UFC 3 -430-11
14 February 2001
CHAPTER 7
CONTROL LOOPS
7-1
GENERAL. Provide controls in accordance with applicable codes. Refer to
paragraph 3 -15 for applicable codes. The codes take precedence over the control
requirements shown in this handbook. A typical control loop is shown in Figure 7 -1 .
7-2
CONTROL LOOP TYPES. A single control loop includes a controlled
variable sensor, controlled variable transmitter, controller, automatic-manual control
station, and final control element. Control loops used for boilers may be of the
pressure, temperature, liquid level type, or flow type.
7-2.1
Pressure. Pressure control loops may be used for the control of boiler
pressure or fuel oil pressure. For the control of boiler pressure, the final control element
regulates fuel flow to the boiler in response to boiler drum steam pressure. For the
control of fuel oil pressure, the final control element is usually a pressure reducing
control valve that regulates in response to downstream pressure. A typical pressure
control loop is shown in Figure 7 -2.
7-2.2
Temperature. Temperature control loops may be used for the control of
steam temperature from boilers or fuel oil temperature from fuel oil heaters. A typical
temperature control loop is shown in Figure 7 -3.
7-2.3
Level. Liquid level control loops may be used for the control of boiler drum
water level. A typical level control loop is shown in Figure 7 -4.
7-2.4
Flow. Flow control loops may be used for the control of fuel flow into the
boiler burners, burner draft airflow, feed water into a boiler, or steam flow out of a boiler.
A typical flow control loop is shown in Figure 7 -5.
7-3
AIR TO FUEL-RATIO. Furnish controls to automatically provide the proper
fuel to air ratio over the entire boiler operating range from maximum turndown to
Maximum Continuous Rating (MCR). Provide cross -limited (lead-lag) controls between
air and fuel to increase airflow before increasing fuel flow and to decrease fuel flow
before decreasing airflow.
Consider full metering controls, which measure directly both airflow and fuel
flow, for all boilers with capacities greater than 5.28 gigajoule/h (5,000,000 Btu/h).
Consider an oxygen analyzer, for trim only, for all boilers with capacities greater than
25.32 gigajoule/h (24,000,000 Btu/h). Consider CO trim for all boilers with capacities
greater than 52.75 gigajoule/h (50,000,000 Btu/h), especially coal fired boilers.
Evaluate the energy savings of these measures and provide all those that are life cycle
cost effective per Executive Order 13123.
A typical cross -limited (lead-lag) boiler control system with oxygen trim for a
single fuel is shown in Figure 7 -6 .
7-1
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