Principle Of Operation; Installation - Emerson EZH Instruction Manual

SPRING
CASE VENT
PORT B
EXHAUST
M1055
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LOADING PRESSURE

Principle of Operation

A pressure relief valve is a throttling pressure control
device that opens and closes to ensure the inlet pressure
does not rise above a predetermined pressure. Fisher™
relief valves cannot be used as ASME safety relief valves.
A backpressure regulator is a device that controls and
responds to changes in the upstream pressure. It functions
the same as a relief valve in that it opens on increasing
upstream pressure.
As long as the inlet pressure is below the set pressure, the
pilot control spring keeps the pilot valve plug closed. Inlet
pressure passes through the restrictor and registers as
loading pressure on the main valve diaphragm chamber.
Force from the main spring, in addition to pilot loading
pressure, provide loading pressure to keep the main valve
diaphragm and plug assembly tightly shut off. 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 main valve diaphragm chamber.
The pilot continuously exhausts gas when the inlet pressure
is above the set pressure. The inlet pressure unbalance
overcomes the main spring force and opens the diaphragm
and plug assembly.
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
L
RESTRICTOR
S
A
Figure 3. Type EZH with Type PRX/182 Pilot
with pilot loading pressure, pushes the diaphragm and plug
assembly onto the knife-edged seat, producing tight shutoff.

installation

!
Personal injury, equipment damage or leakage
due to escaping gas or bursting of pressure-
containing parts may result if the Type EZH
is installed where its capabilities can be
exceeded or where conditions exceed any
ratings of the adjacent piping or connections.
To avoid this, install a Type EZH relief valve or
backpressure regulator where:
• Service conditions are within unit
capabilities (including those in the
Specifications 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
gas. To avoid such injury or damage, install
the unit in a safe location.
Type EZH Relief
Warning
3