A vital part of any building, whether commercial or residential, is the fire sprinkler system. Too often the fire sprinkler system is overlooked or simply considered a step to meet code in commercial buildings. These systems save lives and property. A fundamental knowledge of the systems and the devices needed to monitor them is essential. It helps everyone from the engineer to the contractor to specify and install the correct product.
There are two main classifications of fire sprinkler systems, the wet pipe system and the dry pipe system. The wet system is most commonly used when there is no threat of pipes freezing or breaking. Wet systems are further divided into three basic categories: straight pipe riser, wet system with an alarm check valve and wet system with maintained excess pressure.
Straight pipe risers (Figure 1) do not have alarm valves. There is either a check valve or backflow preventer that isolates the sprinkler water from the supply water. The main riser, fed directly from the water supply, branches throughout the premises. These branches
(Figure 2) have sprinkler heads spaced throughout the protected area. The system pressure is the same as the city water pressure.
Because of fluctuations in the water pressure from demands made upon the city water mains, a vane (paddle) type water flow switch must monitor this system. The water flow switch should have a time delay or retard mechanism to prevent false alarms. The retard time is used to overcome water surges. All water control valves should be supervised.
Wet systems with alarm check valves (Figure 3) are basically systems that contain check valves with an alarm port whose main purpose is to ring a mechanical bell called a water motor gong. Because sprinkler system water often becomes dirty, the check valve is intended to prevent water from the sprinkler system from leaking back into the city water supply. Normally the clapper of the valve is closed, blocking the alarm port and preventing leakage into the city water supply. If a sprinkler head is activated because of fire, or if there is a surge in the city pressure, the clapper of the valve opens. The alarm port is then exposed to the incoming city water supply.
This alarm port may be piped to the outside of the building through a water motor gong.
These systems can use either a vane-type flow switch with a retard or a pressure-type flow switch. The pressure-type flow switch would be installed on the alarm port in line with a water motor gong if one is being used. The switch must be installed on top of a retard chamber. The other option is to use a pressure switch with a built-in retard. These applications are used to prevent false alarms from water surges.
Because these switches are installed on the alarm port, there is no pressure or water present under normal circumstances. This allows for installation of the pressure switches without shutting off the water supply, therefore the sprinkler system is never out of service.
Retard chambers
As shown in the diagram above, this system has a retard chamber. The retard chamber is a metal container that fills with water when there is a surge in city pressure. It absorbs the pressure increase, thereby allowing the pressure switch to operate only in an actual alarm condition. The drip cup at the bottom of the chambers allows the water surge to drain out. Retard chambers require maintenance to make sure the drip valve stays clean and does not get clogged from corrosion and rust. A pressure switch with a built-in retard allows the system to remain operational at all times and eliminates the maintenance required on the retard chamber.Ball valve switches (see bvs in Figure 3) installed on the system allow for supervision of shutoff valves to water flow pressure switches when the system is shut down for maintenance. Per NFPA, the pressure flow switches must be installed before any shutoff valve or the valve itself must be electronically supervised.
A wet system with maintained excess pressure is the third type of wet system. Excess pressure is pumped into the system above the check valve. This pressure holds the clapper of the valve down even when the city water pressure fluctuates. False alarms caused by surges in the water supply are eliminated. Two methods can be used to monitor these types of systems. Each uses different water flow devices. Unlike the first two categories of wet systems, maintained excess pressure systems do not require the use of water flow devices with retards.
The first technique incorporates a straight riser with maintained excess pressure (Figure 4). A pressure water flow device is connected to the system above the check valve. Activation of a sprinkler head in the system results in a loss of the excess pressure. The water flow switch detects this drop in pressure and sends an alarm signal.
The excess pressure pump used on the system should be monitored with a supervisory pressure switch. This switch should be set to trip when the pressure drops 10 psi below normal. The trip point will prevent false alarms because of a drop in pressure from a compressor failure.
The other method of supervising a wet system is with an alarm check valve and maintained excess pressure (Figure 5). A pressure-type flow switch is connected to the alarm port of the alarm check valve. A flow from a sprinkler head in the system results in a loss of excess pressure. This allows the alarm check valve to open. When the valve opens, the alarm port is exposed to the incoming water supply. The water flow pressure switch detects this increase in pressure and sends an alarm signal.
As in the method above, the excess pressure pump should be monitored with a supervisory pressure switch set to trip at a 10 psi drop below normal.
Dry pipe systems
Dry pipe systems are usually installed in unheated buildings or anywhere there is danger of sprinkler pipes freezing. Dry pipe systems (Figure 6) have pressurized air in the sprinkler piping. This allows for supervision of the system and prevents water from entering the pipes. The dry pipe valve must be installed in a heated location. The room temperature where the valve is located should be monitored to prevent the valve from freezing.This type of system uses a dry pipe valve, which has an intermediate chamber or alarm port. Under normal conditions, this section of piping has no water or pressure in it. A pressure-type flow switch would be installed on the alarm port, which in turn could be connected to a water motor gong on the outside of the building. Water flow pressure switches on dry systems do not need retards because there are no water surges.
When a sprinkler head opens because of fire, the air pressure is released from the system. When enough air pressure is removed, city water pressure opens the dry pipe valve and enters the system piping. When the clapper of the dry pipe valve opens, it exposes the alarm port to the incoming water supply. The pressure-type flow switch senses this increase in pressure and sends an alarm signal.
