Design of High Pressure Steam Line on pipe rack
Design of High Pressure Steam Line on pipe rack
(OP)
Dear Colleagues, in a course of review of stress analysis in oil treatment plant I came across of NPS 24” Sch.100 high pressure steam line at 320 deg.C and 11000 kPag. Line is on a pipe rack about 220m long and has 7 expansion loops. The issue is that there are only 4 line stops. Two pairs of loops do not have line stops in between. I was explained that these 4 small loops were designed to act as 2 big loops, so thermal growth will be absorbed by both loops at the same time in proportion to loop size. CII shows 93% expansion stress in one of such loops. Stress is not liberal.
I doubt that relying on friction and not designing proper line stops between loops is reliable theory. In my view result of such stress analysis is just computer numbers based on ideal model with perfectly distributed friction forces that are in balance. In real life there will be start-ups and shut-downs and line will not be evenly heated. Also there is construction tolerance that may result in supporting structural steel being on slightly different elevation that will affect friction and consequently expansion stress.
Can you please let me know your opinion on whether or not such design can be dangerous and are lines stops between expansions loops absolutely required?
Thanks for your expert advice!
Vikoll
I doubt that relying on friction and not designing proper line stops between loops is reliable theory. In my view result of such stress analysis is just computer numbers based on ideal model with perfectly distributed friction forces that are in balance. In real life there will be start-ups and shut-downs and line will not be evenly heated. Also there is construction tolerance that may result in supporting structural steel being on slightly different elevation that will affect friction and consequently expansion stress.
Can you please let me know your opinion on whether or not such design can be dangerous and are lines stops between expansions loops absolutely required?
Thanks for your expert advice!
Vikoll





RE: Design of High Pressure Steam Line on pipe rack
RE: Design of High Pressure Steam Line on pipe rack
Thanks!
RE: Design of High Pressure Steam Line on pipe rack
What it doesn't change is that I agree with DSB123's assessment that such a design is poor engineering practice (although I take umbrage with the Mickey Mouse slam - the Disney imagineers have a remarkable reputation as producing very good quality engineering, but that's beside the point...).
Nevertheless, the choice as to whether it's going to work or not is your call to make. Someone is paying you to make that decision. So, make it. Consider all of the failure modes that any good piping engineer would consider (you do understand the failure mode rationale behind the different allowable stress bases, right?), and make a conclusion. The only one here with all of the applicable information is you, and it is going to take a frighteningly long time period to tease all of that information out of you if you continue to persist in outsourcing this engineering review to the internet. [/rant]
RE: Design of High Pressure Steam Line on pipe rack
I did not intend to make disparaging remarks about Disney Engineers - I was relating to the fact that whoever came up with that design approach was a "clown" or maybe you call them a cowboy (or have I now made an insult to the "home on the range" brigade??)
RE: Design of High Pressure Steam Line on pipe rack
RE: Design of High Pressure Steam Line on pipe rack
I have never been a PC type of guy - I just say it as it is whether people can accept the truth is their problem!!!
RE: Design of High Pressure Steam Line on pipe rack
Thanks!
RE: Design of High Pressure Steam Line on pipe rack
Hope this helps.
The other guys are correct in that you would want to control the design as much as possible with guides, line stops, etc.
RE: Design of High Pressure Steam Line on pipe rack
Your cavalier approach belies a serious misunderstanding of the failure modes of piping systems and the associated design margins against the specific failure modes.
If it were me, I would include in my assessment a sensitivity study into the effect of different values of the coefficient of friction. Friction only obeys one law: Murphy's! If you need it, you can guarantee its not there, and if it "hurts"you, you can guarantee that it's value will be MUCH higher than you expected. Flow-induced vibration will also tend to mess up whatever you expected, in the long term. Proved with extreme caution.
