Selecting Shell Side Design Temperature for Feedwater Heaters

Selecting Shell Side Design Temperature for Feedwater Heaters

Kellen J. Muldoon

American Exchanger Services, Inc.

Hartford, Wisconsin, USA

Abstract:

Operating temperature information was gathered using thermocouples installed on the outside of an existing high pressure feedwater heater (FWH). The operating data was collected to demonstrate the basis for the evaluation of the operational effectiveness of the heater especially during rapid load changes. This information provides insight into the selection of shell side design temperatures of the skirt and shell sections of a feedwater heater. This direct measurement of the temperature was useful in evaluating the appropriateness of both skirt and shell design temperatures selected by standard HEI practice. Also, consideration will be given to upgraded turbines and their impact on the design temperatures and pressures of existing FWH(s) Introduction: Feedwater heater design incorporates many design parameters which are provided by standards and codes. The standards are, as their name implies, a set of guidelines to provide the heat exchanger industry a common set of guidelines which must be followed to allow safe, long term operation. In many states, the ASME standards are legislated requirements for obtaining an operating license for power boilers and appurtenances. While these standards exist, it is the responsibility of the design engineer to verify that the parameters used are verified and re-checked. HEI and TEMA have published standards governing the feedwater heater design temperature. It is the focus of this paper to examine this standard within the context of a larger thermal evaluation of the top extraction feedwater heater in a supercritical power plant. Common practice selects a design shell side temperature for the skirt section of a three zone FWH by the HEI Standards for Feedwater Heaters suggested method of utilizing the Mollier diagram. The Mollier diagram is a graphic approach to depict the relationships between the surrounding air temperature, pressure, enthalpy, entropy, and quality. HEI Standards for Closed Feedwater Heaters Seventh Edition section 3.3 for

Design Temperatures states that shell side skirt design temperature is to be found by entering the Mollier diagram at normal operating steam temperature and pressure and follow the isentropic line to the maximum operating pressure. These pressures are provided to the heat exchanger designer by the purchaser. The normal operating and maximum steam pressures are obtained by collaboration between the turbine manufacturer and the architect engineer derived from various heat balance iterations to obtain the optimum cost and capitol cost for the end purchaser. Once this optimal situation is agreed upon, the turbine manufacturer provides the referenced pressures. The temperature at that pressure rounded to the nearest 10 degrees F is the design temperature. This method is completely dependent upon the maximum allowable pressure, which is provided by the customer. For verification, these numbers, various data samples were gathered near the shell side extraction steam inlet on an existing FWH to offer some indication on actual operation temperatures at the skirt, which is compared to the HEI standard method. These temperature data samples are skewed due to the use of a steam box DHZ shroud inside the skirt where the temperatures were measured with type K thermocouples. To double check the data which could be gathered by the thermal couples on the shell, actual DCS data was provided by the power plant to acquire actual steam temperature. Design barrel temperatures of the shell are selected at the saturation point corresponding to the design pressure provided by the purchaser. This value is a more reliable number versus the skirt design temperature simply due to its direct correlation to analytical steam tables and data which have been established to be effective sources of information over hundreds of years. Shell data was collected with thermocouples on the DHZ exhaust where shell temperature would be large. It is speculated that the design shell side temperature is insufficient at the DHZ exhaust location.

1 Copyright © 2014 by ASME

Proceedings of the ASME 2014 Power Conference POWER2014

July 28-31, 2014, Baltimore, Maryland, USA

POWER2014-32275

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/25/2014 Terms of Use: http://asme.org/terms

Methods: Experimental Procedure

The procedure was to gather analytical research with respect to the scientific method. A large volume of temperature readings were acquired around the DHZ on an existing high pressure feedwater heater. The thermocouples used were Magnetic K type thermocouples accompanied by a Pico 8 Thermocouple USB Data Logger. The logger read the voltage difference noted by the thermocouple and recorded the data in a program which was converted into excel as temperatures data points corresponding to each thermocouple channel.

