Mine shaft piping stress analysis 2

[DGStavlas]

The consulting firm I work for acquired a contract for the reconstruction of an underground mine pumping station. Because we are a small company we asked for the help of a piping stress analysis specialist. Although I don't know the subject so well so as to judge his work, I have some questions that I'd like to share.
- The specialist proposed anchors every 6m for the shaft's pipeline. As far as I know anchors should be placed in elbows, valves etc and not in straight pipelines. If anchors must be placed in straight lines then expansion joints are needed. When I asked him this question he said that due to the nature of the project (maintenance difficulties), any other type of support wouldn't work for the mine workers and that the only result will be a minor oversizing of the supports.
-I tried to check his results only for the shaft's entrance bend with this type

http://www.engineeringtoolbox.com/forces-pipe-bends-d_968.html

but the number's were totally different. I imagine that is the result from the placement of the anchors. As far as I know stress analysis is checked between anchors so the resulting forces from the pipeline's weight are not calculated. The other thing that seems rather strange is that when I asked him to change the operating pressure, the resulting forces at the sustained load case didn't change that much. Probably I'm missing something fundamental here.
-Last question pipe shoes are modelled as anchors at softwares or as an other type of support.

Thank you in advance.

 

[SNORGY]

It sounds to me like the confusion arises from the nomenclature being used. Did he mean "anchor" or "line stop + guide" or was a gap specified, etc? More to the point, did he mean "anchor" as would be defined as "full retraints in X, Y, Z, Rx, Ry, and Rz" or as would be defined as "something to bolt the piping to the shaft wall so it doesn't fall down"?

 

[DGStavlas]

SNORGY first of all thank you for your reply. He defined anchor as full retraints in "X, Y, Z, Rx, Ry, and Rz".

 

[gfdoug]

In my experience, operating pressures are not the driving cause for increasing resultant stresses in most cases - generally the highest stresses are going to occur during your Gravity+Temperature(Max,Min) cases.

 

[miningman]

I always get a kick out of posts like this. I have contributed to a number of mine shaft pumping queries on this site. I have NEVER needed to request a stress analysis for a shaft discharge line, I would recommend supports every 3 metres not6. You need to learn what the maintenance considerations are in a shaft discharge system before you can hope to come anywhere close to an optimum design. Geographically, where is this project , is it a concrete or timber shaft, what other infrastructure has to be installed in the shaft?? All of these are critical questions.

 

[DGStavlas]

Dear miningman the shaft is an existing concrete shaft.Apart from the shaft's discharge line the rest of the infrastructure will remain untouched for the time being.You recommend supports every 3 metres not 6 but my question remains unanswered. Do you mean anchors such as as full retraints in "X, Y, Z, Rx, Ry, and Rz"? Another critical question would be..how do you achieve pipe allignement in the existing shaft ?

Thanks for your help!!

 

[LittleInch]

DGstavlas

The problem we have is that we have about 5% of the information to comment properly. So far we don't know
Size of the pipe
Pipe material
Pressure rating
wall thickness
orientation (horizontal / vertical/ angled)
length
temperature normal / running
pipe connection (welded / jointed / push fit
orientation of pipe (isometric)
Design code of pipe
External loads

I could go on, but I think you get the drift.

Full x,y,z,rx,ry, rz anchors every 6m sounds quite odd to me, but maybe it just made the analysis easy?

If you anchor things then you can get high stresses if you don't allow some movement, but it all depends - the pipe may be able to withstand it espevcially if it's thick and the temperature change is low.

also some aspects of your post are really quite puzzling without all the info you have e.g. "any other type of support wouldn't work for the mine workers" Eh?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DGStavlas]

Dear LittleInch thanks for your reply & your useful comments.

Size of the pipe : 12"
Pipe material : ASTM A-106 B
Pipe Pressure rating : Sch80
Max pressure: 65 bar
wall thickness : 17,48mm
orientation : vertical
length : 350m
delta T : 5 °C
pipe connection : flanged
orientation of pipe : pump,bend,horizontal 50m,bend,vertical 350m,bend,horizontal 20m,discharge tank
Design code of pipe : ASME B31.1
No external loads

 

[LittleInch]

So basically we're looking at a 12" sch 80 pipe with a 350m vertical riser section with very small temperature variation.

