3 replies to this topic

[blooma]

Hello all,

I'm a process Eng. with 2 years of practical experience but this question seems more suitable for this forum as it is a request for general guidance for beginers.

I discovered that I don't realy know when it is necessary to place an expansion bellow on my P&IDs. It seems like an unimportant fitting but somehow I have a feeling that adding the right expansion bellow can prevent a lot of problems in future maintanace.

Is it necessary in plastic lines? metal lines? long lines? is it just recommended?
Before or after a pump?
Are there any rule of thumb for this matter?

I tried to "ask around" in my office but I get a lot of contradictive answers so I would love to learn from your experience or get direction for the right books or articles.

Thanks,

Blooma

[kkala]

blooma:

Expansion joints are not located or specified by Chemical Engineer, it is responsibility of Piping Engineer (or Mechanical Engineer, if there is no piping specialist), as far as I know. The Process Department (I am occupied in) issues PIDs without specifying expansion joints, low point drains, etc. These are left to piping specialists for a later stage and often require visit to the site. Certainly these should be contained in piping arrangements / isometrics, issued after PIDs.

General principle of placing or not an expansion joint can be as follows: Imagine a pipe of length L between two fixed points (that cannot move anyway) at ambient temperature. When a hot fluid of temperature t (above ambient) passes from the pipe, it cannot be freely expanded by ΔL=L*a*t (a=linear expansion coefficient); so the pipe shall suffer a compressive force F (and stress as a result), as if it were contracted by ΔL=L*a*t. Hook's law permits calculation of F (and resulting stress) using Young modulus (matter of Strength of Materials). If resulting stress falls beyond permissible limits (so pipe can be disfigured), an expansion joint to "receive" ΔL is a good solution.

Other solutions may be also applicable, as a non supported pipe loop between the fixed points.

Lateral deflections (if possible) may alleviate mentioned strains; some experience is needed to judge the "fixed points"; Pipe Engineers often run software to specify pipe stresses (which can clarify need and location of suction joints).

There must be rules of thump (not known to me), which could be found in a Piping book. However following notes (subject to comments by others) may be useful.

1. Expansion joints are not usual in short pipes (even though stress is proportional to ΔL/L=a*t, not depending on pipe length).
2. Higher fluid temperatures increase the need for expansion joints (design temperature to be used in calculations).
3. Pump flanges (at either suction or discharge) should not be used as fixed points to "neutralize" displacements (ΔL). They can bear some forces (as defined by pump supplier) but are not intended to be the 'fixed points'. Separate "neutralization" of displacements in (α) suction pipe and (β) discharge pipe is recommended.
4. PVC has a linear expansion coefficient 4 times that of steel (wikipedia) and (I assume) much lower permissible stress. So expansion joints are also needed for PVC and generally for plastic pipes. Calculation and practices may be different (e.g. I have not found Young modulus for PVC).
5. An expansion joint can really prevent future piping problems. A way out is to pull the attention of the relevant Engineer by pointing out the (design) conditions of the "suspected" pipe. On the other hand any traced rules of thump would be welcomed for us to know.
6. For cryogenic liquids ΔL is negalive resulting in elongation (not compressive) stresses. I do not know wheter there are "expansion joints" for this case.

[Art Montemayor]

Blooma:

You are posting in the Student Forum, so your request and post is a little strange.
Expansion bellows are today recognized by most experienced engineers as a very special and rarely used equipment to remedy expansion problems. They ARE NEVER SHOWN on P&IDs (and they should not be shown here). The reason for their not appearing on a Piping and Instrumentation Diagram is that they – as expansion tools – are very site specific. That means that their application is important only at a specific, physical location within a piping network – and a P&ID cannot show this. They are usually regarded and tagged as SPECIAL EQUIPMENT and subject to scheduled, regular inspection and maintenance.

The reason expansion bellows are frowned upon and rarely used is that they are prone to site corrosion within the confines of the bellows and this is considered a very hazardous situation due to the fact that it is difficult to diagnose and to inspect. They are also susceptible to fatigue failure – in other words, they have a certain life-span, after which they are expected to fail (mainly due to fatigue failure). They also are notorious for accumulating trash, debris, and solids in-between their bellows – which immediately defeats their very purpose for being installed in the first place: this prevents their contraction, causes site-specific corrosion, and increases pressure drops – all of which are bad situations that should be avoided in a process plant. They also require detailed engineered structural support in order to ensure that their expansion and contraction is aligned correctly 100% of the time. If not, they will expand and contract in a haphazard manner and cause failure due to unexpected stresses in the bellows.

The only way to diagnose or to make sure required and necessary pipe expansion is in place in the correct and proper manner is by experience or by detailed piping structural analysis. A detailed piping analysis should be done everytime a process piping loop installation is considered, proposed, designed, or installed. This should be clearly drawn and seen in detailed piping drawings at the process plant or unit in question.

You are very correct in giving piping expansion potential problems the importance they deserve. But in my experience the best, most efficient, and direct manner of getting acquainted with and looking for potential piping expansion problems is to actually WALK-OUT the piping runs – preferably with an experienced engineer who is a mentor. I realize that mentor engineers are a thing of the past, but perhaps an older and experienced plant engineer can do you the favor of accompanying you.

Plastic lines – especially PVC – are notorious for their expansion (and structural support) problems and they should be inspected frequently. If you are in a plant or process location, you should have a NAVCO Piping Catalog always with you. If you can’t get this, you should surround yourself with other, similar literature and piping information. I know that piping catalogs, information, and literature are now almost extinct due to the availability of data on the internet, but some old-timers in your plant may still have this literature. It is important for you to familiarize yourself with all piping, materials, types, joints, gaskets, expansion loops (& types).

You should study and familiarize yourself with common and simple “off-sets” and why they are needed and vital in a normal piping layout. That is the usual, normal way of compensating for piping stress and expansion problems. Sometimes, where there are exceptional expansion problems, “Omega” loops are installed. Unless there is no physical space available, these type of loops are much more preferred instead of bellows.

[blooma]

Thank you very much Mr. Montemayor and kkala,
Your answers have been very helpful.