Honeywell Boiler L91B User Manual

L91A,B,D Proportioning  
®
Pressuretrol Controllers  
PRODUCT DATA  
FEATURES  
Models available for pressure systems up to 300 psi  
(2068 kPa).  
May be used with steam, air, noncombustible gases, or  
other fluids noncorrosive to brass or phos-bronze (300  
psi model) bellows.  
When used with steam boilers, a steam trap (siphon  
loop) is recommended.  
Wiper on controller potentiometer moves in response  
to pressure changes.  
L91D has two potentiometers for unison control of two  
motors.  
L91B,D models have adjustable proportioning  
(throttling) range to allow selection of desired  
pressure control range.  
Adjustments made with screws on case top.  
Large, easily-read scaleplates in both U.S. customary  
(oz/sq. in., psi, or in. Hg) and metric (kPa, MPa, or mm  
Hg) units.  
Steel case with clear plastic cover to observe pressure  
settings and potentiometer action.  
APPLICATION  
Fitting with 1/4 -18 NPT threads allows rapid  
installation.  
®
L91 Proportioning Pressuretrol Controllers are Series 90  
Modulating Pressure Controllers that provide direct control of  
modulating motors or valves used on automatic burners or  
steam heating systems.  
Surface mount with screws through knockout holes in  
case back.  
Contents  
Application  
Features  
Specifications  
Installation  
Wiring  
Checkout  
Service Information  
1
1
2
5
6
8
9
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
Electrical Connections: Internal screw terminals. Hole inside  
23176CF Potentiometer: For all L91D models and most L91B  
models (see Table 1); 140 ohms (nominal); length of active  
winding is 1/4 in. (6.4 mm).  
of case for 1/2 in. conduit.  
Dimensions: See Fig. 1; see Fig. 2 for mounting steam trap  
(siphon loop).  
Accessories:  
50024585-001 Steam Trap (siphon loop): 1/4 in. brass pipe  
with 1/4 -18 NPT external threads on both ends.  
14026 Steam Trap (siphon loop): 1/4 in. steel pipe with 1/4 -18  
NPT external threads on both ends.  
Weight: 1 lb, 15 oz (0.88 kg).  
Finish: Gray.  
33312B Knurled Adjustment Knob: With setscrew; fits on main  
scale pressure adjusting screw.  
4074BWJ Limit Stop Assembly: To limit setpoint ranges;  
includes 129564 Range Stop 107194 Range Stop Screw,  
and 23466 Wrench.  
Replacement Parts:  
129178 Thermoplastic Cover.  
23176CB Potentiometer: For all L91A models and a few L91B  
models (see Table 1); 140 ohms (nominal); length of active  
winding is 7/64 in. (2.8 mm).  
Table 2. Approximate value of each division (A to B, B to C, etc.) on proportioning  
(throttling) range scaleplate (L91B,D,F only)  
Operating Range  
Value of Each Division  
U.S. Customary  
U.S. Customary Units  
0 to 10 oz/sq in.  
Metric Units  
0 to 7 kPa  
0 to 103 kPa  
Units  
6.2 oz/ sq in.  
