A key way to reduce a power plant’s heat rate – and fuel costs – is to ensure accurate level control within its feedwater heaters. Although many physical anomalies can degrade feedwater heater performance, many older power plants use outdated level technologies.
Older level technologies simply cannot achieve a performance level sufficient to manage controllable losses due to instrument-induced errors. The following are two primary sources of instrument-induced errors:
- Mechanical or electronic drift due to aging instrumentation, moving parts or intrinsic design. Torque tube/displacer technology requires calibration between shutdowns to achieve reasonable accuracy and prevent nuisance deviation alarms between multiple level transmitters. In addition, this technology may take too long to respond to rapid level changes due to dampening effects that are fundamental to its principle of operation.
- Measurement technology that is vulnerable to process conditions. Shifts in specific gravity and the dielectric constant of media related to variations in process pressure and temperature affect the accuracy of Differential Pressure, Magnetostrictive, RF Capacitance and Torque Tube/Displacer technologies. As a result, these technologies cannot provide accurate level from start-up to operational temperatures without applying external correction factors – or can only deliver the specified accuracy at operational temperatures. To compound the issue, calibrations performed on these technologies during a shutdown often require adjustment when the process reaches operating temperature to maintain acceptable control and prevent unnecessary deviation alarms.
Lower than expected final feedwater temperature occurs when a feedwater heater is out of service due to unreliable level input to the control system or when level is too high or low. If the condition is due to high feedwater heater level, the operator would note a decrease in feedwater heater temperature rise, a decreasing DCA temperature difference and an increasing TTD. The inverse is true if feedwater heater levels are too low. In either of these scenarios, risk of damage to hardware increases, heat transfer is impaired and feedwater entering the economizer is not at the required temperature.
The probable responses – and effects – of a low final feedwater temperature include:
- Over-firing boiler to increase temperature (level too high/low or out of service):
- Increases in fuel consumption and emissions.
- Increases in gas temperature exiting the furnace – reheat and superheat sprays, premature fatigue of hardware.
- Flows through IP and LP stages of turbine increase 10% (HP heater out of service).
- Flashing, which can cause damage to drain cooler section.
- Thermal effects on tubes.
- Opening emergency drains to lower level (level too high):
- Loss in efficiency.
- Potential damage to hardware if water enters extraction tube.
- Potential flashing due to sudden pressure drop.
- Turbine Water Induction Protection (TWIP) trips unit, which can result in lost production, start-up and unscheduled maintenance costs.
Deploying measurement technologies immune to common sources of instrument-induced errors provides operators with the reliable process feedback needed to manage controllable losses – and prevents the ripple effect these errors have on plant operations and maintenance. We’ll discuss Guided Wave Radar, which is an ideal solution for plant operators who want to reduce their heat rate, in our next blog post.