Pinhole above bonnet - HF service

[Danlap]

Good day,

I was wondering if someone have similar experience with pinhole and maybe have done some root cause analysis. Subject is Monel globe valve for HF service.
Fully aware that Monel valves have common challenges such as casting shrinkage or micro porosity which later susceptible to be 'attacked' by HF. UOP and MSS SP-160 committee relentlessly trying to incorporate additional testing criteria to tackle this issue.


Green spot is beginning of the leakage

black remarks

What struck me the most is that we have two incidents on two different brand however on identical location above the bonnet.
Have dealt with pinhole several time but mostly on vena contracta body area due to flow/turbulence induced erosion, now I am clueless as bonnet/yoke is not typically subject for internal errosion.
Is this more about the nature of foundry process wrt residual stress, thinner wall thickness on this part, etc.?

Valves are 2-7 years old, creeping of HF into cracks are like timebomb. And cracks as well as weld repair sometimes cannot be spotted even with 100% Xray

Thank you for the brainstorming in advance.

Regards,
D

 

[Krausen]

HF acid wormhole through the casting it appears. Very common in HF acid service, although I've seen it more on Carbon Steel castings. Monel castings, as you mentioned, are the issue. A very difficult material to cast properly. Many companies require the M35-1 type Monel casting for this reason. Do you have the material spec of this Monel casting? As an alternative, you could upgrade to Hastelloy C276 castings in the future. This is what Descote does on all of their HF acid valves as a standard. They don't even mess around with Monel castings anymore (unless customer requires it).

 

[Danlap]

Thanks for reply Krausen, quite eye opening. No we don't have material spec aside from what is prescribe/approved by UOP (Phillips??). Have to say bluntly following approved types, material (carbon steel and Monel only??) and manufacturers by adding extra assurance such as 100% X-ray, no welding repair, etc. Basically emphasizing what UOP says. Not always a successful task though, been trial and error with handful for the last 30 years.
Quote from MSS-SP 160 "No single inspection or test can detect all casting defects, so the task group started at the foundry level to specify the use of casting simulations and other quality requirements for the foundry to improve overall casting quality and minimize casting defects from the foundry." Unquote

Like the 'new' ideas stipulated in SP-160 over casting simulation, sophisticated RT techniques, Helium testing, etc. but seems cumbersome to be performed by foundry and checked by end user. And seems like a price adder for already expensive valves in this competitive HF Alkylation industry.

Rarely touch Descote (they're on the higher side of price, having said that they always deliver superb quality) and didnt know that we can apply C276. Will take this into account.
Thanks