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Dirt in gas turbines

In this bit deeper article, we will discuss the problems that gas turbines have due to dirt, since one of the less known uses of our brushes is the cleaning of these types of machines, using grinders or other similar tools.
Gas turbines – both the ones used in generation and the ones used for jet propulsion – are a type of machine particularly sensitive to dust and other types of dirt.

We will also mention the alternatives for cleaning the turbines and what role industrial brushing plays in this type of machinery.

Basically, the operation of a turbine is as follows:

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30 MW General Electric turbine, used on the Queen Mary 2 vessel. Source: DSIC Marine

– The cold air enters the turbine and increases its pressure in the compressor.
-In the combustion chamber the fuel is burned, raising the temperature of the air when mixed with the hot gas.
-The hot air passes through the stages of power generation (turbine), moving the shaft while reducing its pressure. This shaft movement is used to generate power and also to power the compressor.

The dirt in the turbines can significantly degrade their efficiency and capacity, creating a number of important problems.

The compressor has a series of small holes that help with the cooling. These holes can be clogged by dust by increasing the inlet temperatures and reducing the efficiency of the assembly. In a turbine for generation less energy would be generated, while in a propulsion such as an airplane, more fuel would be consumed to move at cruising speed. In addition, high temperatures can end up damaging the compressor blades.

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Small holes used for film cooling.

Oil leaks can mix with the dust and create a paste that produces truly problematic obstructions. Possibly the most common point of conflict are the compressor bearings, both because of the relatively low temperatures (insufficient to evaporate the oil) and because of the proximity of the cooling holes mentioned above. In the A-10 aircraft this problem happened due to the propellant of the machine guns, which when used was ingested by the turbines (located behind the weapon) and when mixed with the dust of the desert in flights at low altitude caused obstructions in the compressor.

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A-10 Thunderbolt II plane firing. The propellant used to fire the machine gun is vaporized and ingested by the turbines.

Dirt in gas turbines

In the previous part we have commented the obstructions in the compressor due to dust, but mineral deposits can also form. This is common when turbines are used in environments with dust and/or sand, forcing to use a very complex and expensive filtration systems. One of the best known cases is that of the M1 Abrams tanks of the American army, which uses a Honeywell AGT1500 turbine as an engine.

Although turbines are theoretically more reliable than diesel engines by having fewer moving parts, use in the desert means that any problem in the intake air filters quickly translates into turbine failures, something that has caused the M1A1 recieve numerous criticisms for the chosen propulsion.

Dust and dirt deposits can also cause imbalances in the turbine and compressor blades, which would cause increased vibrations when turning.These vibrations wear the bearings prematurely and raise the cost of maintenance, not to mention that the breakage of the bearings could cause a catastrophic failure of the turbine and stop it completely.
Another of the possible problems is that the dirt can be embedded in the root of the blades, moving them slightly and causing them to operate in a different position from the design. This would increase tensions in that area, contributing to the wear of the blades. In addition, flow problems occur when changing the geometry of the turbines. This problem is typically caused by cement dust, coal, and fly ash.
Erosion problems can also occur when there is dirt in the turbines: as they spin at very high speeds, the particles that are stuck can get laid off. The shocks that produce these small particles can end up eroding and wearing down the blades.

This would modify its profile, spoiling its aerodynamic behavior, in addition to reducing its resistance when removing material. Changes in the natural frequency of the blades due to erosion have also been observed, something very critical because this frequency must always be known: the vibrations at the natural frequency can destroy machines in a matter of seconds if they enter resonance.

The turbines are precision machines, in which despite being designed to be very resistant, the maintenance is critical. In addition to the reliability problems that may arise from dirt, the impact on performance is very noticeable. To get an idea of the magnitude of this impact, in a small gas turbine, a layer of dirt of 0.1 millimeters in the blades can reduce the flow by 10% due to the turbulence it causes, also negatively affecting performance of the compressor by 5%.

Alternatives for cleaning the turbines

and what role does industrial brushing play in this type of machinery.

As we mentioned before, the dirt in the gas turbines is guilty of many problems, which can be avoided with proper maintenance. Regular cleaning of the turbines is vital as part of maintenance work. The biggest challenge of this cleaning is the huge amount of holes, which requires that any cleaning instrument that is used is able to enter the small holes that dust can clog, effectively limiting the instruments that can be used to those based in water, air, or brushes with fine hairs.

The simplest way is the air pressure cleaning. In the same way as cleaning electronic components in which the biggest problem is the dust and that have small holes in which it can be deposited, air compressors are also used to clean turbines. The problem with this method is that if there is dirt formed by dust mixed with oil or something that is very embedded, the pressurized air may be insufficient. In this case, water is used.

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Siemens Class H turbine. The large number of small holes and holes can be observed.

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Equipment for pressure washing of gas turbines.

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Grinder Brush

Pressurized water is another method to clean gas turbines which is very similar to cleaning with air. Both are based on a pressurized fluid that carries dirt particles from the turbine. Water, however, has a greater cleaning power than air, of course. This method has a major problem: the steam in the turbine can obstruct the flow and cause a malfunction, so (although it is possible) it is not usually done when the machinery is turned on. In turbines for generation, the machine is usually stopped for eight or ten hours, with the economic inconvenience that this implies.

The brushes are usually used for hand cleaning of small areas, given the good portability of a grinder with a brush installed. It has the advantage of being able to eliminate the dirt embedded by the abrasive power of these brushes, which are usually made of high-strength steel forming very fine wires. The problem with this cleaning method is that it is not recommended for cleaning complete turbines but only for localized areas. The reason is that abrasion of the brush can produce long-term negative effects on the operation of the turbine, so it is only recommended to remove the most difficult dirt that cannot be cleaned with a less aggressive methods such as air.

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