describes for a new dry system. Black steel supervised with air corrodes at the rate the standing water and the oxygen supply allow, which is the condition that produced the service history the other options answer. An internal coating or stainless steel removes the steel from contact with the water at a material cost that a large system or a corrosive supply may justify. (8.3)

NOTE A black fitting on a galvanized pipe puts bare steel at the threaded joint, which is the location where water stands last and drains least, so the one unprotected component sits at the one place the protection was most needed. (8.6)

NOTE Pipe listed for wet service only is listed on the assumption that its interior stays wet and its exterior stays warm. Light-wall steel corrodes through sooner in a wet-and-dry interior, and a nonmetallic pipe that is not listed for dry service has not been tested for the drying, the cold, or the pressure surge of a trip. (8.8)

NOTE The datasheet defaults are the NFPA 13 minimums for a dry system. A steeper pitch drains faster and leaves less water behind after a trip, at the cost of headroom lost across a long run and more hanger elevations to set. (9.1.3)

NOTE Pipe that reads level on the plan and sags between hangers holds water in each sag, and the sag is invisible once the ceiling is up. Verifying pitch at every section while it is still open is the only time the correction is inexpensive. (9.1.6)

NOTE A drum drip holds the water that collects in its condensate nipple between an upper valve and a lower valve, and is emptied by closing the upper valve, opening the lower, closing the lower, and reopening the upper, so that the system is never open to atmosphere. An automatic drain does the same on a float without an operator, and adds a device with its own listing, its own leak path, and its own maintenance. (9.2.4)

NOTE Auxiliary drains located in unheated space hold water in the condensate nipple and freeze if they are not drained, so the drain that protects the piping is itself the component that most needs the seasonal drain-down NFPA 25 requires. (9.2.9)

NOTE A freezer is the dry system's hardest service. The gas condenses on contact with the pipe wall, the condensate freezes where it lands, ice grows inward from the wall at the point the pipe enters the cold space, and a trip fills the piping with water that has minutes before it becomes ice. (10.2)

NOTE The ice plug forms where the warm supply pipe meets the cold air, and it closes the pipe from the wall inward with no change in supervisory pressure and no signal to any device. The only way to find it is to look. (10.5)

NOTE A standard pendent sprinkler on dry piping hangs below the branch line with its inlet at the low point, so the water left after a trip collects in the sprinkler body and freezes there. An upright sprinkler drains back into the branch line, and a dry pendent carries its seal up at the branch line with an empty barrel below it. (11.2)

NOTE The pressure switch on the alarm outlet sees full supply pressure the moment the clapper opens, so the alarm on a dry system is transmitted before water has reached the sprinkler rather than after. A vane in dry piping is struck by the slug of gas and water that a trip drives through the pipe, which NFPA 13 prohibits because the vane and the alarm both fail under it. (12.3)

NOTE The trim of a dry-pipe valve, the quick-opening device, and the air maintenance device pass gas and water through small orifices that a chip of pipe scale can close, and the first trip after construction is when the debris that entered during construction arrives at those orifices. (13.3)

NOTE NFPA 13 permits an interim air test at 40 psi for 24 hours where freezing weather prevents a hydrostatic test, with the hydrostatic test to follow when the weather permits. Accepting the interim test keeps the construction schedule through the winter; refusing it keeps the sequence simple at the cost of holding the system until it can be filled. (14.1.2)

NOTE A dry system leaks air through joints that held water. Water does not pass a thread imperfection that air passes freely, and the hydrostatic test proves the joint against the wrong fluid. The 24-hour air test is the one that finds the joints the compressor would otherwise have spent its life keeping up with. (14.2.3)

NOTE A tested delivery time longer than the calculated one usually points to one of a few causes: a supervisory pressure set higher than the calculation assumed, an orifice in the trim or the accelerator partly closed by debris, trapped sections holding gas the calculation assumed was pipe, or an air supply that could not be isolated during the test. (14.3.5)