HRSG Performance and Evaluating Field Data

When a new waste heat boiler is installed, its operating data may differ from that stated in the proposal. There can be several reasons for this such as the following:

• The inlet flue gas flow and temperature may be different from that used in the proposal or guarantee statement. An unfired waste heat boiler simply responds to the inlet gas conditions, and hence, the steam parameters may be different from those expected.

• The plant’s operating feed water temperature or pressure may be slightly different

from that used for generating the proposal due to plant’s limitation, deaerator or water treatment plant’s availability. Hence, steam generated or exit gas temperature from boiler may differ from that stated in the proposal.

The question then arises: how can the plant engineer ensure that the boiler has been sized correctly, and will it generate steam at the rated conditions when provided with the rated gas flow and temperature?

Let us consider an unfired HRSG as shown in Figure 4.21a. It was noticed that the operating data were completely different from that predicted by the HRSG supplier. As discussed earlier, one has to investigate the reasons for this. It is likely that the gas turbine is operating at a different load or ambient temperature resulting in variations in exhaust gas flow and temperature from that used for guarantee purposes; the plant may not be able to operate the gas turbine at guarantee load conditions; also, portion of the exhaust gas may have been bypassed as steam demand may not be there during the testing phase and plant does not wish to waste the treated water. While testing a boiler, plant engineers typically shut off the blowdown line and ensure dampers and valves do not leak.

Then the question arises: are the differences between operating data and guaranteed data reconcilable or out of line and whether the HRSG itself was designed properly? If the performance test is not carried out within a few months of installation and if the plant cannot operate the HRSG at the rated conditions, it may not be able to find out if the HRSG was designed correctly or not. The HRSG supplier can blame the plant for not providing proper conditions for testing the HRSG and can walk out without proving if the HRSG design was adequate or not. This is not good from the plant’s perspective. The bottom line is how to check if the HRSG was properly sized or not even if the HRSG operates at different parameters from those guaranteed. The following example shows how one can evaluate the performance of an operating HRSG. The procedure may be used for any waste heat boiler. Figure 4.29 shows the logic or flow diagram for off-design performance calculations of waste heat boilers. There are several iterations, and for each change in gas temperature or steam temperatures, the properties have to be evaluated at each component. If there are multiple modules or if the HRSG is fired, the calculations become more tedious.

Example 4.2

An unfired gas turbine HRSG has the data shown in Table 4.7. The design shows over 15.3

t/h of steam at 463°C at 40 kg/cm 2 g with 105°C feed water. However, in operation, it makes only 11,000 kg/h at 442°C and at 35 kg/cm 2 g with feed water at 120°C with zero blowdown.

The water temperature leaving the economizer is measured as 242°C. The plant is not able

to operate at 40 kg/cm 2 g with feed water at 105°C for various other reasons. Inlet and exit

gas temperatures are measured as 500°C and 190°C. Is this performance acceptable? The HRSG supplier says that due to variations in exhaust gas flow and temperature, they are not able to get the desired steam flow of 15.3 t/h at 460°C and that his HRSG design is perfect.

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

Input Wg, tg1, gas, analysis, steam

tg4

pressure, tw1, A1, A2, A3

tw2

Assume Ws

tw1

Assume ts2 Compute Qa

Assume tw2

Compute tg2

compute Qa, tg4

Compute Qt

Sh. Q1 = Qt Evap. Eco. No

compute Qt

(Qa–Qt)/Qa

Notes: Unﬁred performance, NTU method

used for solving all surfaces Yes

Yes

Q3 = Qt

A1, A2, A3=surface areas of superheater, evaporator, Economizer

Compute Q2, tg3

Compute Wc using

Wg, Ws = gas, steam ﬂows

Q1, Q2, Q3, ts2

Q1, Q2, Q3=duty of superheater, Evaporator and economizer

Wc = Ws

(Ws–Wc)/Ws <0.01

Yes End

FIGURE 4.29

Logic diagram for HRSG performance evaluation.

TABLE 4.7

Gas Turbine HRSG Tube Geometry Data

Tube OD

Tube ID

Fins/in. or fins/m

Fin height

Fin thickness

Fin width

Fin conductivity

Tubes/row

Number of rows deep

Transverse pitch

Longitudinal pitch

Note: Streams are the number of tubes carrying the steam or water. This is explained in Appendix B.

Waste Heat Boilers 201

This type of situation often arises in plants operating HRSGs. The HRSG in all likeli- hood will operate at parameters slightly different from those stated in the proposal docu- ment. Hence, the steam output and the gas–steam temperature profiles will be different from the proposal data. If questioned, the HRSG supplier will cite the gas turbine exhaust conditions for the field performance results, and the gas turbine supplier will suggest that the plant look into the HRSG design. The helpless plant engineer will be better off if

he knows how to evaluate the performance himself and get an idea of what is going on. Tube geometry data is shown in Table 4.7; thermal performance data as shown in Table 4.8 should be obtained from HRSG suppliers for each operating case such as full load and part load of gas turbine and at a few different ambient temperatures, before the HRSG is ordered! If it is a multiple-pressure unit, then such geometric and thermal performance data should be provided for each component in the high- and low-pressure sections. Without a clear performance and geometric data, one cannot evaluate the performance of the HRSG independently. Even if the U values are not given by the HRSG supplier, using the basic equation Q = UAΔT, one may compute the ΔT (from the gas, steam temperature data), and based on surface area and duty shown, compute the U value. Then one may adjust the U value for gas temperature and gas flow variations as follows (this method of adjusting the U value for off-design conditions is also discussed in Chapter 5 on simulation):

where F g is a factor depending on gas properties = (C p 0.33 k 0.67 /µ 0.32 ). Subscripts f and d refer to field and design conditions. Gas properties are computed at the average gas temperature of each section as finned tubes are typically used. W refers to gas flow in mass units.

