DRY ICE BLASTING ON HEAVY

DUTY GAS TURBINES

Pars palayesh co.

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09128490452افشین مىمنی پىر : واحد بازرگانی

The Gas Turbine

Gas turbines use the hot gas produced by burning a fuel to drive a turbine. They are also called combustion turbines or combustion gas turbines. The main components of a gas turbine are an air compressor, several combustors (also called burners) and a turbine. The air compressor compresses the inlet air (raises its pressure). Fuel is mixed with the high pressure air in burners and burnt in special chambers called combustors. The hot pressurised gas coming out of the combustors is at very high temperature (up to 1350° C). This gas then passes through a turbine, giving the turbine energy to spin and do work, such as turn a generator to produce electricity.

Maintenance

The gas turbine has several categories of maintenance. These Maintenance are scheduled

based on operation hours (EOH). Most of the components at the hot section are changed

at 8000 EOH intervals by stages. However, during every 40,000 hours, the gas turbine

will be scheduled to a major overhaul which last about 20 to 30 days. During this time,

removal of contamination and cleaning works will take up 5 to 7 days of their schedule.

For this, the use of cryogenic cleaning is much effective to remove contamination from

the components.

Components which can be cleaned with Dry Ice Cleaning

1. Compressor Blades

2. Compressor Casings

3. Seals

4. Heavy duty Bolts & Dowels

5. Gears (Gearbox)

6. Auxiliary Components

Compressor Blades

The Compressor blades is divided in 2 parts:

• Stationary Blades (attached to the casings)

• Rotating Blades (attached to the Rotor Body)

Types of contamination usually found - fowling, soot, hardened carbon, oil & rust.

The contamination on the blades can cause power augmentation. Although online cleaning with detergent is done regularly, the last stages of the blades suffers from heavy deposit of dirt.

Bearing Housing & Seals

Compressor Blades (Rotating)

Compressor Blades (Stationary/Casing)

Turbine Blades

Bearing Seals & Casings

Casing

Types of Contamination usually found - fowling, soot, hardened carbon, oil & rust.

Bearing Housing and Seals

Types of Contamination usually found – heavy carbon deposit, oil and grease.

Compressor top half casing Bearing Housing

Photo shows before & after of labyrinth seals on bearing.

The grooves are difficult to clean by hand and with dry ice, it takes less than 15 minutes.

Before

After

Photo shows before & after of compressor blade. Please note the fowling. Traditional ways to remove fowling is by using scotch pads and thinners. This method takes approx 4 – 5 days to clean the blades.

Dry ice will remove the hardened carbon content and dirt. As for rust, it will only remove the hard face on the material.

Will not give the surface the cosmetic finishing, since dry ice cleaning is non abrasive.

Compressor Rotor Blades -

Before

Compressor Rotor Blades -

After

Cleaning with Dry Ice

Compressor Blades

The operator needs to be advised on the type of blades he is cleaning. There are coated blades and un-coated steel blades. On coated blades, it’s much easier to remove the contamination; however, the operator needs to be advised to use non aggressive cleaning method of cleaning.

Coated blades

Nozzle to use: Ice Splitter + 50mm Flat or 8mm round nozzle

Pressure @ Ice Flow Rate: 6-7 bar @ 30kg/hr

Uncoated Steel Blades

Nozzle to use: Ice Splitter + 50mm Flat or 8mm round nozzle

Pressure @ Ice Flow Rate: 8 – 10 bar @ 30-40kg/hr Turbine Casing. The casings are divided on to 2. Top half and bottom half. The top half is usually removed during any major maintenance work and the bottom half stays in place. Once the rotors are removed, this will expose the bottom half which most rectification and maintenance done in-situ. Since cleaning is a part of any major maintenance, dry ice cleaning is excellent to be included on their routine shutdown. Use same combination as blades.

Cleaning the Generator

Cleaning Method

For the cleaning of large Turbo Generators, a typical size between (100 –270MW). Requirements 1. Machine: Either a KG50, KG30 or KG12 Dry Ice Cleaning Machine 2. Compressed Air Supply : Clean and Dry Air (5-6 bar and 195cfm) 3. Dry Ice: Stator =400kg / Rotor =100kg Nozzle Configuration: Dry ice consumption is set to 30kg/hr. Use the ice splitter together with the flat nozzle to clean the larger surface area on stator/rotor. Replace the flat nozzle with a 8mm nozzle to clean the cooling holes inside the stator core. Type of contamination: Usually, carbon, grease, oil or soot.

Introduction Dry Ice Cleaning on electrical windings is a non-intrusive, non-damaging and efficient cleaning procedure for rotating electrical machines. No solvents and no organic materials are used which could lead to aging or damage or residues.

Key Advantages

• Faster cleaning time compared to using solvents • No organic residues in inaccessible locations. • Partially disassembled machines can be cleaned as well • Improvement of insulation resistance • Reduces Partial Discharge values on slots & endwinding

• Eliminated Rotor Earth Fault due to contamination.

Proven Effect of Cleaning Measures Partial discharge testing before and after cleaning proves the cleaning efficiency:

Effects on Insulation Resistance (IR) Test Subject : General Electric DC Motor 4MW, 700V, 5800 Amps Excitation Volts: 125V 150/375 rpm

Insulation Resistance Value (Measured after 1 min)

Before Dry Ice Cleaning

After Dry Ice Cleaning

Main Field 600 Kilo Ohms 4,500 Mega Ohms

Interpoles & Comp Windings 850 Kilo Ohms 70,000 Mega Ohms

Armature 850 Kilo Ohms 2 Mega Ohms

1 2

3 4

1. Kinetic effect: Dry ice pellets make impact on the layer provoking fractures

Thermal effect: dry ice penetrates and generates a sudden local temperature decrease on both the dirt layer and the substrate. The different materials contract unequally so that the adherence between them diminishes. The layer cracks even more and begins to loosen off the substrate

Sublimation: after making impact, the dry ice turns into gas, expanding its volume by a factor of 800. The expansion acts as an explosion that detaches completely the layer from the substrate.

The substrate remains clean and completely intact.

BLASTING EFFECTS

THE PROCES – DRY ICE BLASTING

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