Pitting was observed on the rotary blades of last stages of low pressure turbine. What can be reasons and consequences of pitting? How can we avoid it?
erosion...to some degree it is a consequence of operation. some designs use an harder (yet expendiable) insert to midagate. raising the exiting steam's temeperatur, thus lowering moisture content, would result in a loss of efficency
It means that we have to live with moisture pitting because we have LP turbine exhaust temperature & condenser vacuum limitations. We replaced the rotary blades of sixth stage of other same steam type turbine of Sister power plant due to cracks revealed during NDT inspection.
By the way, I am quite new in Turbine maintenance.
We have seen similar pitting on the leading edge of the last stage of some of our larger, condensing turbines. The most recent one was repaired by coating the blade edge with HVOF Chrome Carbide to extend blade life. We have used this successfully on other turbines with similar damage.
Thanks J. Pellin for valuable input. Can pitting lead to developing cracks in blades ?
I feel that pitting may be one of reason of building cracks in rotating blades of sixth stage in our 1st power plant . Still, no certain reason has been found for those blade cracks
"Pitting" of rotating blades in the latter stages of a condensing turbine may be caused by two different mechanisms. First, "pitting" may be caused by mechanical damage due to impact of water droplets on the inlet edge of a rotating blade. This type of damage is referred to as "erosion". It usually occurs near the tip of a rotating blade. It is very rare that erosion would cause a crack in a blade unless the erosion is extremely severe. Some erosion in the latter stages of a condensing turbine is normal. Erosion may be aggravated by low inlet temperatures to the turbine or by extended operation at low loads.
Second, "pitting" may be caused by a chemical reaction due to corrosive contaminants in the steam coming out of solution. "Pitting" due to corrosion may occur at any location along the blade and in many cases results in cracks. Eventually, these cracks may cause loss of a portion of the blade. This type of "pitting" is most likely due to poor steam chemistry.
It would be helpful to see some photos of the "pitting" to determine whether it is due to erosion or corrosion.
Thanks, We have water chemistry issues problem in our previous plant. We observed concentration of chlorides to get high sometimes in condenser hot-well. It might be due to condenser tubes leakage. But on the other hand,those low pressure turbines experienced cracks in last stages leading to blades replacement.In previous reply, .@Pellin advised to use HVOF chrome carbide coating to protect blades from erosion. Is it normal practice ? How can we reach to conclusion to use this coating ?
We have another similar turbine that experienced cracks on the tip of a blade on the last stage. This crack was initiated at the hole where the tie wires connect the blade tips. I tried to find a picture of this cracked blade, but could not. They reevaluated the design, working with the OEM and made a change to the design of the connecting wires. I have attached a picture that shows the original configuration of these interconnecting wires. If you look closely at this picture, you can also see extensive pitting erosion on the last stage blades. We hard coated the edge of these blade to address that.
Thanks Pellin, I will share pictures of cracked blades once I get from office. Sudden condensation of flash steam on last stage can create large void that is sometimes filled rapidly by adjacent condensate. This process can result in shock waves known as hammering. Rapid collapse of the flash volume and associated shock caused by high velocity condensate filling the void can lead to damaging erosion. So erosion can be significany factor as you said.
Some manufacturers offer so called "last blade protection" control logic system. For condensing turbine type, the logic basically looks at the steam pressure at the turbine outlet and on the drain line flow coming out of the turbine and considering predefined trip/alarm set points, the system would alarm or shut down the equipment. By the way, could you provide details on your type of turbine: condensing, with how many injection/extraction if any? rated power ? just to have an idea which kind of beast we are talking about?
