Field Data Evaluation

Field Data Evaluation

In Chapter 4, we saw the example of an HRSG operating at conditions different from those stated in the proposal, and we made an evaluation of its performance and found that the off-design performance was acceptable and that the HRSG would perform as stated in the proposal guarantee within margins of error. We shall evaluate the same problem using HRSG simulation methods so that plant engineers can appreciate how useful this tool is.

Example 5.9

An HRSG is designed to generate 15,313 kg/h of steam at 40 kg/cm 2 g and at 463°C with

105°C feed water using 110,000 kg/h of gas turbine exhaust at 550°C. The exit gas tem- perature was stated as 183°C.

The plant is unable to operate at the earlier parameters for several reasons. The gas turbine is not operating at full load as the plant does not need power. The feed water

temperature is 120°C not 105°C, and steam pressure required is only 35 kg/cm 2 g. The

plant is generating 11,000 kg/h of steam at 442°C. Plant engineers want to know if this

HRSG Simulation 285

performance can be accepted or whether the gas turbine or HRSG supplier should be questioned further regarding the performance.

Solution

Here is a simple way to check if the HRSG performance is reasonable. Let us do a heat balance and find out the exhaust gas flow. From Chapter 4, the gas flow required to gen- erate 11,000 kg/h of steam at 442°C from 120°C feed water was shown to be 90,400 kg/h.

Gas analysis used % volume CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15. The HRSG was simulated in the design case. Then, in the off-design case, the lower gas flow and temperature conditions were inputted. Results are shown in Figure 5.9a and b.

HRSG performance—Design case

Sh. Evap. Eco. Project—eg4 Units—Metric case—eg4 Remarks -

Amb. temp., °C = 25 Heat loss, % = 1 Gas temp. to HRSG C = 550 Gas flow, kg/h = 110,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 68.89 tot duty, MW= 12.4

Surf. Gas temp.

Flow Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty Pres.

kcal/h °C Sh.

in/out °C MW kg/cm 2 a kg/h

Gas–steam temperature profiles 550

(a) Design information for an installed HRSG. (Continued)

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

HRSG performance—Off—Design case

Sh. Evap. Eco. Project—eg4 Units—Metric case—eg4 Remarks -

Amb. temp., °C = 25 Heat loss, % = 1 Gas temp. to HRSG C = 500 Gas flow, kg/h = 94,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff., % = 64.03 tot duty, MW= 8.6

Surf. Gas temp. Wat./Stm. Duty Pres. Flow Pstm. Pinch Apprch. US Module no.

°C kcal/h °C Sh.

in/out °C in/out °C MW kg/cm 2 a kg/h

FIGURE 5.9 (Continued)

(b) Field performance of the installed HRSG.

The simulation results show that 11,000 kg/h of steam at 439°C can be expected from 90,400 kg/h of exhaust gas at 500°C. The exit gas temperature is expected to be 191°C, and steam temperature is about 439°C. The actual field data show 11,000 kg/h of steam at 442°C, which is also obtained by physical performance evaluation (see Tables 5.9 and 5.10). These results also compare well confirming that simulation can be used to obtain a lot of information about an yet-to-be-built HRSG or an operating HRSG.