Frequently asked questions and areas of concern that users have regarding this innovative and cost-effective power boosting fog technology.
Gas Turbine Applications
Not a single FOD problem has been experienced to date with MeeFogT Systems. Extensive safety wiring and analysis of the fog nozzle array for airflow-induced vibration ensures that the structure is strong and cannot break.
The control system automatically terminates fogging whenever there is any chance of inlet icing due to the static temperature depression that occurs in the bellmouth due to the acceleration of the air to mach number of about 0.5 for heavy-duty gas turbines and 0.8 for aeroderivatives.
This is only a concern on systems that are being fog intercooled (i.e. those in which a small quantity of overspray is allowed). In most cases the extent of the overspray is a little over the amount that is allowed for on-line water washing. Water injection into the compressor will cause a mismatching of the successive stages as the flow coefficients will increase and the design temperature rises will not be met. This will cause the points on the compressor map to move towards the surge line and increase the stage loading, possibly pushing the later stages nearer to stall. However, the small excessive mass flow rates envisioned have not been a problem in any unit to this date. We have some Frame 7 Units that are operating with heavy overspray for over one year. In general no surge related problems are experienced especially when overfogging by numbers such as 0.6 -1%. Normally, on-line water wash numbers are themselves in the region of 0.4-0.5% on most heavy-duty gas turbines. We do not foresee any problem unless excessive overfogging is done or large water slugs are being ingested into the compressor. Axial compressors have an adequate surge margin and unless the units are operating under frequency there should be an adequate surge margin unless there is severe deterioration of blade condition or tip solidity.
Intake Temperature Distortion
This is a concern as axial compressors have stringent temperature and pressure distortion criteria. The MeeFogT System is designed in multiple stages and each stage has manifolds distributed within the inlet duct to minimize intake distortion. To date no problems have occurred in this area.
Fouling and unloading of severely loaded filters has been promoted by high ambient humidity and ambient fog that often occurs during the nights and causes at times high filter delta P trips. However, if the air filtration system is working well, the increased humidity at the inlet itself does not increase fouling. Fouling is a situation that is so site specific that it is very difficult to predict the behavior at times if the No.1 bearing is leaking oil then this may combine with the high humidity to create some fouling. An important issue is to wash the silencers thoroughly to avoid dirt that has been accumulated here to be washed into the compressor by the fogging system. We also recommend a crank wash of the engine after operating with fog during start-up.
The droplet sizes are relatively small and CFD studies have shown that the flow will tend to follow the air stream. There is some issue of larger water particles forming on the trash screen and inlet cone of the gas turbine, but with proper design and drainage this can be minimized.
Corrosion In The Inlet Duct
The use of demin water can deteriorate inlet ducts that are already in a deteriorated state. The increased humidity is clearly a corrosion factor. With proper maintenance and painting, this problem can be mitigated significantly. The use of SS 316L as the duct material is gaining in popularity as life cycle studies have indicated that while the first cost is a little higher, the life cycle costs are significantly lower.
Compressor Coating Distress
Some gas turbines that have undergone overfogging have experienced coating distress in the first few stages of the axial flow compressor. In most cases, this can be minimized by careful location of fogging nozzles, avoidance of excessive water accumulation on ducts and inlet cones, and the use of appropriate drain systems at the nozzle manifold locations, intermediate ductwork, and at the floor of the bellmouth section of the gas turbine. In spite of these precautions, overspray will, over time, create some coating distress. In a few rare cases, there has been some coating distress on units that are undergoing evaporative fogging only.
There can be several causes of this situation:
- Excessive or improper use of overspray.
- Improper orientation of fog nozzles.
- Lack of drains or inappropriately located drains.
- Corrosive ambient conditions that will cause acidity and hence coating damage.
This is an issue that must be resolved by proper inlet air filtration, especially aggressive industrial environments.
- Combinations of the above.
In some cases, if some leading edge coating distress already exists prior to fogging then this would progress with fogging due to the reasons mentioned above.