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Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/30/2017 11:30 AM

Pitting was observed on the rotary blades of last stages of low pressure reaction turbines at our power plant. Can we avoid pitting keeping in mind that we have condenser vacuum limitation and exhaust temperature limitation of low pressure turbine ? Can pitting lead to development of cracks in blades ? Can we coat HVOF chrome carbide on pitted blades to avoid any risk ?

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#1

Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/30/2017 2:45 PM

Have you ruled out impurities in the steam giving rise to this pitting? Silica?

What steps have you taken to eliminate condensing within this last row of blading due to low back pressure, cold exhaust, or excessive back-pressure due to non-condensibles?

Insufficient information has been provided thus far. Your limitations mentioned, are you referring to OEM limitations, or your own inability to maintain conditions within the guidelines provided?

Eventually the blades could thin out enough (especially if the pits extend into blade roots) to result in stress fractures.

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#2
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/30/2017 9:39 PM

Thanks, Steam purity is upto mark.There is no evidence found of chemical impurities causing "corrosion" leading to pitting. It is water condensation i.e erosion". Temperature & pressure parameters are with in design range. For your information, we have replaced the rotary blades of other same time of turbine in our sister plant due to excessive cracks revealed in NDT inspection. In that case, all pareters were in range.

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#10
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 10:14 AM

If the parameters are "right" and you still have experienced condensation erosion on this and one other turbine, you need to fire your instrument technician and get someone who can calibrate the instruments to the correct actual readings.

There is one other thing, but I doubt it to be of any significance: does your source water for boiler feedwater contain boron? How much, and are you sure you are eliminating this as well as silica in the water treatment?

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#11
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 12:05 PM

Stewart ! We have borated water on the tube side of Boiler. Chemistry of feed water on shell side is maintained.

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#13
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 2:39 PM

So this is a nuke plant? OK. Obviously, any intrusion of dissolved solids into the boiler feed-water that can carry over in steam is disallowed. You have a requirement for clean water on the shell side, correct? You don't want the whole place glowing in the dark, do you?

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#14
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 8:31 PM

Yes true. . Atomic power plant for electricity generation. Therefore chemistry parameters are controlled through strict guides.

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#15
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

02/01/2017 11:28 AM

How much radioactivity is in prime mover steam supply? Any? If so, could this ionizing radiation have an effect on corrosion? I think so. Maybe we need to turn one more page in the corrosion book to answer this one.

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#16
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

02/02/2017 11:12 AM

I am feeling dizzy after getting so diverse thoughts

Thanks friends for kind co-operation.

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#3

Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 2:30 AM

A1) Yes

A2) Yes

A3) Yes

Whether the <...we...> is able to do it is not known as the abilities of the <...we...> cannot be judged from here.

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#4
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 2:57 AM

I need experience and technical fees back on my questions

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#6
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 4:07 AM

It looks like there will be many more questions before that happens.

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#5

Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 3:30 AM

What is the blade material?

What is the steam plant chemistry regime being used?

Exactly where is the pitting located on the last stage blades? Can you post pictures or provide a detailed desription of the pitting?

Can you describe the condenser vacuum limitations you noted in more detail?

Can you provide some information on the typical operation/ frequency of power change, start up shut down?

You say chemistry is within spec... how many times has it beed reported put of spec? Too often would be concerning (though, never might be more so).

.

Don't let them sell you on a solution before you have some confidence you understand the problem well. No coating or other solution will allow you to 'avoid risk' in totality.

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#7

Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 5:00 AM

1.- YES

2.- YES

3.- YES

"Cavitation pitting" can occur from the sudden shock wave impacts caused by the implosion of small liquid-free zones within condensate. Cavitation erosion occurs because flash steam can occupy a very large volume, but then suddenly and rapidly condense after a portion of its heat is lost, having been transferred to the adjacent fluid.

Due to specific volume differences between steam and condensate, the sudden condensation of flash steam can create a large void that is rapidly and often violently filled by adjacent condensate, thereby causing shock waves known as water hammer. The rapid collapse of the flash volume and associated shock caused by high velocity condensate filling the void can lead to significant erosion and damage.

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#8

Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 7:50 AM

Thanks to all mates for valuabe input.

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#9
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 9:07 AM

How much superheat do you carry in the exhaust section?

1. Your turbine temperature specs were poorly set to allow for some wear?

2. cool spots in the shell casing, shaft gland leakage? Your exhaust might be plenty hot, but maybe some small scale condensation & reflashing in the passages could be masked by your instrumentation. The loss in power output from this stage may not be measurable.

3. How do you measure the exhaust temperature, what errors are possible?

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#12
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Re: Pitting on Rotary Blades of Last Stage of Low Pressure Reaction Turbine

01/31/2017 1:34 PM

As in huge air leak by at the shaft? Hmm...

I still think it is instrument malfunction, or whosoever falsely calibrated the instrument to a bad level by mistake.

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