In Part 2 we will look a the exit of the compressor through the diffuser access ports and we will also review the combustion chamber insprection requirements.

Just aft of the compressor is the diffuser. It has four borescope plugs, giving access to the rear of the compressor exit guide vanes.

You may have to remove some of the thermal blancket around the turbine to gain access to the ports.

A 6 mm flexible borescope is used for this inspection. The 6 mm flexible borescope is the most commonly used, because it has a good light source, while being small enough to get into most areas.

The smaller diameter scopes are great getting into tight spots, but if used in a large area such as the combustion chamber, you will not be able to see very much until the tip is very close to the item.

Any metal parts liberated from an earlier stage of compression, or if some solid item enters the front of the turbine, it will pass this last stage vane. If the part has enough mass it will dislodge other parts of blades and vanes along the way. This can end in what is known as corn cobbing. It is common when carrying out a quick inspection of the turbine, to check this location, as if anything serious happened to any of the compressor blades, you will see some damage at this location.

Combustion chamber

Moving back along the turbine we have the combustion chamber.

There are obvious differences between the standard combustion and SoLoNOx combustion chambers, however the general defects you will come across are similar. In fact the type of issues discussed here are typical of any gas turbine.

There are fourteen SoLoNOx injectors, or twenty one standard injectors fitted to the Mars. It is important that the injectors have their location marked when removing, to ensure they are replaced in the same location.

The condition of each injecter needs to be documented for the report. The condition of the injectors will depend much on the cleanliness of the fuel used on site.

If you are using liquid fuel there should be ongoing control, of the supply and storage of the fuel. Liquid fuel degrades the turbine hot section quicker that gas fuel.

Contaminants in the fuel will be seen on the injector and the combustion chamber and hot section. These normally discolour the metal where it is deposited.

If the fuel supply is coming from a wellhead or production area you can get all sorts of contamination in your fuel. Pipline gas for example is ideal, but not always available.

Because of the small diameter orifices in injectors they often block up, especailly the SoLoNOx injectors. This will have an effect on the flow to that injector, often supplying less fuel than the other injectors. If this condition is identified you will need to clean or change the effected injectors. The T5 spread profile will change and is the primary indicator of injector problems. We will discuss this shortly.

A common proble with gas fuel is sulfur in the fuel. This can be very difficult to remove as it often passes through filters before “coming out of the gas” as the gas expands entering the combustion champer. This deposit of sulfur can effect the movement of fuel valves. Sulfur when mixed with other chemicals will casue hot corrosion which will be describel later.

Removal of the injectors is the primary access, to carry out a borescope inspection of the combustion chamber, and first stage nozzle guide vanes.
There are also four borescope plugs that can be removed from the combustion casing. These can be used for a preliminary inspection if need be, but the main inspection is through the injector mounting location.

Access to the combustion chamber:

We will normally access the combustion chamber via the fuel injectors.

Here we see a diagram of one of the guide tube field tools that are used to guide the borescope right to the first stage nozzle guide vanes.

A shorter guide tube is also available which allows the external inspection of the combustor, liners and baffles that protect the combustion casing from high temperatures by directing layers of relatively cooler air to protect it.

The principle guide tube, bolts to the injector mounting holes. A copper pipe-bending-spring is then passed through the guide tube, and finally the borescope flexible lead is passed through the spring. The spring bends going around the ninety degree turn entering the combustor, and internally guides the borescope lead to exit in the correct direction of the nozzle guide vanes.

This makes it much easier for inexperienced technicians to carry out the inspection.

We are now going to discuss the damage commonly found in the Combustion Chamber:

Fretting due to combustor vibration.

Fretting is when two or more pieces of material slide over one another repetitively causing wear.

Because of the need for the combustion chamber to expand and contract, it is hard to avoid surfaces where one slides over the other.

There will always be a certain amount of vibration present in the combustion chamber due to combustion pressure pulses.

And these pressure pulses can become a major concern during SoLoNOx operation, when the flame is running very lean.

These pressure pulses on newer Mars untis are monitored using a system called “Burner Accoustic Monitoring (BAM for short)”.

Cracking:

Cracking is a common problem on turbines with high operating hours.

The cycle of expansion and contraction of metals, vibration, stresses from distortion, erosion of surfaces make cracking quite common.

There are no published limits from Solar regarding allowable tolerances with respect to cracks.

Any cracking of structural material is a cause for immediate action, whereas cracking of the annulus and heat shields are less critical.

The length and location of the cracking should be recorded and then seek the opinion of Solar. The recommendation may be for a reduced time interval between inspections.

The danger with cracking is the possibility of a flame pattern change as the cooling flow of air may be effected. There is the added obvious danger of metal being liberated into the turbine.

Bulges and deformations.
Bulges and deformations are often caused by the combustor metal not being able to expand and contract normally. Once this happens the airflow is disrupted, which can lead to other issues such as local overheating, cracking and material liberation.

Erosion.
Erosion often eats away the leading edges of blades and vanes. The particles which cause the erosion flow along with the gases, and impinge on the metal surfaces, eroding it. This can be material such as salts, sand, pollution etc, entering the air intake. The erosion can be from soot and coke, the result of fuel quality. Or from particles liberated from thermal coatings.

Burning.
Burning is often caused by the combustor flame not being kept in the center of the combustor. The fuel exiting the injector is swirled and mixed with air, to improve the mixing of the two, and to help keep the flame in the center. If any of the injectors becomes partially blocked or is damage, it will not be able to provide a proper mixing of the air and fuel. This has the effect of heating some areas more than others. It will cause buckling and burning of the combustor metals, and hot spots on the nozzle guide vanes.

Sulfidation – is not so much an issue in the combustion area so it will be covered in the turbine section.

Injectors

Just as with the spark plug of an internal combustion engine, the injector condition can give you much information, as to what is happening inside your combustion chamber.

All injectors are not equal, and you will find that SoLoNOx injectors are much more complex components, than the basic gas or liquid injector. They are more likely to crack, erode, overheat, form blockages etc.

Technology has made amazing advances, but it often comes at a price. Solonox injectors are costly and complex items to manufacture, and will not tolerate poor quality fuel or air.

It is difficult to clean injectors that are very dirty. The high temperature in this location burns anything inside the injector flow orifices. Sometimes the only way to clean the injector is to heat the injectors to a very high temperature in a special oven, that converts the dirt into dust. At the same time the oven injects gases through the injector, to remove the dust. The injector has to be heat treated after this procedure. The injector is normally flow tested after this process, to ensure it has not changed significantly and is within specification.

If the fuel being supplied is not very good quality, you might consider an additional inspection, at less than 4000 hours. This could be done by removing a few injectors, to allow access with a borescope, to inspect the rest in place.

While not a part of the borescope inspection, the T5 profile gives an early warning to pending issues in the hot section.

The T5 measurement for each thermocouple is monitored for deviation, from its normal profile. Changes to the profile means a possible problem upstream.

For example if one injector is partially blocked, due to dirt in the fuel. Some of that fuel, will then be diverted to the other injectors, which will in turn, run a little hotter and the blocked injector will run cooler.

As well as seeing changes to the temperature profile, the flame at that partially blocked injector, may move position. This may cause localized heating of a casings etc, with its knock-on effects. This can be seen as a shadow or change of color on the combustor.

The way the T5 profile works, is that when new, the temperature profile is recorded on a graph, similar to the one in the presentation. This is the, “as new” condition, which is typically when the turbine is new, or just overhauled. What you are subsequently looking for is a change from the, “as new” profile. Sometimes the change in profile is understandable, such as when the injectors are replaced.