Need a Plunger Pump primer (sorry!) 4

[Guest102023]

I am grappling with the failure by cracking of a cast steel plunger pump casing (see attached photos). The mechanism is obvious enough, but I want to get a better understanding of the basic operating principles of these pumps. Links to internet resources would also be useful; I've done much googling, finding lots of operating manuals and parts dealers but not much theory. I understand these pumps are subject to considerable vibration, so the possible effect of that is of particular interest.

Thanks in advance.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[desertfox]

Hi Again

Just looking at the crack close up and a comment made earlier by DubMac about machined and un-machined areas, that could be the site for initiation of the fatigue crack i.e. a tooling mark, change in section.
In addition if it is the sealing area/bore of the pump the fatigue cracks are running in the correct orientation for the hoop stress due to pressure cycles of the pump operation.

 

[ccfowler]

brimstoner,

You have sought and received advice from a group of engineers who each have extensive practical experience with their own obviously varying educational and experience background. They have all shared potentially helpful advice with you, and I find it most unfortunate that you have found it necessary to post such an unpleasant comment in response to their kind and well intended advice.

I agree with you that steel castings can generally be very consistent in quality and reliability, but I cannot agree that steel castings can actually be without flaws. Everything that is manufactured invariably contains flaws, and the quantity and severity of the flaws determine whether or not the specific casting is adequate for its intended purpose. The imperfections, however slight they may be, typically determine the location of failure initiation due to fatigue, over stress, corrosive attack, or whatever failure mode is actually experienced.

Personally, although not a major factor in my experience base, I have had some modest experience with the manufacture of products involving steel or iron castings ranging from fairly small to very large size. Most castings, whether iron or steel, were of suitable quality for their needs, but in general, iron castings were used for purposes where their flaws would generally be less crucial to their function. Steel castings were of generally much better and more consistent quality, and their quality was invariably of considerable significance to their suitability. During the manufacturing process, it was uncommon, but not profoundly unusual, for fairly extensive (read very costly) machining processes to have been completed before unacceptable flaws become apparent despite multiple testing processes intended to minimize such pointless work on castings with unacceptable flaws.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[Guest102023]

desertfox,

You are correct about hoop stresses in the cylinders; I believe that is what drove the crack.

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Once again, casting defects were RULED OUT early in the investigation, and the fracture mechanism was corrosion fatigue. Fairly coarse machining lines can be seen on surfaces near the crack, so we are not dealing with as-cast surfaces.

Thanks again to all for your assistance, the link to the tekna presentation was especially helpful in distilling down the critical issues of plunger pumps.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[desertfox]

Hi brimstoner

Your welcome.

Desertfox

 

[Guest102023]

One more question ... this is an FMC pump, and they do not reveal any more information about the casting than that it is "low alloy steel". Anyone know what is specified for these pump cases?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"