Need a Plunger Pump primer (sorry!)
Need a Plunger Pump primer (sorry!)
(OP)
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.
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?"





RE: Need a Plunger Pump primer (sorry!)
Think of the Triplex Reciprocating Piston Pump in two parts; you got your power end, and you got your fluid end. Your power end converts the rotational motion of the motor into the reciprocating motion required by the pistons. Due to the nature of this mechanical process, there is inherent constant acceleration or deceleration of the piston rod; it is never at constant velocity. This not only adds peculiarity to the vibration signature, but also complicates the fluid hydraulics on the suction side of these pumps.
The fluid end is assigned the task of introducing the low energy, incoming fluid, to the high pressure, outgoing fluid. The most common of "recip" pumps, and looking at yours, it is most common...:).... use energy from the piston's suction and pressure strokes to open and close suction and discharge valves. The fact that there are (3) pistons in a triplex pump, and that they are all 120 degrees off from each other in phase, produces pressure pulsations = vibration.
My wife just called me to dinner, I gotta go. There are plenty of knowledgeable folks that can finish off what I've started. Good luck to you.
RE: Need a Plunger Pump primer (sorry!)
I expect that pressure increases at each stage(?) - that certainly would not help the vibration problem.
My question now is (and I have only been shown the part you see in the picture): is the surface that is cracked exposed to process fluid? Which is a mixture of BFW, condensate & stripped water (~50% of mixture) from the sour water stripper; the stripped water has a few ppm each of H2S and NH3.
Anyone have maintenance / failure analysis experience with this refinery equipment?
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
The pressure does not increase at each stage. There are no stages. One Rotation of the crankshaft means one suction and one discharge stroke of each plunger. The plungers run with a 120° crank offset.
Vibrations can occur for several reasons: Worn or damaged suction or discharge valve (in this case there will be backflow and non-uniform output from the pump), broken pulsation dampener bladder (if there is any installed), too high suction pressure that opens the suction valves too early. Can you tell us a bit more about the vibrations?
Triplex plunger pumps create pulsations, much less than duplex or simplex piston/plunger pumps but still there are pulsations. A pulsation dampener can reduce these pulsations down to whatever is required. It depends upon stroke length, plunger diameter and pressure whether or not a dampener is required at all but your photo seems to be from a rather big pump. So, I would definitely recommend a dampener.
RE: Need a Plunger Pump primer (sorry!)
Also, there may be some issues introduced by the actual geometry of the pump's crank linkage. The plungers do not move according to simple harmonic motion. Somewhat surprisingly high accelerations may be found at some points in the cycle. These are generally very rugged machines, but all do have their limits for endurance.
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.
RE: Need a Plunger Pump primer (sorry!)
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
RE: Need a Plunger Pump primer (sorry!)
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
RE: Need a Plunger Pump primer (sorry!)
Cavitation damage typically appears more like erosion, uneven pitting like moon craters or acne.
RE: Need a Plunger Pump primer (sorry!)
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
Your cracks are not caused by cavitation.
RE: Need a Plunger Pump primer (sorry!)
Based on a failure analysis of a plunger pump many years ago, I suspect that your pump likely experienced an unfortunate momentary pressure peak. While this failure appears to show a material flaw, it may actually merely represent a normal material weak point, and the underlying problem is external to the pump. I would want to check the connected system for something that could have caused a pressure excursion at the pump to avoid a repeat of this problem in the future. Plunger pumps are very much subject to the general reality that apparent pump problems are likely to be merely the symptom of a problem originating in the connected system.
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.
RE: Need a Plunger Pump primer (sorry!)
a slice through the crack may reveal somehting, but i suspect this a metalurgists expertise not a piping / pipeline engineers realm so you way want to link a question on one of those forums to this one and see what they think.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Need a Plunger Pump primer (sorry!)
This was fatigue, with lots of corrosion on the fracture surfaces. Pitting at the origin acted as stress raisers. Anyway, I think I've sorted this one out, so thanks to everyone.
p.s., I'm not sure why so many people assume that castings must all have defects.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
Information exchange works in both directions.
I don't assume casts in general have defects, but without all the knowledge you have on your own bit of kit then it's a good place to start. Plunger pumps are like any recicrocating unit going to generate vibrations and pulsing and may then fail by fatigue.
WOuld be good to know your conclusions.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Need a Plunger Pump primer (sorry!)
RE: Need a Plunger Pump primer (sorry!)
page 25 of this paper looks like the area where the packing glands are on your pump case, it gives a reason for erosion in that area etc so it might be worth while finding out about the operating conditions the pump was under prior to failure.
http://www.tekna.no/ikbViewer/Content/22983/Steve%...
RE: Need a Plunger Pump primer (sorry!)
That is true but I've heard this kind of stuff for many years.
I suspect it comes from engineers who were obliged to sit through one materials or manufacturing course and mentally link cast steel with cast iron. As they say, a little knowledge is a dangerous thing.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Need a Plunger Pump primer (sorry!)
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.
RE: Need a Plunger Pump primer (sorry!)
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.
RE: Need a Plunger Pump primer (sorry!)
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?"
RE: Need a Plunger Pump primer (sorry!)
Your welcome.
Desertfox
RE: Need a Plunger Pump primer (sorry!)
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"