1.8 psi  
Metric Units  
2.7 kPa  
0 to 15 psi  
12.4 kPa  
5 to 150 psi (L91B)  
5 to 150 psi (L91D)  
0.03 to 1.03 MPa 7.8 psi  
0.03 to 1.03 MPa 6.7 psi  
0.054 MPa  
0.046 MPa  
0.113 MPa  
0.095 MPa  
10 to 300 psi (L91B with 7/64 in. potentiometer) 0.07 to 2.07 MPa 16.4 psi  
10 to 300 psi (L91B,D with 1/4 in. potentiometer) 0.07 to 2.07 MPa 13.8 psi  
3
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
(114.3) (COVER)  
(110.3) (CASE)  
4-1/2  
L91A,B,D  
4-11/32  
2-3/4 (69.9)  
3-57/64 (98.8)  
3-13/32 (86.5)  
1-3/16 (30.2)  
1-1/16 (27.0)  
1-5/32 (29.4)  
3
4
2
2
3-23/32  
(94.5)  
1-3/32 (27.8)  
3-7/8  
2-5/64  
(52.8)  
(98.4)  
ALL OTHER  
MODELS  
DIM C (SEE  
TABULATION  
TABLE)  
7/8  
1-13/16  
(46.0)  
(22.2)(2)  
13/16  
(20.1)  
10 TO 300 PSI  
0 TO 15 PSI  
0 TO 16 OZ/IN 2  
1/4-18 NPT  
1
1-1/16  
(27.0)  
2-1/8 (53.5)  
DIM A (SEE TABULATION TABLE)  
DIM B (SEE  
TABULATION  
TABLE)  
1
EXTERNAL THREADS ON 0 TO 15 PSI MODELS; INTERNAL THREADS ON ALL OTHER MODELS.  
SOME MODELS ARE ALSO AVAILABLE WITH 1/4-19 BSP-TR INTERNAL THREADS; SEE TABLE 1.  
2
3
PROPORTIONING RANGE ADJUSTING SCREW ON L91B,D MODELS ONLY.  
33312B KNURLED ADJUSTMENT SCREW KNOB, 7/8 IN. [22.2 MM] DIAMETER,. KNOB IS INCLUDED WITH 10 TO 300 PSI [0.07 TO .07 MPa] MODELS;  
OPTIONAL ACCESSORY FOR OTHER MODELS.  
4
FOR 10 TO 300 PSI [0.07 TO 2.07 MPa] MODELS. DIM C INCLUDES THE KNURLED ADJUSTMENT KNOB.  
TABULATION OF DIMENSIONS A, B, AND C  
OPERATING RANGE  
DIM A  
DIM B  
DIM C  
CUSTOMARY UNITS  
METRIC UNITS  
0 TO 103 kPa  
IN.  
MM  
61.9  
41.3  
31.8  
IN.  
1-7/32  
13/16  
5/8  
MM  
31.0  
20.6  
15.9  
IN.  
MM  
2-7/16  
1-5/8  
1-1/4  
6-7/8  
5-3/4  
6-1/16  
174.6  
146.1  
154.0  
0 TO 15 PSI  
5 TO 150 PSI  
10 TO 300 PSI  
0.03 TO 1.03 MPa  
0.07 to 2.07 MPa  
4
4
M29781  
®
Fig. 1. Mounting dimensions of the L91 Proportional Pressuretrol Controllers, in in. (mm).  
60-2152—08  
4
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
INSTALLATION  
When Installing This Product...  
L91  
1. Read these instructions carefully. Failure to follow them  
could damage the product or cause a hazardous  
condition.  
PRESSURETROL  
CONTROLLER  
2. Check the ratings given in the instructions and on the  
product to make sure the product is suitable for your  
application.  
PRESSURE  
GAUGE  
3. Installer must be a trained, experienced service  
technician.  
PRESSURETOL  
4. After installation is complete, check out product opera-  
tion as provided in these instructions.  
TEE  
2
WARNING  
Electrical Shock Hazard.  
Can cause severe injury, death or property damage.  
1. Disconnect power supply before beginning  
installation to prevent electrical shock and  
equipment damage.  
1
STEAM TRAP  
(SIPHON LOOP)  
4-1/2 (114) TO  
5-1/2 (140)  
2. When using the controller with a compressor, install  
a dampening device (such as a needle valve,  
header, or surge tank) to dampen pulsations that can  
damage the controller or reduce its life.  
BOILER  
IMPORTANT  
1. Locate the controller where the ambient temperature  
will not exceed 150° F (66° C).  
2. When the controller is used on a boiler, be sure to  
connect a steam trap (siphon loop) between the  
controller and the boiler.  
3. Before installing controller, be sure the siphon loop  
has enough water in it to fill lower trap.  
4. Use pipe compound sparingly to avoid clogging the  
hole in the pipe or bellows fitting.  
