Once again, a failure has occurred on the steam turbine that drives a large compressor train in your plant.
Broken parts are everywhere. As the maintenance manager, you have been called to determine the root cause of the problem. The steam supply system has been problematic, so you immediately suspect the supply has been disrupted by the various systems that generate the steam. In the worst-case scenario, your plant has to import steam from outside the fence line to meet the demand. Because the unit tripped out on high vibration, it was too late to avoid the damage in the steam turbine. At the time of the failure, the entire unit was unstable, likely due to a thunderstorm in the area.
In your particular plant, steam comes from waste heat in the process, boilers, heat recovery steam generators (HRSG) and/or is imported. The utilities department handles the balance of steam within the plant, monitoring the quality of steam, the demand, as well as the conditions required for its release. During a normal day, the steam supply is fairly predictable. However, malfunctions within the system usually result in unplanned shifts in the steam supply. To keep the plant efficient and guard against such disruptions, many production facilities rely on using as much waste heat as possible from the process system to generate steam.
These conditions and challenges are common in many facilities. An approach to diagnosing and solving the problem may be as follows.
In determining the root cause, several factors must be considered. First, since the steam turbine failed, the steam conditions must be evaluated. Because the instrumentation is not available to measure the steam conditions, other data can be used to determine steam conditions at the time of the incident. In this case, the compressor data was available. A compressor performance study was conducted based on the actual process condition to determine the horsepower consumed.
Through a heat transfer analysis based on the flow conditions, an estimated steam turbine inlet temperature was calculated. Because there were many users on the system, each user had to be evaluated to determine the total flow conditions. Iterations based on the heat transfer were performed to converge on a heat and material balance that would equate to the horsepower developed versus consumed. In this case, it was determined that wet steam probably slugged the turbine or initiated high vibration due to the development of water in the internals in a region or regions not designed for liquid fallout. There was also a question of the aerodynamic flow-field due to the increased demand of steam caused by lower superheat.
The following steps are recommended in solving a problem such as this is.
- A material and metallurgical analysis should be conducted to evaluate the failed components. In this case, we would be looking for evidence of high erosion due to water and sudden failure due to liquid slugs.
- The process and mechanical data should be collected and reduced.
- A mechanical evaluation of the train should be conducted.
- A heat and material balance of the steam system should be performed.
- The consumed horsepower should be calculated from the process side.
- The steam and process sides of the system should be reconciled so that an accurate flow rate of steam can be determined.
- A root-cause failure analysis should be developed from the above.
It is important to look at transients in these systems that may have occurred, such as, in this case, many users of the steam. Some could have tripped in and out causing transient/momentum, leading to unwanted pulsation in the system. As with many of these complex systems, the design and root cause analysis should be led by a professional engineer competent to do the work using a multidisciplinary approach.
KnightHawk has been involved with numerous projects involving steam systems over the last 30 years, including but not limited to: pressure let down devices, steam turbine failures, waste heat boiler failures, steam pipeline failures, valve control issues, stop valve failures, turbine rerates and steam turbine blade design.
For more information, visit www.knighthawk.com or call (281) 282-9200.
