HSDs are mounted on the overhead of the protected
space and are connected to the manifold of the PRP
valve by individual 1/8-inch transmission lines. A
circle seal check valve is installed in each transmission
If an active fire occurs in the protected space, heat
from the fire will be transferred to the air within the
bellows, causing the air to expand and create a pressure.
This pressure will be transmitted to the rear of the
release diaphragm of the PRP valve, thereby creating
the differential pressure necessary to trip the valve.
If a smoldering fire occurs, the pressure within the
bellows will increase too slowly to trip the PRP valve.
Therefore, the temperature will continue to increase. If
it reaches 1 60°F (±3°F), the fusible link in the end of the
collet will part, removing the restraint holding the
bellows. The bellows will collapse under the tension of
the spring and create a pressure impulse that will be
transmitted to the rear of the release diaphragm of the
PRP valve. This pressure impulse will create the
differential pressure necessary to trip that valve.
Figure 1-5.--Heat-sensing device (HSD).
The lines that connect the thermopneumatic
elements to the PRP are called transmission lines and
are Rockbestos®- or Rockhide®-covered seamless
Heat-Sensing Device (HSD)
Vented Check Valve
spring-loaded bellows designed to create a pressure in
response to either a rapid or slow rise in temperature.
The vented check valve (fig. 1-6) is a brass, spring-
The spring and bellows are held in the
loaded check valve designed to check against a rapid
compressed/expanded positions, respectively, by a
change of air pressure in one direction and to open when
fusible link that connects the bellows to the HSD
air pressure is applied in the other direction. One vented
housing. The fusible link is designed to part when the
check valve is installed in eachtransmissionline (above
the PRP, with a maximum of 12 per PRP) from an HSD
link temperature reaches 160°F (23°F).
Figure 1-6.--Vented check valve.