C
O
MP
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LOADING PRESSURE
TYPE 112
RESTRICTOR
TYPE MR98H
PILOT
OPTIONAL TRAVEL INDICATOR
MAIN SPRING
DIAPHRAGM AND
PLUG ASSEMBLY
INTERNAL
STRAINER
TYPE LR128 WITH TYPE MR98H PILOT AND TYPE 112 RESTRICTOR
Figure 2. Type LR128 Operational Schematic
Introduction
Scope of the Manual
This Instruction Manual provides installation,
startup, adjustment, maintenance and parts
ordering information for Type LR128 relief valve or
backpressure regulator, 1/2 NPT Type MR98H pilot
and Type 112 restrictor.
Product Description
The Type LR128 pilot-operated, relief valve or
backpressure regulator is used for liquid applications
and include a Type 112 restrictor and a 1/2 NPT
Type MR98H pilot.
Pilot Type Description
Type MR98H – High-pressure relief pilot for 25 to
375 psig / 1.7 to 25.9 bar set pressures. Designed to
handle inlet pressures up to 450 psig / 31.0 bar.
Principle of Operation (Figure 2)
A pressure relief valve is a throttling pressure control
device that limits pressure build-up, it opens to prevent
the rise of internal pressure in excess of a specified
value. Fisher™ relief valves cannot be used as ASME
safety relief valves.
A backpressure regulator is a device that maintains
a desired upstream pressure by varying the flow
in response to changes in upstream pressure. It
functions the same as a relief valve, i.e., it opens on
increasing upstream pressure.
Relief Valve
As long as the inlet pressure is below the set pressure,
the Type MR98H pilot control spring keeps the pilot
valve plug closed. Inlet pressure passes through the
Type 112 restrictor and registers as loading pressure
on top of the Type LR128 diaphragm and plug
assembly. Force from the main spring, in addition
3
Type LR128
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