TABLE 4.8

HRSG Performance Data

Gas Flow = 110,000 kg/h Design Case

Process Data Surface

Gas temp. in, ±5°C

Gas temp. out, ±5°C

Gas spht., kcal/kg °C

52.39 30.96 39.11 LMTD, °F

U, kcal/m 2 h °C

Duty, MM kcal/h

Surface area, m 2 268

Gas press. drop, mm wc 16.53 87.42 41.82 Foul factor, gas

0.0004 Steam Side

Steam pres, kg/cm 2 a 41 42 42

15,773 Fluid temp. in, °C

Steam flow, kg/h

Fluid temp. out, ±5°C

245 Press. drop, kg/cm 2 1.5 0.0 0.6

Foul factor, fluid

0.0002 Spray, kg/h

Gas analysis: % volume CO 2 = 3, H 2 O = 7, N 2 = 75, O 2 = 15.

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

If U is not provided, the duty Q and the temperature profiles will be known from which ΔT and U may be estimated for each section such as superheater, evaporator, and economizer. Let us assume that the customer has provided them as shown in Tables 4.7 and 4.8 in our case. One may also use the method discussed in Appendix E for finned tubes and evaluate U for each section and check the values provided by the HRSG supplier. They also should be aware of the fact that fin geometry influences U significantly.

Let us estimate the exhaust gas flow from the inlet and exit gas temperatures and the steam parameters, which are presumed to be more accurately measured than the gas flow.

The total energy absorbed by steam = 11,000 × (792.4 − 120.9) = 7.38 × 10 6 kcal/h (792.4 and

120.9 kcal/kg refer to the enthalpy of superheated steam and feed water, and blowdown was neglected). Gas specific heat at average gas temperature of (500 + 190)/2 = 345°C is 0.266 from Table F.6. Hence, exhaust gas flow = 7,380,000/[0.266 × (500 − 190)× 0.99] = 90,400 kg/h (0.99 refers to casing heat loss of 1%). As discussed in Chapter 6, one may estimate the casing heat loss based on provided insulation thickness; however, for this exercise, 1% heat loss is a good assumption.

Since some variations in exhaust gas temperature profile are present, one may apply

correction factor F g while estimating U from the design data. (The accurate method is to

develop a program that estimates U using the detailed procedure discussed in Appendix E on finned tubes after computing the inside and outside heat transfer coefficients and foul-

ing factors.) F g at average gas temperature of 366°C in design case is 0.4143 and at oper-

ating case is 0.4112. Hence, a correction of (0.4112/0.4143) = 0.992 may be applied to U; however, for this exercise, we are not applying temperature correction factor as the main purpose is to show how much steam the HRSG should be making and see if the field data are reasonable. Since the steam flow is significantly lower, the superheater, U, may be

reduced by a factor (11,000/15,300) 0.15 = 0.952 (see Chapter 5 or Ref. [1]). For superheater, U = 52.39 × 0.952 × (90,400/110,000) 0.65 = 43.9 kcal/m 2 h °C

For evaporator, U = 30.96 × (90,400/110,000) 0.65 = 27.25 kcal/m 2 h °C For economizer, U = 39.11 × (90,400/110,000) 0.65 = 34.43 kcal/m 2 h °C Superheater Performance

One may refer to the NTU method discussed in Appendix A. The specific heat of steam between 442°C and saturation of 245°C is (792.4 − 669.3)/(443 − 245) = 0.622 kcal/kg °C (specific heat is enthalpy difference divided by temperature difference). The specific heat of exhaust gas may be taken as 0.273 in the superheater based on the exhaust gas analysis and data in Appendix F on gas properties (and can be checked in the second iteration). C max = 90,400 × 0.99 × 0.273 = 24,432. C min = 11,000 × 0.622 = 6,853.

C = C min = 6853 = . .( 0 28 1 − C ) = . 0 72 C max 24432

{ 1 −− exp ( − . 1 717 0 72 × ∈= . = ) }

 1 − exp NTU { − ×− ( 1 C ) } 

C exp NTU { − ×− ( 1 C ) }  ( − 1 0 28 . × exp ( − . 1 717 0 72 × . ) )

The Q = 0.772 × 6823 × (500 − 245) = 1.349 × 10 6 kcal/h

Waste Heat Boilers 203

Exit gas temperature of superheater = 1,349,000/90,400/0.99/0.273 = 444°C. Exit steam temperature = 245 + (1,349,000/6,823) = 442°C

Evaporator Performance See Appendix A for evaporator performance evaluation.

HRSG Performance and Evaluating Field Data