Dry pipe systems should also have a supervisory pressure switch installed to monitor the system air pressure. This switch should be adjusted to send a low air signal when the system pressure drops 10 psi below normal. This will prevent accidental tripping of the dry pipe valve because of low air pressure from a compressor failure. These systems should also be monitored for high air pressure as too much pressure will delay the time it takes to bleed enough pressure off the system to allow the water pressure to open the dry pipe valve. A bleeder valve should be installed in line with the air supervisory switch for test purposes.
Other types of supervision are necessary to properly monitor systems. Loss of water supply pressure in the sprinkler supply mains can be monitored with a supervisory pressure switch. When using the pressure switch, care must be taken to adjust the pressure switch below the lowest pressure available during peak demand periods to avoid nuisance alarms.
Fire pumps & water tanks
Fire pumps are used to supply more water pressure to the sprinkler system. These are automatically started in the event of water flow in the system. The power to an electric pump should be supervised for loss of phase or phase reversal. A pump running signal can be obtained by installing a pressure switch in the discharge line of the pump.Pressure tanks are pressurized water reservoirs used to supply a limited amount of water for sprinkler systems. The tanks are usually located above the highest sprinkler heads under the main roof. Another location for the pressure tanks is in a heated room on the roof. Each tank is kept two-thirds full of water with a maintained pressure of approximately 75 psi in the tank. These tanks must also be monitored by using water level, low and high pressure, low temperature, and water control valves.
Gravity tanks are tower- or roof-mounted reservoirs and can immediately supply a large volume of water for sprinkler systems. The tanks may have a heating element to prevent freezing. Low temperature, water level and water control valves are the types of supervision used in this application.
In premises protected by wet sprinkler systems or the room in which a dry pipe valve is located, low temperature (40° F) monitoring is recommended. This prevents sprinkler pipes or dry pipe valves from freezing in the event of a building heating equipment failure.
Special hazards
There are a few specialized applications where the common wet system or dry pipe system is not feasible. In these instances, deluge systems or pre-action systems are used. These are considered special hazards applications.Deluge systems (Figure 7) deliver water immediately to the protected area. They wet down an entire area by dispensing water through sprinklers or spray nozzles that are normally open. Because the operation of a deluge system does not rely on the opening of sprinkler heads, they are suitable for extra-hazard occupancies in which flammable liquids are handled or stored.
The water for a deluge system is held back by an electrically operated valve. The valve is connected to a compatible releasing panel. The panel monitors initiating devices such as smoke or heat detectors and pull stations, and when the panel detects an alarm condition, the release circuit is energized. This allows the valve to open and water to flow into the system piping and out of the normally open sprinklers or spray nozzles.
In order to prevent an accidental discharge of water, deluge systems are sometimes cross-zoned. This requires two initiating zones with one device in each zone to be in alarm before the panel will energize the release circuit. A manual station will override the cross-zoning feature and cause the release circuit to be energized immediately.
Pre-action systems are usually broken into two categories-single interlock and double interlock. They are basically dry pipe systems, except they use an electrically operated valve to hold back the water supply instead of a dry pipe valve. They are designed to overcome the operational delay of conventional dry pipe systems and to eliminate the possibility of inadvertent water damage caused by accidental damage to the sprinkler heads or system piping.
The water supply valve is operated independently of the opening of sprinklers. The valve is opened by the operation of a releasing panel that is monitoring initiating devices and not by the fusing of sprinkler heads.
Single interlock systems (Figure 8) use closed sprinklers attached to a piping system containing a small amount of supervisory air pressure, usually less than 10 psi. The water is held back by an electrically operated valve, which is connected to a compatible releasing panel. The panel monitors any smoke or heat detectors and manual stations that are located in the same area as the sprinkler heads. These devices are connected to the releasing panel's alarm initiating zones. A low air pressure supervisory switch is used to monitor the supervisory pressure in the piping system and is connected to a supervisory zone on the releasing panel.
If the system pressure drops because of an open sprinkler, damaged pipe or air compressor failure, the pressure switch will cause a supervisory condition on the release panel and the valve will not open. If the smoke or heat detectors operate, or a manual station is activated, the release panel will go into an alarm condition, causing the release circuit on the panel to energize. This will allow the valve to open and water to flow into the system piping, even if no sprinklers have opened.
Double interlock systems
Double interlock systems (Figure 9) are designed for applications such as refrigerated areas requiring maximum protection against inadvertent operation. Double interlock systems use closed sprinklers attached to a piping system containing supervisory air pressure.The water is held back by the system pressure by an electrically operated valve. The structure of the double interlock system is the same as with the single interlock, except in one basic area. In order to actuate the system, two independent events must occur. The sprinkler system piping must lose air pressure due to the operation of one or more heads, and the release panel must detect an alarm from one of the initiating devices.
As mentioned earlier, these systems are sometimes considered merely a necessary part of a commercial building. To the general public, fire sprinklers are just for commercial property. While smoke detectors warn of potential hazards, the sprinkler system stops or contains the fire, preventing catastrophic damage to property and aiding to save lives. Education as to the benefits of commercial protection and residential fire sprinkler systems is a key to preventing losses.
