A sprinkler system is system designed to automatically distribute fire preventing chemicals to an area for premises protection. In simpler terms, a sprinkler system pumps a lot of water (usually) over a burning or hot area to protect property. While the primary purpose of a fire alarm is life safety and then premises protection, for a sprinkler system those priorities are reversed. They get fire extinguishing chemicals on a fire far quicker than the fire department, and can do a great deal to limit damage to property in event of a fire. In the United States, sprinkler systems are regulated under section thirteen of national fire code put out by the National Fire Protection Association. Sprinkler systems must be built according to any local codes but mostly out of NFPA 13. No building may be occupied before the system is inspected. They suppress and/or put out a fire without a finger being lifted by anyone inside. All new systems are monitored, which means a local fire alarm panel supervises the system and can summon the fire department or a technician as needed.
But what is a sprinkler system? In most applications it is nothing more than a a piping system designed to carry water all over a building, and when the area gets too hot to spray the water over the affected areas. Let's begin with a very simple sprinkler system designed to handle a small one storey building.
Water will be brought into the piping system from city water, a cistern, storage tank, or some other type of reserve through a pipe calculated as adequate to handle the needs of any likely fire. For even modest buildings main pipes of four or six inches are not uncommon (10-15 cm). If the system is supplied by city water pressure there will be a shutoff valve, a backflow prevention device. then another valve to isolate the backflow. This may be followed by a check valve. The check valve comes before the pipes that raise the pipes above ceiling level. This pipe (or pipes) is called a riser, Any building of size will have more than one. If there is more than one, then a sectional valve will allow the shutoff of each riser or major branch. Pipes will then branch out from there and terminate in a sprinkler head, which is where the water actually sprays out. The heads are rated by flow capacity and temperature. Sprinker heads work because they are plugged buy a glass ampule known to break at a predetermined temperature. There will be a main drain pipe provided to drain the system, and an inspector's test valve provided at an appropriate location so the building inspector can determine if adequate water is being provided throughout the system. If high pressure is needed (such as in a multi-storey building) or the supply is low pressure a fire pump will be provided to move the water.
So what does all this mean? And why? Let's begin by breaking down the parts a bit more.
The systems operate because whenever a sprinkler head gets too hot a small glass plug filled with liquid cracks releases whatever is inside the pipes. The plugs are colored. Each color represents a different boiling temperature (and expansion) of the liquid contained within the plug. When the liquid gets hot enough to boil, it cracks the glass releasing water. A plug's glass is necessarily very thin, thus very susceptible to damage. But once a head 'pops' water flows which reduces pressure in the system. When water pressure in the system falls below local water pressure, the check valve opens and water flows in. If there is a fire pump, the pump activates when the pressure gets too low.
Water enters, and because sprinkler systems must be maintained, there must be one or more main valves to shut the system off. There are two families of sprinkler valves. OSNY valves operate by dropping a flat plate to block the valve. They have a tall A-frame like structure and a threaded rod rising out of them to a large wheel with actuates the valve. Butterfly valves rotate until they block the pipe, sort of like the chokes on a carburetor. OSNY valves are more common on large pipes, because they are mechanically very efficient. Butterfly valves are more common on pipes below three inches in diameter. Their operation is enclosed in a (typically) orange box operated by a small wheel like for a hose.
Water in sprinkler pipes doesn't move unless someone opens a valve or a sprinkler head pops, which often means a fire. So the water is often rusty. Sprinkler pipes are often threaded on the job site, so it's quite common for them to be oily inside. When you drain a sprinkler line the water is usually black or discolored. Because cities don't want that water entering their drinking water system most require a backflow device on all connected systems. A backflow is nothing more than a one-way valve, that allows water to flow in only one direction. They require maintenance, and must be tested at least annually. Which is why you put a valve on either side of the backflow, so it can be isolated if replacement is needed.
The check valve is another type of one-way valve. The reason for a check valve is that sprinkler systems are pressurized at night because daytime demand lowers the water pressure of utility water systems. Pressure that is 100 PSI at night might drop to 40 by day, so you keep a check valve in place so that the water pressure in the branches and risers are held constant. Most operate on the differential surface area, where the low-pressure side is wider than the high pressure side so as to equalize the force held on the valve. The most common type is a clapper valve where a flap held by water pressure can open in only one direction. A backflow is really a doubled-check valve in one body.
At this point depending on the system we may have a fire pump. Fire pumps are common where utility pressure is low, or the building is tall. Fire pumps are quite powerful pumps that are fused fire much higher than their melting current. Fire pumps are allowed to burn up and die in service of their building. The pump operates whenever water pressure falls too low, which might be caused by a sprinkler head flowing water. Of course all systems face small pressure losses. Sprinkler pumps are large enough to push a lot of water, that"s their job. So a smaller jockey pump is provided to maintain pressure in event of small losses. Both are electrically controlled and must be monitored. Fire pumps may be powered by electrical, gas or diesel engines.
A main drain is provided so the system can be drained for servicing. Occasionally that drain will serve as an inspector's test.
