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To achieve optimum control, both steam and feedwater flow values should be corrected for
density. This provides tighter drum-level control. It's appropriate if a system suffers from fluctuating feedwater pressure or flow, or if a more sophisticated control is required.
To stabilize control over a wide range of steam demand, the three-element mode acts during high steam demand, but can drop back to two-element mode if the steam-flow measurement fails. The module reverts to single-element level control if the feedwater flow measurement fails or if steam demand is low.
A primary goal in operating a boiler plant is ensuring the working steam pressure (or temperature in a hot-water system) is constant for any load demand placed on the plant. Some valuable features of demand-load scheduling include base load or modulating operation, parallel or serial demand sharing, boiler banking and programmable sequence selection.
In a multi-boiler plant, demand-load management optimizes the distribution of steam demand among the units and adjusts the overall output to meet working requirements. This ensures that boilers are fired only when required, thus reducing operating costs. Alternatively, demand-load management can allow each boiler to operate for the same amount of time.
A demand share arrangement should allow each boiler to operate in either base-load or modulating service to achieve the best distribution of load. In base load operation, the total demand is shared among the base-loaded boilers in accordance with user input. The modulating mode of operation, on the other hand, automatically enforces the load allocation without operator intervention. The total demand, less that satisfied by the base-loaded boilers, is shared among the modulating boilers in proportion to their capacities. Effective load allocation is based on real-time calculations that account for operating safety margins, load fluctuations, shutdown characteristics and boiler capacities.
Figure 6:With series demand sharing, the modulating boiler's firing rate increases until the load requires an additional boiler. Then the additional boiler starts and becomes the modulating boiler.
Demand sharing also can be implemented in parallel or series. In parallel, the boilers share the total demand using equal firing rates. On load increase, the firing rate of each increases equally until the load requires an additional boiler. Then, the firing rate of the active boilers decreases to compensate for the firing rate of the additional boiler.
Steam boilers generally use parallel modulation. It offers the most effective control for relatively steady process loads. The controller provides a smoother response to changing loads by modulating the boiler plant to maintain header pressure.
Series demand-sharing allocates loads by modulating one boiler at a time to satisfy demand (see Figure 6). On load increase, the modulating boiler's firing rate increases until the load requires an additional boiler. Then, a new boiler starts and becomes the modulating boiler. The other boilers have already been ramped to their optimum firing rate. Series modulation is generally implemented for hot-water systems or fluctuating steam loads. It allows faster individual boiler response to plant conditions as the boiler pressure is adjusted to the required setpoint.
The boilers that always operate are referred to as lead boilers, while the others are called lag boilers. The most effective boiler always starts first and the least effective always stops first.
Boiler banking keeps boilers in hot standby mode by intermittently firing unused boilers, thus maintaining a required pressure. Boiler banking acts as a warm-start facility, improving the plant's response to sudden load changes.
Remember that demand-load management is an optimizing function that augments, but doesn't replace, the combustion-control system.
Taken together, burner modulation, air/fuel cross-limiting, excess air regulation, oxygen trim and total heat control can provide excellent control and fuel efficiency for most boiler systems.
Rob Kambach is senior technical specialist at Invensys Foxboro in Foxboro, Mass. Contact him at (508) 543-8750.
Figures: Invensys Foxboro