One of the most common causes of tube failures in a feedwater heater (FWH) is the improper control of the internal liquid level, which also can cause operational and maintenance costs that might lead to premature replacement. These problems are not new, they have been experienced by many utility plants throughout the industry during the past 50 years. However in many cases, the resulting damaging phenomenon has seldom been totally understood, and the loss of corporate knowledge and failure of some utilities to identify and rectify level control problems continues to bring this issue to the forefront of root causes of FWH operational failures.
In general, the performance of the Drain Cooler (DC) Zone is tied to the operational parameter of Drain Cooler Approach (DCA). DCA is defined as the temperature difference between the drains leaving the heater and the feedwater entering the heater. Most FWHs are designed with a DCA of approximately 10°F. While DCA is a good indication of whether the DC Zone is operating properly, it is not the only parameter that should be
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The pressure of the drains also must be known in order to determine the degree of subcooling and whether there is a potential for flashing, either within the DC itself or the downstream piping before the level control valve. Flashing and two-phase flow in either of these areas can cause significant damage to the heater.
It is important to remember that the drain cooler is designed to be a water-to-water exchanger. It must remain that way to function properly. Any admission of vapor into the zone typically results in problems. This might be a result of a low liquid level in which steam is admitted directly from the condensing zone into the DC zone, the result of flashing within the DC zone itself, or can be the result of leakage into the zone via the endplate or shroud cracks. In most cases, the OEM designs the DC zone such that the linear velocity of the liquid within the DC zone remains a reasonable 2´-4´ per second. When velocity increases, the pressure drop increases exponentially (approximately a square function). When flashing occurs, the localized velocity can be much greater than designed and tube vibration and/or tube OD erosion might occur, as well as damage to the carbon steel cage components.
Flashing, by definition, is the change in state of liquid to vapor. While in most cases this change of state results from the addition of heat (as in the boiler), in a FWH the most common cause of flashing is a result of a reduction in pressure (or pressure drop). Pressure drop might be a result of the geometry of the Drain Cooler Entrance window, the fact that the drains must travel around the tubes and change direction many times due to the baffling arrangement, and also due to changes in elevation and elbows in the downstream piping. If the liquid drains are not subcooled enough, any one of these pressure drops could result in flashing and two-phase flow. Two-phase flow is known to cause problems to piping, tubing, the cage and the shell, especially in the case of carbon steel components.
This flashing phenomenon is typically more problematic in LP heaters than in HP heaters, although both are susceptible. To understand this, one need only to consult the steam tables and look at the specific volume of saturated liquid vs. saturated vapor. Let’s consider a HP heater operating at approximately 250 psia and a LP heater operating at approximately 10 psia. From the steam tables, we observe the following:
It can be seen that in the case of the HP heater, when the liquid flashes within the drain cooler, it wants to occupy a volume that is approximately 100x the volume that the liquid previously occupied. As mentioned above, this drastically increases the localized velocity (and can result in further pressure drop and more flashing). In the case of the LP heater, the same amount of flashing liquid now wants to occupy more than 2,000x the volume, which can lead to significant failure mechanisms.