Industrial Gas Turbines

Typical task on site is inspection work already mentioned in the previous Chapter 4.1. The operator needs to carry out maintenance work such as cleaning and /or washing of the compressor ( "Ill. 4.2-1.1"), according to the manufacturer’s (OEM) instructions. Precisely in this process, some things have to be considered. If the compressor shows a clear deterioration (see manufacturer’s details) not connected to a malfunctioning of the bleed valves, a visual examination of the inlet area should be undertaken. If there are signs of oil residue, the source of this contamination should be detected.

If found, one must provide remedies. It is important to keep in mind, that modern compressors are already strongly influenced by impurities that are not recognizable at first sight (e.g., oil film). An examination with clean paper can be helpful. When in doubt, if the manufacturer does not suggest otherwise, a cleaning of the compressor should be undertaken. A modern computer aided engine monitoring (gas path analysis, Chapter 5.1) allows to identify the optimal point of time to wash the engine depending on the compressor data ( "Ill. 4.2-1.2").

The cleaning of the compressor ( "Ill. 4.2-1.1" and "Ill. 4.2-2") can follow in different ways, but always with reference to manufacturer’s instructions. The kind of cleaning with abrasive dry materials and/ or liquid wash solutions depends on the type and consistency of the impurities and deposits to be removed (Chapter 3.1.2.2). Accordingly, one decides the cleaning procedure and the washing or cleaning material together with the amount and method of work. Compressors that run under especially unfavorable conditions must be more frequently cleaned. Here devices offered by the manufacturer (OEM) have to be available.

If the blades show dry, hard deposits, one cleans with light abrasive dry materials such as nut shells, rice shells or polish powders. During cleaning with abrasive materials, in case the manufacturer does not give any details, one has to definitely take care that no erosion sensitive coatings are damaged in the compressor ( "Ill. 3.1.2.4-4"). These are, e.g., organic paint on blades and casings soft abradables and unprotected Mg and Al materials. In case of doubt, ask the OEM.

For oily and sticky deposits one recommends liquid wash solutions. Even here, only permitted original media from the manufacturer should be used ( "Ill. 4.2-1.1"). Not only the direct main gas passage is to be considered, but also the air removals/bleeds and tubes belonging to it, measurement systems and units. If, the producer demands it, there are, e.g., bleed tubes in the lower compressor casing half, to be so treated that the wash solution can indeed emerge, but is not further transported by the tubes. If it is foreseen in the design, tubes are to be drained at fluid drainages. All drainage tubes are to be opened according to manufacturers instructions. If demanded, openings for pyrometer, fire detectors or similar probes are to be sealed.

As usual, the wash process starts with a sufficient water cleaning of 5 to 10 minutes at low compressor speeds. This wash process must balance the drying effect of the air current. One has to pay attention that all air meeting surfaces are entirely wet. Here, variable inlet guide vanes must be entirely opened. The injection of the aqueous cleaning solution follows with a suitable device in the area of the blade roots during the slow down of the rotor, up to a speed with which the cleaning solution cannot be transported through the entire compressor. Finally, a sufficient soaking time during static makes possible that the stuck coatings soften and loosen. Those can be removed during low speed, through a further double- so- long water flushing, together with the cleaning solution remnants. An examination of the emitted water impurities, e.g., can throw light on the origin and cause. Aimed measures against the causes of air impurities are made possible. After washing, the engine should run further for 20 minutes in order to dry. An especially good cleaning effect can be reached through a follow up cleaning process with dry abrasive media during the first operation with low power.

Depending on conditions such as accessibility, size of engine and available infrastructure, extensive maintenance and repair work is carried out on site by the technicians of the manufacturer or a firm recommended by him. Here, however, it is necessary that the operator himself or a technical consultant is present. It is important that the operator forms his own impression of the condition of the engine. According to experience, decisions with a certain margin are necessary. He is then questioned on limited further usage of operation influenced components.

One assumes the successful cooperation of manufacturer, operator and consultant. This is even more feasible the more the consultant is recognized by the manufacturer (OEM). As overhaul locally often does not have the optimum conditions (e.g., accessibility) for personnel, one has to proceed with care, thoroughness and expertise.