1
2
50024585-001 STEAM TRAP IS A 1/4 IN. BRASS PIPE WITH  
1/4 - 18 NPT EXTERNAL TRHEADS ON BOTH ENDS.  
FITTING ON BELLOWS HAS 1/4-18 NPT EXTERNAL THREADS  
ON 0 TO 16 OZ/SQ. IN. AND 0 TO 15 PSI MODELS; INTERNAL  
THREADS ON ALL OTHER MODELS. SOME MODELS ARE  
ALSO AVAILABLE WITH 1/4-19 BSP-TR INTERNAL THREADS;  
SEE TABLE 1.  
M23884  
5. Do not tighten the controller by hand by holding onto  
the case and turning it.  
Fig. 2. Approximate dimensions in in. (mm) for mounting  
L91 Pressuretrol Controller beside Pressure Gauge on  
®
Steam Trap (Siphon Loop).  
Location and Mounting  
Mounting Beside a Pressure Gauge  
When used with steam boilers, always mount the controller  
above the water line in the boiler. A steam trap (siphon loop)  
must always be connected between the controller and the  
boiler (Fig. 2) to prevent boiler scale and corrosive vapors from  
attacking the bellows. Before installing controller, be sure the  
siphon loop has enough water in it to fill lower trap.  
To mount the controller beside a pressure gauge (Fig. 2),  
remove the gauge. In its place, install a steam trap (siphon  
loop) with a tee on top. Using elbows and pipe nipples, mount  
the controller and pressure gauge on the ends of the tee.  
Mounting on a Boiler  
The controller can be mounted (1) beside the pressure gauge,  
(2) in a fitting on the boiler provided by the manufacturer, or (3)  
at a remote location in case of excessive vibration.  
If it is not convenient to mount the controller beside the  
pressure gauge, install a steam trap (siphon loop) in the fitting  
provided by the boiler manufacturer. If there is no fitting, mount  
the steam trap at a location recommended by the boiler  
manufacturer. Screw the controller directly to the steam trap, if  
possible.  
Make all pipe connections in accordance with approved  
standards. Use only a small amount of pipe compound to seal  
the connection joints. Excess pipe compound may clog the  
small hole in the fitting and prevent the controller from  
operating properly.  
Mounting at a Remote Location  
If there is excessive vibration at the boiler that may adversely  
affect the operation of the controller, mount the controller at a  
remote location. All piping from the boiler must be suitable and  
solidly mounted. The piping must be properly pitched to drain  
all condensation back to the boiler. If this causes the controller  
to be located at an inconvenient height (on large boilers), it  
may be mounted at a lower level if the connecting piping is  
filled with clean water. A steam trap (siphon loop) must be  
mounted between the remote piping and the controller.  
To avoid leaks and damage to the case, use a parallel jaw  
wrench on the controller fitting. Do not tighten the controller by  
hand by holding onto the case and turning it.  
5
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
6. Refer to Fig. 4 for typical hookup. W and B connections  
may be interchanged at the motor for reverse action  
(cooling). Follow the burner or boiler manufacturer’s  
wiring diagram if provided. Also refer to the wiring  
diagrams in the motor instructions.  
ACTION ON  
PRESSURE FALL  
W
B
7. Replace the front cover when wiring is completed.  
R
W
B
1
SERIES 90  
MODULATING MOTOR  
L91 PROPORTIONING  
CONTROLLER  
M9484  
M9494  
CLOSED  
R
W1  
R1  
B1  
(LOW FIRE)  
1
W
OPEN  
(HIGH FIRE)  
B
2
L91A,B  
T2  
T1  
ACTION ON  
PRESSURE FALL  
TRANSFORMER  
W
B
1
WIPER MOVES TOWARD W END OF  
POTENTIOMETER ON PRESSURE RISE,  
AND TOWARD B END ON  
3
R
W
B
PRESSURE FALL.  
(HOT) L1 L2  
2
3
TERMINALS ARE LABELED.  
