hi stress engineers

how calculate span length?
in which way give support to pipe in casear modeling.

by srini

Span length is usually limited by the lesser of the spans associated with allowable bending stress, allowable mid-span deflection and local stress at the support (indentation).

The most effective way to to give support to a pipe is to apply it in the direction opposing the forces of gravity. By default this is the Y direction in Caesar.

Dear Shaber,

In addition like mentioned by MoverZ, limitation of vertical deflection is based on to avoid resonance. For resonance issue, pipe quite stiff for small deflection = high natural frequency. It purpose is to avoid large amplitude due to small disturbing force.

fn = 1/2*phi *(g/deflection)^0.5 = 3.12/(deflection)^0.5
Deflection = 3.12/fn ; fn = natural frequency ; g = acceleration of gravity.

Let say that 4 cps is the sufficient natural frequency. We can solve the deflection = 0.78 in. Then by using formula deflection in the middle of pipe then solve for L = distance of span = (deflection*E*I/22.5w)^0.5

Please note that the shorter span will give higher natural frequency. But natural frequency not need to be so high, 4 cps is quite good. You can take as reference "Introduction to Pipe Stress Analysis" by Sam Kannapan Chapter III - Pipe Span Calculation.

Samsul,

What end conditions are you considering when deriving your natural frequency ?

Dear MoverZ, Shabeer,

I should include the end condition when deriving natural frequency in my posting before. Thanks MoverZ. It is quite clear in that book. Below is the condition.

Although this seem too low, in practice the natural frequency will be higher because (1) end moments, neglected here, will raise frequency by more than 15%; (2)the critical span is usually limited by stress and is rarely reached; and (3) the piping weight is assume is often larger than the actual load.

My resposne is to the original post.

Shabeer, I understand that you are new to pipe stress and like most companies these days your supervisor must have given you caesarii and has assigned you with some task.

Pl. note that every job requires two things: Training and experience. Experience comes with time and we all are learning. However in my opinion a formal training or at least working under a supervisor ( who knows his job) is must. In your college you have studies subjects like vector mechanics, strengthy of materials, fluid mechanics, thermodynamics etc. all of which will be in use in pipe stress analysis. Beyond that what is required is reading the codes, understanding their intents , taking a formal couse on pipe stress analysis or at least working under a supervisor. Pl. note that this forum can help you when you are stuck up with some problem but cannot substitute a formal and proper learning module which is required.

I don't know how you will take my advise but in my humble opinion it is for your best.

Also I feel that as an engineer your sentence construction in english should follow the basics of english grammer.

Samsul,

You don't score brownie points by quoting from a book, think about the question.

The fact is that the average pipe span (usually in chart form in individual companies) is based on the criteria I outlined, not frequency response. Span limits are based on an assumption of end conditions, usually around midway between fixed and pinned end. This is because the real spans may see either condition, depending on support and geometry.The aim is to cover all situations. Frequency response of a pipe span is greatly affected by end conditions, particuarly rotational stiffness and this would only be properly reflected in a Caesar dynamic analysis or similar.

Dear MoverZ,

Thanks for your explanation. As you mentioned before Span length is usually limited by the lesser of the spans associated with allowable bending stress, allowable mid-span deflection and local stress at the support (indentation).

What is the basis to limit mid-span deflection? Thank you very much.

Mid span deflections are limited in part becuase a perfectly safe but sagging line looks bad. Partly because it can lead to dangerous pooling of liquid in for instance, a steam line. Pooling can cause build up of slugs, or larger and unsafe midspan deflection, due to the increased weight.

Dear MoverZ,

Again, thanks for you explanation. I quote this from Sam Kannnapan's Book. "The maximum allowable spans for horizontal piping system are limited by three main factors: bending stress, vertical deflection, and natural frequency. By relating natural frequency and deflection limitation, the allowable span can be determined as the lower of calculated supports spacing based on bending stress and deflection."

I would like to know the reference for the factor you mentioned. I don't know if Sam Kannapan already added in the new edition. I have the old one.

When I write assumption (1) end moment neglected; I do really understand that this will give most conservative result. For simply supported beam +Y support for both ends is the example which cause moments are zero at the both ends, higher deflection which will produces lower fn instead of ANCH.

