Dehumidifiers for Thermal and Hydraulic Plants

With the boom in renewable energy, conventional power generation plants began to have very long downtimes. On seeing the problems faced by power plants, we began to develop high precision equipment to achieve maximum product quality.

FISAIR works with large turbine companies, equipment companies and owners of power plants to achieve the optimum product to be used in each process, and has thus acquired great experience in these types of products.

For more information contact our experts.

Power generation plants are very subject to corrosion during a shutdown. If the appropriate conservation measures are not taken, corrosion can cause great damage to different components in a system and greatly disrupt the start-up.

Due to the importance of the problem, great efforts have been made to find appropriate solutions. Different existing methods have been compared, and the dry air method, using drying rotor dehumidifiers, has proven to be the most effective.

Advantages of the Dry Air Method with Dehumidifiers for Preservation of Thermal and Hydraulic Plants

The dry air method has replaced other previous methods, as it is the simplest, cheapest and fastest.

One advantage of the dry air method over others is the simplicity of its control, as it can be carried out with a simple hygrostat.

Prevent Corrosion and Plant Shutdowns with Fisair Dehumidifiers

When using the dry air method with dehumidifiers external environment, very good results are obtained:

01. It is easy to check if there is a risk of corrosion by simply checking the humidity in the air vents.

02. Very good in humid climates.

03. It allows acces to the conserved parts, while protection is performed.

04. Removes small areas of trapped water quickly (in a matter of hours).

05. It requires less energy than the heating method, and does not require the dangerous or expensive chemicals needed in wet conservation.

06. Water pockets or conservation failures are detected quickly.

07. Chemical products are not used.

08. Cleaning intervals increase from 3 to 12 years, when dry air circulation is introduced in existing power plants.

09. Simple and effective.

10. Avoids costly repairs and lack of availability due to the sudden entry of moisture.

Fisair Specializes in Equipment Preservation during Plant Shutdowns

The Main Goal in Power Plants is to Prevent Corrosion Both in Operational and Non-operational Times. Mainly Non-operational.

At a high relative humidity of 60-100% corrosion rates are 100-2.000 times higher than at moisture values than 30%.

Equipment to be Preserved in Plants

HRSG water/steam side

This is the most complicated system as it requires a precise procedure to ensure all equipment and pipes subject to water and/or steam are emptied and drained before introducing dry air through them. You should realise that, as long as pockets of water remain, the relative humidity will not fall.

Due to the large number of valves operated in a position different to that of Operation and equipment needing to be drained, a system needs to spend a lot of time being conserved for a quick start-up.
This requires a very accurate conservation procedure to minimise time.
The established procedure prepares the equipment for conservation at the same time or staggered to facilitate operation. At the end of the procedure, all the equipment and subsystems have been communicated with.
Dehumidifiers are used interchangeably for several subsystems and equipment to be conserved. This is done by providing them with short individual runs to allow this versatility.
Sections furthest from the dehumidifier in each circuit, or in the upper part of the equipment in conservation, should have a continuous “escape” of dry air.
The humidity of the air in conserved systems should be periodically measured to verify proper application.

– Relative humidity should be kept at 30%.
– The variation of absolute humidity with respect to the temperature difference with the outside should be taken into account when dimensioning.
– Operating in an open system (all outside air).
– Drive the air to the parts most sensitive to humidit.
– The air conditions must be measured at the outlet.

HRSG gas side

• Corrosion is due to the presence of sulfuric acid. This is less aggressive when the concentration increases, which it does upon dehumidifying the environment. According to the diagrams, the relative humidity required is 5%, although experience tells us that 20% is sufficient.

Basic operation: The main principle is to introduce dry air on the compressor side where it goes to the gas turbine, and from there it flows freely through the exhaust to the gas side of the boiler, which will be “bottled” (short, closed runs through the flue and manholes).

For effective conservation in this system, according to the environmental conditions of pressure, temperature and relative humidity, the equipment is supplied with 3,000-6,000 m3/h. FISAIR has them in its catalogue (DFRC). FISAIR will provide you with the optimal solution.

The air loss through the short flue runs is enough to keep the RH in the boiler and gas side at around 20-30%.

The conservation of this system can be approached from two locations:

Through the inlet plenum, by means of shields.
Through compressor bleed valves.

Steam turbines

The blades of the turbine are subject to high stress during operation and are designed for maximum efficiency. Attack when not under load comes in the form of general corrosion or pitting under humid conditions in the presence of aggressive ions, particularly chloride, with austenitic steel also being susceptible.

General corrosion decreases the efficiency when restarting and pitting on the roots of the blade can cause fracture with catastrophic results.

Dry air supplied to the turbine casing and discharged via the condenser eliminates these risks and also preserves the condenser; this, in turn, shortens the restart cleaning of the condensation system. This can save at least $200,000 per day for a 500 MW unit. Heating does not provide a good solution, as the moisture absorbed initially re-condenses when the air comes into contact with colder machine parts; thus re-establishing the danger of corrosion. To avoid this, the entire turbine has to be heated at considerable cost.

Gas turbines

Storing turbine/compressor units not under load in dry air is recommended to prevent potentially dangerous contamination of highly stressed alloy steel blades and to minimise corrosion of lower alloy components. Corrosion of the turbine shaft itself can cause some imbalance and the corroded blades heat unevenly, which leads to unacceptable vibration, especially during the normal fast start-up of gas turbines.

The proven method is to close the air inlet valve (shock absorber) and admit dry air, which is allowed to escape through the hot gas system.

Condenser and tanks

Equipment that needs to be coated should be dried and sometimes heated before surface preparation. For some tanks, especially square ones, heat alone is not enough, as corners are difficult to heat at the proper temperature.
The new flue-gas desulphurisation plants must be covered on site with materials that have to be applied under rigorously controlled humidity and temperature conditions to be successful. Failure of the coating causes carbon to accumulate, which causes moments of anxiety during operation and even close down, until the ramps are released to allow carbon to flow to the mills and boiler. For condenser water boxes, the drying equipment can dry a complete unit in a week, compared to the heat of a fan, which can easily take 2-3 weeks.

All that is needed to supply dry air around the volume of the tank here.

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Pipes

At each scheduled maintenance or shutdown of a power generation system, the pipes used for cooling water have to be dried. Water is removed from the system, but there is always some remaining water due to the structure of the pipes.

And this moisture, if left unchecked, causes corrosion inside the pipes. To dry this system, the piping volume must be known and a unit with the exact air flow selected.