A frangible pellet sprinkler (fig. 8-16) has a rod
between the orifice cap and sprinkler frame. The
rod is held in place by a pellet of solder under
compression. When the solder melts, the rod moves
out of the way of the orifice cap. The cap is pushed
off by the water pressure in the piping network.
The bimetallic element sprinkler (fig. 8-17)
uses a disk made of two distinct metals as a heat-
sensitive element. When the sprinkler is off, the
disk maintains pressure on a piston assembly.
When a fire occurs and the temperature reaches
the sprinklers rating, the disk flexes and opens,
releasing pressure on the piston assembly and
allowing a small amount of water to bleed out of
the piston chamber faster than it can be replaced
through a restrictor. The water pressure in the
piping network pushes the piston down and allows
water to discharge from the sprinkler. When the
temperature of the heat-sensitive element is
reduced, the element returns to its normal position
and allows water to pass through the restrictor,
filling up the piston chamber, forcing the piston
into the closed position, and stopping water
discharge. This sprinkler can be used to
automatically cycle on and off as necessary; for
example, to put out a rekindled fire.
Other sprinkler heads that do not have release
mechanisms include the dry pendent sprinkler, the
open sprinkler, and water spray nozzles.
Figure 8-17.Bimetallic element automatic sprinkler.
Figure 8-16.Frangible pellet automatic sprinkler.