Piping support from ground for a vent line 4

[somacast]

hello,

I posted this earlier in another forum and one helpful member there gave me some good answers however I still seek more please ...

We are facing a problem with a vent at our plant, and it was agreed by all parties that it needs to be re-routed to a further point.

The vent pot is 12 meters high, small in size and weighs about 60 Kgs, and in order to maintain the vent and due to pressure drop its vent pipe shall be enlarged to 6 inch pipe.

so to make things clearer its a 6 inch pipe starting from 6" x 3" reducer, and running horizontally for 6.5 meters , and at the 6.5 meters we shall introduce a support from the ground made of 10" sch 40 carbon steel pipe, this support is 12 meters high , and the vent line will be anchored to it by U-bolts..

my questions are if someone can answer them please:

1- I did basic calculations considering a wind speed of 92 mph, and assumed worst case scenario that a resultant force of this wind load will act at the top of the 10" sch 40 pipe, and the bending moments were fine at its base taking an allowable value of 21000 psi (approx. 0.6 Fy), so my question is will the 10" pipe stand by its own from ground given that a proper foundation will be provided underground with good pedestal?

2- do I have to worry about the thermal expansion of the 6" pipe stressing the 3" end ?

I know that the second question might not be so relevant to the forum but I hope someone might find a way to do it.

Note: my company lives in stone age, no software, the only software I have is a casio calculator , and Iam a general Mechanical Engineer and no one else to do this and they throw such non-related works on me once a year at least, so I have to give an answer , thnx

 

[pennpiper]

somacast,
Thank you for including a sketch (nice job).
Now some additional information that may help.
- Is this Vent a continuous or intermittent flow?
- Is this HC & STM (V) Toxic?
- How close is the nearest platform (Horizontal & Vertical)from the vent outlet?
- What is the direction of the prevailing wind relative to the Vent outlet and the nearest platform??

Sometimes its possible to do all the right things and still get bad results

 

[chicopee]

Looking at the diagram, I see that the 6" horizontal run has only end supports at nearly 20ft. apart. I view this set up inadequate and I would proceed with caution and that is to have two additional supports all spaced equally. You should also pitch, in a downstream direction, the horizontal run at about 1/4" per 15 ft. Just make sure that if you have significant reaction forces from the vent relieving that the system is designed accordingly. As far as thermal expansion, the vertical 10" pipe at about 36' long may be able to flex enough, however, I would rather choose a self standing, 4 or 3 leg tower and devise an anchoring system allowing thermal movement of the horizontal run.

 

[somacast]

@pennpiper , thanks for the reply and your words, it is continuous vent, however its not a big stream, and its not toxic. platforms are approx. 5 meters away horizontally, and vertically its just the ground level is the nearest which is 11.8 meters, and prevailing wind I don't have it and Iam not sure if it is available in the as-built documents, but I can tell you its an unclassified area, and the location was mutually selected by concerned sections as no other alternative places near by... so kindly tell me what do you have on mind ?

@Chicopee thanks for the reply, regarding the unsupported span, I have noticed it as well, however by referring to table 121.5 of ASME B31.1-2001 it suggests 6.4 meters for gas services, and they assume a much higher temperature than what we really operate at, so I assumed that 6.5 meters is safe, am I right? and for the 1/4" per 15 ft pitch, could you please explain more? and if you can refer me to an example of those self standing towers also?
regarding the anchoring I was thinking of anchoring the 6 inch pipe on the H-section by U-bolts at first, but that would restrict it in all direction, so a slider foot would do as it will allow the movement in the thermal expansion direction.. kindly feel free to correct any of that

 

[LittleInch]

Somacast,

As this is your first post I'll cut you a bit of slack, but double posting the same question in more than one forum is not allowed. At the very least, proved the cross reference to the other forum so people can see what has already been said -

http://www.eng-tips.com/viewthread.cfm?qid=409224

You make the point about some mysterious table saying you can get 6.5m between supports - Seems a long way to me and you'll find this is based on a continuous pipe on many supports, not two point loads. Also it will be based on supports close to the ground, not a pipe dangling in mid air exposed to wind. You may well find you will run into vortex induced oscillations at this diameter and a reasonable wind size. Quite an interesting phenomena to see in practice.

It's usually better to let pipes move when they need to expand due to thermal effect and introduce vertical and horizontal guides to stop the support jumping off in the event of sudden shock.

TBH, I don't understand why you're using pipe. A decent H beam or even channel is much stiffer and you have better flat surfaces to weld supports and strengthening beams.

For things like this, calculation provide part of the answer, but engineering judgment and a bit of might is right fills in the rest.

