Ball valve seat

[kclim]

Haven't worked with ball valve failures too much, so I thought I should ask for some thoughts on this forum.

Came across a recent failure of a 1" NB ball valve, the downstream set had experienced severe gouging in a 180 deg arc. The gouging was sharp and distinct. Outside the gouge, the material appeared to be in good condition.

The seat material looks to be a stainless of sorts (not an austenitic - was magnetic). Upstream conditions of 180deg C, 18 MPa pure water (boiler feed) so chance of entrained solids is very low.

See attachment for image.

Any thoughts on mechanism? Erosion? Contact fatigue?

TIA

 

[PeterIgg]

kclim,

I have not seen a failure quite like that before, either.

A few questions:
Is the valve normally in the full open or full closed position? Or does it throttle?
Can you define the movement of the bore across the seat - i.e. if the valve were in a partially open position, would only the failed area of the seat been in the flow path? It helps if you consider the stem to be in the 12 o'clock position, so I would ASSUME (!) that he ends of the arc of failure would be from 12 to 6 o'clock?
Is the seat area an overlay or is it integral with the remainder of the component? (I.e., just a hardened area?)

 

[zdas04]

With that fluid, pressure, and temperature I'd guess that both erosion and cavitation are out. I would bet a lot that it is a manufacturing defect. It just covers too much area for a jet cut and is too deep for short-duration erosion.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[btrueblood]

David,

Why do you rule out cavitation? 180 C and 18 MPa - that's pretty high pressure, and temperature way up in the steam range, so a partially opened ball valve could easily cavitate, depending on flow rate and d/s pressure. I'd agree that a properly designed boiler feed valve would have d/s pressure well up above cavitation limits, even in a partially opened valve. But, maybe the d/s pressure is low during startups, or drops during blowdowns, i.e. under transient conditions?

 

[zdas04]

The temperature is 356°F (I don't have easy access to metric steam tables). Saturation temperature at 2600 psia is 674°F. I'm just not buying 320°F J-T cooling through a valve with that much seating surface exposed. Saturation pressure for 356°F is 145 psia, I'm also not buying 2455 psi pressure drop through a valve with that much seat exposed.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rajarathinam285]

The picture indicates cavitation caused by high pressure,turbulance and soft material.

 

[bcd]

Could be cavitation damage, but it appears to be localized to one area of the seat. For cavitation, the vapor bubbles would have to be formed in the high velocity region where the flow passes through the opening between the ball and upstream seat. Then the vapor bubbles would have to collapse on the surface of the downstream seat where the fluid pressure recovers. I would expect to see more damage around the seat and possibly some in the ball port as well if it is cavitation.

Cannot be flow erosion because there are no flow lines or paths of erosion that are along the flow direction.

Looks like a material defect to me. Usually there is a welded on hard facing applied to the seat and ball. The base material may have cracked. If the base metal is highly ductile, and the applied coating very brittle, this can cause the top coat to break off and expose the softer base metal. Once the softer material is exposed, cavitation, corrosion, erosion, etc. can further eat into the base metal.

 

[gerhardl]

Hello all!

Cavitation will give an uneven, pitted surface, in cases somewhat similar to casting faults failure cracking, but with more and deeper local pittings. Abrasion will grind and smooth out surfaces. With crack opening fo a ballvalve, even hairline crackings in sealing surfaces, the pressure drop will be large enough against lower pressures on downstream side, to cause cavitation.

(You can 'forget' temperature. If you look at the steam table, look at extreme low-pressure points for 'boiling', and dont forget that it is the collapse of steam-bubbels when pressure increases again by expanding that gives the high-pressure implosion shock needle-points, said to be some 1000 bars (or more!), that is the definition of cavitation)

I have several times seen damages in pipelines for steam and feeding water ballvalves, quite similar to the picture here, all caused intially by hairline crackings by damages and/or not 100% closed valves. 180 degree damages indicate partially opened valve or partial sealing damage.

As there is limited reports on damages on other parts, it is difficult to be 100% sure of initial cause.

I believe personally in initial hair cracking and cavitation, later worn down a bit over prolonged exposure to by then a larger opening (to large to give sufficient low pressure to cause cavitation) to restricted flow, maybee even started or assisted by material faults or weak materials before, after or during leakage and flow damage.

Maybe cutting, grinding and microscopic material analysis of samples near damaged area could assist in testing material quality, to rule out or include this.

 

[kclim]

Finished up the investigation. Turns out the valve was (unintentionally) throttling, which lead to the damage observed. On closer inspection, damage was consistent with flow-accelerated corrosion.

The seat was a thermally sprayed chrome carbide (on what amounted to a carbon steel substrate), which had spalled off in service. Presumably due to the violent throttling conditions.

Unusual to say the least. Thanks for all your comments.