REAR POTENTIOMETER  
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
M8520A  
R
W
B
1
®
ACTION ON  
PRESSURE FALL  
Fig. 4. Hookup of L91 Proportioning Pressuretrol  
Controller to Series 90 Modulating Motor.  
B
W
Setting  
In all models, the proportioning range (also called throttling  
range) extends above the main scale setpoint (Fig. 5). The  
proportioning range is fixed on L91A models, but is adjustable  
on L91B,D models. (For values, refer to Table 1 in the  
Specifications section.)  
FRONT POTENTIOMETER  
TERMINALS ARE LABELED.  
L91D  
1
M8522A  
Main Scale Set Point (All models)  
Fig. 3. L91 terminal blocks and internal schematics.  
Adjust the main scale setpoint for the desired operating  
pressure by turning the main scale adjusting screw (Fig. 7) or  
knurled adjustment knob on 10 to 300 psi (0.07 to 2.07 MPa)  
models, on the top of the case, until the main scale setting  
indicator is at the minimum pressure desired. The  
proportioning range extends above this value. The scaleplate  
is marked in both customary (oz/sq in., psi, or in. Hg) and  
metric (kPa, MPa, or mm Hg) units.  
WIRING  
1. Disconnect power supply before beginning installation to  
prevent electric shock and equipment damage. All wiring  
must comply with applicable electrical codes,  
ordinances, and regulations. Use NEC Class 1 (line  
voltage) wiring.  
Use of L91 Proportioning Controller with  
Limit Controllers  
The L91 main scale setpoint plus the value of the differential  
(proportioning range) must be less than or equal to the limit  
controller’s (L404) main scale setpoint.  
2. For normal installation, use moisture-resistant No. 14  
wire suitable for at least 167°F (75°C) if you are using  
the controller with a flame safeguard primary control, or  
at least 194°F (90°C) if you are using it with a program-  
ming control.  
3. For high temperature installations, use moisture-  
resistant No. 14 wire, selected for a temperature rating  
above the maximum operating temperature.  
4. Disconnect the power supply before beginning wiring to  
prevent electrical shock and equipment damage.  
5. All models have a terminal block inside the cover (Fig. 3)  
and two 7/8 in. (22.2 mm) holes in one side for 1/2 in.  
conduit, cable, or wires. Remove the front cover by loos-  
ening the screw at the bottom of the scaleplate.  
For example, to control system pressure between 70 and 80  
psi: select an L91B, 5-150 psi operating range, adjustable  
differential (proportioning, throttling range) of 5 to 23 psi and an  
L404A, 10-150 psi, adjustable differential of 8 to 16 psi. Set the  
L404A main scale setpoint at 80 psi and its adjustable  
differential at 10 psi. The L404A settings will then provide boiler  
pressure limit control between 70 and 80 psi. An L91B  
differential (proportioning range) pressure of 5 psi is desired.  
Therefore, an L91B main scale setpoint of between 70 and 75  
psi is required (L91 main scale setpoint plus its differential  
must be less than or equal to the limit controller main scale set  
60-2152—08  
6
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
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.  
MAIN SCALE  
ADJUSTING  
SCREW  
PROPORTIONING RANGE  
ADJUSTING SCREW (L91B,D ONLY)  
CONTROLLER  
POTENTIOMETER  
Due to device tolerances, the scaleplate settings are  
approximate and, therefore, the settings should be fine-tuned  
with the system operating.  
1
WIPER ARM(S)  
Proportioning Range (L91B,D Only)  
PROPORTIONING  
RANGE  
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.  
SCALEPLATE  
(L19B,D ONLY)  
PROPORTIONING  
RANGE SETTING  
INDICATOR  
(L91B,D ONLY)  
TERMINAL BLOCK  
MAIN SCALE  
SETTING  
Typical Operation  
INDICATOR  
BELLOWS  
HOUSING  
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.  
