Effectiveness of 440C stainless steel in erosive environment 2

[ashaari]

Dear all,

I've recently serviced two desuperheater control valves (boiler feed water service) which were found passing, and the immediate cause was erosion at plug and seat. Currently we suspect that the erosion is accelerated by continuous small opening of the control valves, due to the control valve oversized during design. Both the plug & seat material are 440C.

Upon cross-checking with my plant's piping specification, I find that the piping designers specified that all gate & globe valves at boiler feed water service are to be hard-faced with stellite as according to API 602. The valve trims are of material A-216 WCB.

I can't help comparing between the control valve plug & seat 440C material with gate & globe valves hard-faced specs.

Boiler feed water parameters:
Design press: 80 barg
Operating press:60 barg
Design temp: 150 degC
Operating temp: 135 degC

My question:
1. Is 440C material comparable to hard-faced with stellite for erosion resistance?
2. Is 440C a suitable material for this service?
3. How can I improve erosion resistance for the plug & seat? For instance, is it acceptable practice to perform hard-facing on 440C? Or, if I were to upgrade the material, what would be a suitable material choice?

Thanks in advance for your feedback!

 

[EdStainless]

Nothing will touch a Co alloy like Stellite for erosion resistance.
I doubt that you can weld overlay the 440C as it is so brittle.
You might try a tungsten carbide HVOF coating, but it will peal off eventually.
Replace with the right stuff.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

Change out the 440C valve plug and seat to Stellite overlaid carbon steel or Cr-Mo steel substrate material. This is what we use in our plant boiler feedwater systems. No way would I even consider an HVOF coating in this environment.

 

[ashaari]

Thanks guys for your prompt replies. Just need some clarification:

EdStainless:
Really sorry, I don't understand your first sentence i.e touch a Co alloy like Stellite. Do you mean to say that stellite is great for erosion resistance?

metengr:
May I know what Cr-Mo steel you use in your plant's boiler feedwater systems? Also, do you think that 440C is an inferior material to be used in boiler feedwater, esp. with the parameters I provided?

 

[metengr]

ashaari;

RE; Cr-Mo alloys - Grade 22 (2.25% Cr-1% Mo) and Grade 11 (1.25% Cr - 1/2% Mo). The 440C will not provide the erosion resistance that Stellite 6 provides for valve plugs and seats. Also, using Stellite 6 overlays are much cheaper in comparison to a solid 440C stainless steel.

Last, but equally as important, toughness of 440C is poor and can result in sudden (brittle) failure from fatigue or other damage mechanism. Using the lower strength Cr-Mo will have better toughness and will be much less prone to cracking in service.