Demand sharing
In boiler plants, the most effective load allocation is not based on a simple operating decision but on real-time calculations taking into account the following
A further important decision involves the demand sharing methodology, which can be either parallel or series, depending on plant requirements. The Eurotherm control module allows for both configurations.
In parallel, the available boilers share the total demand simultaneously by taking up an equal firing rate to meet the load. On load increase, the firing rate of all modulating boilers will increase equally until the load requires an additional boiler. At this point, the firing rate of the active boilers decreases to compensate for the firing rate of the newly started boiler. Figure 1 explains the process for an increase of load.
Parallel modulation is generally implemented for steam boilers. It offers the most effective control when relatively steady process loads are available. As the system modulates the boiler plant to adjust the common header pressure to the required setpoint, a smoother response to changing load conditions is performed by the controller.
Series demand sharing allocates loads by normally forcing one boiler at a time to modulate in order to satisfy the demand and is most effective when used with the Eurotherm Process Automation demand schedule control module. On load increase, the firing rate of the modulating boiler will increase until the load requires an additional boiler. At this point, a new boiler is started and becomes the modulating boiler. The other active boilers are ramped to their optimum firing rate. Figure 2 explains the process for an increase of load.
Series modulation is generally implemented for hot water systems or fluctuating steam loads. This mode allows faster individual boiler response to plant conditions as the boiler pressure is adjusted to the required setpoint.
The boilers that are chosen to always run are referred to as the 'lead' boilers. All the other boilers are 'lag' boilers but are prioritized such that a boiler with a high priority always runs before a boiler with a low priority and so on. E.g. the most effective boiler is always started first and the least effective one is always stopped first.
Boiler banking
This functionality is achieved by keeping the available boilers in hot standby mode until required to fire. This is achieved by intermittently firing the unused boilers, thus maintaining a required pressure by use of upper and lower banking thresholds or by recirculation of return water through the boilers to keep them hot. The main advantage of boiler banking is that it acts a 'warm' start facility improving the plant response to sudden load changes.
8-day timer
Further enhancement to the man machine interface is achieved by the 8-day timer facility depicted in Table 1. Boiler banking is tabulated according to daily upper and lower banking threshold pressures with an additional user definable 'Today' schedule to be loaded if and when required. Up to four optional session settings can be pre-configured and stored at the supervisory computer.
Multi-sequence programme selection
In order to meet the plant demand with savings on fuel consumption, the boiler dispatching can be automated via a multi-sequence programme selection. The boiler duties can then be scheduled according to configurable daily sessions or sequences of events. The effects of this feature are
Demand load management is an optimizing function that augments, but does not replace, the combustion control system.
