Fluid transient loading scenarios

[cougeng21]

Hello,

Am having some fun designing piping systems to ASME B31.3 and I have to address fluid transients. I am applying loads to the piping per code, however there is no distinction between static and dynamic loading cases or any guidance as to what a good dynamic impact factor would be for the piping. Does anyone know a good reference for a dynamic impact factor for fluid transients on a piping system?

 

[BigInch]

+/-25 to +/- 50% of operating pressure would be reasonable, not to say that wider ranges are not possible under some circumstances.

What would you be doing, if you knew that you could not fail?

 

[stanier]

I suggest the following books for a start.

Pressure Transients in Hydraulic Pipelines ARD Thorley

http://books.google.com.au/books/ab...ulic_pipelin.html?id=iXbiZwEACAAJ&redir_esc=y

Pressure Transeints in Water Engineering Ellis.

The subject of fluid structure intereaction is complex and there is no specific softare for same. One approach is to use AFT Impulse to determine the hydraulic loading at nodes. Tjs spftware allows the export of a file to be used in Caesar II as an input to loads.Refer this white paper

http://www.aft.com/training/white-papers

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[cougeng21]

Thank you for the replies! We are trying to get by without having to resort to software (therefore the "quasi-static" approach). What I was trying to do is determine a conservative dynamic load factor (DLF) to apply to the peak force to bound impact forces due to fluid transients to convert the dynamic load to a quasi-static load. From what I have read I acknowledge that every dynamic scenario is different, however I was hoping there would be something tabulated somewhere that had a specific DLF for specific cases. I have found that the maximum dynamic factor that can be applied is 2 (i.e. F_dyn = DLF * F_peak_static).

25-50% seems reasonable to me, but due to the nature of the calculation I have to be able to have a basis for my factor, not just engineering judgement.

 

[stanier]

Cougeng21,

There is no such thing. The use of the joukowsky equation to predict dynamic pressure increases can be out by a factor of 6 to 10 if column separation occurs.

Design codes, particulalry ASME, require you to take surge into account in establishing the design pressure. So for every line you need to do the analysis whther by similarity or sophisticated software. The investment in software is economic when you think of the engineering hours saved.

You cannot just say I have allowed a design factor to cover for surge.

Not doing the engineering is a foolish approach and could give grounds for you insurer to walk away from your cover. If someone dies you could go to jail for your negligent actions! Waterhammer does occur, failures do happen and people die. You dont see much in the media and on the internet because of the confidentiality clauses inserted by insurance companies. Add tot hat the view that no one wants their mistakes aired too much. Judges do not view the defence of "The software was going to cost too much money" too highly. USD7,000 is a lot less than the consequential damage and your lawyer's fees!

“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."

Lord Kelvin [PLA, 1883-05-03]


"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[cougeng21]

I am not trying to use a design factor to cover the surge; we are already using the Joukowsky equation to predict the pressure rise in the system in an over-conservative pump start-up (i.e. void filling) scenario.

We were asked to use an additional dynamic factor on top of the predicted pressure rise to account for the dynamic loading (possibly) experienced by the piping.

I found this: (Figure 3.1-5, page 47 of 373)

http://my.epri.com/portal/server.pt...RaiseDocID=TR-106438&RaiseDocType=Abstract_id

problem solved.

 

[stanier]

You have not taken into account that the joukowsky equation does not cover all scenarios in terms of dynamic analysis. Adding a factor is grossly simplifying a situation.

You could of course just build a 12" to the foot model, test it and wear the consequences. Your risk and licence.

There is no such thing as a design factor in ASME B31.3 of the type you seek. There is a clause to cover occssional loads but you need to know what that load is before you can use the clause. Dynamic loads could be considered an occassional load if they fall within the boundaries of the clause. That is an engineering judgement. Again your risk and your licence. BY the way the clause also needs the Owner's approval to be used. Companies such as Shell do NOT allow this clause to be used in general.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[stanier]

Clause 301.1 is below. Do you qualify? from the tone of you submissions I would guess not.

The Designer is the person(s) in charge of the engi-
neering design of a piping system and shall be experi-
enced in the use of this Code.
The qualifications and experience required of the
Designer will depend on the complexity and criticality of
the system and the nature of the individual’s experience.
The owner’s approval is required if the individual does
not meet at least one of the following criteria:
(a) Completion of an engineering degree, requiring
four or more years of full-time study, plus a minimum
of 5 years experience in the design of related pressure
piping.
(b) Professional Engineering registration, recognized
by the local jurisdiction, and experience in the design
of related pressure piping.
(c) Completion of an engineering associates degree,
requiring at least 2 years of full-time study, plus a mini-
mum of 10 years experience in the design of related
pressure piping.
(d) Fifteen years experience in the design of related
pressure piping. Experience in the design of related pres-
sure piping is satisfied by piping design experience that
includes design calculations for pressure, sustained and
occasional loads, and piping flexibility.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[cougeng21]

I thank you for such enlightened conversation. Good day sir.

 

[BigInch]

Assuming you are filling a pipeline, Joukowsky isn't valid for such scenarios.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[vpl]

stanier, You can't teach people who refuse to learn. He evidently considers the problem solved. If it doesn't fail, I wouldn't be surprised if he keep on doing the same thing. If it does fail, I wouldn't be surprised if he comes looking for people to blame.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[cougeng21]

Not refusing to learn, it's as he says an engineering judgement. He does not know the system nor the configuration or the extent of the calculations. Thanks for the help.

 

[cougeng21]

That being said I will discuss using dynamic analysis software. Once again thank you for the input gentlemen.