Boiler Controls; Boiler Output Control; Combustion Control; Flame Safeguard Control - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS

BOILER CONTROLS

BOILER OUTPUT CONTROL

There are three ways to control the output of a commercial
boiler:
1. On-off (cycling) control.
2. High-fire, low-fire control.
3. Modulating control.
On-off (cycling) control is most common for small boilers
up to 300 kW capacity. The oil or gas burner cycles on and off
to maintain steam pressure or water temperature. Cycling
control causes losses in efficiency because of the cooling of
the fireside surfaces by the natural draft from the stack during
the off, prepurge, and postpurge cycles necessary for safety.
High-fire, low-fire burners provide fewer off cycle losses
since the burner shuts off only when loads are below the low-
fire rate of fuel input.
Modulating control is used on most large boilers because it
adjusts the output to match the load whenever the load is greater
than the low-fire limit, which is usually not less than 15 percent
of the full load capacity. Steam pressure or hot water
temperature is measured to control the volume of gas or oil
admitted to the burner.
Boiler firing and safety controls are boiler manufacturer
furnished and code approved. A BMCS usually enables a boiler
to fire, provides a setpoint, controls pumps and blending valves,
and monitors alarms and operation.

COMBUSTION CONTROL

Combustion control regulates the air supplied to a burner to
maintain a high gross efficiency in the combustion process.
More sophisticated systems use an oxygen sensor in the stack
to control the amount of combustion air supplied. Smoke density
detection devices can be used in the stack to limit the reduction
of air so stack gases stay within smoke density limits. A
continuous reading and/or recording of flue gas conditions
(percent O , stack temperature) is usually included in the control
2
package of large boilers.
A simple combustion control system contains a linkage that
readjusts the air supply from the same modulating motor that
adjusts fuel supply (Fig. 44). There may be a provision to stop
flow of air through the fluebox during the off cycles.
PRESSURE
MANUAL
REGULATOR
VALVE
GAS
SUPPLY
TO FLAME
MODULATING
SAFEGUARD
SYSTEM
OIL
SUPPLY
FILTER PUMP SAFETY
SHUTOFF
VALVE
Fig. 44. Combustion Control for Rotary Oil Burner.

FLAME SAFEGUARD CONTROL

Flame safeguard controls are required on all burners. Controls
for large burners can be very complicated while controls for
small burners such as a residential furnace are relatively simple.
The controls must provide foolproof operation, that is, they
must make it difficult or impossible to override any of the safety
features of the system. The controls also should be continuous
self check. For commercial and industrial burners, the flame
safeguard control goes through a series of operations. The
following sequence is an example:
– Purge firebox of unburned fuel vapor (prepurge).
– Light pilot.
– Verify that pilot is lit.
– Open main fuel valve.
– Verify that flame is present as soon as fuel is introduced.
– Cut off fuel supply promptly if the flame fails.
– Purge firebox of any unburned fuel after each on cycle
(post-purge).
The key to any flame safeguard system is a reliable and fast
means of detecting the presence or absence of a flame. Methods
of detection are:
– Response of bimetal sensor to heat (slow response).
– Response of thermocouple to heat (slow response).
– Flame conductivity (fast but can be fooled).
– Flame rectification (fast, reliable).
– Ultraviolet flame detection (fast, reliable).
– Lead sulfide (photo) cells (fast, reliable if flame
frequency check included).
Some sensors can potentially cause improper operation
because of shorts, hot refractories, or external light sources.
Other sensors, like flame rectification and ultraviolet, respond
to flame only. Flame safeguard systems must be approved by
UL or Factory Mutual for specific applications.
Figure 45 shows a flame safeguard system often applied to
small gas boilers or furnaces. The flame of the gas pilot impinges
on a thermocouple which supplies an electric current to keep
the pilotstat gas valve open. If the pilot goes out or thermocouple
fails, the pilotstat valve closes or remains closed preventing
gas flow to the main burner and pilot. The pilotstat must be
manually reset.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
330
PILOT GAS
STEAM
PILOT
VALVE
PRESSURE
SENSOR
VALVE
AIR
CONTROL
LINKAGE
METERING
BURNER
VALVE
FLAME
C2932