Expansion Joints: SS Bellows vs. Elastomeric 2

[KernOily]

Hi guys. I'd like your opinion on the following piece I wrote to document the selection of expansion joints for a recent plant project. The project is a crude oil treating plant expansion. Joints in question are all operating at low pressure, fluids are crude oil and produced water, pressure is 40 psig and less, max temperature 220 F, and not in 31.3 lethal service.

Please read, and fire away. Thanks!

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For joints that are located up in elevated piping, metal (SS) bellows-type joints were selected because, in those locations, they are protected from damage by impact. The bellows element is fragile, so bellows-type joints are not a good idea in plant areas subject to frequent visits by operators and areas that will receive frequent maintenance/repair events by personnel and maintenance equipment and tools. On this project, SS bellows joints are located up in the overhead pipeways.

In the areas subject to frequent maintenance and operating events, elastomeric expansion joints were selected for the following reasons:

(1) the elastomeric element is flexible and resistant to damage by accidental impact and bumping because the bellows “gives” on impact. The SS bellows is thin, highly stressed and brittle, and they are easily damaged and dented, which affects their service life and increases the chance of a crack or rupture and subsequent spill.

(2) The elastomeric joints will be located at pump suction and discharge connections. At these locations, the joints will be flexed and moved around during removal and reinstallation of the pumps in and out of their foundations, especially the vertical pumps, and the joints will be subject to accidental impact by maintenance and operating personnel. The nature of these joints makes them more maintenance and operator-friendly compared to a SS bellows.

(3) The location of the elastomeric joints on equipment flanges enables them to be easily inspected for cracks or other damage or degradation, providing the operators a chance to implement a planned shutdown of the pump to replace the expansion joint.

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Thanks!
Pete

 

[unclesyd]

I personally don't like expansion joints in piping systems especially the elastomeric kind. I've seen too many problems like the last one I was involved in where is change in process fluid killed the joints. This resulted in a very large spill.

Does your piping specifications allow elastomeric joints as they have a fire rating of near zero?

What is wrong with metal flex hose with braid if there is no work around in the piping design?

 

[KernOily]

Right on Syd, thanks for that, good comments all.

Spec does allow 'rubber' (these are Viton) joints. Nothing in this plant is fire-rated.

Famous last words, but process fluids won't change. This is a crude oil dehydration plant and it is a purpose-built plant. Process flexibility is not one of the design criteria.

SS braid hose would work IF I could find stiffness values/spring rates for them for use in my stress analysis. Without spring rates I can't justify the use of SS braided hose. Thanks! Pete

 

[stanier]

Avoid flexible joints by piping design. If you cannot do this for hydrocarbon service use metallic bellows. Elastomeric bellows are not firesafe and also degrade with UV if outdoors. Do not use for hydrocarbons.

If your maintenance crews are so dumb as to damage equipment sack them or train them. If this fails mechanically protect the bellows with a shield bolted to the flange. If the dumb fitters leave them off sack them or retrain them. If this fails fit a limit switch or inspect more often. If this fails leave and join a more professional company for the one you belong to will soon burn down and it will be your fault.

Go visit a refinery by Shell or BP and see if they use bellows on pumps.

Elastomeric flexible bellows belong in the low cost industries where they do not necessarily have the resources with sufficient skills to align a pump or build the piping accurately.

 

[unclesyd]

If you do consider flexible metal hose you might want to check these 2 sites.

http://www.tycoflowcontrol-pc.com/ld/Selection Factors.pdf

At one time these people at Senior Flexonics were quite helpful.

http://www.ushosecorp.com/

 

[KernOily]

Stanier - Thanks for your comments. I'll provide some replies.

> Avoid flexible joints by piping design.
Amen to that. Believe me when I say I have beat the design team about the head and shoulders that expansion joints are the absolute last resort when addressing flexibility problems. However, as you know, they must be used in some cases, and this is one of those cases. I used every trick in the book to gain as much flexibility as possible with what I had to work with, and we still ended up with a handful of expansion joints. But, short piping runs + buried lines + high temperatures + restrictive piping layouts caused by gravity flow with little driving force + design constraints set by process units in the plant = flexibility problems. All of life is a trade-off.

> Elastomeric bellows are not firesafe and also degrade with UV if outdoors. Do not use for hydrocarbons.
As I said in a reply above, fire-safe is not one of the design requirements for this plant. That was the owner's call. This is not a refinery/chemical plant and is also not an OSHA 1910/PSM facility. It is a heavy crude dehydration plant, the crude having a very high flash point. There is not even a plant firewater system. If it catches fire, they are going to let it burn. The elastomer we have selected for the bellows (Viton) is fully chemically compatible with the process.

> crews are so dumb as to damage equipment sack them or train them. If this fails mechanically protect the bellows with a shield bolted to the flange. If the dumb fitters leave them off sack them or retrain them. If this fails fit a limit switch or inspect more often. If this fails leave and join a more professional company for the one you belong to will soon burn down and it will be your fault.

Good comments. As the consulting engineer I have no stroke or input in the area of craft skill/competency. But as I said, sh*t happens, and a shield over the bellows might not be adequate. Judgment call.

> Go visit a refinery by Shell or BP and see if they use bellows on pumps.
As I said above, this plant is not a refinery. If it was, the owner would have different requirements and so would I.

> Elastomeric flexible bellows belong in the low cost industries where they do not necessarily have the resources with sufficient skills to align a pump or build the piping accurately.
This plant belongs in that category, on all fronts: low cost, labor skill, owner staff skill, etc.

Thanks! Pete

 

[joeswoes]

Maybe a cure-all, at least if you must use expansion joints, is to use PTFE and not rubber. It will give you better chemical resistance, no aging problems, and are available with protective T-bands to protect the PTFE bellows from exterior impacts.

 

[PaulDBerg]

Metal, rubber, PTFE, and other expansion joints are my business. See

http://www.flexicraft.com.

I could go on for hours on this topic, having worked with these issues for many years.

On the damage: if this is a concern, specify a cover for the metal joints. A carbon steel cover protects the bellows from misc. damage when that is a concern.

On rubber vs. metal: there are a few issues impacting this decision. Mainly it is a temperature issue. Even though the elastomers can be rated to 250F and above, they will deteriorate faster the hotter the service. The elastomer must also be chosen to be compatible with the chemical. The advantages are they are cheaper, absorb sound and vibration better, and have a lower spring rate.

Braided hose connectors will last longer than rubber, but they do not move axially. This limits their effectiveness to absorb thermal growth from the piping.

The PTFE joints are limited in the pressure they can take, but of course are important for certain chemicals.

I'll stop here.