Emerson 249B Datasheet download pdf (Page 12)

2500-249 Controllers and Transmitters

D200037X012

Product Bulletin

34.2:2500

July 2012

Anti‐Reset Windup

2502F and 2502FR controllers additionally have

anti‐reset windup to minimize the delay in returning

the controlled variable to the set point. This capability

is provided by a reversible differential relief valve with

adjustable spring. As shown in figure 6, proportional

pressure registers rapidly on the spring side of the

relief valve diaphragm as well as in the proportional

bellows. Reset pressure registers slowly on the

opposite side of the diaphragm. As long as the output

pressure changes are slow enough for normal

proportional and reset action, the relief valve spring

prevents opening of the relief valve diaphragm.

A large or rapid decrease in controller output pressure

decreases the pressure in the proportional system, and

on the spring side of the relief diaphragm. If the

decrease on the spring side of the diaphragm is greater

than the relief valve spring setting, the diaphragm

moves off the relief valve orifice and permits reset

pressure on the opposite side of the relief valve

diaphragm to bleed rapidly into the proportional

system. The differential relief valve can also be

reversed to relieve with an increasing output pressure.

On‐Off Controller With Proportional

Valve

This construction has the same flapper, relay, and

proportional valve responses to a level or density

change as does a proportional 2500 controller.

However, the Bourdon tube is constructed (figure 1) so

that output pressure change feedback moves the

nozzle in the opposite direction from the way the

flapper is moving. This reinforcement completely

opens the relay valve either to full supply pressure or

to full exhaust of output pressure, allowing no

in‐between throttling.

On‐Off Controller Without

Proportional Valve

As long as vessel level or density remains above the

lower snapping point on a direct‐acting controller (or

below the upper snapping point on a reverse‐acting

controller), the flapper remains far enough away to

keep the exhaust port of the Bourdon tube valve

closed and prevent any pressure escape from the

Bourdon tube. The relay valve remains closed at the

exhaust end and open at the supply end, allowing full

output pressure into the control valve actuator.

When level or density sufficiently decreases with direct

action or increases with reverse action, the flapper

pushes the Bourdon tube valve in enough to seal the

inner Bourdon tube channel (figure 7). This opens the

exhaust port of the valve and permits exhaust of

pressure from the actuator, initiating the appropriate

control action. This control action continues until the

level or density change again moves the flapper away

enough to permit closing of the Bourdon tube valve

exhaust port and the full application of output

pressure to the actuator.

Options

n Stainless Steel Heat Insulator Assembly—Refer to

figure 8. Available for mounting between the torque

tube arm of any 249 sensor and the instrument.

Recommended for applications where combination

of process and environmental temperatures would

result in controller temperatures in excess of safe

limits (figure 4).

n Jerguson™ Gages—Permit direct observation of

process level and other relevant characteristics.

These gages are described in the Jerguson Gages

supplement. The 249 sensor cage comes standard

with suitable bosses that can be tapped for gage

installation. All other sensors require the gages to

be installed at the factory. When specified, the

bosses will be tapped 1/2 NPT on the CL125 249

sensor, and 3/4 NPT on the CL250 249.

Installation

Although it can be shipped alone for separate

installation, a 249 sensor usually is shipped with a

controller or transmitter installed. During shipment,

displacers are detached from cageless sensors and

optional tubular gauge glasses are detached from

caged sensors.

Equalizing piping, stillwells, or other equipment may

be required for installation. Emerson Process

Management does not provide this equipment.

Complete dimensions and case connection

information for all 249 constructions can be found in

Fisher product bulletin 34.2:249 (D200039X012).

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Emerson 249B Datasheet Page 12

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