by Sanjeev Jolly, P.E. – Senior Engineer, Engineering Services, NAES Corporation
A number of conditions can cause fouling in a heat recovery steam generator (HRSG): deposits plugging the finned tubes; sulfur in the fuel reacting with ammonia used for NOx control, causing salts to form; and corrosion resulting from tube leakage, acid and water dew point condensation.
An HRSG with fouling directly impacts plant performance in two ways: it increases exhaust gas side-pressure drop, which reduces gas turbine (GT) performance; and it decreases steam production and temperature, which reduces the steam turbine output. According to GE bulletin 3567-H, every 4-inch increase in exhaust gas side-pressure drop reduces GT output by 0.42 percent, increases heat rate by 0.45 percent and boosts stack temperature for Frame 7EA gas turbines by 1.9° F. This does not include loss of steam turbine output caused by the decrease in steam production and temperature in the HRSG.
The majority of this degradation can be mitigated by cleaning the HRSG tubes. Since most of the deposits occur at the back end of the boiler, this discussion will focus on cleaning that area to restore HRSG performance and reduce the gas side-pressure drop. (In rare cases, deposits can also occur at the front end if the tubes become plugged by loose insulation.)
HRSG Cleaning Methods
Several approaches are used to clean HRSG tubes: high-pressure water jets, chemicals, CO2 ice blasting and pressure-wave cleaning. Water-jet cleaning unfortunately introduces water into the liner panels and wets the insulation; it can also cause rusting of the carbon-steel liner panels. The major drawback with chemical cleaning is having to dispose of the chemicals afterwards. The other two methods are therefore used most commonly these days.
CO2 Ice Blast Cleaning
With this approach, frozen CO2 (dry ice) pellets are blasted against the fouled surfaces using a high-pressure compressor. The ice blast sublimates as it goes directly from a solid to a gaseous state. This rapid expansion of the CO2 – to 750-800 times its initial volume — loosens the debris, which is then removed with a vacuum device. Scaffolding is required within the unit to access the full height of the tubes and for bore scope inspection after the cleaning. To access the full depth of all tubes in a module, special spreading tools are used for tube spreading (figure 1a). The modules are accessed from both side to ensure cleaning of both the front and back sides of the tubes (figure 1d). If necessary, a chemical solution is used after the blasting to loosen any stubborn deposits that remain.
Fig. 1: a) tube spreading; b) before blasting; c) after blasting; and d) modules accessed from both sides for front-and-back tube cleaning. (Photos courtesy of PIC)
Pressure Wave Cleaning
This approach was developed by BANG&CLEAN Technologies AG of Switzerland in 2001 and is licensed in the United States by GE. A ball filled with a combustible mixture is ignited near the tubes to be cleaned. The pressure wave and vibration shake the tubes, loosening the debris, which then falls to the floor. Since a series of explosions – 100 or more – is used to complete the process, it is remotely controlled to prevent injuries.
The photos in Figure 2 below – taken during a recent pressure-wave cleaning at a NAES-operated facility – show the condition of tubes before and after cleaning, and the Figure 3 photos show the amount of debris collected after 100 shots. (The term ‘shots’ refers to the process of repeatedly exploding the mixture in the ball to generate pressure waves and shake the tubes.) As a result of the cleaning, the exhaust gas side-pressure drops decreased by 1.8 inwc (4.5 mbar) and 1.73 inwc (4.3 mbar) for the two HRSGs, respectively.
Figure 2: a) baffles removed; b) tubes before cleaning; c) tubes after cleaning; and d) close-up of tubes after cleaning.
Figure 3: a) debris fallen to the floor; b) 8.5 inches of debris after 100 shots.
Impact on GT Performance
The compressor discharge pressure and flow are not affected by the exhaust gas pressure loss. The expander pressure ratio goes down because the inlet pressure does not change, and the exhaust pressure has gone up. The decrease in pressure ratio causes a decrease in temperature ratio, which causes the exhaust temperature to increase.
If we assume an ideal gas relationship:
where T1 is the turbine inlet temperature, T2 is the exhaust gas temperature, P1 is the shell pressure and P2 is the exhaust gas pressure,
then the work done by the expander = mgas Cp (T1-T2 ) Equation 2
If the HRSG tubes are plugged, the exhaust Δp goes up, so P2 goes up. P1 does not change, so T1/T2 goes down. T1 does not change, so T2 increases. Therefore, the output goes down because (T1-T2) in Equation 2 decreases. The mass flow does not change.
After the tubes are cleaned, the exhaust Δp goes down, so P2 decreases. If T1 remains the same, T2 will decrease. In some rare cases, the exhaust gas temperature control may require some adjustment, especially if the exhaust temperature has gone down after cleaning. The GT manufacturers often calculate the turbine inlet temperature (T1 above) separately as a precautionary measure. This ensures that this temperature remains below the design temperature to protect the hot gas path components, even though the exhaust temperature control curve itself is representative of turbine inlet temperature.
We get the same deduction if we introduce expander efficiency into the equation:
HRSG cleaning is an effective way to restore lost performance because it reduces the back-pressure on the gas turbine, thereby increasing the net power output. For the example above, the net increase in the GT power output for one 170MW turbine was 1 MW. However, not all of this resulted from the HRSG cleaning; some of it can be attributed to the offline pressure water wash. As for the other turbine, the output gains are not obvious because of other modifications performed on the GT during the outage. The results for this GT are still being discussed with the OEM.