Steaming in Economizer

Economizer steaming is a common problem in gas turbine HRSGs. This arises at low or part load operation of gas turbine when the exhaust gas temperature is low while the gas mass flow is nearly the same. As an example, Figure 5.7 shows the results from the simulation program when the exhaust gas temperature is 385°C instead of 500°C, other parameters remaining the same as in Example 5.1. It is seen that due to the lower amount of steam generated, the economizer reaches saturation temperature and some steam is also formed.

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

HRSG performance—Off—Design case

Sh. Evap. Eco. Project—eg3 Units—Metric case—eg2 Remarks -

Amb. temp., °C = 25 Heat loss, % = 1 Gas temp. to HRSG C = 385 Gas ﬂow, kg/h = 100,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eﬀ., % = 47.4 tot duty, MW = 5.3

Surf. Gas temp.

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

Wat./Stm. Duty

Pres.

°C kcal/h °C Sh.

in/out °C MW kg/cm 2 a kg/h

Steaming economizer.

When the gas inlet temperature is low, the steam generated in the superheater and evaporator is significantly reduced, and hence, a small amount of steam is generated compared to the design case. Here, we see only 7,253 kg/h of steam versus 12,700 kg/h in the unfired normal case. As the gas mass flow is unchanged, the heat transfer coefficient in the economizer is unchanged as gas-side flow governs U (see Appendix A). Hence, the enthalpy pickup on water side is rather high, and the exit water temperature reaches the saturation temperature and causes steaming. One can estimate the surface area involved in steaming (see Figure 5.8) and ensure that the flow of steam in those tubes is not from top to bottom but from bottom to top so that the bubbles can flow smoothly. There are a few methods to handle steaming economizer that can cause tube vibration or deposition of salts inside tubes or flow blockage inside tubes.

HRSG Simulation 283

Water in Water in/

Steaming portion of economizer has upward water flow direction.

Methods to Minimize Steaming in Economizer

1. Increase the blowdown rate and hence the mass flow through the tubes to minimize the amount of steaming. This can be done if the steaming occurs for a short period and not a prolonged affair.

2. Increase steam flow through the HRSG by supplementary firing. Once the exhaust gas temperature is increased, more steam is generated in the evaporator and more flow through the economizer. This reduces the enthalpy pickup of water, and hence, steaming is avoided.

3. Place the feed water flow control valve between the economizer and the evaporator so that the economizer operates close to the feed pump discharge pressure, which has a higher saturation temperature, and hence, steaming is avoided.

4. Calculate the surface area in which steaming occurs. The HRSG supplier should do this exercise. Then, ensure that the water flow direction in the steaming surface is upward and not downward as shown in Figure 5.8.

5. Provide bypass on gas side at the economizer so that the duty of the economizer

is reduced. This is a loss of energy, and the system is easy to install in small HRSGs. Bypassing the entire HRSG is another method, but the energy loss is much more.

In practice, one has to have an idea if the gas turbine is likely to operate at part loads for a long time and check the performance at this condition before the HRSG is designed and fabricated. If steaming is likely, then the economizer size can be reduced (resulting in loss of efficiency). If part load operation of the gas turbine is for short duration only, then we may use methods 1–3 suggested earlier to overcome this problem. Steaming in economizer is a nuisance and may cause vibration of tubes or flow blockage and deposition of salts present in feed water.

In the economizer mentioned earlier, the gas flows horizontally. The final section of the economizer is likely to steam. Hence, the water flow direction is from bottom to top in that portion. However, such an arrangement of economizer may not be always feasible particularly in large HRSGs. The simulation program results as well as results from physi- cal design are shown in Figure 5.7 and Table 5.8, respectively. The amount of steaming is slightly different due to the consideration of nonluminous heat transfer.

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

TABLE 5.8

Performance of a Steaming Economizer

Evap Econ

Process data Gas temp. in, ±5°C

349 259 Gas temp. out, ±5°C

259 211 Gas spht., kcal/kg °C

0.2649 0.2601 Duty, MM kcal/h

0.15 0.31 0.50 2.36 1.24 Surface area, m 2 40 93 98 2152

1770 Gas press. drop, mm wc

2.22 3.49 3.02 35.20 23.66 Foul factor, gas

0.0002 0.0002 Steam side

Steam press., kg/cm 2 g 41 41 41 41 41 Steam flow, kg/h

7270 7343 Fluid temp. in, °C

251 105 Fluid temp. out, ±5°C

252 251 Press. drop, kg/cm 2 0.5 0.0 0.0 0.0 0

Foul factor, fluid

0.0002 0.0002 General data Exhaust gas flow

40.00 kg/cm 2 g Exhaust gas temp.

kg/h

Steam pressure

338 ±5°C Exhaust gas pressure

±5°C

Steam temp.

7270 kg/h Heat loss

1.00 kg/cm 2 a Steam flow

105 ±5°C Firing temperature

Feed water temp.

0 kg/h Burner duty, LHV

0 ±5°C

Process steam

1 % Burner duty, HHV

0.00 MM kcal/h

Blowdown

2.94 % Flue gas flow out

0.00 MM kcal/h

Eco steaming

kg/h

Steam surface

521 m 2

Steaming in Economizer