Xylem MM 221G User Manual

INSTRUCTION MANUAL

!

MM-221G

Model 157S-RBP-MD

(Snap Switch)

Low Water Cut-Off/Pump
Controller

Applications:
For bi-level pump control applications such

as multiple boiler level operation.

CAUTION

WARNING

• Before using this product read and understand instructions.

• Save these instructions for future reference

• All work must be performed by qualified personnel trained in the proper application, instal­lation, and maintenance of plumbing, steam, and electrical equipment and/or systems in
accordance with all applicable codes and ordinances.

• To prevent serious burns, the boiler must be cooled to 80˚F (27˚C) and the pressure must be
0 psi (0 bar) before servicing.

• To prevent electrical shock, turn off the electrical power before making electrical connections.

• This low water cut-off must be installed in series with all other limit and operating controls
installed on the boiler. After installation, check for proper operation of all of the limit and
operating controls before leaving the site.

• We recommend that secondary (redundant) Low Water Cut-Off controls be installed on all
steam boilers with heat input greater than 400,000 BTU/hour or operating above 15 psi of
steam pressure. At least two controls should be connected in series with the burner control
circuit to provide safety redundancy protection should the boiler experience a low water
condition. Moreover, at each annual outage, the low water cut-offs should be dismantled,
inspected, cleaned, and checked for proper calibration and performance.

• To prevent serious personal injury from steam blow down, connect a drain pipe to the control
opening to avoid exposure to steam discharge.

• To prevent a fire, do not use this low water cut-off to switch currents over 7.4A, 1/3 Hp at
120 VAC or 3.7A, 1/3 Hp at 240 VAC, unless a starter or relay is used in conjunction with it.

Failure to follow this warning could cause property damage, personal injury or death.

®

Model 157S-RBP-MD

WARNING

Pump or Motorized Valve

Circuit Rating (Amperes)

Voltage Full Load Locked Rotor Pilot Duty

120 VAC 7.4 44.4

345 VA at

240 VAC 3.7 22.2

120 or 240 VAC

Approximate

Distance Above

Cast Line Differential

Setting In. (mm) In. (mm)

Pump Off 5

1

/2 (140)

1

1

/2(38)

Pump On 4 (102)

Pump Off

15

/16 (24)

3

/8(16)

Pump On

9

/16 (14)

Burner Off 0 N/A

Pump Off 17/16 (37)

3

/4(19)

Pump On

11

/16 (17)

Burner Off -

3

/8 (-16) N/A

2

PUMP OFF

CAST LINE

3

/8"

(35mm)

9

/16"

(16mm)

PUMP ON

BURNER OFF

CASTING LINE

PUMP ON

PUMP OFF

;

;

;

11/2"

(38mm)

4"

(102mm)

OPERATION

Maximum Pressure: 150 psi (10.5 kg/cm2)

Electrical Ratings
Float Control

Probe Control (750BM-P-120-CI)

157S-RBP-MD

Settings and Differential Pressures:

Probe Sensitivity:

- 26,000 ohms

Probe Input Power

- 120 volts

Pump or Motorized Valve

Circuit Rating (Amperes)

Voltage Full Load Locked Rotor

120 VAC 7.2 43.2

240 VAC 3.75 21.6

Probes Any

Pressure

Float
0 psi

(0 kg/cm

2

)

Float

150 psi

(10.5 kg/cm2)

Pressure

Alarm Circuit Rating

Voltage Amps

120 VAC 1

240 VAC 1/2

Motor Horsepower

Voltage Amps

120 VAC 1/3

240 VAC 1/3

* Values are ± 8" (3.2mm).

Probe Settings

Float Settings (0 psi)

NORMAL BOILER WATER LINE

“CUT-OFF” LEVEL AT CAST LINE

11/2"

(38mm)

MINIMUM
SAFE
WATER
LEVEL

BLOW DOWN VALVE

BLOW DOWN VALVE

Size the steam (top) and water (bottom)
horizontal equalizing pipe lengths so that
the horizontal cast line on the body is 12"
(38mm) below the boiler’s normal water
level, but not lower than the minimum

safe water level, as determined by the
boiler manufacturer.

STEP 1 - Determine the Elevation at Which the
Low Water Cut-Off/Pump Controller Must be Installed

INSTALLATION –

TOOLS NEEDED:

Two (2) pipe wrenches, one (1) flathead screw
driver, and pipe sealing compound.

3

IMPORTANT: Follow the boiler manufacturer's
instructions along with all applicable codes and
ordinances for piping, blow down valve and water
gauge glass requirements.

a. Using a pipe wrench, unscrew the plastic float

blocking plug (A) from the float block tapping
of the low water cut-off body (B).

Install pipe plug (provided) to seal port.

STEP 2 - Installing the Low Water Cut-Off

b. Mount and pipe the low water cut-off (C) on a

vertical equalizing pipe (D) at the required
elevation level, as determined in Step 1.

Install full ported blow down valves directly below
the lower cross of the water equalizing pipe (F).

NORMAL BOILER
WATER LINE

C

D

F

BLOW DOWN VALVE

BLOW DOWN VALVE

The plug must be reinstalled before control is
shipped installed on the boiler, and removed
when boiler is installed after shipment.

Failure to follow this caution may damage
float and operating mechanism

!

CAUTION

B

A