MAIN  
SCALEPLATE  
1
KNURLED ADJUSTMENT KNOB ON 10 TO 300 PSI  
[0.07 TO 2.07 MPa] MODELS.  
M8525  
®
Fig. 6. Setting L91 Proportioning Pressuretrol Controller.  
W
LOW-FIRE  
(CLOSED)  
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.  
PRESSURE  
RISE  
PROPORTIONING  
RANGE  
POTENTIOMTER  
2
R
1
(WIPER)  
MAIN SCALE  
SETPOINT  
HIGH-FIRE  
(OPEN)  
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.  
B
1
2
FIXED ON L91A; ADJUSTABLE ON L91B,D.  
L91D HAS TWO POTENTIOMETERS OPERATING IN UNISION.  
M8519A  
Fig. 5. L91A,B,D operating points.  
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.  
7
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
CHECKOUT  
MAIN SCALE  
PROPOTIONING  
RANGE ADJUSTING  
SCREW  
ADJUSTING SCREW  
After the controller is installed, wired, and set, it should be  
tested with the system in operation. First, allow the system to  
stabilize. Then, observe the operation of the controller while  
raising and lowering its setpoint. Pressure should increase  
when the setpoint is raised and decrease when the setpoint is  
lowered. Use accurate pressure testing equipment when  
checking out the controller. Do not rely on inexpensive gauges.  
The controllers are carefully calibrated at the factory.  
1
(L91B,D ONLY)  
MAIN SCALE  
SETTING  
INDICATOR  
MAIN  
7
DIFF.  
16  
14  
12  
10  
8
6
4
2
0
F
E
6
5
4
3
2
1
0
D
2
Make sure the modulating motor or modulating valve actuator  
reaches the low- and high-fire positions at the proper points. If  
the motor or actuator runs in the proper direction when the  
setpoint is adjusted, assume that the controller is operating  
properly. If it runs in the wrong direction, reverse the B and W  
wires. Observe the action of the motor or actuator until it  
stabilizes. If the motor or valve is moving constantly, widen the  
proportioning range (not adjustable on an L91A) incrementally  
until the system is stable.  
C
B
R
A
MIN  
W
kPa  
OZS  
DIFF. IS  
ADDITIVE  
2
IN  
B
If a Controller Seems to Operate  
Improperly  
OHMMETER  
If the controller is suspected of operating improperly, it may be  
further checked by:  
OUTPUT  
0
INPUT  
1. Leaving the controller installed where it is, but discon-  
necting all power to the controller motor or valve.  
2. Loosening the cover screw below the main scaleplate  
and removing the cover.  
SETPOINT  
INCREASE  
135  
3. Disconnecting the wires from the controller.  
4. Connecting an ohmmeter between controller terminals B  
and W to measure the resistance for the potentiometer in  
the controller. The ohmmeter should read about 135  
ohms for an L91A,B, or D.  
R
B
W
ZERO  
ADJ  
5. Connecting the ohmmeter between controller terminals  
W and R (Fig. 8) and raising the setpoint of the controller  
above the actual pressure being measured. The ohmme-  
ter should read the full value of the potentiometer mea-  
sured in step 4 (135 ohms for an L91A,B, or D).  
6. Slowly lowering the setpoint of the controller while  
observing the ohmmeter reading. The resistance should  
drop to zero at some setpoint below the actual pressure.  
7. Making an approximation of the proportioning range by  
observing the change in setpoint required for a resis-  
tance change from zero to full value.  
INCREASES TO  
3
135 OHMS  
1
KNURLED ADJUSTMENT KNOB ON 10 TO 300 PSI [0.07 TO 2.07 MPA]  
MODELS.  
2
3
TERMINALS ARE LABELED  
135 OHMS ON L91A,B OR D.  
M8546A  
®
Fig. 7. Checking L91 Proportioning Pressuretrol  
Controller.  
8. When the controller is operating properly, reconnecting  
the wires, replacing the cover, tightening the cover  
screw, and resetting the controller to the desired value.  
9. Reconnecting power to the controlled motor or valve.  
CAUTION  
Equipment Damage Hazard.  