At this point the water is pumped or sent above ceiling through a network of pipes of ever decreasing size. Pipe is made of galvanized iron, and is either threaded together or grooved and sealed with compression connectors and seal rings. The object of the piping system is to get water to all sprinkler heads distributed throughout the building. To that end a system may have many risers and sectionals, or places where they system is divided into branches. The term "sectional" is only used if the pipes fork and the different branches are controllable by valve. Sprinkler heads are replaceable and open at a predetermined temperature and flow a known quantity of water. Once a head has opened it must be replaced. Heads may not be painted or covered with any substance that might interfere with their operation. They are usually distributed in a grid pattern throughout the building (or floor) with a minimum spacing of six feet (two meters) and a typical maximum of fourteen feet (4.75 meters). The minimum spacing is to prevent a problem called cold solder effect. Water sprays out from a head, and the next head is closer than six feet it might get hit by the water stream. That would cool it and perhaps keep it from flowing when needed. For example, if we had a fire in a corridor where the heads were too close the fire might pop every other head, and leave gaps in the coverage.
At the end of the line is an inspector's test, which the inspector will use to prove that the system will deliver what's promised. That really is a valve with a drain that the inspector can open to see if the system will flow the goods. It's located at the extreme end of the piping system and sized to simulate one open sprinkler head. Inspectors figure that if opening that valve will set off the monitoring system, the monitoring will work anywhere.
Monitoring
Sprinkler systems must sound some kind of alarm. The earliest (and some current) systems used the water movement caused by water flowing through the system to operate a bell. Those systems persist, but most alarms are sounded electrically. At the very least the main line must contain a flow switch to actuate alarms. Most flow switches have a flapper inside the pipe that moves in the event of water flowing. Most are time delayed from 40 to 120 seconds so that normal fluctuations in water pressure will not cause an alarm. It is traditional to have one flow switch per riser, and often after every sectional valve.
Flow switches are wired normally open, which means that water flowing closes the switch and activates the alarm. Simple systems flow current through a the switch to power a bell. Monitored systems may have a bell, but are also connected to a local fire alarm panel or a sprinkler dialer. In each case when any flow switch operates any notification devices (horns, strobes and speakers) are activated and the monitoring agency called so that the fire department may be dispatched. All monitoring systems are required to use two completely discrete phone lines, like a fire alarm panel, so that if one fails the alarm may be sent out the other.
In addition valves must be monitored and/or chained open. The first thing an arsonist does is try to disable the sprinkler system. The valves that allow maintenance can also prevent the system from operating. In the old days valves were required to be chained open. With a sprinkler monitoring system a tamper switch can be attached to the valve so that the fire alarm or sprinkler monitor will sound a supervisory whenever any of such valves are less than totally open. Tamper switches are external on OSNY valves, and built into butterfly and PIV valves. They can be wired either normally open or normally closed depending on the application.
Dry systems
Water is a wonderful fire extinguisher and in most places is both cheap and plentiful. But it has one notable drawback: it freezes. When water freezes it expands, and that shatters pipes. Sprinklers often protect places that aren't heated, or are run through unheated spaces like attics. If the temperature is not kept above freezing pipes will break and the first notice a building owner may have is when things warm up a bit and water drips through the ceilings. Or in some cases floods the entire place.
Dry systems are designed to protect such places. Above the check valve the system contains no water but air. A compressor is installed in order to maintain air pressure at reasonable levels. Pressure switches are used to trigger the alarm, but one set at a higher pressure is also used to trigger the compressor. The idea is that when a sprinkler head melts and opens air pressure will rush out too quickly for the compressor to keep up. When pressure drops too low, a special check valve opens flowing water into the system. These systems must be tested annually, and that includes flowing water. Which means they must be filled and drained annually. Still, they are the best way available to maintain sprinkler protection when cold weather is expected.
Pre-action systems
When a sprinkler system protects an area loaded with expensive electronics other types of protection are chosen. One alternative is to go without water, instead using halon gas. But in either case you don't want a discharge unless there is really, truly a fire.
A pre-action system will not flow water unless a sprinkler head has opened and either a pull station or smoke detector has tripped. Pre-action systems have their own special controllers that electrically open the check valve in event of smoke alarm or pull station activating. The smoke alone will not flow water; a smoke head must still open. But if you really don't want your data center flooded a pre-action system and/or a halon system is the way to go.
Deluge systems
Let's say you make fireworks. A fire at Joe's fireworks doesn't just send everyone outside for a break. Fire among flammables must be put down immediately or the building turns into a bomb. Deluge systems are designed so that if any one head goes, the entire place floods, right now! Every head goes off, and drenches everything. Deluge fire pumps are oversized and deluge smoke heads are designed to flow water like there's no tomorrow.
Sprinkler systems can do a fair amount of damage when they break. But in the event of a fire nothing is more effective at minimizing damage. Big fires can be slowed enough to allow people to escape. They get firefighting started even before the fire truck leaves the station. A sprinkler system is often a property owner's best defense against fire.
some valves, like an inspector's test, are not required to be locked open or monitored. The general rule is that closed valves may be left unlocked as the flow switch will give alarm.