Data Required for Performing Steam Generator Analysis

Plant engineers often face problems with their steam generators and seek the help of the boiler suppliers or consultants to understand and resolve the problem. Often, the basic data of the steam generator required for performing the analysis may not be available with the plant as the boiler supplier often does not provide them at the time of purchasing the steam generator. Hence, the following discussion will help plant engineers obtain the desired data from the boiler supplier (before ordering the boiler) for performing an analysis at a later date.

140 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

During my visits to several plants, I have seen that boiler suppliers provide bare minimum information on the boiler tube geometry or arrangement, while a lot of information on nonpressure parts, painting, weights, shipping details, materials, and welding insula- tion is provided. The plant engineers are also unfamiliar with thermal design aspects and do not ask for the required information. This results in a lot of running around to get the tube geometry data when a problem has to be investigated. Hence, this discussion will enable plant engineers to collect the required information before the boiler is ordered as often boiler suppliers may not be available to help particularly if some problem arises a few years after the warranty period.

Hence, one should be able to analyze the performance of a given boiler to understand if the steam generator is properly sized or likely to have problems as discussed in the pref- ace. The following pages provide some information on how plant engineers may perform these calculations.

Example 3.7

A steam generator is designed to generate 100 t/h of steam at 42.2 kg/cm 2 g and 400°C.

Feed water temperature is 116°C. Fuel fired is natural gas with the following analysis by volume: methane—83.4, ethane—15.8, and nitrogen—0.8. Furnace dimensions: furnace length—9.75 m, width—2.44 m, and height—3.96. Furnace is fully water cooled

as shown in Figure 2.2. Furnace projected area is 142 m 2 . Typical boiler arrangement is

shown in Figure 3.17. The number of streams is very important information particularly for superheaters. This is discussed in Appendix B.

Using 15% excess air, determine the boiler performance at 25%–100% load. The boiler furnace and tube geometry details are given in the following text. This boiler has a good screen section ahead of the superheater, which has several advantages as discussed earlier. The boiler supplier has provided some basic information. However, it is possible for plant engineers to evaluate the boiler performance independently using some of this information. LHV = 11,653 kcal/kg and HHV = 12,879 kcal/kg (see chapter on combustion calculations). Enthalpy of flue gas at 1360°C = 412 kcal/kg based on the analysis (see Appendix F). Enthalpy of air at 21°C (reference 15 °C) = 1.3 kcal/kg.

This steam generator has a water-cooled furnace, followed by a screen section that shields the superheater from external radiation from the furnace, a two-stage superheater with intermediate spray desuperheater, and an economizer.

The data in Tables 3.12 through 3.14 shown are the minimum information a plant should obtain from any boiler supplier. If not routinely provided, they should demand them from the boiler supplier before placing the order. Else the plant will be facing lots of difficulties later on when a consultant is asked to evaluate the boiler thermal performance. Even if the boiler is operating at a different load, one can apply the methodology discussed here and evaluate the complete boiler performance. Critical field data such as % oxygen in flue gas, exit gas temperature, and any other measured data such as steam, water flows, and temperatures should be available to verify the calculations. The plant engineer can perform the following studies:

1. Predict the boiler performance at any load or with any other fuel firing or excess

air or even changes in feed water temperature or steam pressure.

2. Analyze any heat transfer surface such as evaporator, economizer, or superheater

for problems such as under- or overperformance.

Steam Generators 141

TABLE 3.13

Detailed Boiler Performance—From Computer Program

Surface

Screen

Final SH

Pry SH

Evaporator Economizer

465 Gas temp. out, ±5°C

Gas temp. in ±5°C

154 Gas sp ht, kcal/kg °C

0.2815 Duty, MM kcal/h

11.78 7.16 4.44 11.60 10.78 U, kcal/m 2 h °C

2395 LMTD, C

Surface area, m 2 105

111 Gas press. drop, mm wc

55.22 83.47 54.63 97.00 45.10 Max gas vel., m/s

51 49 43 33 16 Tube wall temp. ±5°C

123 Fin tip temp. ±5°C

136 Weight, kg

16,071 Fluid temp. in °C

116 Fluid temp. out ±5°C

222 Press. drop, kg/cm 2 0.00 0.50 0.71 0.00 0.93

Fluid velocity, m/s 0.0 24.4 26.9 0.0 1.7 Fluid ht. tr. Coefft.

9390 Foul factor, gas

0.0002 Foul factor, fluid

0.0002 Spray, kg/h

3. Check the tube wall temperature of superheater, steam temperature, and spray quantity.

4. Check boiler efficiency. Table 3.14 shows the summary performance of the boiler at various loads, as provided by

boiler supplier. For example, during operation, if the exit gas temperature at full load is not close to 154°C based on the fuel, excess air, and water and steam conditions provided earlier, one can review the complete performance and see if fouling on tube side could be an issue. Heat transfer coefficients may be calculated by the methods discussed in Appendices B through E and then checked against the values indicated by the boiler supplier. Large variations may be challenged. Some variations in heat transfer coefficient may be expected as there are several correlations available for flow inside, outside tubes. However, the boiler supplier should have fine-tuned these correlations based on field data from several of his boilers in operation.

If superheater tube wall temperatures from the field data are much higher than indicated, then tubeside fouling is likely; carryover of water particles by steam is possible from the drum. The superheater could also have been oversized. Gas-side flow maldistri- bution also may be looked into. The screen section is a good insurance against superheater failures as discussed in this chapter earlier, and hence, a design such as that mentioned earlier with a large screen section followed by superheaters is desirable.

142 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 3.14

Performance at Various Loads

Boiler Load, %

Boiler duty 64.68 48.51 32.34 15.98 MM kcal/h 34.79 Amb. temp.

21.1 Relative humidity

21.1 21.1 21.1 21.1 °C

60 Excess air

15 Flue gas recirculation

Fuel input (HHV) 77.58 57.89 38.52 19.36 MM kcal/h 94.35 Heat rel. rate (HHV)

kcal/m 3 h 1001,179 Heat rel. rate (HHV)

kcal/m 2 h 665,206 Steam flow

kg/h 100,000 Steam pressure

42.2 42.2 42.2 42.2 kg/cm 2 g 42.2 Steam temp.

±5°C 399 Feed water temp.

±5°C 116 Water temp. lvg eco

±5°C 116 Blowdown %

1 Boiler exit gas temp.

±5°C 512 Eco exit gas temp.

±5°C 512 Air flow

kg/h 142,180 Flue gas flow

kg/h 149,506 Spray flow

0 kg/h 8540

Flue gas analysis, losses, efficiency, %

Dry gas loss

16.51 Air moisture

0.31 Fuel moisture

13.19 Casing loss

0.5 Unacc/margin

0.5 Efficiency, LHV

76.25 Efficiency, HHV

69.00 Furnace back press.

85.88 27.92 mm wc 481 % vol. CO 2 8.56 8.56 8.56 7.38 8.56

H 2 O 17.30 17.30 17.30 15.12 17.30 N 2 71.65 71.65 71.65 72.51 71.65 O 2 2.48 2.48 2.48 4.99 2.48 SO 2 0 0.00 0.00 0.00 0

Fuel flow

7326 Note : The last column shows the performance without the economizer.

Data Required for Performing Steam Generator Analysis