Energy-Tech Magazine

Operating feedwater heaters without paying attention to proper heater level is sort of like operating your car without keeping track of the air pressure in the tires. Things will seemingly be just fine, but if the air pressure is not correct the car performance will be affected, and eventually significant damage to the tires will result. The importance of routine vehicle maintenance is relatively well understood and we attempt to take good care of our vehicles. It is a good idea to take the same type of care when it comes to feedwater heaters in a power plant. Significant damage to feedwater heaters can result from long-term heater level issues that have gone unnoticed. Proper adjustment of level is a form of preventative maintenance that can support years of reliable heater operation.

So why use feedwater heaters?
Before we get into the problems caused by low level operation, let’s look at what heaters do for us.

Most steam power plants use closed shell-and-tube type feedwater heaters. Feedwater heating plays a significant role in improving

References ASME PTC 12.1 – 2000 Closed Feedwater Heaters. Heat Exchanger Institute, Standards for Closed Feedwater Heaters, 8th Edition, 2009. EPRI Report CS-3239, Recommended Guidelines for the Operation and Maintenance of Feedwater Heaters, 1983. EPRI Report TR-102923, 1992 FW Heater Technology Symposium. Feedwater Heater Survey, EPRI Report GS-7417. ASME PTC 19.2 -2004, Instruments and Apparatus: Pressure Measurement. EPRI Report PE-0010.0-141, Feedwater Heater Performance Prediction Calculation Procedure.

the efficiency of Rankine power cycles. Feedwater heaters, as their name implies, pre-heat the feedwater before it enters the boiler. So how do feedwater heaters improve the cycle efficiency? They add what is called reversibility to the cycle. A reversible cycle is a process that can be “reversed” by using infinitesimal changes in some property of the system, in this case temperature, without the loss or dissipation of energy. A completely reversible cycle is impossible, of course. But using feedwater heaters to transfer heat across small temperature differences to the feedwater (or condensate) instead of a single large temperature difference increases reversibility. Although steam is robbed from the turbine train to heat feedwater in this fashion, it provides increased efficiency.

You might look of it this way: The temperature entering and exiting the boiler drum and evaporator essentially doesn’t change. However, changing subcooled water to steam takes enormous energy.

Bringing the temperature up reduces the energy needed to change the water to steam, thus reducing fuel consumption. So heating the water using steam extracted from the turbine, which has already converted part of its available energy to useful work, decreases the energy to change water to steam.

Feedwater heaters levels
Now that we see the importance of feedwater heaters in the cycle, let’s discuss maintenance.

Operating a feedwater heater with a low level during a long period of time can damage the heater’s internals. The type of damage is dependant on the design of the heater and its operating environment. In sever cases, tube leaks, damage to drain cooler shroud and other internal problems can result in the need to operate at non-ideal conditions or even remove the heater from service. Many power plants have experienced forced unit power reductions or shutdowns, and eventually premature replacement or major repair of the heater, all of which are expensive.

Because of this, the feedwater heater level controls should be monitored closely to ensure the proper operating level is maintained. One of the most damaging environments in a steam cycle is when steam and water are introduced together and must pass through a torturous path. Normally, on the shell side of the heater, steam enters and slightly subcooled condensate exits the heater drain. Internally, steam enters a desuperheating zone (if applicable), then a condensing zone and finally a drain cooling zone. If steam was to pass into the drain cooling section and subsequently the drain outlet, then this destructive environment of steam and water combination occurs. As the steam condenses on the cooler tube surfaces, the bubble collapse in a water environment can induce high vibrations and erosive damage similar to cavitation, which will wear the heater tubes, drain cooler baffles, tie rods, shroud and the drain cooler end plate.