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?

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.

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