With increased attention on improving bottom-line performance, heat rate is a key concern for every power plant. Many operators have found that focusing on effective feedwater heater level control – and its role within a power plant’s basic power cycle – is an effective way to reduce heat rate and fuel costs.
Basic Power Cycle
Although the Rankine Steam-Water cycle for a typical steam plant will vary to some degree depending upon whether it is a reheat or nonreheat unit, the basic flow diagram shows how the cascading feedwater heater stages fit into the general process layout.
The process flow begins at the condenser, where condensed steam from the feedwater heater drains and LP Turbine is routed through each successive stage of feedwater heaters. At the same time extraction steam from the HP, IP and LP turbines is sent to the appropriate feedwater heaters where the transfer of energy takes place. Maintaining accurate level controls throughout these stages is critical to achieving the required final feedwater heater temperature before water arrives at the economizer.
Feedwater Heater Level Control
The most important aspect of feedwater heater performance is precise and reliable level control under all operating conditions. Accurate level control ensures the unit is operating in the area of greatest efficiency (straight condensation) to optimize heat transfer while preventing undue wear and tear on the feedwater heater and other system components.
Aging level instrumentation, as well as use of technologies that are vulnerable to instrument-induced errors, limit your ability to manage controllable losses associated with feedwater heater level control. Many plants operate their feedwater heaters at levels higher or lower than design – trading efficiency to accommodate the shortfalls of the instrumentation in exchange for mitigating the risk of damage to the expensive hardware.
This trade-off has an effect on performance and ultimately the net unit heat rate. The need for additional fuel and over-firing of the boiler to recover the lost energy has immediate financial ramifications. Conversely, if the level fluctuates to the extremes of the envelope, activation of protective measures to bypass a feedwater heater is the minimum response, with the outside possibility of a unit trip. Each scenario, in one way or another, negatively impacts the profitability of the plant.
If the heater level is higher than the intended set point, the active condensing zone decreases and tubes in the heater that should be condensing steam are sub-cooling condensate. Exacerbating the problem is the risk of turbine water induction from the feedwater heater. Although fail-safe measures are in place to prevent such occurrence, the impact on efficiency is a valid concern.
In addition to exposing the tubes to excessively high temperatures that can cause premature wear or worse, a lower than acceptable level introduces excessive amounts of high temperature steam to the drain cooler, which causes the condensate to flash to steam. The resulting damage to the drain cooler section increases maintenance cost and unscheduled downtime. Another issue tied to low heater levels is having a mixture of steam and water blown through the heater. The subsequent reduction in heat transfer will cause the heat rate to rise.
The design of the feedwater heater itself (horizontal versus vertical) and the drain cooler section (snorkel inlet versus full length) can challenge some level technologies. Level control on horizontal heaters and those with full-length drain cooler sections is easier since more volume is required for a given change in level. These subtleties need to be considered during the instrumentation selection process.