Page 3 of 4
When calculating DCA, the available instrument locations should be evaluated. In some cases the feedwater inlet temperature to a heater is taken as the outlet temperature from the previous heater. This reading might be prior to any bypass lines that will decrease the actual inlet temperature in the presence of a leak. Also, stratified flow can sometimes lead to inaccurate readings. This is particularly so with tube-side outlet temperatures.
The TTD is equal to the feedwater heater shell-side steam saturation temperature (at the operating shell pressure) minus the tube-side outlet temperature, Tsat,shell – TFW,out. An increase in TTD indicates reduced heat transfer. TTD increases results from several internal issues such as tube sheet bypass, fouling, etc. A high drain level begins to cover tubes in the condensing section and will increase TTD. However, to increase the TTD resulting from level, the level would need to be quite high. DCA is the best index for level monitoring.
In summary, DCA is an indicator of level issues and TTD is an indicator of heat transfer
Determining an optimum normal operating level
A level control test may be performed to determine the optimum operating level for a feedwater heater by manually altering the heater level and recording the DCA and TTD responses. Figure 2 shows what is called a knee curve relating these parameters. The curve illustrates that lowering the level results in increased drain temperatures and subsequently DCA. When the drain cooler is uncovered, there is a sudden step up in drain temperature and DCA resulting from steam entering the drain cooler, which is shown on the left side of the graph. TTD can be seen to have a lesser impact from level. The figure shows that for this particular heater, the drain cooler entrance (snorkel) became uncovered when the level drops to 5.5?. It also shows the condensing section tube coverage begins around 13?. Based on these test results, normal level control can be adjusted to an optimum operating range between 7?-9? based on actual level measurement, regardless of the “design” level. Selecting a new optimum operating level control range includes consideration of the existing high-level dump control valve, alarm and bypass switch settings, as well as their range of adjustment and control, both prior to performing the level control test, as well as prior to establishing a new normal control range. The knee curve in Figure 2 is a generic example. Each feedwater heater will have unique behavior relating level, location of level indicators, and the DCA and TTD performance indices.
It is important for the optimum operating level to also consider the impacts of periodic changes to normal or full-power operating conditions, such as start-up and shutdown, transient operation, heater strings out-of-service, low load operation and similar conditions. Adjustments or tuning of level control valve feedback or gain settings might also be appropriate to reflect the dynamics of the system response to a new normal operating control range. Heater level tests should be considered whenever there are indications of level related problems with the feedwater heater, as well as if there have been significant changes made to the heater or its operation, such as tube plugging or uprates.
Performing a feedwater heater level control test should include careful preparation and planning, as well as detailed communications with plant operations personnel. This preparation should provide confidence that changes and their potential consequences are evaluated for station response to normal and transient conditions. In the event a heater has sustained drain cooler shroud damage, the knee on the knee curve will shift to the right in proportion to the extent steam in-leakage is occurring, until the damaged area has been covered. In extreme cases the heater level might need to be as high as the mid-line of the feedwater heater to cover the drain cooler if damage to the shroud or endplate is higher on the drain cooler. Operating at such a level is likely to be impractical from an instrumentation control range and operations would need to carefully consider the potential consequences of a tube leak and flash-back on turbine water induction and/or overspeed protection measures. Obviously, a high heater level decreases thermal performance.
After the level tests
In the event level tests indicate operation with a higher than normal level and control range is warranted, additional actions beyond the changes to the level control parameters should be considered. Visual inspection might be warranted in cases where damage is indicated to the drain cooler shroud. The use of other non-destructive inspection methods or data, such as Eddy Current testing, should be evaluated to determine if it might yield an adequate understanding of the damage to avoid a shell cut to support a thorough visual inspection (see Figure 3).