Optimization of feedwater heater performance

In the feedwater heater operation, it is important to maintain the liquid level at inlet of the drain cooling zone at the design level specified by the manufacturer.

If the liquid level is much higher than design, some additional heat transfer surface in the condensing zone may be flooded. This flooding may reduce the heat transfer capability and increase the TTD. If the liquid level is much lower than design, steam may enter the drain cooling zone and significantly increases the DCA.

185 Theoretically, the optimum feedwater heater level will be the point preventing the steam from entering the drain cooling enclosure and simultaneously preventing tubes in the condensing zone from being flooded. Figure 128 shows typical change of DCA and TTD with variance of the internal liquid level and the knee point.

Decreasing the liquid level to the knee point will maximize the feedwater heater performance, but the possibility of steam in-leakage into the drain cooling zone also increases. This damages tubes in the drain cooling zone.

A typical result of steam in-leakage is steam flashing inside the drain cooling zone and resultant oscillation of the heater drain temperature. It also significantly increases tube vibration in the drain cooler resulting in tube and baffle plate wear.

Figure 129 shows oscillation of the feedwater heater drain temperature when the liquid level is maintained too low and steam enters and flashes inside the drain cooling zone. Figure 130 shows the change of the heater drain temperature when the liquid level oscillates.

FIG. 128. Typical DCA and TTD vs. internal liquid level (courtesy ASME PTC 12.1 [4])

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FIG. 129. Oscillation of feedwater heater drain temperature

FIG. 130. Oscillation of heater drain temperature

The example feedwater heater had been operated at inadequate liquid level for a long-term period and during this time damage on the drain cooling zone became worse as shown in Figure 131. The peak to peak variations of the heater drain temperature had been gradually increased and in the long run all internals within the shell had to be replaced.

187 The lesson learned from this case is that in order to optimize the turbine cycle performance, rapid increase of DCA or oscillation of the heater drain temperature need to always be checked.

Also, liquid level needs to be increase if this phenomenon occurs.

FIG. 131. Trending of heater drain temperature for 5 years

The original feedwater heater operating level may not be suitable for continued use due to:

 Error/mistake in establishing the level;

 Tube plugging in lower rows increases ΔP before drains are sub-cooled (flashing);

 Flow increase – uprate, turbine changes, lower FW inlet temperature;

 Debris at drain cooler entrance increases ΔP before drains are sub-cooled;

 Unstable level control allowing level to fluctuate below minimum.

Improvement of liquid level control may improve heater performance by lowering the DCA.

Correct level control is more important for heater reliability and to maximize service life. Level control improvement could include:

 Level test to determine optimum operating level.

 Increased operating level may lower elevated DCA and control valve cycling.

 Possibly require changes to normal control and alarm set points.

 Possibly require change to instrument elevation, pending adjustable range.

 Replacement or upgrade of worn, degraded, or obsolete control instruments.

 Insulating steam legs of level instruments.

6.1.5.2. Heat exchange coefficients approach

Knowing the design information listed in Section 5.9 allows more precise calculation. Under that condition, the following approach calculates the heat transfer coefficients of the heaters, instead of using the manufacturer’s data to determine the outlet fluid temperatures. Some inlet conditions cannot be known by permanent sensors, so in addition to the measurements mentioned above, some hypothesis may need to be taken:

 The steam mass fraction in the inlet vapour pipe can be determined using the turbine’s reference efficiency (see Section 5.4.2) and turbine exhaust pressures or by using a constant mass fraction obtained by a recent measurement (tracer technique).

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 The incoming condensate flow from the upper stage of heaters needs to be measured.

In case the emergency letdown line separates from the main letdown pipe downstream of that measurement, it needs to be carefully confirmed that no leakage through these valves is occurring.

 For HP feedwater heaters, two main feedwater lines drive the flow to the heaters. The distribution of the main feedwater flowrate between these two lines can eventually be measured by using ultra sonic flowmeter technology. This measurement is not compulsory since if all the other inputs are known, the flowrate can be calculated along with the heat transfer coefficients with NTU method.

Under these conditions, the NTU method as the one described in ASME PTC 12.1 [4] can be used or computed to determine separately:

 The heat transfer coefficient from the drain cooling zone;

 The heat transfer coefficient from the condensing zone;

 The estimated drain cooling zone exchange surface, which is directly linked to the estimated water level inside the heater.

The calculated coefficient can be compared to manufacturer’s coefficients, or to the best values of the coefficients calculated over the heater’s life cycle. In case of symmetric degradation of both drain cooling and condensing coefficients, fouling is assumed. And the electric loss can easily be calculated with a thermal calculation code yielding results shown in Figure 132.

FIG. 132. Electrical losses vs fouling factor curve

Thus, the fouling factor of each heater can be monitored over the component’s life as shown in Figure 133.

This approach uses relatively complex calculations. It relies on an important number of measurements and hypothesis, which causes the results to suffer from a high uncertainty.

189 FIG. 133. Fouling factor for HP heaters

Though, a trend analysis over long periods of time gives a good overview of the heater’s performance, the TTD and the DCA do not deviate significantly from their reference values.

Knowing the fouling rate is essential to decide whether or not heat exchanger tube cleaning is justified:

 Performance gains from a HP heater tubes cleaning varies from 0 to 1 MW/heater depending on the heater’s fouling factor. Most common gains are below 0.5 MW.

 Cost of a tube cleaning is also quite low: from 20 000 to 40 000 €/heater and can be realised in two to four days during a shutdown period.

Therefore, the return on investment time varies from less than four months if applied to a heavily fouled heater to several years if the heater is already clean.

6.1.6. Optimization of MSR excess steam vent flow (for 4-Pass tube arrangement)