I am analysing a PSV piping . This piping is in fact boiler vendor's supply, however I need to verify the analysis due to contractual terms..
The vendor has attached a Pressure balanced expansion joint to the discharge elbow and the size is increased to vent pipe size after the EJ. This makes this one combined system.
The question I am concerned is DLF factor.The vendor has calculated DLF as per B31.1 appendix II. However I understand that since the outlet elbow is further connected to the vent pipe by EJ the PSV reaction forces can cause deflections on that piping also.So the simple calculation of DLF will not be applicable in this case. I modelled the piping from run pipe to silencer with the PSV branch as anchor and found the natural frequencies of the system. I used The time period calculated from this for DLF calculation.
Now the problem is that I get a higher DLF (close to 2) with this method. The vendor says they have always used this design and nobody complained (they have some big names in their client list including some international power majors). They have shown similar drawings used in their past projects approved by clients without any comments.Although they are not sure if the stress analysis was verified by clients .
Am I missing some thing? Any suggestions /Criticism are welcome.

Goodling's paper has a good discussion of how DLF's can be calculated. I will post it as a separate thread. But I recommend using a DLF of 2 unless you are very sure of what you are doing. It's the transient load cases that can get your name onto the evening news; as a stress analyst that's the last thing you want.

B31.1 Appendix II has a subtle note - if "...the safety valve installation can be idealized as a one degree-of-freedom system...", then you can use their approach.

You said you calculated the natural frequencies of this (sub)system. If you believe more than one of these natural frequencies could be "activated" by your applied load, then your system is not behaving like a single degree-of-freedom (DOF) system. Look at your mode shapes, if more than one mode shows that elbow rocking up and down, this Appendix II method may not be appropriate for this system.

But... should only one mode be activated by this thrust load, then use THAT frequency to calculate the DLF. Maybe that will help.

The idea here is that the mode shape activated by the jet load will be similar to the shape produced by a static analysis of the (amplified) point load on your piping.

One more issue - you say you started your subsystem with an anchor at the PSV branch. That's good of that point cannot move in the field. But should there be deflection on the main, then the Appx. II method is questionable as you probably do not have a single DOF system. The lead in to the B31.1 quote above says "If the run pipe is rigidly supported," .

Mr. Dave and Craig B

Thanks for the replies.I will go for DLF 2.

I am waiting for the paper.

Regards

Siv.

Mr. Craig B

Thanks for the paper.

Regards
Siv.

Can you share Goodling paper on DLF?

pradeepdarji@lntchiyoda.com

See http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=14930

Regards,
M

Thanks a lot mariog.

I seek experts view /advise on Dynamic Load Factor (DLF) for closed discharge system.

I am analyzing closed discharge PSV piping and applying reaction force at PSV exit point itself.

Initially vendor provided PSV Reaction force of 4000 Kg and we did analysis with DLF=2 and Reaction Force F=2 x 4000 = 8000 Kg.

Now vendor has revised Reaction force to 9000 kg and using the same with DLF=2, creating structural failure as PSV's are located at height and two sets of PSV's imposing 36000 kg loads horizontally.

Hence I want to use realistic value of DLF instead of using 2 straight away.

I am proposing below method which is presented in LC Peng book (Chapter-12 , Un-damped system subjected to impulse loads - FIG.12.5 / Ramped force profile) and in ASME B31.1 Appendix.

1. Valve opening time period = t = 0.08 sec.

2. First natural frequency of PSV piping is = 8 Hz(obtained from CAESAR-II model analysis results).

3. So natural period = T= 1/8 = 0.125 sec.

4. Ratio = t /T = 0.64 .......By using this ratio , LC peng, Fig. 12.5 & ASME B31.1 gives DLF close to 1.5.

5. With this PSV reaction force = F = 1.5 x 9000 = 13500 Kg.

Do you think above method is reasonable to use and no major concern.
I noticed ASME B31.1 graph is for Open discharge PSV system but could be conservative for closed.

Seeking experts opinion on above.

IMO, it is fully unreasonable to consider that ever two PSVs would synchronize opening together in 0.08 sec.

Second, Vendors specify the reaction force at the PSV's flange outlet neglecting the existence of the piping attached; they claim that, having no knowledge on your piping, it would be impossible to say more.
Of course this is a conservative approach, the only problem is when you try to say something more realistic about the subject. But in case you have no trouble dealing with a conservative approach, you don't need to comment it.

Thanks mariog.

Yes , I agree that both will not open at a same time and there would be time lag.

However approach shown above to arrive realistic DLF against direct use of 2 is suitable or not, is of my interest as well.

Any comments !

You may calculate it using the DLF Spectrum Generator; after that you can see your procedure is conservative or not and how much conservative is.

Be sure you are collecting the proper "first natural frequency of PSV piping" - your step 2.
CAESAR II calculates natural frequencies of the entire piping system and the first natural frequency of your entire piping system may not be fundamental frequency of your PSV piping. Be sure the mode shape associated with this frequency shows response in your PSV piping.

Thanks Dave.
Ya, system has lower modes as well but PSV piping fundamental frequency comes out to 8 Hz.

Thanks!