Cast 316 SS in Boiler Water Service?

[dkoenig]

We have installed CF8M (cast 316 SS) ball valves in 125# boiler blowoff service. The valves are threaded Class 800, rated at 2500 psi and will be exposed to relatively untreated boiler water (portable drilling boiler c/w chlorides) at around 125 psi, 150C.

Everything that I have read would indicate that CSCC will not be a problem for the following reasons:

1. Estimated maximum tensile hoop stress in the valve is less than 5 MPa. Several publications, including MTI and ASM make reference to a threashold tensile stress value of 100 MPa that is necessary to have CSCC.
2. Valves are cast - increased ferrite content will help prevent SCC.
3. Valves are stress relieved as they are to NACE MR0175. Residual manufacturing stresses in a cast and stress relieved valve should be nil.

Any input from others who may have an opinion on this material selection, or even better relavent past experience with such an installation would be appreciated. Thanks!

 

[hacksaw]

stainless is fine, but if your supplier cannot supply convincing documentation of the valve in that service don't use it.

the issue as i see it is that ball valves are generally unsuited to boiler blowdown service

 

[stanweld]

Depending on chloride content and opening frequency, I would be concerned with crevice corrosion on the seating surface of stainless components.

Also agree with hacksaw.

 

[EdStainless]

The stresses are probably low enough to avoid CSCC. But Stan is right, crevice corrosion will eat you alive. The valve might not leak, but when you cycle it it will tear the seals and not re-seal.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[unclesyd]

hacksaw made a point about ball valves in steam service.

We only allowed them in trace steam (15 psig) and only if the lever operator was replaced with a round operator. We had two injuries, one recordable, in 25 psig steam service blow downs where operators accidently opened lever handled valves with their legs and got a rubber shoe full of hot water.
The plant standard was changed to require all ball valves to have round operators. If you want a lever for quick operation you had to get a exception.

Ball valves were never considered for use in any steam service other than the trace system.

This was prior to any change in regulations.