Dry Pipe Systems

Dry SystemIn US building codes dry pipe systems can only be used (by regulation) in spaces in which the ambient temperature may be cold enough to freeze the water in a wet pipe system, rendering the system inoperable. Dry pipe systems are most often used in unheated buildings, in outside canopies attached to heated buildings (in which a wet pipe system would be provided), or in refrigerated coolers. Dry pipe systems are the second most common sprinkler system type.

Water is not present in the piping until the system operates. The piping is pressurized with air, at a "maintenance" pressure which is relatively low compared with the water supply pressure. To prevent the larger water supply pressure from forcing water into the piping, the design of the dry pipe valve (a specialized type of check valve) intentionally includes a larger valve clapper area exposed to the maintenance air pressure, as compared to the water pressure.

Operation - When one or more of the automatic sprinklers is exposed to sufficient heat, it opens, allowing the maintenance air to vent from that sprinkler. Each sprinkler operates individually. As the air pressure in the piping drops, the pressure differential across the dry pipe valve changes, allowing water to enter the piping system. Water flow from sprinklers needed to control the fire is delayed until the air is vented from the sprinklers. For this reason, dry pipe systems are usually not as effective as wet pipe systems in fire control during the initial stages of the fire.

Some view dry pipe sprinklers as advantageous for protection of collections and other water sensitive areas. This perceived benefit is due to a fear that a physically damaged wet pipe system will leak, while dry pipe systems will not. However, dry pipe systems will only provide a slight delay prior to water discharge while the air in the piping is released prior to the water filling the pipe.

Disadvantages of using dry pipe fire sprinkler systems include:

  • Increased complexity - Dry pipe systems require additional control equipment and air pressure supply components which increases system complexity. This puts a premium on proper maintenance, as this increase in system complexity results in an inherently less reliable overall system (i.e., more single failure points) as compared to a wet pipe system.
  • Higher installation and maintenance costs - The added complexity impacts the overall dry-pipe installation cost, and increases maintenance expenditure primarily due to added service labor costs.
  • Lower design flexibility - Regulatory requirements limit the maximum permitted size (i.e., 750 gallons) of individual dry-pipe systems, unless additional components and design efforts are provided to limit the time from sprinkler activation to water discharge to under one minute. These limitations may increase the number of individual sprinkler systems (i.e., served from a single riser) that must be provided in the building, and impact the ability of an owner to make system additions.
  • Increased fire response time - A maximum of 60 seconds is allowed by regulatory requirements from the time a sprinkler opens until water is discharged onto the fire. This delay in fire suppression results in a larger fire prior to control, producing increased content damage.
  • Increased corrosion potential - Following operation or testing, dry-pipe sprinkler system piping is drained, but residual water collects in piping low spots, and moisture is also retained in the atmosphere within the piping. This moisture, coupled with the oxygen available in the compressed air in the piping, increases pipe internal wall corrosion rates, possibly eventually leading to leaks. The internal pipe wall corrosion rate in wet pipe systems is much lower, in which the piping is constantly full of water, reducing the amount of oxygen available for the corrosion process.

 

 

Source: http://en.wikipedia.org/wiki/Fire_sprinkler_system