For all steam applications you need to talk to steam specialists.

What you see in the attached picture is in my opinion a typical cavitation damage. In this case no hardfacing will improve or alter the conditions. You will (as assumed) need a correct dimensioned (sized and type/form) valve for the application.

Search for blowdown valves, go for the oldest and most experienced worldwide companies, as for instance Gestra, Spirax Sarco, Armstrong or others, and ask for preliminary offers. Note: with all and detailed information in the inquiry.

And yes, the valve (or total solution) will cost a lot more than you think, but probably also likely save you more than you think in trouble and downtime.

I think that when the blowdown valve is open, discharges wáter that if its vapor pressure is above 14.5 psi, flashes and this is the cause of the damage.
Depending of the piping layout and the valve position, the flashing may begin in the pipe before the valve, in the valve or in the downstream pipe. The best design is to have the flashing in the valve or in the downstream pipe.
If the vapor pressure is much higher than 14.5 psi, probably there are critical conditions in the valve and at the end of the outlet piping.
In my experience, the Yarway valves are appropiated for this service. I think that at present the Yarway valves belong to Emerson.
Also take into account that if the outlet piping is of Carbón Steel, it is convenient to chek its thickness. The erosion produced by the flashing also may damage the piping.

Sounds like you need a choke valve to me.

Have you got a section view through the valve available?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

We need more specifications here!

Are you going through the valve from 1740 psi to 14.5 psi? (120 bar to 1 bar)?

Casflo is of course right, missing drainage/steamtraps or faulty layout is a common problem with all steam pipe and vessel constructions.

There is a multitude of possible solutions and valves. See my original answer.

Examples (not necessarily correct for you) https://www.flowserve.com/sites/default/files/2016...
https://www.boilerwarehouse.com/images/Everlasting...

Your problem could also stem from unprecise opening (crack opening) before full opening.

Good luck!

Thanks for your answers. They confirm my assumption that the valves used in the past are unsuitable for this application. I will set up a basic specification and inquire a valve fromt he proposed suppliers.

BR

You really just need the right type of blowdown valve designed for the service. The Velan Powerball is specifically designed for blowdown. Inconel ball and stem with HVOF coating. Rated up to 4500psi and 1,000 degrees F.