Failure of SS 316 stem in tension 5

[Phil121324]

Hi,

Hopefully i have posted this in the correct area, apologies if not- Send me in the right direction...

I am looking at the failure of a valve stem from a control valve, the stem has sheared during valve operation, the failure has occurred at the first thread at the bottom of the stem, the first weakest point, the thread connects the valve plug to the stem. In short the stem connects the plug within the valve body to the actuator, this then allows the control valve to control the pressure in the system. The stem material is SS 316, the valve works on a FWKO system (

https://www.petropedia.com/definition/1681/free-water-knockout-fwko)

at around 40 bar.

In normal operating conditions the stem sees very little tension (i am assuming failure in tension), if the valve is incorrectly setup the stem can be subject to tension, if the stem receives the full tension that the actuator can deliver the yield strength of the material is surpassed by around 20% however the UTS is still well within. As the stem is seeing tension my initial thoughts were not to be too concerned with the yield strength of the material and look at the UTS.

I have attached a photo of the failed shaft- can anyone shed some light on the type of failure or point me in the right direction. Could the failure be hydrogen embrittlement, a brittle fracture of a relatively ductile material.

thanks

Phil

 

[Maui]

Thank you very much for your permission!
I am using that freedom to NOT offer my opinion, even though I have one. It would be professionally irresponsible to do so, and given that safety is involved, unethical.

The fact that you are compelled to talk down to and denigrate anyone who offers any practical or useful advice to the OP, including those like myself who did recommend that it would be wise to perform a thorough metallurgical failure analysis, should serve as a red flag to others participating in or reading this thread. I am done here.

 

[Tmoose]

OP Phil ain't been back for 10 days.

I'm still interested in Pictures and cross sections of the valve and whether it is ever required to close completely when "controlling the pressure" .

 

[arunmrao]

@ Maui, Always enjoyed and learnt from your posts over the years. A pink star comes your way a small token of appreciation.

 

[mfgenggear]

Hey To @Maui
and all Gentlemen
I heave learned so much over the years from all of you, I want to say thank you so much.

 

[Phil121324]

Hi all,

I really appreciate your comments on this and the time taken to reply.

In total i have five failures now on the same model of valve, all stems the same design, materiel, size etc

All fracture faces are vert similar.

I went to site to witness a valve in service that had reported vibration and noise, i wasn't disappointed. There was a lot of vibration from the valve, originating from inside the body and during certain flow conditions the vibrations were amplified to a point "knocking noise" could be heard inside the valve- the knocking noise is most likely to be the plug within the cage rattling around due to the constant (excessive)change in flow conditions, Note the plug is held within the valve by the stem.

The other noticeable aspect was the valve was continually "hunting" for position and was operating (small moments), up and down every second or so.

The next day, this valve stem became failure number five.

Ok, so, my thoughts.(without the use of a matlab yet)- relating to failure five.

If the valve was in service for 100 days, just an example.

Total valve moments due to adjusting position: 8.7 million (60 x 60 x 24 x 100)

Total valve moments due vibration (vibration estimate 2 Hz, low ball- i don't have any monitoring equipment): 17.4 million (60 x 60 x 24 x 100 x 2)

Failure: low stress, high cycle fatigue.

Cause: vibration, cyclic loading

Root cause: To be established- what is causing the vibration, solve the vibration issue the stem issues probably goes away.

I might get the stems to a lab in the next month, ill keep you posted on the results they come back with.

 

[desertfox]

Hi

Off the top of my head how close is the whole valve to it’s natural frequency? Also if the valve with given flow conditions might have cavitation issues and hence the knocking noise.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[mrfailure]

Your observations of vibration are consistent with physical signs of high cycle fatigue as mentioned above. I wonder if this was occurring because the valve plug was unscrewing from the stem and, in turn, whether the situation was caused by insufficient tightening in the first place when the plug was installed on the stem. Under this scenario, the vibration could be a symptom rather than a cause of failure. In any case, you should check the plugs to see if they are fully tightened in the valves.

 

[mfgenggear]

Kindly please advise the configuration and how this valve body functions.

Generally vibration is because of inadequate tolerancing, poor design, or bad manufacturing.
Poor workmanship of dimension
It would be condusive to obtain a new assembly.
Disassemble and inspect and record
To engineering drawings
Tear down a failed valve look damaged components and do a failure analysis.
Is this an in house design or purchased?

Make a fish bone diagram apply possible reasons for failure. And eliminate what's not and drill down to root cause.

 

[weldstan]

How long have the valves been in service < a year or many years? Since 5 have now failed and it appears from your observations a sixth will soon fail, the design would seem inadequate for the service or the valves have been poorly designed and or poorly manufactured (all manufactured by the same company?). If that is the case, selecting a stronger stem may not be a satisfactory repair.

One other question, have the operating system process parameters been changed from the original design by the owner/operators operating the unit.

 

... how close is the whole valve to it’s natural frequency?

Good point. This would necessitate an assessment of the mounting ... I have seen similar equipment whose steel legs were just bouncing on the concrete floor. More shocking than the design was the fact that no one noticed a very audible problem!

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."