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Type LR128

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 specied

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 ow

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

to inlet pressure bleeding through the Type 112

restrictor, provides a downward loading pressure to

keep the main valve diaphragm and plug assembly

tightly shutoff.

When the inlet pressure rises above the set pressure,

the pressure on the pilot diaphragm overcomes the

pilot control spring and opens the pilot valve plug. The

pilot then exhausts the loading pressure from the top

of the main valve diaphragm and plug assembly. The

inlet pressure unbalance overcomes the main spring

force and opens the diaphragm and plug assembly.

The pilot continuously exhausts process uid when the

inlet pressure is above the set pressure.

As the inlet pressure drops below the set pressure,

the pilot control spring closes the pilot valve plug and

the exhaust to atmosphere stops. Force from the main

spring, along with pilot loading pressure, pushes the

diaphragm and plug assembly onto the tapered edge

seat, producing tight shutoff.

Backpressure Regulator

As long as inlet pressure remains below set pressure,

the Type MR98H pilot control spring keeps the

pilot valve plug closed. Force from the main spring,

in addition to inlet pressure bleeding through the

Type 112 restrictor, provides downward loading

pressure to keep the main valve diaphragm and plug

assembly tightly shutoff.

When inlet pressure rises above the set pressure,

pressure on the pilot diaphragm overcomes the control

spring to stroke the valve plug open. The pilot then

exhausts loading pressure from the top of the main

valve diaphragm. Inlet pressure unbalance overcomes

the main spring force to open the main valve

diaphragm and plug assembly.

As inlet pressure drops below set pressure, the pilot

control spring overcomes the diaphragm force to

stroke the valve plug down to close. Force from the

main spring, along with pilot loading pressure, pushes

the diaphragm and plug assembly onto the tapered

edge seat, producing tight shutoff.

Installation

WARNING

Personal injury, equipment damage or

leakage due to escaping process uid

or bursting of pressure-containing

parts may result if the Type LR128 is

overpressured or is installed where

service conditions could exceed

the limits given in Specications

section or where conditions exceed

any ratings of the adjacent piping or

piping connections.

To avoid such injury or damage,

install a

Type LR128 relief valve or backpressure

regulator where:

• Service conditions are within unit

capabilities (including those in the

Specications section).

• Service conditions are within applicable

codes, regulations or standards.

Additionally, physical damage to the

relief valve or backpressure regulator

could break the pilot off the main valve,

causing personal injury and property

damage due to escaping process uid.

To avoid such injury and damage, install

the regulator in a safe location.

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