Expansion loops OR bellows in pipeline? 1

[SFuller]

Dear All,
We are installing a 8" new steam line on site. The line is a branch from the site main header and will contain 7 barg saturated steam. The end points for the line are slightly off-set from each other so there are two 90º elbows in the line with a short section running between them.

When we had stress analysis done on the line the engineering consultant indicated that we needed to allow for some movement in the line due to expansion. They gave us two options:

1 Install a bellows in the short section that runs at 90º to the general direction of the line to allow for lateral movement of the bellows.

2 Install an expansion loop in the line by fitting a few more elbows and allowing some extra pipe.

Obviously, either solution should work if properly designed and installed.

As a process/chemical engineer I am inclined towords using the second option (an expansion loop) as any refernce book on industial disasters is full of references to bellows failing. My mechanical colleagues wish to install the bellows as it is a cheaper solution and will save them from having to re-design the line to accomodate the loop.

Do any of you have an opinion or good/bad experience of either solution that you could share with me?

Thanks,
Steve

 

[GregLamberson]

Check out this thread, apart from deteriorating into a discussion on the English language, there may be some help.

http://www.eng-tips.com/viewthread.cfm?qid=160013&page=7

I have had no experience with bellows, so obviously expansion loops are the way I would tend to go unless you have some restrictions (space) that would preclude them.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BigInch]

That is not a choice. Use the expansion loop and always use an expansion loop, unless you have to protect critical sensitive equipment from pipe that can not be otherwise contained, or constrained by using as many expansion loops as you can fit into the plant.

Every expansion JOINT you DO NOT use now will allow many hours of maintenance free operation which will easily pay for the extra pipe it takes to make a few loops.

http://virtualpipeline.spaces.msn.com

 

[JohnBreen]

Hello SFuller,

I am also with BigInch on this - if you have room for the expansion loop you should go with that. Expansion joints have their place in design but the loop will eliminate many other issues.

Sometimes designers need to be reminded that the bellows expansion joint is one third of an expansion compensation SYSTEM. The entire system also includes "anchors" and guides (near the joint to assure axial compression). Of course this is also true for the expansion loop to some degree. The piping designer will always have to consider the axial thrust due to internal pressure and axial thrust due to expansion (this is what compresses either the expansion joint or the loop) regardless of the choice between a loop or an expansion joint. These forces act upon the "anchors" and these "anchors" must be designed to accommodate them. The moments acting on these "anchors" can be managed by placing guides near the expansion joint or near the loop. The Standards of the Expansion Joint Manufacturing Association (EJMA) should alway be consulted in designing a piping system that employs an expansion joint.

Regards, John.

 

[MikeHalloran]

Bellows are used in marine exhaust systems solely because loops won't fit in the boat. They are treated as a consumable.



Mike Halloran
Pembroke Pines, FL, USA

 

[SFuller]

Dear All,

Thank you for your opinions and expertise. The message that I am getting from your posts is that the expansion loops are the way to go and that bellows are only used when nothing else will fit.

We will re-design the line with a loop in it.

Regards,
Steve

 

[Montemayor]

Steve:

Too bad you won't name the "consultant" who gave you the choice between a potential hazard and the safe way to go without stating it as such. That is one bad or unethical professional. If we knew his/her name, they wouldn't be allowed to give consultation on these Fora.

Thanks to your initiative and common sense, you consulted with our guys. The advice you got is 100% right on the button. Experience tells you to always go with the natural expansion loop unless you don't have that option. If you use a bellows know clearly up front that it has a definite life span and is subject to plugging and corrosion - among other things.

 

[BenThayer]

i am concurring with everyone else with regards to the use of the loop and not the bellows.

but two things are bugging me:

1) What pressure steam was this for? I might be inclined towards less condemnation for the designers if it was for 15 psig for HVAC on an unaccessible trestle and not 300 psig for distribution in a building(but it was an 8" line).

2) Do you trust them to design the expansion loop after they made a recommendation of the bellows?

 

[MikeHalloran]

The pressure was specified as 7 bar, gage, saturated.

As the OP wrote it, the recommendation was to put one bellows in the short dog-leg between the two long sections of pipe, so that the bellows would be subject to lateral displacement as the line changed temperature.

Since that's just about the worst way to install a bellows, and it says so on page two of the catalog, I'd be _real_ careful about using that consultant.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ashereng]

I would prefer to use expansion loops.

I would use bellows when I can't use expansion loops.

Using an expansion loop is neither good nor bad. It has it's uses and when done properly, work as designed - like everything else.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[RossABQ]

JohnBreen's comments are the heart of the matter, this should be an engineered system from start to finish. That the engineer has a "short section" between the two straight runs, indicates they are pretty clueless IMO. That they then suggested putting the bellows in the short section, perpendicular to the growth, is your second clue that they are unaware of what is proper steam system design. Just curious, do they have a slope and driplegs on the run?

 

[BronYrAur]

I couldn't believe the suggestion of bellows placement either! Perpendicular to the run of pipe??? I think the decision has been made, but just one more footnote and a question of my own.

With bellows, the anchor loads are substantially larger than with pipe loops. This can be easily overlooked with comparring costs of the two options. You may need substantially more steel, concrete, labor, etc. for the bellows anchors.

Now, what are everyone's thoughts on the flexible jopints, such as Metraflex's Metraloop?

http://www.metraflex.com/metraloop.php

I used them once in the past with no problems ... that I know of.

 

[BigInch]

Bron, Please make a new thread. I'm tired of this one.

http://virtualpipeline.spaces.msn.com

 

[tigny]

Hi,

I don't think the expansion joint placed in the small perpendicular section is such a bad option: if it is a universal tied expansion joint, its purpose is to allow for lateral displacement. [better knowledge of overall dimensions would ease the choice between loop and EX joint!]

You guys who do not agree with such a design, are you talking of just a bellow without tie-rods?

yours,

tigny

P.S. I would also first propose a pipe loop.

 

[RossABQ]

Expansion bellows are not meant to take up lateral displacements beyond a fraction of the axial movement.

 

[jistre]

Since this horse has been out for rebeating, my problem with this is that the joint is being considered for steam service. In any potentially lethal service, bellows joints designed for a life on the order of thousands of cycles scare me. Preventative maintenance of the joints by replacing them before they begin to get into the end of their "safe" life mitigates the problem. However, consider the fact that the maintenance crew could easily bang the bellows portion of the joint against steel and dent it (or cut or abrade it if it's elastomeric) then not notice or not care. Then you have a situation where you have an expected life on the order of thousands of cycles, but due to the damage to the joint have far less. Then Joe P. Worker is walking by the line when the joint is going through an excursion and ruptures and then gets nailed with live steam or caustic or HF acid, or any of a number of horrible fluids. In any of these situations, my response has always been to use a loop, and anyone who disagrees is more than welcome to take the design and take responsibility and sign off on what I won't. They rarely fight when you put it in those terms.

I'm not saying I haven't used these joints before; they have their place, and I've used them on non-lethal services where the worst that would happen in a rupture is a low-grade thermal burn and a big, stinky mess. In practice, I limit it to services that won't kill workers in a spill, won't explode in a spill, won't catch fire in a spill, won't disfigure workers in a spill, etc. If the plight of the workers doesn't move you to think hard about where and how these things are used, the plight of you if your design kills someone should.

 

[zerosum]

I have designed steam lines up to 40 barg and also used bellows and universally tied expansion joints for other hot process fluids. I strongly concur with the other opinions here. Use piped expansion loops for steam unless there is a very good reason not to.