My question is on bracing of branches.

I have seen many consultancies and clients ask for bracings of small bore bracings to the header for header pipe pressure rating >= 600 Lbs or when the small bore branch is close to some source of vibration like connection to reciprocating or rotary equipments etc.

However when I have referred to the MTD guideline ,besides other parameters like location of small bore on header, branch length, no. of valves on the branch , header thickness etc. there is no reference to the pressure rating of the line. In fact I have made some sample calculations to find that the branch bracing is not required for a 900 lb rating line but required for a 300 lb rating line ( one simple reason is that , for the 900 lb. rating line, the header wall thickness is higher thereby having a less LOF or likelihood of failure). This is in contrast to the view in paragraph 1. Personally I have full faith on the MTD guidline and I feel that view in paragraph 1 is not having a solid scientific rigor, which the MTD guideline is having.

I would like to get the opinion of forum members on this issue.

This is not in context of high frequency acoustic vibration where bracings need not help rather can create problem due to asymmetry.

Regards

Anindya,

In my understanding...

Even though MTD guidelines does not address the issue of pressure rating directly, there is an indirect reference in para 2.3 of appendix A3 regarding unsupported mass. 600# and above valves and flanges are naturally heavy; consequently a heavier lumped mass at the free end will create vibration problems. Hence ur first statement!

And regarding asymmetry, I guess u r referring to the paper “AIV in high capacity pressure reducing systems” by Carucci and Mueller. My inference from this paper regarding asymmetry is; in case of small bore connections braced back to the main pipe, we are not restraining the vibrating main line pipe wall. Hence this case cannot be treated as an asymmetric discontinuity.

Pl. correct me if my reasoning is not correct.

Abdul,

Yes, unsupported mass ( i.e. no. of valves on the branch) is one of the parameters for LOF. But my question is " is the practise of using branch bracing for lines with 600 lb rating irrespective of other considerations,based on scientific vigor?". I guess not as you will not need bracing for 150/300 lb rating, where based on the MTD guideline , i.e. lesser wall thickness of the header, it may be required.Personally I agree with the MTD guideline.The reason for asking this question, is to get answer to the above as I felt that such specifications can result in vibration problem as all the parameters involved have not been considered, although apparently it appears that it is a conservative approach.


In AIV, the nature of vibration is circumferential i.e. shell mode, hence we require circumferential stiffeners to minimize this problem. In this context, I feel that the brach bracing is a source of asymmetric discontinuity.

Regards

If I recall correctly, the body of work that resulted in MTD guidelines for branch connections originated following a large number of failures in duplex stainless, which was a newish material offshore at the time.

Failures were mostly at the header weld toe, where it was thought excessive weld heat or mass caused localised hardening. Systems were commonly seawater, which due to DSS properties were run at higher velocities and in smaller diameters than other materials, and consequently were at risk of flow / turbulance induced vibration. I believe that comparisons with reinforced and simple stub-in branch connections showed lesser problems therein.

I personally doubt that there is much more than an risk / economic decision to limit bracing to 600# and above. However as pointed out already, branch valve mass increases significantly with pressure, so does header wall thickness and consequently stiffness and this may also be a contributory factor in development of excessive local stress.

There is a strong argument for bracing all free standing small bore connections ... for higher pressures, reasons as above and lower pressures to add mechanical strength, if for no other reason than protection from that 240lb hairy ar$ed fitter climbing up the pipe rack.

On my current project, the client has asked us to brace ALL header branches less than 2" NPS per NORSOK L-CR-003 regardless of rating.

With reference to MTD Pub. 99/100 " Guidelines for the Avoidance of Vibration Induced Fatique in Process Pipework", the copyright of the original Guidelines was transferred to the Energy Institute and the Guidelines have been re-written to increse the scope and be much more user friendly. The final draft is out for review now with publication scheduled near the end of the year (I'm told).

As a good engineering practice, it is better to brace all <2" free branches, which are closer to a vibration source.

But beware .... vibration sources are not always reciprocating machines. Purely flow in duced vibration has caused many branch failures in the past. Many such problems are only evident after start-up, when fitting some sort of bracing is a real pain.

Flow Induced Vibartion can be a very broad terminology. However by this term we mainly refer to vibration caused mainly due to pressure pulsations due to high turbulence and boundary layer separation at elbows, valves etc.The lines susceptible to the same is best identified by screening using a LOF parameter and a cut off velocity of 0.5 Mach is recommended.The solution methodologies , besides branch bracing, also involves the computation of pulsating drag force , which is the dynamic pressure tims area times acoustic and mechanical magnification factors and a form factor. The bending stresses due to this pulsating drag force is computed and it is recommended to keep below an allowable limit, say endurance limit.

Regards