casing materials for steam turbine 1

[ANNEESMALIK]

Hi
Subject: Material for turbine casing
I want your expert opinion regarding the application of Nodular Cast Iron GGG40 for LP turbine inner casing and IP turbine outer casing in a power plant. The boiler will operate at subcritical pressure( 245 bar and temperature 540C).What are the shortcomings of using Cast Iron in Casing. Which will be the best material in these conditions.

 

[metengr]

For the LP turbine inner casing rings, I would use cast carbon steel. Look at ASTM A 356 Grade 1 for cast carbon steel. The reason is that cast carbon steel is weld repairable. I have seen nodular cast iron as inner casing rings for old GE large LP steam turbines. However, weld repair from steam or water droplet erosion can be difficult on casing rings.

For an IP outer shell you need a low alloy steel like 1Cr-1/2 Mo or 1.25Cr- 1/2 Mo (A 356 Grade 5 or 6) for elevated temperature strength and ductility to guard against thermal/mechanical fatigue cracking at shell transitions.

 

[EdStainless]

toughness, resistance to fatigue (mechanical and thermal) and repairability are why steels win out over irons.

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[ANNEESMALIK]

Ed Stainless
Thanks for your response and useful advice . However, I will highly appreciate if you kindly answer specifically about my queries and your recommendation/suggestion.
Thank you very much.

 

[arunmrao]

Aneesmalik,

Ed has plainly and correctly stated the necessary conditions for selecting a grade of material . It is for you
to form an opinion about its suitability. In a forum like this with limited information, it is impossible to provide such specific information and no one is obliged to .

Good Luck, hope the matter rests .

 

[EdStainless]

There are many smaller, older, lower pressure (and temperature) turbines with alloy cast iron cases. There is Cr in the alloy for strength and erosion resistance. These are small enough units that they are considered disposable since they cannot be repaired if the wear or crack.
I don't know of anyone that would currently choose such materials over cast alloy steels.
Today even many IP housings are made from cast 400 series SS grades.

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[arunmrao]

Ed, I am uncomfortable in processing iron castings for such applications. If there is an accident, the consequences on the casting manufacturer will be severe. I have always recommended and used cast steels for the reasons stated earlier.

 

[EdStainless]

Like I said, the only ones that I have seen were older (1950-60's) and low pressure.
I don't recall the exact sizes but I would guess that none of them were over 500kW.

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