Types 66R and 66RR
Instruction Manual
D100248X012
February 2020
W1935
W1908
Types 66R and 66RR Relief Valves
TYPE 66R RELIEF VALVE
TYPE 66RR RELIEF VALVE
Figure 1.
Typical Constructions
Introduction
Scope of the Manual
This manual describes and provides instructions, installation,
and parts list for Type 66R direct-operated relief valve* and
for Type 66RR pilot-operated relief valve* complete with
Type T208RR pilot. Instructions and parts list for other
Fisher™ equipment used with these relief valves are found
in separate manuals.
Product Description
Types 66R and 66RR throttling relief valves (Figure 1) are
used to help protect a system against overpressure, or to
maintain an inlet or backpressure. The Type 66R direct-
operated construction is used for set pressure ranges of
2 in. w.c. to 5 psig / 5 mbar to 0.34 bar, while the Type 66RR
pilot-operated construction with the Type T208RR pilot is used
for set pressure ranges of 4 in. w.c. to 7 psig / 10 mbar to
0.48 bar.
Principle of Operation
Type 66R Relief Valve
Refer to Figure 2. Inlet pressure registers under the diaphragm
and is opposed by the spring. When the inlet pressure
increases above the spring setting, the valve plug opens in
a throttling manner and relieves the inlet pressure. As inlet
pressure drops back to set pressure, the spring closes the
valve plug.
Type 66RR Relief Valve
Refer to Figure 2. Inlet pressure registers on the bottom
of the pilot diaphragm through the upstream control line
and bleeds through a fixed restriction in the pilot to provide
loading pressure that helps the main valve spring keep the
main valve plug tightly shut off. When inlet pressure exceeds
the setting of the pilot spring, the pilot diaphragm moves
upward, opening the pilot valve disk and relieving some of
the pressure from the top of the main valve diaphragm. At
the same time, the inlet pressure increase registers on the
bottom of the main valve diaphragm.
*Relief valve defined in ANSI standard B95.1-1972. Not all codes or regulations permit these valves to be used as final overpressure protection devices.
