304 stainless poppet valve cracking 3

[plowguy2]

Stamped valve,304 stainless, 1/8 inch thick, temperature cycles from 40° to 140° quickly. Valve in pump head assembly. valve cracks at approximately 1000 hours, can anyone suggest a higher quality stainless that will still handle my chemicals and is less resistant to cracking.

 

[btrueblood]

What pressure, what chemicals? Where are the cracks appearing? Is there a center post on the poppet, or is it just a dish-shaped piece?

 

[plowguy2]

the Max design pressure is 2000 psi, I operated it at 1500 PSI, there is no center post on the poppet, on the bottom of the dish there is a small concave dish, reverse going up. The crack will form on the outside of the poppit, forming a triangle to the center. The chemical I use is a caustic, the pH is slightly over 12. The pump flows approximately 3.4 gallons per minute. The stainless 304 poppet is heat-treated, I can only assume it is heated and then rapidly cooled. Thank you very much for your inquiry and response. PS the poppet is stamped.

 

[btrueblood]

Okay, so this is just like a Cat plunger-pump head check valve, right? You say the poppet is "heat treated"...is that information you got from the manufacturer (i.e. new valves are heat-treated), or are you seeing a color change to the material (i.e. a new valve is shiny, and an old valve is discolored)?

The only heat treatment a 304 material can get is a solution anneal, which tends to reduce the yield strength over a stamped (cold-worked) part. Hopefully, the mfgr. is NOT heat treating the valves. Leave them in the cold-worked (as-stamped) condition for better fatigue strength. Maybe even shot-peen the surface...though you'd have to re-lap the seal face, so it might not help much.

140 F is no big deal for a CAT pump, we routinely operated these pumps, and similar ones, for thousands of hours at temperatures of 190 F/170 F cycling. Even a rapid chill from 140 F to 70 F (e.g. a temperature relief valve suddenly opening) would not affect the poppets much.

But, cavitation might - are you seeing a shiny valve become mottled in appearance on the downstream surface/outer edge? Is there something in the system that suddenly dumps the downstream pressure, allowing the pump to cavitate?

More likely, the valve is suffering from low NPSH at the hot condition (the pump may seem to "suck air" or have low output flow/pressure until the temperature drops again). Try improving the suction head (bigger and shorter suction line, tank elevated above pump), or (better) find a pump that has a larger bore diameter for the plungers - this has the effect of reducing the piston max. velocity and reducing the acceleration/deceleration that the fluid undergoes as it is pumped, with resulting better NPSH characteristics. If possible, consider a larger-capacity pump with reduced rpm, for the same reason (doesn't always help as much as #1 though).

Finally, a change in materials for the check valve may help, since you are using a high-pH solution - look at the precip.-hardening steels, 17-4 or 17-7 PH alloys, which can be heat treated to fairly high strength.

 

[plowguy2]

incredible insight, I think you hit the nail on the head. Thank you ever so much.

 

[EdStainless]

are the cracks from fatigue or CSCC? You might consider a duplex SS like 2205. It is stronger and more resistant to CSCC. Your other choices of alloys are higher in Ni and much more expensive.

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Plymouth Tube

 

[dgallup]

Don't think you really want an alloy "less resistant to cracking", do you?

Contrary to the above, 304 can be heat treated, we do it to million of parts every year. We do a plasma nitriding process (commonly called ion nitriding) that gives us a case about 50 microns deep with a surface hardness of 750 HK / 100 gf. Treated parts are in a precision valve that does not leak or wear for over 1 billion cycles. Your mileage may vary.

 

[plowguy2]

gentleman, thank you for your replies, you've given me so much to think about. I appreciate being able to attack this problem from such a diverse and multitude of angles. Each of you to have helped me make great strides in building a better mousetrap, thank you.