Failure to follow checkout instructions can damage  
components or systems.  
Do not put the system into service until you have  
satisfactorily completed all applicable tests described in  
this Checkout section, all tests in the Checkout section  
of the applicable instructions, all tests for the flame  
safeguard control, and any tests required by the burner  
and boiler manufacturers.  
60-2152—08  
8
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
Replacing the Controller  
Potentiometer (Fig. 8–10)  
SERVICE INFORMATION  
IMPORTANT:  
CAUTION  
1. Only qualified service technicians should attempt to  
service or repair flame safeguard controls and  
burner systems.  
2. Disconnect power supply before cleaning the  
potentiometer windings or wiper, or before replacing  
the controller potentiometer.  
1. Replace the controller potentiometer only when nec-  
essary to obtain proper operation.  
2. When replacing the potentiometer, be very careful not  
to bend or damage the wiper arm, and not to change  
the wiper tension. Any damage or change in tension  
will decrease the life of the new potentiometer.  
Electrical Shock Hazard  
1. Disconnect all power to the controller.  
2. Loosen the cover screw below the main scaleplate and  
remove the cover.  
3. Mark the wires to the external device (motor or valve  
actuator) and disconnect them from the terminal block.  
4. Remove the screw holding the terminal block bracket to  
the top of the case (Fig. 8). Put this screw in a safe place  
because it will be needed later.  
Calibration  
All controllers are carefully tested and calibrated at the factory  
under controlled conditions. If the actual operating pressure  
does not match the setpoint, move the main scaleplate slightly  
up or down until the setpoint agrees with the actual pressure.  
5. While careful not to damage the potentiometer wiper or  
any of the internal wiring, lift out the terminal block and  
bracket.  
6. Before removing any potentiometer wires, carefully note  
and record (sketch) the position (off-center) of the active  
winding on the potentiometer and the location and con-  
nections of all wiring terminals. The new potentiometer  
must be inserted and connected the same.  
Maintenance  
Keep the cover of the controller in place at all times to protect  
the internal components from dirt, dust, and physical damage.  
Perform routine maintenance occasionally by inspecting and  
blowing or brushing away any accumulated dirt and dust. To  
assure proper functioning of the controller at all times, perform  
an operational check of the entire system during routine  
maintenance checks. Be sure to handle controllers carefully at  
the time of installation, during actual use, and during  
maintenance.  
Example: In Fig. 9, the active winding is on the left half of  
the potentiometer; the wire from the left end of the wind-  
ing is connected to the (W) terminal on the terminal  
block, and the wire from the right end of the winding is  
connected to the (B) terminal on the terminal block.  
Cleaning the Potentiometer Windings  
or Wiper  
7. Loosen the (W) and (B) screws on the terminal block,  
and remove the two wires to the active winding of the  
potentiometer. Leave the wire to the wiper arm intact.  
8. Carefully unscrew the bolt that holds the potentiometer  
to the bracket. Make sure the potentiometer wires do not  
entangle with the wiper and bend it.  
9. Carefully slide the old potentiometer off the bolt.  
10. Carefully slide the bolt through the new potentiometer.  
Make sure that:  
Occasionally, the windings or wiper on the potentiometer (two  
on the L91D) may need cleaning. Disconnect the power supply  
before removing the cover from the controller and before  
cleaning the potentiometer.  
IMPORTANT  
1. Use an electrical contact cleaner that does not con-  
tain solvents.  
2. Use extreme care to avoid bending the wiper arm,  
changing the wiper tension and damaging the potenti-  
ometer windings.  
a. The off-center position of the winding on the new  
potentiometer is the same as the old potentiometer.  
(Consult sketch in step 6.)  
b. The wiper will contact bare wires. (Rotate the potenti-  
ometer on the bolts so the surface of the winding  
where the brown enamel was removed is toward  
you.)  
3. Do not use an abrasion or burnishing tool to clean the  
potentiometer windings or wiper.  
