How to determine maintenance requirements in steam turbines

July 11 2013 - William E. Forsthoffer

Condensing and extraction steam turbine efficiency determination requires the calculation of the driven equipment power. This calculation can be inaccurate due to the many variables involved. Installing a torquemeter during the project phase will ensure accurate efficiency calculations for all condensing and extraction condensing steam turbines.

Efficiency calculations will be erroneous and non-conclusive in determining maintenance requirements when torquemeters are not installed. Torquemeters can be installed in existing trains but will require rotor response study, torsional study, and coupling and coupling guard modifications. The installation of torquemeters has been recommended as part of project specifications since 1995. This has resulted in the accurate determination of critical (un-spared) steam turbine maintenance to be carried out during turnarounds only and during planned operation.

How is energy extracted from steam? Power output is determined by the following factors in a steam turbine:

  •   The energy available from the vapor
  •   The external efficiency of the turbine
  •   The steam flow rate

The energy available from the vapor is determined by the steam conditions of the particular application i.e. the pressures and temperatures at the turbine inlet and exhaust flanges. Steam conditions determine the energy available per pound of steam. Frequently, proper attention is not paid to maintaining proper steam conditions at the flanges of a steam turbine. Failure to do this will affect power produced, and can cause mechanical damage to the turbine internals resulting from blade erosion and/or corrosion.

Turbine efficiency

It is relatively easy to determine the efficiency of any operating turbine in the field if exhaust conditions are superheated. All that is required are calibrated pressure and temperature gauges on the inlet and discharge, and a Mollier diagram or steam tables. However, for turbines with saturated exhaust conditions, the actual exhaust condition cannot be easily determined. This is because the percentage of moisture must be known. Instruments (calorimeters) are available, but results are not always accurate.

Performance testing

When purchasing large steam turbines that do not use proven components, it is not cost effective to performance test the turbine prior to field installation. If the turbine does not meet predicted output horsepower values, the field modifications will be lengthy and costly in terms of lost product revenue resulting from reduced output horsepower. In some cases, the output power predicted may never be attained.

Closing hand valves for low horsepower loads increases the efficiency of the turbine. Hand valves are not modulating – they are either fully open or fully closed. Throttling a hand valve will destroy the valve seat and may damage the valve stem, thus rendering it immovable. Normally, hand valves are manually actuated, however, modern electronic governor systems provide outputs to open or close hand valves based on power requirements.

When selecting an extraction turbine, care must be taken to be sure the turbine produces the horsepower required during the start-up of the process. The cost of an extraction steam turbine can be significantly reduced if the size of the exhaust section (LP steam section) is reduced. Usually, extraction turbines are sized to only provide the process start-up horsepower with zero percent extraction. These values may be as low as 50-60 percent of full load horsepower.