Twelve metres up is quite a long way - relatively small forces at the end will translate into some big moments at the foot of the support

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[somacast]

@LittleInch

Hi , and thanks for the reply, I did mention that I did post it in another forum, I did not know that forums rules states that I should mention the link, and I said the person over there was kind and helpful as well, but I still seek more answers so I posted over here as well as I thought its relevant as well, sorry for any inconvince due to this.

now for the topic, I tried to give as much data as I could, and I don't understand why the table is mysterious, its a table from ASME B31.1-2001 page 46, kindly correct me if its not applicable, as I find it quite a common reference in many related pages on internet, however that is the reason behind my post in Eng-tips, I need good opinions about it, and appreciate yours and shall consider it,

Regarding usage of pipes, its an oil & gas company and since its establishments decades ago they have always followed day-one practices no matter they were right or wrong, and this includes using pipes for supports everywhere... I will check a suitable H section to satisfy the bending moments at the base..

I did perform basic calculations, taking a wind speed of 92 mph , which is approximately equals to wind pressure of 126 kg/m2 , and assumed it acts on all surface area of vertical support and horizontal pipe, and as a worst case ever took the equivalent force (pressure x area) as if it acts on the top of the 10" support, and a 10 inch sch 40 seems to have enough section modulus to withstand the bending moments acting on the foot of the support, and although its a slender column as you can see in the attachment, the direct loads are not much, however it seems to have quite a big deformation at top if such load happens, which would lead to excessive loads on the 3 inch side.

H-section instead of a pipe seems like an excellent idea, I will try to find a suitable section for the same.

again I would appreciate any further discussion from you, thanks a lot.

 

[Tmoose]

What is the 400 mm long 4" X 6" H beam (item 20) going to attach to via the welded 6 mm thick flanges on the end?

 

[chicopee]

The pitch that I mentioned for the horizontal run is to drain condensate that may form within the pipe. The pitch would also assist in the removal of debris when venting takes place. Debris may come from pipe internal corrosion.

 

[somacast]

@Tmoose item 20 was added assuming additional supporting to the up rising vent pot, nothing welds to it just was intended to support the extended part of 6inch pipe before the vent pot.


@chicopee ok I got your point now, its valid of course, however if such line is not connected to a tube all the way to the ground no one is gonna eveven care about due to elevation, however would consider that thnx.

Iam still looking for an opinion in my reply above, is the approach as a basic correct here? thanks again

 

[somacast]

still looking for help , and if I may ask what is the allowable sag in a horizontal pipe run? as this pipe will deflect under its own weight at the mid span (3.25 m) ... thank you in advance..

 

[LittleInch]

Ok,

Will this stand up - yes, no problem.

Is it a good idea - don't think so. My issue isn't deflection or force but simply the inherent flexibility of both the support and the pipe. Wind isn't steady, nor is vortexing so the key issue will probably be vibration and fatigue at the connection of the pipe (3") onto the nozzle of the vessel. Working out what the natural frequency of this thing is will be difficult. Far better to simply make the support more secure - Basically an H frame or A frame would be much better as would an intermediate support. I looked at the reference and it is a maximum span based on something like a 5 or 10mm midpoint deflection, but is clearly based on ground level supports which are not subject to wind loading or vortexing, either of the main 6" pipe or the 10" pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[somacast]

@Littleinch

many thanks again, that was helpful, I will see what can be done accordingly..

 

[SNORGY]

I'm not sure if I should chime in without actually crunching the numbers or doing the calculations.

What I will state is this:

(1) Calculations are warranted to verify acceptability of design.
(2) It's not so much the pipe support that is at issue, as it is the pipe it is supporting.
(3) People have been putting 10"-D SCH 40 free standing flare stacks (for example) of this and greater height, c/w flare tip, ignitor, etc. for some time without having them topple over in the wind - even in places like Waterton, Alberta.

My gut check tells me that this is probably OK - but without the calculation, it's not engineering. Kudos for going the extra mile to prove adequacy via calculation. Sadly, too many out there wouldn't bother.

 

[somacast]

@SNORGY

Thank you very much for the excellent reply, I understand that without detailed analysis it might be difficult to tell,

I tried to judge this with basic calculations as I posted in an earlier reply, I can summarize it as follows:

taking a wind speed as 92 miles per hour (this is the wind speed taken by all EPC works when this plant was built), and corresponding wind pressure would be 126 kg/m2 approx. , and as this wind pressure acts on all surfaces of the support, the vent line 6 inch, & the vent pot, the area is known , so the resulting force acting in the middle is pressure x area , so to make this a worse case scenario I assumed that this resulting force acts on the top of the 10 support , and moments around its base were 6789 kg.m , so section modulus of 10" sch40 was good enough, so it was selected.
Also I was advised to check the deflection of both the 6inch and 10 inch pipes, and the 10 pipe according the assumption of the wind force I made above will deflect quite a lot approx. 240 mm, and the 6 inch pipe under its own weight wont deflect much only 2.5 mm.

Those were just basic checks but they were not enough to me to advise anything to me boss so I posted in 2 forums in eng-tips.