4. Do not use hard paper, such as a business card, or  
abrasive materials (sandpaper, emery boards, file,  
etc.) to clean the windings or wiper.  
11. Screw the bolt into the potentiometer bracket. Make sure  
the wiper is contacting bare wires (step 10.b); then  
tighten the bolt.  
12. Connect the two potentiometer wires to the (W) and (B)  
terminals on the terminal block and tighten the screws.  
Make sure these wires are connected to the same termi-  
nals as in the old potentiometer. (Consult sketch in step  
6.)  
13. Carefully fit the hole in the bottom of the terminal block  
bracket over the screw protruding upward from the bot-  
tom of the case (Fig. 10). Insert the screw (removed in  
step 4) through the hole in the top of the case (Fig. 8)  
and into the top of the bracket, and tighten it.  
14. Reconnect the wires from the external device (motor or  
actuator) to the terminal block.  
Solvent-type electrical contact cleaners can deteriorate plastic  
components and wire insulation and leave an oily residue that  
accumulates particulate matter (dust, etc.). The residue can  
break down to form various carbonaceous substances that  
cause early potentiometer failure.  
Use of abrasive materials results in wearing of the  
potentiometer windings and accumulation of particulate matter  
that changes the resistance between the windings and the  
wiper.  
15. Replace the cover and tighten the cover screw.  
16. Reconnect power to the controller.  
9
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
REMOVE BRACKET SCREW  
FROM TOP OF CASE  
TERMINAL  
BLOCK  
DIFF.  
MAIN  
7
TERMINAL  
BLOCK  
16  
14  
12  
10  
8
6
4
2
0
F
E
6
5
4
3
2
1
0
BRACKET  
BRACKET  
D
MAIN  
7
DIFF.  
16  
14  
12  
10  
8
6
4
2
0
C
F
E
6
5
4
3
2
1
0
B
A
TERMINAL  
BLOCK  
D
MIN  
R
W
B
C
B
kPa  
OZS  
DIFF. IS  
ADDITIVE  
2
TERMINAL  
BLOCK  
IN  
A
MIN  
kPa  
OZS  
DIFF. IS  
ADDITIVE  
2
IN  
FIT HOLE IN BOTTOM  
OF BRACKET OVER SCREW  
PROTRUDING UPWARD  
M8521A  
FROM BOTTOM OF CASE  
M8548A  
Fig. 8. Removing terminal block bracket.  
Fig. 10. Replacing terminal block bracket.  
ACTIVE WINDING  
BARE WIRES  
POTENTIOMETER  
BRACKET  
(ENAMEL REMOVED)  
1
POTENTIOMETER  
BOLT  
WIPER ARM  
MAIN  
7
DIFF.  
16  
14  
12  
10  
8
6
4
2
0
F
E
6
5
4
3
2
1
0
D
C
B
A
TERMINAL  
BLOCK  
MIN  
R
kPa  
OZS  
DIFF. IS  
ADDITIVE  
W
2
IN  
2
B
POTENTIOMETER WIRES  
THE POTENTIOMETER IN AN L91A IS MOUNTED AT A 45 DEGREE  
ANGLE INSTEAD OF HORIZONTALLY. AN L91D HAS TWO  
POTENTIOMETERS.  
1
2
TERMINALS ARE LABELED.  
M8547A  
Fig. 9. Replacing potentiometer in L91.  
60-2152—08  
10  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
11  
60-2152—08  
 
®
L91A,B,D PROPORTIONING PRESSURETROL CONTROLLERS  
Automation and Control Solutions  
Honeywell International Inc.  
1985 Douglas Drive North  
Golden Valley, MN 55422  
Honeywell Limited-Honeywell Limitée  
35 Dynamic Drive  
® U.S. Registered Trademark  
Toronto, Ontario M1V 4Z9  
customer.honeywell.com  
© 2009 Honeywell International Inc.  
60-2152-—08 K.K. Rev. 12-09  
Printed in U.S.A.  
 

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