If one day there is a equipment which imposed 1000 cps dynamic load to our piping system, do we need to revise tabulation of pipe span length?

Anindya,

Although your advice to young ones is excellent. They should do like exactly as you suggested. But I feel if somebody has asked some question, it should be answered. People learn by asking (their supervisors as you suggested), learned people like you, studying, experience, internet and sometime the hardway i.e. failure.

Some of our seniors did not have internet and they learned from all above (minus the internet). They wanted the juniors to do all the exercise they did when they were young. This reminds me of one of my seniors. He used to tell us that in old times, we did hardwork and learned, you people have many facilities for learning, you have easy jobs. And that gentleman had the habit to put us in (physically) difficult situations (unnecessarily). Once he made me count / check all the supports received from fabrication shop in the open yard while it was raining and the temperature was minus four. But I dont put my juniors in situations like this. While they should know the source from where they can get the information, the answer to their immediate query MUST be given.

Samsul,

I am getting confused here. Are we discussing one special span or spans in general ?

If it's one span with 1000cps then do some serious dynamic analysis.

If you are covering spans in general, as most engineering companies do, then create a chart that covers MOST situations, and is simple to apply. Kellogg Design of Piping Systems section 8 details the equations normally used ... and that book is dated 1955, still used by most of us today. It covers stress and deflection limitation. Local stress is an obvious further consideration, usually based on data from Roark.

Dear Moverz,

I am sorry, I am just kidding . Of course that is a big big frequency for dynamic load . We know that the rotating equipment impose the dynamic load in low frequency. Again, Thank you very much for the reference.

Dear Shafiq Rafiq,

Yes, the first time I want to learn Caesar was when have a college. I had a little reference, not enough money to enter training, no access to people who work in such engineering consultant, etc. Learning alone. For the such situation it was very easy for me to stuck. I really remember the first email I send to Coade the same like question addressed by Oackdarcy. "Dear Sir, why there is no allowable stress for OPE load cases for ASME B31.3 code but I already checked it in input piping?"

Coade answered me. For ASME B31.3 the primary load (W+P) and secondary load (T) cause pipe fail in different mechanism yield and fatigue which assumed has no interaction each other. That's why need to check separately. But if the SUS and EXP passed, it will be fine during operation. Thanks to them. Sometimes I think if we want to give the minimum answer, by giving the reference or a link is good one.

Samsul,

Besides the three parameters mentioned by you (with additional notes from MoverZ), there is an additional parameter also ie high temperature service.

Dear Anindya,

Thank you very much for reference relate to this additional parameter. I would like to revise:
fn = 1/2*phi *(g/deflection)^0.5 = 3.12/(deflection)^0.5
Deflection = (3.12/fn)^2 ; fn = natural frequency ; g = acceleration of gravity. Let say that 4 cps is the sufficient natural frequency. We can solve the deflection 0.608 in.

Dear Shabeer/Srini

you can calculate the Maximum allowable support span in two way
based on Mid-span deflection and Based on stress limitation,
above this two method which value of the result is minimun that value you can say the (safe) maximum allowable support span.
Based on limitation of stress- Span L= SQRT(0.4*Z*Sh)/W
Based on limitation of deflection span L= 4SQRT{(Delta*E*I)/13.5*W}
where,
L=Maxi allowable support span.
Z=section modulus
Sh= Allowable stress in hot condition (as per your ope temp.)
W= total weight of pipe (including insulation,content)
Delta= mid span deflection
E=young's modulus
I=Moment inertia.

Thanks.

Jernas Sam,

Did you not read the trail of submissions and replies to this topic ?

You have merely repeated stuff from above.

What is the point of that ?

good evening!friend!
I want to ask you some doubts.I am new to learn caesarii, i encounter many difficulties. such as :i dont have any imformation about caesar ,so i learn very hard,i only can read caesar help .
what is your experience in learning it ,do you learn it in your colledg.
are there some books about caesar in the libary when you are in colledg?
i now have nothing,so it is hard for me ,i want to ask you for some caesar
imformation,do you have some ?could you please give any?thank you !
waiting for your help!

Don't double post.