How “hot spotting” helps rotors get straight23 May 2014
What causes rotor bowing in steam turbines and how can it be corrected? Ansaldo has developed a process it calls “hot spotting” that is proving highly effective in the field.
One of the problems that might have to be faced when operating steam turbines is shaft inflection, or rotor bowing, a condition that can be either temporary or permanent.
Temporary bowing is usually corrected when the turbine cools down, so that no further action is required. Permanent bowing, on the other hand, requires analysis of the root causes and implementation of countermeasures.
There are a number of conditions that can cause bowing of a rotor, for example, a long standstill of the rotor when hot (requiring the rotor to be turned) or uneven steam flow, with uneven temperatures in one or more admission branches.
Perhaps the most serious cases of permanent bowing are due to water ingress into the hot turbine, with quenching of part of the rotor, and rubbing, which may cause plastic deformation of a small portion of the rotor.
According to our experience, in the case of rotor bowing, if the "total indicated run out" is above 0.2 mm, it is very difficult to recover the situation by balancing.
As a consequence, it is not possible to run the rotor above critical speed and the turbine cannot be operated. At this point there are two possible choices: replace the rotor with a new one; or straighten and rebalance the rotor.
Rebalancing the rotor in situ
Ansaldo Energia has developed a method to carry out this second option in situ, supervising the power plant operator's personnel as well as deploying some portable equipment and special tools manufactured ad hoc.
We have tried various different methods for straightening rotors, using a single technique or techniques in combination, for example cold mechanical straightening using jacks and thermo-mechanical straightening. Both options have been used successfully on very small rotors with slight bowing.
Depending on the inflection line and the amount of inflection, there is also the possibility of machining the rotor shaft (offsetting the shaft centreline).
Peening, or hammering, can also be employed in the case of small rotors with limited bowing.
However a much more effective procedure has been developed by Ansaldo Energia, which we call "hot spotting". This essentially entails the controlled application of a thermal source to a well-defined point, followed by controlled cooling.
The 'hot spotting' process
The hot spotting procedure consists several steps.
At the outset an in-depth analysis of the rotor is carried out, before embarking on any countermeasures. Hardness measurements must be done and replication carried out, to assess the material structure and whether it has suffered any modification.
In preparation for the hot spotting process, special tools have to be manufactured in situ or shipped in.
Also, before hot spotting can start, the rotor needs to be stress relieved by heat treatment (to relieve stresses arising from the bending).
The hot spotting process itself basically consists of rapid warm up of a point on the "extrados" (outer surface of the bent area), paying careful attention to keeping the adjacent areas cool. The warm material begins to expand elastically, but is axially constrained by the adjacent cold material.
Thus, the hot material plastically expands outside the boundary of its former shape.
When cooling down, the material tries to shrink back to its former shape, but the material that has been plastically expanded outside the border of the former shape, when cooled, cannot occupy the same space, thus inducing a shrinkage that is greater than the expansion.
After the hot spotting, the stress arising from the process needs to be relieved through a post heat treatment (which may be also be used to recover metallurgical characteristics that may have deteriorated during spotting).
A final lathing to finish the rotor shaft may also be applied.
Tests are then carried out to guarantee that the rotor can safely operate again, and rotor balancing is carried out.
As already noted, some of the special tooling must be prepared in advance, eg up-righting equipment, equipment for holding the rotor during heat treatment, special torches and water-cooled shields.
Other equipment needed can usually be found on site, eg hoisting equipment, slings, inspection equipment, a lathe that is capable of accommodating the rotor, balancing equipment, and heat treatment equipment.
100% sucess rate
Although it is not possible to guarantee the outcome of the straightening procedure, so far, hot spotting has achieved 100% success with the rotors treated. And all the rotors treated have been successfully straightened within one month.
The team typically involved in the operation consists of five people from the power plant operating company, two Ansaldo supervisors, and three skilled specialists, in lathing, balancing and heat treatment.
In terms of outage time and costs, our experience to date suggests that this hot spotting solution is the most competitive of available rotor straightening techniques.
About the authors
Enrico Foglino (head of steam turbine field engineering) and Paolo Tolomei (power plant field engineer), Ansaldo Energia, Italy