Forging or casting?

[carletes]

Dear all,

Usually small steel valves (NPS<2&quot I have seen are forged , but lastly a manufacturer has quoted me casting body valves for those sizes. Do you think is allowable? Having a look at ANSI B16.34 it seems that, for example, materials like A-105 or A-216 WCB are exactly the same. Is forging better than casting (as I have heard sometimes) or is it just a topic? Perhaps casting require more non destructive testing? any good link or book about this?

Any help will helpful.

Best regards

 

[bljnv]

Forging is better than a casting bcos internal defects like blow holes, shrinkage will not be there in a forging.
Also the dimensional conformity and finish of a forging will be better.
Generally smaller size valve bodies are made from forging because of its size and good production volumes.
As the size increases the forging becomes difficult.
Depending on the severity of application different NDE techniques will be used. For forging UT and casting RT is generally employed.

 

[RXH]

Forgings are better than castings as described by &quot;bljnv&quot; above. API 602 for small compact gate valves allows both. Lately, I have also noticed that many ball valve and needle valve manufacturers are offering cast bodies - especially in stainless steel. I believe it is a question of economics.

 

[JNieuwsma]

&quot;Better&quot; depends on the application and the relative importance of cost and benefit. It's true that forging are not susceptible to shrinkage, porosity, etc. But you don't want to pay for more performance than you need.

API 602 defines small valves (<= 4&quot in Classes 150, 300, 600, 800, and 1500, and allows both cast and forged material (but from my experience I associate API 602 with forged valves). API 602 is used carbon steel (eg. WCB or A105), alloy steel (WC9 or F22) and stainless steel (eg CF8M or F316). However, the wall thickness requirement for all valves from Class 150 to Class 800 is the same. If your application is low pressure you end up with more valve than you need.

For stainless steel only, API 603 defines valves from 1/2&quot; to 24&quot; in Classes 150, 300, and 600--with different wall thicknesses for each class (corresponding to B16.34). API 603 also allows both cast and forged, but at least from my experience I associate API 603 with cast valves.

JN

 

[carletes]

Thak you all for your help.

So, in a critical service do you think that a casting valve with radiographic examination is as &quot;good&quot; as a forged one? Or even in that case you would not rely on them?
Thanks

 

[andyenergy]

Good points made by your previous correspondants.

Sorry to confuse things even more, but valves can be very difficult to radiograph successfully due to their complex shape. We have had experiences with cast feed control valves on boilers that have contained defects even though they were radiographed following production. Sometimes vlaves are so complex in shape that forging isn't possible.

In our industry, our 'big' steam valves have to be shown to have a very low probability of failure. Cast valves come under intense scrutiny for such features as evidence of repairs but then we have to because of the plant we operate.

Really I think you have to put the choice into context. Provided a manufacturer is making the components fully in accordance with the specification, provides all the required documentation and has demonstrated that the required NDT has been carried out and that there are no specific requirements for YOUR plant, and if there are that you are satisfied that the NDT will detect the required defect types, then either a cast valve or a forged valve may be acceptable (provided its the right thickness and material).

If you are worried about failure then you get into a whole different ball park when you have to be concerned about whether it would leak before breaking or fracture.

I can't recall that many instances when a valve (body) has failed dramatically, but there are several instances where plant conditions (erosion etc) has caused the valve to bypass due to erosion of an existing casting defect.

I'm not familar with ANSI B16.34 so I can't be more specific about your particular application. Generally (I hope someone can back me up here) if the standard doesn't say you can't use a particular manufacturing method then it is OK to go for either and the choice then is down to either economics (along with the points I made earlier) or any specific requirements your plant or regulator might have.

Hope this hasn't made things more difficult.

Cheers

Andy

 

[AESINC]

at that size stay with forge unless you use investment cast.

look at

http://www.aesvalves.com

they make the small valves in cast.