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3. Operation of an excessive number of auxiliary pumps. This results in throttled, inefficient operation.

Requirements for Maximum Economy

Establish a Boiler Loading Schedule. An optimized loading schedule will allow any plant steam demand to be met with the minimum energy input. Some general points to consider when establishing such a schedule are as follows:

1. Boilers generally operate most efficiently at 65 to 85% full-load rating; centrifugal fans at 80 to 90% design rating. Equipment efficiencies fall off at higher or lower load points, with the decrease most pronounced at low-load conditions.

2. It is usually more efficient to operate a lesser number of boilers at higher loads than a larger number at low loads.

3. Boilers should be put into service in order of decreasing efficiency starting with the most efficient unit.

4. Newer units and units with higher capacity are generally more efficient than are older, smaller units.

5. Generally, steam plant load swings should be taken in the smallest and least efficient unit.

Optimize the Use of High-Pressure Boilers. The boilers in a plant that operate at the highest pressure are usually the most efficient. It is, therefore, desirable to supply as much of the plant demand as possible with these units provided that the high-grade energy in the steam can be effectively used. This is most efficiently done by installation of back-pressure turbines providing useful work output, while providing the exhaust steam for low-pressure consumers.

Degrading high-pressure steam through a pressure reducing and desuperheating station is the least efficient method of supplying low-pressure steam demands. Direct generation at the required pressure is usually more efficient by comparison.

Establish an Auxiliary Loading Schedule. A schedule for cutting plant auxiliaries common to all boilers in and out of service with rising or falling plant load should be established.

Establish Procedures for Maintaining Boilers in Standby Mode. It is generally more economical to run fewer boilers at a higher rating. On the other hand, the integrity of the steam supply must be maintained in the face of forced outage of one of the operating boilers. Both conditions can sometimes be satisfied by maintaining a standby boiler in a "live bank" mode. In this mode the boiler is isolated from the steam system at no load but kept at system operating pressure. The boiler is kept at a pressure by intermittent firing of either the ignitors or a main burner to replace ambient heat losses. Guidelines for live banking of boilers are as follows:

1. Shut all dampers and registers to minimize heat losses from the unit.

2. Establish and follow strict safety procedures for ig-nitor/burner light-off.

3. For units supplying turbines, take measures to ensure that any condensate which has been formed during banking is not carried through to the turbines. Units with pendant-type superheaters will generally form condensate in these elements.

Operators should familiarize themselves with emergency startup procedures and it should be ascertained that the system pressure decay which will be experienced while bringing the banked boiler(s) up to load can be tolerated.

Guidelines for Day-to-Day Operation

1. Monitor all boiler efficiencies continuously and immediately correct items that detract from performance. Computerized load balancing may prove beneficial.

2. Ensure that load-balancing schedules are followed.

3. Reassess the boiler loading schedule whenever a major change in the system occurs, such as an increase or decrease in steam demand, derating of boilers, addition/decommissioning of boilers, or addition/removal of heat-recovery equipment.

4. Recheck parameters and validity of established operating mode.

5. Measure and record fuel usage and correlate to steam production and flue-gas analysis for determination of the unit heat input relationship.

6. Keep all monitoring instrumentation calibrated and functioning properly.

7. Optimize excess air operation and minimize boiler blowdown.

Computerized Systems Available

There are commercially available direct digital control systems and proprietary sensor devices which accomplish optimal steam/power plant operation, including tie-line purchased power control. These systems control individual boilers to minimum excess air, SO2, NOx, CO (and opacity if desired), and control boiler and cogenera-tion complexes to reduce and optimize fuel input.

Boiler plant optimization is realized by boiler controls which ensure that the plant's steam demands are met in the most cost-effective manner, continuously recognizing boiler efficiencies that differ with time, load, and fuel quality. Similarly, computer control of cogeneration equipment can be cost effective in satisfying plant electrical and process steam demands.

As with power boiler systems, the efficiencies for electrical generation and extraction steam generation can be determined continuously and, as demand changes occur, loading for optimum overall efficiency is determined.

Fully integrated computer systems can also provide electric tie-line control, whereby the utility tie-line load is continuously monitored and controlled within the electrical contract's limits. For example, loads above the peak demand can automatically be avoided by increasing inplant power generation, or in the event that the turbines are at full capacity, shedding loads based on previously established priorities.

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