RE: Design of High Pressure Steam Line on pipe rack
Vikoll has already mentioned that delays with be very costly and delay start up. His analysis passes... is it the best design practice? No. Is your partner the hottest person in the world? No, but he/she is good enough as is Vikoll's design. Is it "cavalier"? Yes, but I believe the "Cavaliers" won the NBA finals last year... short point, they were good enough. I have had to design similar systems that have two loops with no support in between the loops. This is on reformer piping operating up and down consistently of various ranges extended up to 1700 F. They have been in operation for 15 years with no failures...
Will increasing cost, upsetting the client, etc. be worth adding two supports for a piping system that is already passing? This does not make financial sense at all.
A lot of young piping engineers spend thousands of extra dollars for supports that are not needed and they never think about additional pipe supports reducing the flexibility of a piping system which cause higher stress areas and decrease stress relaxation of the system. Not to mention additional pipe supports could mean additional structure steel, engineering time on structure steel etc. (more money) You have to consider your Engineering Economics courses as well as Engineering materials, etc...
Last point.. the pipe is going to look disfigured after years of operation regardless of how you support the system unless you cold spring every location to make it appear uniform in the hot position, but again no one does this and it doesn't make economical sense either. B31.3 design is for 15-20 years. Most plants only have a 10-15 year contract before they recertify/replace piping and equipment.
Adequate design is and adequate design... It is not a perfect design.
RE: Design of High Pressure Steam Line on pipe rack
I agree with TGS4, especially in the sensitivity study on the last paragraph of his latest post. If the sensitivity analysis gives you no problem you should try it. If the analyses show you there are some problem areas available you should report them. At least the PE or the owner of the job will know where the higher expected stresses (including failure if available), displacements, and support/nozzle forces are. I suppose PE and the owner will be very happy if they saw the problems earlier in case there are some. This way the operation/maintenance/failure of the system will be more cost effective.
rhino27,
You would be careful with the code issues in your posts with TGS4. First check his background on the code involvement, especially B31.3.
You remind me my supervisor, that I worked with 25 years ago for about 8 years, with your wording and several subjects in your latest post that I do not want to get into discussion here.
I am also sure that TGS4 was very honest with that word "cavalier" as well if you read it fully and carefully. I believe you misunderstood him (cultural clash on the use of word), and you should not be offended.
Kind regards.
RE: Design of High Pressure Steam Line on pipe rack
I also take serious umbrage with your statement
I have more experience than I would like to have with have with B31.3 (and B31.1) piping that is older than 30 years, and some approaching 50 years. A design for 15-20 years, based typically on design corrosion rates, is only at the EPC-stage, and it is for economic purposes only. Actual installed life will be much much longer.
The OP's design may indeed be "good enough", but that is their call to make, after sufficient due diligence. Neither you nor I can make that assessment without all of the details. But to make that decision needs a complete and correct understanding of the different failure modes applicable.
RE: Design of High Pressure Steam Line on pipe rack
RE: Design of High Pressure Steam Line on pipe rack
RE: Design of High Pressure Steam Line on pipe rack
RE: Design of High Pressure Steam Line on pipe rack
Except for materials designed for (and operating in) the creep range, there is no inherent design life in any of the piping design Codes. This imagined limit of 15-20 years (or some magical 175,200hr limit) may be typical of your experience with EPCs, but it is not found anywhere in any published Code or Standard. And it is most certainly not a fact - perhaps an alternative fact in your world-view.
The only thing that limits the operating life below the creep threshold is corrosion (and erosion), which is based on a corrosion rate that is highly dependent on the process fluid - and (with a few notable exceptions) the ASME Codes are agnostic with respect to process fluid; and the ASME Codes don't even provide any guidance on how to select said corrosion or erosion allowances.
DSB123 - thanks for the support. That is a good description, too. It's ok - from a business perspective I don't mind designers doing the wrong thing with the wrong mentality - the owners typically end up calling me in after 5-7 years of operation to fix the original deficiencies anyway. I'm maybe halfway through my carer and I've already seen a career's worth of screw-ups caused by this type of mentality.