Steam Turbine Maintenance Reliability and Service Life

Steam turbines offer great reliability, ease of maintenance, and long service life. Excluding consideration of the steam generation and distribution systems, steam turbines usually have relatively few maintenance requirements. This is due to simplicity of design, minimal moving parts, construction with durable materials, and operation at fairly low speeds. Turbines can operate for decades without requiring major overhaul and can last for 50 years with proper maintenance. When a high-pressure steam system is required for other purposes, steam turbines can be effectively applied with a relatively low incremental maintenance cost.

Causes of steam turbine performance deterioration include leakage, control damage, solid particle erosion, moisture erosion, steam path deposits, and foreign object damage. The quality of the turbine components and construction and the type of design are all important, particularly with respect to the ability of the blades to withstand the abuses of wet steam. More complex multi-staged turbines require greater care and attention than rugged single-stage turbines. Turbines designed with small clearances, as is common with reaction turbines, also require greater maintenance. Simple design produces greater reliability, less maintenance, and longer service life.

Routine procedures include monthly visual inspections that consist primarily of inspecting for oil or steam leaks and reviewing monitoring data. Any problem will show up in pressure and temperature readings or in measurement of vibrations.

Annual inspections and maintenance procedures include the following:

• Pull the casing to check the turbine wheel for blade wear

• Check nozzle alignment to ensure that steam is not cutting the turbine blades; re-balance and remove scale as needed

• Open packing box to check condition of the valve packing and spindle

• Inspect shaft seals for wear

• Check thrust bearing end play

• Inspect bearing clearance and end play

• Inspect gear tooth wear pattern, if applicable

• Check foundation

• Check and re-calibrate gauges

• Remove and clean steam strainer

• Drain water and clean foreign material from oil reservoir

• Drain oil from governor and flush clean, if applicable

• Drain small quantity of oil from system and conduct an oil analysis; change oil as needed and, if applicable, change filter element at same time

• Inspect grease in ball or roller bearing housings and renew if necessary

Premium turbines, with oil lubricated bearings, will typically run 3 to 5 years without requiring anything but the most minor adjustments: oil and some repacking. Provided there are no easily detected problems, such as blade wear, vibration, steam leaks, or contaminated oil, the turbine should continue to operate for years with no need for repair or lengthy interruption of operation.

As noted, steam quality is an important consideration for any turbine system. Moisture can erode the turbine blades, increasing maintenance requirements and decreasing efficiency.

Steam supply to the turbine always has some impurities in the form of various organic and inorganic compounds. As steam expansion proceeds and pressure and temperature decrease through the various stages, these impurities are likely to deposit on the turbine blades. This causes a reduction in flow area and a roughing of the passage walls, reducing capacity and efficiency.

Special design features are available to minimize erosion. Integral moisture separators or stainless steel moisture shields can be used. Steam purity indicators and modern day water purifiers can provide adequate means of control.

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