Power plant feedwater heaters (FWHs) make the most of the heat from condensation to preheat water destined for the boiler. In doing so, they reduce the amount of fuel required to bring the water up to temperature.

Unlike their haughty turbine or boiler counterparts, however, FWHs seem rather boring. After all, they don’t make steam, generate electricity or rotate at 3600 rpm. Seldom do they cause forced outages or demand teams of maintenance personnel. Similar to piping supports, they do their job without fanfare. Although essential to power plant operation, it’s easy to understand why FWHs suffer inattention and neglect.

(Find information on feedwater heater equipment, suppliers and standards at Engineering360.)

With ever-increasing market pressures, it has become imperative for generating companies to operate and maintain their facilities in the most cost-effective way possible. Now more than ever, it’s critical to understand the FWH's risk potential as well as its value to the bottom line.

Maximizing Efficiency with Regenerative Feedwater Heating

The process of regenerative feedwater heating (that is, making use of existing energy from the change in saturated steam to saturated liquid) allows the water to be raised to saturation temperature very gradually. Within the Rankine cycle, this minimizes the inevitable irreversibility associated with heat transfer to the working fluid (water).

Placement of high pressure and lower pressure heaters in the power cycle. Improved thermodynamic efficiency reduces unit operating costs and eases boiler thermal stress. Most plants use both low-pressure (LP) and high-pressure (HP) heaters, and many also have the intermediate-pressure (IP) variety. LP heaters initiate the process by heating condensate with low-pressure turbine extraction steam prior to the boiler feed pumps. Some LP heaters are placed within the condenser at the turbine exhaust throat. IP and HP heaters are located downstream of the boiler feed booster and boiler feed pumps, respectively. Typically, the HP heater tube side design pressure exceeds 1500 psig with the high-pressure turbine supplying steam.

Additional components, although technically different, serve similar functions by adding heat to the process and maximizing efficiency.

• Deaerator heaters, open to the atmosphere, heat water and remove dissolved oxygen.

• Economizers, instead of using steam, employ furnace flue gas as the final heating step before the water enters the boiler drum.

Design and Components of Feedwater Heaters

These types of FWHs are of the closed, shell-and-tube type and are of similar design and function across the industry. They are unfired (ASME Section VIII pressure vessels) since the heat transfer does not occur by means of combustion, but by convection and condensation.

Multiple stages usually exist among separate heaters. Each heater stage corresponds to a turbine extraction point. As extraction stages increase in number, the amount of thermal energy required to generate a given amount of electrical energy is reduced.

Feedwater flows within the tubes and the extracted steam condenses on the shell side. Condensed steam from each heater drains sequentially to the next lower pressure heater and is returned to the feedwater through the condenser or heater drain pumps. FWHs can be mounted vertically or horizontally, with each configuration having unique advantages.

A schematic showing key points of a closed feedwater heater. In terms of tubing materials, copper alloy is prominent among LP heaters. Stainless steel is popular among HP heaters. Monel metal and carbon steel are also used to varying degrees. FWHs find homes among all types of steam generators including fossil, nuclear and combined-cycle heat recovery steam generators.

Value

FWHs play a substantial role in the unit’s thermodynamic cycle as they enhance thermal efficiency. Approximately 33% of modern plant cycle efficiency savings can be attributed to feedwater heaters, and so can be directly responsible for fuel cost savings. Practically speaking, even though extraction steam is stolen from the turbine to heat the feedwater, it’s an efficiency gain. That’s because the water temperature going into the boiler drum vs. the steam temperature coming out is essentially the same. Conversely, heating subcooled water to steam requires much more energy than any work which could be obtained from the exhausted turbine steam being utilized. Heating water this way is more efficient than relying upon combustion alone because it takes advantage of energy already available to bring water up to temperature.

In practice, feedwater heaters serve four purposes:

1. Reduce fuel required for combustion by increasing the initial water temperature to the boiler

2. Reduce heat losses in the condenser

3. Minimize thermal transients within the boiler

4. Lower emissions as fuel use is reduced due to improved heat rate

Operational Risks and Challenges with Feedwater Heaters

Operating FWHs without paying attention to their heater level is like driving your car 100,000 miles without checking the motor oil. Significant damages result from long-term heater level issues that have gone unnoticed. Tube leaks, drain cooler damage and other internal problems diminish performance and can result in removing the heater from service.

Tube leaks can lead to costly repairs and downtime for power plants. One of the most damaging conditions is when steam and water mix in a non-design condition. Consider that when an FWH (or group of FWHs) is isolated the water enters the next-stage FWH at reduced temperature, and that FWH will extract additional steam. Overloading up to three times the design steam flow is possible. Massive steam flows and their resulting elevated drain flows yield flashing steam, hammering of the components and causing potentially catastrophic damage.

Some plants attempt to increase generator output by isolating the high-pressure (HP) FWHs to overfire the boiler. But overfiring exposes the boiler to thermal shock and overloads the remaining FWHs as well as the turbine; permanent system damage far outweighs any short-term gain. Other major problems include steam impingement, flow-assisted corrosion (FAC), tube vibration and difficulty plugging failed tubes. Many plants have experienced forced outages, major repair or premature heater replacement, all expensive propositions.

Life Management

A life management program can determine an accurate cost of operating and maintaining FWHs to determine whether replacement makes more economic sense than repair. A physical condition assessment is the first step in determining remaining useful life. To do this successfully requires information including original design data, heater operating data, detailed heater inspections and knowledge of current repair practices and outcomes.

Glamorous it is not, but a well-maintained feedwater heater is a thing of beauty. As heater integrity conditions worsen and failures occur, understanding the mechanism(s) and root cause(s) of failure are perhaps more important than the stopgap activities of the immediate failure. Because of this, failure cause analysis (FCA) is the chief objective in FWH life-cycle management and, as such, must remain the primary concern throughout the corrective maintenance process. The organization also must be committed to periodic condition assessments and fastidious record-keeping.

FWH failures adversely affect power plant efficiency and availability. Lack of attention or maintenance accelerates issues, causing costly leaks, component failures and even operational shutdowns. Mindful monitoring, timely maintenance and effective repairs combined with optimal operational practices prevent expensive repairs and downtime.

Unfortunately, the lack of comprehensive standards, guidelines and procedures continue to plague plant personnel in their efforts. Operations must keep a trained eye to identify heater level issues while maintenance is obliged to enact comprehensive repair approaches instead of the “fix-it-and-forget-it” mindset. Corrective repairs usually address only the immediate failure. Determining the root cause of failure offers the only way to stop repeat failures.