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Failure or Fatigue Failure of a Valve Shaft

10/12/2017 7:47 AM

we were working in a butterfly valve design.i would like to know why the failure occurs in the stem(i.e)shaft,

so I would like to calculate the fatigue failure, bending moment, deflection, fatigue life in cycles

kindly advise me on these formulae and theory, so that it will be useful for me.

awaiting for your reply

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#1

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 9:35 AM

The type of failures you will get will in a high degree be depending of the quality of the valves different parts and quality and how suited they are compared to the process media and conditions.

As the process seems to have a larger number of different smaller valves, one way could be to build on existing experiance and routines, and change complete valves and check types of wear/failures after a preplanned periode and system.
I gather your purpose is to preplan maintenance. This again to have as few unplanned stops as possible and select valve and parts quality, maintenance or new valves to have best cost/lifetime value.
part of this would also be to chart how much wear is acceptable before the process get disturbed.
Apart from this, you will probably have a failure rate, a bit different on the different types of valves, but perhaps mainly (if valves materials are selected to suit process)
Sealings: wear of stem sealings and seat sealings.
Seats: wear of seats.
Wear of stem (depending of valve/stemtype) mostly caused by penetration of seals and deposits on stem.
Same as above for balls.
Corrosion (only by uncorrect or poorer quality of (inner?)part of body (material and/or protection) compared to fluid requirements.
Cavitation (by wrong selection or throtteling of valve comared to flow and delta p)
Remember that dirt, deposits, and particle abrasion and 'non optimum clean' fluid condition often are the strongest contribution to wear.

API standard 609 covers design, materials, face-to-face dimensions, pressure-temperature ratings, and examination, inspection and test requirements for gray iron, ductile iron, bronze, steel, nickel-based alloy, or special alloy butterfly valves. The following two categories of butterfly valves are included:

a) Category A—Manufacturer’s rated cold working pressure (CWP) butterfly valves, usually with a concentric disc and seat configuration. Sizes covered are NPS 2 to NPS 48 for valves having ASME Class 125 or Class 150 flange bolting patterns.

b) Category B—ASME Class and pressure-temperature rated butterfly valves that have an offset seat and either an eccentric or a concentric disc configuration. These valves may have a seat rating less than the body rating. Sizes covered are listed below.  For lug and wafer, Class 150: NPS 3 to NPS 48.  For lug and wafer, Class 300 and Class 600: NPS 3 to NPS 48.  For double-flanged long pattern, Class 150, 300, and 600: NPS 3 to NPS 36.  For double-flanged short pattern, Class 150 and Class 300: NPS 3 to NPS 48.  For double-flanged short pattern, Class 600: NPS 3 to NPS 24.

The ASME B31.1 code lists three valves standards, excluding cast iron and bronze valves. The standards are: 1) ASME B16.34, Valves–Flanged, Threaded, and Weld End; 2) MSS SP67–Butterfly Valves; and 3) MSS SP68–High Pressure Butterfly Valves with Offset Design. As stated in B31.1, para 107.1, “Valves not complying with above [the list] shall be of a design, or equal to the design, that the manufacturer recommends for the service as stipulated in para. 102.2.2. Such valves shall be pressure tested in accordance with MSS SP-61.

The AWWA C 504 establishes minimum requirements for rubber-seated butterfly valves, 3 in. (75 mm) through 72 in. (1,800 mm) in diameter, with various body and end types, for fresh water having a pH range from 6-12 and a temperature range from 33º-125ºF (0.6º-52ºC). This standard covers rubber-seated butterfly valves suitable for a maximum steady-state fluid working pressure of 250 psig (1,723 kPa), a maximum steady-state differential pressure of 250 psi (1,723 kPa), and a maximum fully open fluid velocity of 16 ft/sec (4.9 m/sec) based on nominal valve size.

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#2

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 9:47 AM

1. If you are really, truly designing your own butterfly valves, that information may be available from the stem material supplier, or your mechanical engineer.

2. If you, for some reason, want to analyze a failure of an existing valve, I'd start by contacting the valve supplier and asking their engineer about the failures.

3. Not all answers are derived using calculations. Not all calculations yield correct answers, or solutions.

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#6
In reply to #2

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 1:51 PM

We are the valve manufacturer we are started to analyze the failure in the stem, ya your right sir not all the calculations will lead to a perfect answers. But it would help us to better understanding in. Root cause of the problem so kindly help me

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#8
In reply to #6

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 6:00 PM

Look into getting a material analysis done on the shaft, chances are the problem is related to out-of-spec materials resulting in premature failure.

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#9
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Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 7:05 PM

Was the failure premature? When did we get this info?

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#10
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Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 8:51 PM

My comments were based on reading between the lines of what little information has been supplied. The fact a failure has occurred on a part of a device that (if designed correctly) would seem to indicate either operation outside expected parameters (user abuse) or material failure.

From this it would be wise to eliminate the possibility of third-party supplied component material failure early on in the investigation due to the high potential cost of future failures in the field and damage to company reputation due to a faulty product line.

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#11
In reply to #10

Re: Failure or Fatigue Failure of a Valve Shaft

10/13/2017 2:25 AM

I agree with you there.

Just thought some more details might have been wheedled out of the OP.

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#3

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 10:28 AM

<...why the failure occurs in the stem...> is because the stress it experiences exceeds the level that the material is able to withstand.

Any formula is merely a simplified mathematical abstract of real-life. What happens if practice indicates the theory is inadequate (rhetorical question - NNTR)? That's why testing is carried out; this particular valve stem failed the test.

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#5
In reply to #3

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 1:42 PM

To add to what you said, "The twisty torquey thing-a-ma-jig twisted more than the fracture strength of the shaft in a radial moment.

It might have happened in a New York second, though, and you should take that into account.

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#4

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 11:02 AM

In the advent of 3D CAD and simulation software, you can easily simulate or run a test there with out getting into so much trouble and results would be precise. That is if you know how to do the drawing and set boundary conditions.

In the kind of your questioning, you are not in a position to ask since, you are not a mechanical engineer I guess, since you have no idea about fatigue strength, fatigue failure, dynamic and impulsive loading. If you are an undergrad the best thing to do is to go get a Machine Design book read and study about fatigue and those things that I told you about. There are factors on shaft design depending on geometry, direction of loading, operational temperature, type of loading. I have not memorized it all, but do advise to consult the Handbook if in doubt, like all the others did.

Obvious failure, without going the actual conditions and just by scientific guessing, butterfly valves fails due to, it could be cavitation and excessive pressure difference and temperature beyond operational specification.

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#7

Re: Failure or Fatigue Failure of a Valve Shaft

10/12/2017 2:30 PM

Maybe somebody slammed the valve shut abruptly.

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#12

Re: Failure or Fatigue Failure of a Valve Shaft

10/13/2017 5:22 AM

Vibration/flutter at certain angles?
Del

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