Compound Cylinder

[rrossey]

I am constructing a compound cylinder to withstand an internal pressure of 6,000 psi.
The construction is from two pipes
Inner pipe material seamless Stainless Steel 316
ID = 4.00”
OD = 4.25”
Wall thickness = 0.125”
Outer pipe material seamless Steel 1018
ID = 4.375”
OD = 6.625”
Wall thickness = 1.125”
Both ends of the 48” long cylinder to be closed by 1.25” thick flanges with proper seals.
There is a gap of 1/16” between the inner and outer pipe. I propose to fill this gap with a high strength epoxy.
As per the Barlows formula, the outer pipe bursting pressure works out to be 24,000 psi. This gives a safety factor of 4 for a working pressure of 6,000 psi.
I request your suggestions regarding the construction and any design flaws.
Thanks

 

[TGS4]

Do you have to design this to any particular Code or Standard, per local legal requirements?

 

[rrossey]

It is not designed to any code or standard.
It is for internal use as an accumulator for the working of other components in a prototype system.

 

[TGS4]

Is there any cyclic service? There is more to the design of this type of pressure equipment than just "pressure".

 

[tbuelna]

The first question that comes to mind is why not just make the 316 cres wall thick enough to handle the pressure? As for design issues, one thing is the relative stiffness mismatch between the thin inner 316 cres tube, the epoxy filler, and the thick outer mild steel tube. Under sustained pressure loading, the epoxy filler will gradually creep and separate from the outer wall surface of the 316 cres tube when the fluid pressure is removed.

Another problem I can see is the large change in section thickness where the 1.25" thick end flanges attach to the .125" thick walls of the inner cres tube.

Lastly, with a high pressure hydraulic cylinder the stiffness of the cylinder walls can be of more importance than the simple stress in the cylinder walls. If there is significant deflection in the cylinder walls from high pressures, it can cause problems for the piston seals.

 

[rrossey]

The cylinder will be under constant pressure for approx 3 hours, then to atmospheric pressure for 30 minutes and then back to 6,000 psi.

The bore or outer dia need not be machined, so making one piece from 316 was the cost consideration and availability of standard hollow bar, tube or pipe.

The alternative I considered was to fill the gap with hydraulic oil and pressurise from the same source as internal pressure, so that the inner and outer pressure of the stainless steel pipe always remain the same.

 

[TGS4]

At that loading frequency, fatigue is likely going to be a governing design consideration. In your case, I would highly recommend that you design your cylinder to the rules in ASME Section VIII, Division 3. Compound cylinders such as fiber-wrapped cylinders are permitted and encouraged.

I make this recommendation because it is highly likely that there are failure modes that you have not considered that could (potentially) come back to bite you in the proverbial... If/when it blows up and injures or kills someone or does some significant property damage, you want to be able to tell either your insurance company or the courts that you followed an internationally-recognized Code/Standard as opposed to just winging it.

 

[rrossey]

I studied the applicable code ASME Sec viii, Div 2
As per this code, the wall thickness required is 0.8”, whereas my outer pipe is 1.125” thick.
Please allow me to ask another question:
For the inner stainless steel pipe with a thickness of 0.125”, the bursting pressure is 4,000 psi.
If I pressurize the inner pipe slowly, it will start expanding on reaching the yield pressure of 500 psi and with further increase of pressure the circumference will start expanding.
As the original gap between the pipes is 1/16”, the circumference will increase by 2.95% only before it touches the inner surface of thick pipe.
In your opinion, will the inner pipe burst before touching the outer pipe?
If not, will the stainless steel pipe act as lining to the outer thick pipe?

 

[tbuelna]

Just to keep things simple, let's just consider what happens when your .125" thick inner wall yields from the 6000psi hydraulic pressure applied. The epoxy resin filler will not help much since it will creep when exposed to high sustained compressive pressure forces. And once the .125" thick inner cres tube wall elastically yields, the hydraulic pressure seals will no longer be able to perform their function.

 

[rrossey]

Assuming that there is no leakage at the ends and sealing is perfect. There is no scope of epoxy or any other filler to creep out, In that case I feel there should not be any problem.

 

[LittleInch]

Still having difficulty working out why you need this inner liner. A sketch of the end connection would be useful, but what you seem to trying to make is a lined cylinder. Much better to ignore the inner cylinder strength and just make sure the epoxy fills the gap completely and can't go anywhere. This is similar to epoxy repair sleeves on pipelines where they do this for internal defects.

leaving a gap for the inner liner to yield and then collapse would break it in a few cycles and is a bad idea IMHO

My motto: Learn something new every day

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

 

[EdStainless]

Is this a hydraulic cylinder or merely an accumulator?
Why do you want a stainless Liner?

If this is an accumulator and you need SS for some reason I see two options.
1. make it solid stainless, much easier and less prone to issues
2. use a thin SS liner and expand it tightly into the outer cylinder

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