Honeywell L91B, L91D Typical Operation

L91A,B,D PROPORTIONING PRESSURETROL® CONTROLLERS

760-2152—08

point). The L91B settings provide system modulation between

70 and 75 psi or between 75 and 80 psi, depending on the

exact setting of the L91B main scale setpoint.

Due to device tolerances, the scaleplate settings are

approximate and, therefore, the settings should be fine-tuned

with the system operating.

Proportioning Range (L91B,D Only)

Adjust the proportioning range (throttling range) by turning the

proportioning range adjusting screw (Fig. 6) on the top of the

case until the proportioning range setting indicator is at the

desired value. The proportioning range scale is graduated from

A to F with a MIN (minimum) value below A. The value of each

division depends on the operating range of the controller. Refer

to Table 2 in the Specifications section.

Typical Operation

Pressure variations cause the bellows to expand or contract.

Linkage between the bellows and the potentiometer wiper

causes the wiper to move across the potentiometer windings.

This varies the resistance between R and B, and between R

and W, causing an unbalance in the circuit connected to the

controller. See Fig 7.

Fig. 5. L91A,B,D operating points.

Fig. 6. Setting L91 Proportioning Pressuretrol® Controller.

A proportioning controller is used most often to regulate the

firing rate of a burner by controlling a modulating motor (Fig. 4)

or a modulating valve. The controller potentiometer, the

feedback potentiometer in the motor (or in the valve actuator),

and a balancing relay in the motor (or actuator) form an electric

bridge circuit. As long as the pressure of the controlled medium

remains at the setpoint of the controller, the circuit is balanced;

that is, equal the relay contacts are open. When the circuit is

balanced, the motor (or actuator) does not run.

If the pressure of the medium rises, the wiper in the controller

moves toward W. This unbalances the circuit, so a larger

current flows through one side of the balancing relay. The

close contacts in the relay make, causing the motor (or valve

actuator) to drive toward its closed position. As the motor (or

actuator) runs, the wiper on the feedback potentiometer moves

in a direction to balance the circuit. When the circuit is again in

balance, the balancing relay contacts open and the motor (or

actuator) stops. The valves and dampers connected to the

motor or actuator will be partially closed, decreasing the firing

rate and reducing the pressure.

Similarly, if the pressure of the controlled medium falls, the

wiper on the controller potentiometer moves toward B, and the

open contacts in the balancing relay make. The motor (or

actuator) drives toward its open position until circuit balance is

achieved. The valves and dampers will be opened wider and

the firing rate will increase, thus increasing the pressure.

A small change in the pressure of the controlled medium will

cause a change in the firing rate to compensate for it, thus

keeping the pressure constant. This process is called

modulation.

M8519A

PROPORTIONING

RANGE

(WIPER)

POTENTIOMTER

LOW-FIRE

(CLOSED)

HIGH-FIRE

(OPEN)

MAIN SCALE

SETPOINT

PRESSURE

RISE

FIXED ON L91A; ADJUSTABLE ON L91B,D.

L91D HAS TWO POTENTIOMETERS OPERATING IN UNISION.

M8525

PROPORTIONING RANGE

ADJUSTING SCREW (L91B,D ONLY)

CONTROLLER

POTENTIOMETER

MAIN SCALE

ADJUSTING

SCREW

WIPER ARM(S)

PROPORTIONING

RANGE

SCALEPLATE

(L19B,D ONLY)

PROPORTIONING

RANGE SETTING

INDICATOR

(L91B,D ONLY)

TERMINAL BLOCK

BELLOWS

HOUSING

MAIN

SCALEPLATE

MAIN SCALE

SETTING

INDICATOR

1 KNURLED ADJUSTMENT KNOB ON 10 TO 300 PSI

[0.07 TO 2.07 MPa] MODELS.