A single support at the top with a 350m length hanging off it would seem to be stretching things (literally), but a full anchor support every 6m would also seem to be excessive. However, it will reduce the size of the support as the vertical pipe load will be shared amongst many others and any support failure would not be catastrophic. It's the full anchor bit that surprises me - I would have thought vertical supports every 15 to 25m and horizontal guides would have been enough, but maybe the difference is not worth bothering about and makes the vertical support too big.

there is a practical aspect also - as you say this is flanged I can only assume that you're planning on building this pipe in sections so multiple supports would be safer and easier to support. Getting a multiple flanged pipe in a dead straight line is wholly dependant on how accurate you weld the flange on. It's difficult to make up any angular misalignment on a flange

Also how do you attach the support to the concrete shaft? do you have space limitations?

did you design this pipe and then ask the stress man to "stress it" or did the specialist also design the pipework?

I can't see any big issue in having full anchors in terms of stress given the low temperature variation and really quite thick pipe, so maybe the specialist did this as a worst case run, it turned out OK so he stopped doing any other runs?


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DGStavlas]

You write..."I would have thought vertical supports every 15 to 25m and horizontal guides would have been enough, but maybe the difference is not worth bothering about and makes the vertical support too big.".
This is what I was trying to ask but probably I wasn't that clear when I wrote "any other type of support wouldn't work for the mine workers". I was thinking something like your proposal (anchors+guides) but he said to me that guides need maintenance and that is something that the maintenance department doesn't want.

As I can understand you think that the piping is oversized.
Finally the support to the concrete shaft is assigned to a structural engineer so I can't say really much.

Thanks again for your valuable help.

 

[LittleInch]

I doubt it will change anything, but for your education (and mine) just ask him to detail what maintenance a vertical pipe guide needs and how often? I can't think of anything.

I have no idea about the piping, but full anchors every 6m looks excessive.

However, the devil is in the detail which you and the stress man have and I don't so it's difficult to be any more positive than that.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DGStavlas]

As I said in my first post I don't have the experience to confront him. I'm asking those questions in order to understand as much as I can the problem. To be honest, I can't think any other detail that I'm not sharing with you.
Let me share a thought with you..
Because as I have said already the shaft is existing and we don't know its strength, full anchors every 6m will reduce the size of the support thus making its attachment to the concrete easier.

How does that sounds ?

 

[SNORGY]

What are the pipe joints? Welded / flanged / mechanical / hammer union? Is the stress analyst aiming to take credit for some axial growth into the joint(s) along each segment of pipe?

Anchors every six metres, if you have any dT at all, probably hurts the system more than it helps, at least until enough of them break that you end up with the number you should have had in the first place.

 

[DGStavlas]

Dear SNORGY the pipe joints are flanged. As far as I know no he doesn't aim to take credit for some axial growth into the joint and yes there will be a delta T of 5 degrees.

 

[miningman]

I have a lot of respect for Snorgy and Littleinch, based on their oil/gas posts they know their stuff but you are all getting out of your area of expertise. The pipe will be prefabbed at nominal 20 foot lengths. It will be flanged to accomodate the approx 450psi from the hyrostatic head( plus the frictional losses that I tend to neglect on this type of job... poor engineering , yes , but its practical and works for me every time). Unless somebody screwed up big time during the shaft sinking there will be threaded concrete inserts that have been cast into the concrete during sinking. You obviously need details of the spacing to design your brackets. And I will guarantee that the miners will have to change out at least three pipes during the first few years of operation........ gaskets fail, pipes get damaged due to falling debris in the shaft etc etc.I forget the weight of a 20 foot length of sched 80 c/w flanges but as a designer do you want to volunteeer to help them if the supports are anything other than two supports per length, so when you pull out the bad one, all the other remain undisturbed. Remember wil be standing on a make shift flatform with perhaps an 800 foot vertical drop below you.

If this is'nt adequate for you, go find an experienced shaft miner.... he doesnt need to be an engineer but he will set you straight. You have admitted to being inexperienced so this is not meant to be a major criticsm , more you are gettingg advice from the wrong people.

