Midco Unipower MPG Series - Manual (Page 11)

11
The Honeywell diaphragm valve supplied on standard burners is of the slow opening, fast
closing type. Closing force combines a gas pressure differential, diaphragm weight and spring.
When the controller is not calling for heat, the coil is de-energized. The plunger of the three
way actuator is in the down position, so that the bleed is closed and the supply port is open. Gas
flows to the top diaphragm, causing the gas pressure and spring to hold the valve closed.
With a call for heat, the flame safeguard energizes the coil. This pulls the plunger to the up
position, opening the bleed valve and closing the supply port. The gas then bleeds off the top of
the diaphragm, allowing the gas pressure below to lift the diaphragm, and open the valve. When
all the gas has bled off the top of the diaphragm, the valve is fully opened. In the event of a
power failure during operation, the valve will close. Normal operation will resume upon restoration
of power.
For general service, tests for gas tightness and proper operation will suffice. If leakage is
observed, replace valve. If the valve will not open, make sure that normal gas pressure is
available at the valve, the bleed line is unobstructed and power is available at the terminals.
If the valve fails to close, make sure power is off at the terminals and gas flow is in the
direction of the arrow on the valve body. When a malfunction is found to be caused by the valve
operator head, the entire valve must be replaced.
If the valve will not open, make sure normal gas pressure is available at the valve, the bleed
line is unobstructed, and power is available at the terminals.
___________________________
Motorized Main On-Off Valves are available on special order burners.
When the actuator is energized, hydraulic fluid is pumped from a reservoir to a metal bellows.
The bellows transfers the resulting pressure through the drive stem to open the valve.
___________________________
MPG burners may feature a redundant solenoid gas safety valve. This valve is an On-Off (2
position) valve.
When the valve operator is electrically energized, a plunger lifts the valve disk off the valve
seat, allowing gas to flow. When the current is broken the valve closes. It will normally require no
service. If the operator malfunctions or the valve leaks replace the entire valve. After replacement
check for leakage.
___________________________
WARNING: Explosion hazard. Do not use this device if it gets wet. It can malfunction
and cause serious injury or death. Replace any device that has been wet.
Standard MPG burners are equipped with a Honeywell RM7895 microprocessor based burner
control, employing a flame rectification system of flame detection. Burner construction for special
codes and/or insurance requirements such as Factory Mutual or Industrial Risk Insurers (GAP)
may require alternate controls. Refer to Section XV Special Equipment. A safe start and run
control sequence is provided with instantaneous response to presence or loss of flame signal.
Flame failure response time is 3 seconds. Pilot Flame Establishing Period (PFEP) is field
selectable from 4 or 10-seconds. The RM7895 features a pre-purge time (30-seconds for On-Off,
90-seconds for 2-Step or Modulating burners) and a plug-in amplifier. An airflow circuit is also
field selectable to allow either lockout or recycle upon loss of airflow. Five LED's (light emitting
diodes) are provided to display sequence information. Refer to the Honeywell RM7895 literature
for detailed operating information, configuration requirements, testing and service.
INITIATE ("POWER" LED is lit).
The RM7895 enters the INITIATE sequence when it is powered. The INITIATE sequence
lasts for ten seconds unless the voltage or frequency tolerances are not met (refer to Honeywell
RM7895 literature for criteria). When tolerances are met, the INITIATE sequence will restart. If
the condition is not corrected and the hold condition exists for four minutes, the RM7895 will
lock-out. Causes for hold conditions in the INITIATE sequence are in the Honeywell RM7895
literature.
STANDBY ("POWER" LED is lit).
The RM7895 is idle in this state of sequencing. When the burner switch, limits, operating limit
controls and all microprocessor monitored circuits are in the correct state for the RM7895 to
continue, sequencing will advance to pre-purge.
1. Diaphragm Main
Automatic Valve
XIV Flame
Safeguard
3. Redundant
Solenoid Gas
Safety Valve
2. Motorized Main
Automatic Valve
Part 1 Installation