Valve stem borken from sectional redcution

[nssjvs]

AISI 410 ( hardness 200 HBW ) valve stem broken immeditaly from the point where achme thread start. It looks like shear cutting ( machine cut). Thread portion OD is 30 MM and seal area is 36 MM. it cut from point where 36 MM become 30 MM. what could be the probable reason ?

is the diameter reduction from 36 MM to 30 MM create root cause to shear break stem ?? can 30 MM stem break like shear cut while it is in valve ?

Regards
Shah

 

[metengr]

Shah;
You have not provided enough information related to service conditions of the stem prior to failure. Also, can you post any fracture surface pictures of the stem?

Most of the stem failures I have seen over the years that have been exposed to fluid flow in service either failed from fatigue crack propagation, tensile overload or stress corrosion cracking.

 

[bcd]

Hello,

You mention a few items which indicate a potential type of failure. A picture and more information about the type of valve and how long it has been installed and how many times it has operated would allow our group a better understanding and an even better answer.

You mention ACME threads, which identifies a linear type valve. Multiple turns from open to close.

You mention the fracture occured at a change in diameter and shape. That is a stress concentration.

Also, you say the break looks like "shear cutting" or machined. I interpret this as a fracture surface that is flat and perpendicular to the stem axis.

From this, it sounds like a fatigue failure. Any type of bending stress caused by misalignment, worn bearings, or no bearing support in the design at all to take away side forces from the handwheel. A possibility to look at and consider. But again, with more detail, our community can give you a more certain answer.