Cold Stretching in Practice

[VPbris]

Hi,

I'd like some advice on performing cold stretching for a vessel intended for cryogenic storage (ASME VIII-1 Mand App 44). I will list some items for discussion.

1- Pc (cold stretching pressure) is said to be between 1.5P and 1.6P(inclusive). Do I need to keep this value constant during the process or any value within this range will do? During the stretching volume will increase, so I'd need to be controlling pressure, If value is to be kept constant it becomes a bit harder to control.

2- The cold stretching pressure shall be maintained until strain rate drops to below 0.1%/hr. This strain rate is calculated by repeated or continuous measurement of the circumference. Are there any recommended good practices / equipment to measure this circumference with minor deviations?

Thank you for your inputs.

 

[DriveMeNuts]

1: It is not an exact science. Anywhere in the range will do. Remember that Design P is being used, not MAWP. This means there could be excess material thickness that prevents the shell from stretching enough for the material to achieve the 270MPa allowable stress.

2: Measure it every 15 minutes or so. Perhaps a small pocket of compressible air in the vessel will make it easier to hold a constant pressure, if the rules allow it.

 

[VPbris]

Hi,

Sorry for the long time to reply,

1-I agree with you that excess material thickness would prevent shell from stretching enough, but as you said P is design pressure, shouldn't it be MAWP? Can anyone clarify on that?

2- Seems like a good interval, i agree with you. However I'm still on doubt if pressure should be mantained within this range during all the process or just set as initial pressure. Because as the vessel stretches pressure will drop.

Anyone else with knowledge on this?

 

[DriveMeNuts]

1 - The code is correct. A combination of experience and thought experiment is required to understand.

2 - Pressure definitely needs to be maintained for a period of time, otherwise all that happens is the weakest component of the vessel is cold worked and the remainder of the vessel remains a conventional non-cold stretched vessel. The whole purpose of the procedure is to cold work all of the materials so that the entire vessel can sustain 1.5-1.6P (for an hour).

 

[victorpbr]

Hi, Sorry for the long time to reply this, got busy with some other stuff.

Regarding pressure, yes the code says P is design pressure, but I also found an interpretation which says that MAWP + static head can be use as P. (

https://cstools.asme.org/Interpretation/InterpretationDetail.cfm?TrackingNumber=14735).

Another interpretation that I found interesting was this one:

https://cstools.asme.org/Interpretation/InterpretationDetail.cfm?TrackingNumber=15001

, it says the thickness can not be less than the calcualated minimum, but this thickness is calculated before cold stretching using the higher allowable stresses. When you carry the cold stretching, diameter will increase and there will be some thinning on the thickness, hence I understandone should always provide some allowance to keep the thickness after stretching above minimum.

Regarding pressure, still not clear to me, see this example (assuming S=PR/t):
Allowable strss: 270 MPa
Nominal Diameter 1500 mm
Design Pressure: 1,5 MPa
Minimum thickness: 4.17 mm

In this scenario, when cold stretching hoop strss would be 404,676 MPa

Now lets say one exagerates and uses actual thickness to be 12.7 mm, in this scneario the hoop stress when cold stretching will be 132 MPa, which would not even plastic regime. That's why I think MAWP is more appropriate. What do you think?

 

[DriveMeNuts]

Using MAWP ensures that the Cold Stretching procedure pushes the rules of Appendix 44 to the 270MPa limit.

Each component on a vessel has its own MAWP. The vessels MAWP is the lowest of all the component MAWP's.

Therefore only one component on the vessel experiences the 270MPa limit during the cold stretch procedure. The remaining components are cold stretched to less than the 1.5-1.6×MAWP (of the component) and therefore less than the 1.5-1.6×270MPa limit. As you point out, some are not cold worked at all. This especially applies to tall vessels where the bottom shell tier is thicker due to liquid head.