 

[miningman]

Rereading some of the posts here , perhaps I need to explain that in mining, a shaft goes straight down vertically for the full depth with zero offsets at any point. This is not a tunnel in the side of a hill, nor is it a ramp at perhaps 15% grade.

 

[LittleInch]

Miningman,

No problem in getting info from those who have seen the realty of life in those particular situations. I gathered it was vertical eventually, but I suppose w all have different practical aspects to deal with. I probably wouldn't have designed it to be made up of 100 or so flanged joints, but then I can't figure out why you would needs to replace sections after a few years either. As said less anchors mean bigger single ones and that may not be practical.

It might sound stupid, but do you not get access down a shaft by a wire and basket type things?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SNORGY]

Does piping in a mine shaft behave differently compared to piping strapped to the side of any other vertical structure?

FL/AE = &L(T2-T1); I get F = {insert your number} kN and an axial compressive stress between anchors (due only to the dT) of {insert your own number} MPa. Are they "Ideal Anchors", or how stiff are the anchors, and what is the stress analyst planning as mitigative measures against what I perceive might be somewhat high loads generated between them?

I'll check my own numbers again, but this is why I am asking the question.

Sadly, my experience in these fora over the years has conditioned me to not offer any calculation results, because in the event an error might be found, the numerous nay-saying "error vultures" will latch onto it and the ensuing thirty posts will degrade into attacks on the competence of the individual trying to offer help.

It actually doesn't matter if its piping in a mine shaft or anywhere else. Can the concrete (and, *especially the concrete*) accommodate the loads?

 

[miningman]

Well in the interests of broadening everyone's general knowledge background , yes Littleinch we descend into the depths on wire and basket type things but I've never heard the systems so eloquently put. We call them cages on ropes. The ropes are typically 1-2 inches in diameter, ( and are of course steel cables) the cages hold anything up to 50 men and travel between 5 an 10 meters per second depending on depth and a few other technicalities. The entire hoisting systems are designed with F of S of 10:1 which is what you want if that 1inch rope is the only thing holding you against a very swift death. There tends to be very little clearance between the cage and infrastructure mounted on the shaft walls... often around 10mm so any movement of the pipes etc is unacceptable... regardless of temperatures. The atmosphere in the shaft is generally very wet , 100% moisture, often with H2S, NO2, sometimes acidic to the point of pH of 4 or less, sometimes with major brine concentrations leaking into the shaft.... each one is unique but you get the idea.

Maybe I'm a very poor engineer but I'm close to the end of my career and its way too late for me to even ask " what does Rx, Ry and Rz mean?? Similarily Fl and AE?? Perhaps that tells you how little structural engineering I've performed over the last 40 years but I'm pretty good with explosives, both theory and practical usage, and I do have some other knowledge which is perhaps almost unique to the underground environment so hopefully you'll cut me a bit of slack on the theory.

I would think that the inserts I would expect to find would take 0.75 inch bolts , possibly 1.0 inch. The brackets might be dedicated to a single pipe column but I would not be surprised to see "multi pipe brackets " which would hold the 12 inch pump line plus perhaps a 8 or 10 inch compressed air line plus a 4or6 inch water intake line and maybe a couple of 4160 or 13.8 kv electric feeders as well. Each shaft is unique but you should be getting the idea by now. And the concrete is nearly always sulphate resistant but perhaps what I would regard as normal deteroriation , you guys would regard as being horrendous. Its all a bit subjective I suppose.

 

[SNORGY]

No disrespect intended towards anyone.
The OP asked if pipe anchors spaced 6 metres apart is a design that ought be be questioned. Pipe anchors, when specified by a piping stress specialist, have a very specific definition with very specific implications. In this very particular instance, my very rough calculations for the anchor force developed between ideal anchors by 12" SCH 80 A106B pipe came to 205 kN (46000 lbf) and a compressive stress of 12 MPa in the pipe. The pipe is fine, but the theoretical anchor force is high. My questions are centered around trying to establish what other assumptions or measures the stress analyst has taken that would suggest that the loads are really much lower than that and that, therefore, the "anchors" are fine.