|The Pros & Cons of Different Types
A wet pipe system is one in which water is constantly maintained within
the sprinkler piping. When a sprinkler activates this water is immediately
discharged onto the fire. Wet systems are frequently installed in office
buildings, hospitals and schools where the potential for freezing does
not exist. In many cases, they are the choice for museums, libraries and
historic building protection.
System simplicity and reliability. These systems have the least number
of components and therefore, the lowest number of items to malfunction.
This produces reliability, which is important, since most sprinklers may
not be needed for many years. The reliability factor is also important
in facilities where system maintenance is lax or not performed with the
Relative low installation and maintenance expense. They require the least
amount of installation time and capital due to their overall simplicity.
Less service time is required, compared to other system types, which helps
maintenance save money.
Ease of modification. Wet pipe systems are advantageous since modifications
involve shutting down the water supply, draining pipes, and making alterations.
Following the work the system is pressure tested and restored. Additional
work for detection and special control equipment is avoided, which again
saves time and expense (NFSA, 1999).
Short term down time after a fire. They require the least amount of effort
to restore. Generally, replacing the fused sprinklers and turning the
water supply back on reinstates sprinkler protection.
· They are not suited for subfreezing environments (NFSA, 1999).
· Piping in warehouses may be subject to sever impact damage.
Dry Pipe Systems
The system operation is similar to a wet-pipe system, except that the
system piping is charged with compressed air of nitrogen instead of water.
This air holds a remote valve, known as a dry pipe valve, in a closed
position. The dry pipe valve is located in a heated area and prevents
water from entering the pipe until a fire causes one or more sprinklers
to operate. Once this happens, the air escapes and the dry pipe valve
releases. Water then enters the pipe, flowing through open sprinklers
onto the fire (NFSA, 1999). Dry-pipe systems often are installed in loading
docks and unheated storage areas.
Ability to provide automatic protection in spaces where freezing is possible.
Should impact damage happen, there will only be a mild discharge delay,
1 minute, while air in the piping is released before water flow.
· Increased complexity. Without proper maintenance this equipment
may be less reliable than a comparable wet pipe system.
· Higher installation and maintenance costs. The complexity of
the system impacts the overall dry pipe installation. It also increases
maintenance expenditure, primarily due to added service labor costs.
· Lower design flexibility. There are strict requirements regarding
the maximum permitted size (typically 750 gallons) of individual dry pipe
systems, which could affect the ability of an owner to make system additions.
· Increased fire response time. Up to 60 seconds may pass from
the time a sprinkler opens until water is discharged onto the fire. This
will delay fire-extinguishing actions, which may increase content damage.
· Increased corrosion potential. After a fire, the systems must
be completely drained and dried or the water may cause the pipe to corrode
or may even result in premature failure.
The pre-action system requires activation of a detection system before
allowing water to flow into the closed sprinkler system piping (EBO, 1999).
This helps prevent unwanted water flow, should a pipe or sprinkler be
broken accidentally. Activation of both the detection system and a sprinkler
are necessary in order for the water to be released and extinguish the
fire. Pre-action systems are used to protect against water discharge or
to speed the action of large dry-pipe systems. Typical applications include
computer rooms, process control rooms, libraries, archival vaults, fine
storage art rooms and museums.
Dual action required for water release. The pre-action valve must operate
and the sprinkler heads must fuse. This provides an additional level of
protection against inadvertent discharge.
Quicker response time than dry pipe systems.
Higher installation and maintenance costs. They are more complex with
several additional components, including a fire detection system. This
adds to the overall cost of the system.
Modification difficulties. Like dry pipe systems, they have specific size
limitations, which may have an affect on future modifications. In addition,
in order to function properly system modifications must incorporate changes
to the fire detection and control system.
Potential decrease in reliability. Pre-action systems are more complex
and there is a greater chance that they might not function properly when
needed especially if they are not well maintained.
Variations of Pre-action:
This style is basically a pre-action system using open sprinklers. Operation
of the fire detection system releases a deluge valve, which in turn produces
immediate water flow through all sprinklers in a given area (NFSA, 1999).
Deluge systems are found in specialized industrial situations including
aircraft hangers and chemical plants, where high velocity suppression
is necessary to prevent fire.
This type utilizes the basic arrangement of a pre-action system, with
the addition of a thermal detector and non-latching alarm panel. In an
on/off system, as the fire is extinguished, a thermal device cools to
allow the control panel to shut off water flow. If the fire should reignite,
the system will turn back on.