Emerson Fisher 98LD - Principle of Operation

Types 98LD and 98HD
3
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
Product Description
The Types 98LD and 98HD (see Figure 1) are differential
pressure relief valves that are suitable for service on
water, steam, air, gas, oil, and other uids. They are
used to maintain differential pressure control on testing
xtures, wash tanks, sterilizers, steam tables, fuel lines,
and plant air supplies.
Type 98LD bodies are available in sizes NPS 1/4 to 1
(DN 25). Type 98HD bodies are available in sizes NPS
1/4 to 2 (DN 50). There are four relief pressure ranges
for each relief valve. For Type 98LD, the ranges are
between 2 and 38 psig (0,14 and 2,6 bar). For
Type 98HD, they are between 15 and 200 psig (1,0
and 13,8 bar).
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The Specications section gives some general
specications for the Types 98LD and 98HD
regulators. The nameplates give detailed information
for a particular regulator as it comes from the factory.
Principle of Operation
See Figures 2 and 3. The Types 98LD and 98HD relief
valves are used to maintain a differential between the
controlled pressure and loading pressure of a system.
The spring setting determines the differential. The
Types 98LD and 98HD can also be pressure loaded to
remotely change the relief setpoint. .
The relief valve responds to both controlled
pressure and loading pressure and opens or closes
as these pressures change. If the loading pressure
increases, pressure on the upper side of the
diaphragm increases. The valve plug moves closer
to the orice and restricts the ow through the relief
valve. When loading pressure decreases, pressure
on the upper side of the diaphragm decreases.
This allows the valve plug to move away from the
orice and allow more ow through the relief valve
(to atmosphere or back into the system). The relief
valve opens and closes in response to changes in
the controlled pressure. In this way, the differential
pressure between the controlled and uncontrolled
pressures is maintained. .
Figure 2. Type 98HD Relief Valve Operational Schematic
LOADING
PRESSURE
VALVE PLUG
ORIFICE
DIAPHRAGM
CL3746-A
A1767-1
CONTROLLED HIGH
PRESSURE
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
LOADING PRESSURE
INLET PRESSURE
OUTLET PRESSURE