To verify the data acquired by the thermocouples on the feedwater heater would be accurate, they were first tested in various conditions. The thermocouples were attached to a cup forging cooling subsequent to undergoing a 200 F̊ heat treatment process. The data acquired provided a gradual decreasing temperature around 165 F̊ which was then calibrated by cross referencing the temperature reading with an infrared temperature gun. In attempt to verify the thermocouples would react appropriate to higher temperatures, a steel plate was heated using a plasma torch. The temperature readings appeared accurate, and were then cross referenced again by the infrared temperature gun noted above verifying the results were both accurate and precise. One last test was performed to verify that the thermocouples would acquire accurate data over a longer period of time. Using the same heated steel plate, the thermocouples were attached, and the specimen was left to cool. A sample size of 180 data points was set with 1 sample every two minutes to be taken in ambient room temperature. The temperature stabilized as expected, verifying the data acquisition unit was working properly.

With approval from the power plant, feedwater heater insulation was removed around the DHZ. Selection of the thermocouple locations was determined from an outline drawing to ensure thermocouple locations properly captured temperature data at the DHZ steam inlet, as well as the DHZ exhaust. All 8 thermocouples were mounted in the locations noted in figure 1. Locations 2, 3, and 5 are hidden by valves and associated piping. The data recorder was connected and set up to record data on 20 second intervals with approximately 3000 samples each data set. This data was taken over twelve days and twelve data sets which correspond to approximately 16 hours per sample. Following all data collection, the data was sorted from seconds to match the approximate time of day that the snapshot was taken. The data was then matched to the data provided by the plant and examined. MS Excel was used to create plots and for data interpolation. A detailed thermal performance was also run for each data point for additional insight into the operation of the heater, particularly during rapid load changes. Methods: Data Analysis/ Collected Data Design temperature was plotted against the actual metal temperature measured by the thermal couples mounted on the feedwater heater’s skirt. For this particular feedwater heater the Shell side design temperature is 584 F̊ for the shell and

780 F̊ for the skirt. Again the focus of this paper is on the shell just outside the desuperheating zone, since the shell is given to be designed at saturation. The temperature of the steam entering the DHZ from the nozzle is going to be the highest temperature and is noted at channel five. The maximum steam temperature at this point was found to be 764.29 ̊F. This is approximately the maximum temperature the shroud could be at, since it is at plant peak load power production. This is represented in the below figure 2. The recorded maximum inlet temperature is very close to the design skirt temperature, but cannot be directly correlated to the data because the zone is shrouded off by the DHZ shroud section. It is important to note that it is not known whether the extraction steam inlet nozzle has a lining. Although there are high temperatures noted in the extraction steam inlet nozzle, it is beyond the scope of this paper for analysis. The shroud which is experiencing the largest extraction steam temperature and pressure is still below that of the design skirt temperature by a minimum of 15.71 ̊F. The sample set provided was during an exceptionally hot summer day, and therefore demand for energy was high. The plot shown below in Figure 3, is a representation of the rapid swing in power output of this super critical coal fired plant on day 5 of data acquisition. This unit, and many super critical coal fired plants similar to this are designed for this type of cyclic operation with large swings in power output. It can be seen in Figure 4, how rapidly the skirt temperatures were affected by this unit load variation, which may lead to various fatigue and creep issues after years of service. This inlet steam temperature correlates to a maximum measured skirt temperature of 576.29 F̊ and may be seen in the figure 4 as captured by thermocouple channel 5. Although this temperature is higher in the desuperheating zone shroud than the skirt, because of the nature of the design, it shows that it is still slightly below that of the design skirt temperature. It is of significant interest to note how close the DHZ exhaust shell temperature is to the shell design temperature. It can be seen in figure 4 below that the DHZ exhaust a full load, which is on the shell not skirt, is noted at approximately 560̊F. The design temperature by standard practice of the existing heat exchanger is 580 F̊. The temperature measured at the skirt can be seen in figure 5. This data is plotted and compared to the constant skirt design temperature line. There is an ample difference between the measured skirt temperature and that of the design temperature. This difference in temperature between the measured value and the design skirt temperature is approximate 220 ̊F. Discussion: From the acquired information it appears that there is an ample difference between the experimental, measured, values and that of the design temperature provided by HEI standards for the skirt but, as indicated, this design utilized a steam box (also known as chest) . As anticipated this 220 ̊F difference is more than ample between design temperature and measured



