D. STEAM CONTROLS (ALL FUELS)
1. Operating Limit Pressure Control (Figure 1-3): Breaks a
circuit to stop burner operation on a rise of boiler
pressure at a selected setting. It is adjusted to stop or start
the burner at a preselected pressure setting.
2. High Limit Pressure Control (Figure 1-3): Breaks a
circuit to stop burner operation on a rise of pressure
above a selected setting. It is adjusted to stop the burner
at a preselected pressure above the operating limit
control setting. The high limit pressure control is
normally equipped with a manual reset.
3. Modulating Pressure Control (Figure 1-3): Senses
changing boiler pressures and transmits the information
to the modulating motor to change the burner firing rate
when the manual-automatic switch is set on “automatic.”
4. Low-Water Cutoff and Pump Control (Figure 1-4):
Float-operated control responds to the water level in the
boiler. It performs two distinct functions:
•Stops firing of the burner if water level lowers below the
safe operating point. Energizes the low-water light in the
control panel; also causes low-water alarm bell (optional
equipment) to ring. Code requirements of some models
require a manual reset type of low-water cutoff.
•Starts and stops the feedwater pump (if used) to maintain
water at the proper operating level (Figure 1-5).