data. In contrast to the skirt, the data provides evidence that at the DHZ exhaust, temperature difference between shell design temperature and actual has minimal margin compared to that of the shell design temperature, at maximum load. With only approximately 20 ̊F of temperature difference between the shell side design temperature and the measured temperature, it is a real possibility that the shell side design temperature should be increased. Increasing the design temperature will increase the difference between the shell side temperature at the exhaust (measured) and the design temperature, allowing for a more conservative design. It is important to note that altering a Rankine Cycle via turbine upgrades may adversely affect the auxiliary components of the cycle such as the feedwater heaters. If the extraction points on the turbine were modified, pulling in more superheated steam at a higher pressure and quality, the desuperheating shroud and skirt may approach the design temperature, and the shell side design temperature at the DHZ exhaust may be exceeded. With 220 F̊ of difference between the skirt design it is unlikely that serious issues may arise. It should be noted that at the DHZ exhaust it is plausible to reach critical metal temperatures where failure mechanisms may begin to propagate, such as creep. Another important observation to note is the speed at which the temperature recorded responds to the power plants

designed load change and pressure. This lag may result in various other DHZ issues such as wet and dry wall situations. This would result in a decrease of longevity over time. Conclusion Design temperatures provided by HEI standards appear to be adequate for the skirt side design temperatures. For the shell, design temperature appears to be too low in the area of the DHZ exhaust. These design temperatures are based upon pressure which shall be verified before heat exchanging equipment is purchased. With 220 ̊F of temperature difference between the measured skirt temperature and that of design temperature, it is clear to say that the design temperature is adequate with a sufficient safety factor. With only 20 F̊ of difference between the shell temperature near the DHZ and the design temperature, it appears the design temperature may at times be insufficient. Design temperature and operating temperature are below the temperature where creep and stress failure mechanism generally occur for the provided materials. When upgrading a turbine, it is important to note the affect of possible increase in temperature and pressure and how this may adversely affect the feedwater heaters Reference: Standards for Closed Feedwater Heaters. Seventh ed. Cleveland, OH: HEI, 1984. Print.



FIGURES

Figure 1 ‐ Thermocouple Location



Figure 2 ‐ Temperature Vs Time (Sat. & Zone)

Figure 3 ‐ MW Output

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80017:04:12

17:41:06

18:18:57

18:58:24

19:38:17

20:18:03

20:59:29

21:35:35

22:12:33

22:46:27

23:27:30

0:08:25

0:53:05

1:40:34

2:22:59

3:08:48

3:55:38

4:33:58

5:21:59

6:05:47

Temperature (°F)

Time

Saturation and Desup Zone Temperature Vs. Time

Saturation

Desup Zone Steam Inlet

Desup Zone Steam Outlet

DZ Steam In ReCalc

DZ Steam Out ReCalc

300

350

400

450

500

550

600

650

700

16:48:00 19:12:00 21:36:00 0:00:00 2:24:00 4:48:00 7:12:00 9:36:00

MW Output

MW Output



Figure 4 ‐ Temperature Vs. Time

Figure 5 ‐ Temperature Vs. Time

480

490

500

510

520

530

540

550

560

570

20:00:00

20:18:00

20:36:00

20:54:00

21:12:00

21:30:00

21:48:00

22:06:00

22:24:00

22:42:00

23:00:00

23:18:00

23:36:00

23:54:00

0:12:00

0:30:00

0:48:00

1:06:00

1:24:00

1:42:00

2:00:00

2:18:00

2:36:00

2:54:00

3:12:00

3:30:00

Temperature (°F)

Time

Channel 8 and Channel 5 Vs Time

Channel 5

Channel 8

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16:48:00 19:12:00 21:36:00 0:00:00 2:24:00 4:48:00 7:12:00 9:36:00

Temperature (°F)

Time

Temperature Vs Time Channel 2

Channel 3

Channel 4

Channel 5

Channel 6

Channel 7

Channel 8

FW Inlet

FW Outlet

Skirt Design Temp



Selecting Shell Side Design Temperature for Feedwater Heaters

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