Support and Lifting Lugs for Low Temperature Service

[nickypaliwal]

For low temperature design service (assuming -40degC), the vessel is proposed of LTCS.

In this case, do we need to propose support legs of LTCS if legs are holding the vessel ?

For lifting lugs, since they will not be used while vessel is online, can we use normal CS ?

 

[metengr]

1.

In this case, do we need to propose support legs of LTCS if legs are holding the vessel ?

Yes, for low temperature service.

2.

For lifting lugs, since they will not be used while vessel is online, can we use normal CS ?

Yes, assuming adequate load carrying capacity.

 

[kingnero]

LTCS = low temperature carbon steel?

As for the lifting lugs:
As I'm only familiar with steel with guaranteed charpy values at those temps (J4, K4, ... ), make sure you have an adequate welding procedure, and have the charpy values tested at service temp, just as a precaution.

http://www.fusionpoint.be

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[nickypaliwal]

Thanks for your views however I have seen vendors offering structural steel without impact test for wither of the supports or lugs. Please also respond to below:


1) As I understand, the low temperature embrittlement is a reversible phenomenon since the material regains its ductility with temperature when coming back to normal temperatures.

2) The material needs impact at low temperature to crack. However as I understand the strength of the material does not change at low temperature so its load bearing capacity should not reduce. In such case the LTCS vessel support of CS plates should be fine since it is not affected from process flow.

However as I understand the tensile stress bearing capacity may not be affected but the shear stress handling capacity is affected. Material may not have enough ductility to arrest the existing cracks which may propagate due to vessel load.

Please provide elaborated views on above.

 

[kingnero]

It depends.

please confirm or elaborate on LTCS.
Then, mention your base material a bit more specific.
The material of the lugs is also important, however I assume it's going to be low carbon as well.
Then, elaborate on your welding procedure (heat input, process, filler metal, ...). Will there be grain growth? Will you perform Post weld heat treatment?

Then, we might have a clue whether or not it may be feasible.

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

 

[metengr]

Thanks for your views however I have seen vendors offering structural steel without impact test for wither of the supports or lugs.

There are vendors that offer many options. You have to evaluate the risks and select the option best for your service conditions.

1) Correct

2). What is the minimum design metal (MDMT) temperature under a worst case scenario or extreme cold weather? Once you decide what this temperature is, you select the material and have impact testing performed to ensure the material meets the MDMT.

 

[moltenmetal]

Low temperature service is a design case, no different than a steam-out or other hot operation. It can occur because of low ambient, autorefrigeration or other conditions- but the mere presence of low ambient is insufficient reason to specify low temperature carbon steel. There must be a design case, i.e. a pressure coincident with the low temperature, for it to be considered in the design of the vessel.

Will the vessel be operated under pressure at -40 C? If yes, then it needs to be designed with materials which meet the temperature requirements of the design code. Example: a propane bullet installed in a cold ambient environment.

Will the vessel be lifted at -40 C? If so, you're going to need to ensure that the lifting lugs aren't subject to brittle failure during the lift.

People often get confused when the ambient temperature may fall as low as -40 C, but any practical pressurized service conditions must be above that. The mere fact that a vessel will be put in an environment at low ambient conditions does not mean it needs to be designed for those conditions. As an exmaple, any vessel containing water is going to be heat traced and insulated, and if it is going to be taken out of service and allowed to cool to ambient temperature, it will be depressurized long before it gets down to anything near -40 C.

Structural steel is another matter entirely. I'm not a structural engineer, but I am unaware of a situation where a static structure (i.e. not a piece of lifting equipment or the like) is specified with low temperature carbon steel. To my understanding, what is usually done is heavier sections of ordinary carbon steels are used and more attention is given to points likely to crack during cold operation.

 

[SJones]

@nickypaliwal

2) Brittle fracture cracking isn't about "impact," it's about the relationship of stress, defect size and material fracture toughness. It's the latter variable that tends to be influenced by temperature that then affects the threshold values of the other two.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[nickypaliwal]

@moltenmetal: The minimum design is agreed due to de-pressurization and we do not design for vessel operating at -40degC. Vessel will not be lifted or moved during de-pressurization so lifting lugs of LTCS may not be required. Same is the response from metengr above. You are right that LTCS is not generally asked for structural steel but in this case the legs are supporting the vessel load so they need to be designed for it.

@metengr: I agree that we design material to withstand the design conditions as min design temperature. We have taken care of the same for the pressure containing parts.

For support legs, why should we ask for LTCS if the vessel is LTCS?

@kingnero: I am not much knowledgeable in welding however that can be taken care by welding engineer once we finalize the requirement for support legs material.

 

[r6155]

Support legs is under AISC. Brittle fracture must be considered.
Seismic and Wind must be part of design.
See AWS D1.1 2015 Structural Welding Code

Regards
r6155

 

[nickypaliwal]

Thanks for clarity r6155. I am not aware of AWS standards so do not know if my below query is already answered in the mentioned standard.

What is the technical reason for it since tensile strength of material is not supposed to be changed at low temperature.

 

[r6155]

@ nickypaliwal
Under sufficiently adverse combinations of tensile stress, temperature, loading rate, geometric
discontinuity (notch), and restraint, a steel member may experience a brittle fracture. All
these factors need not be present. In general, a brittle fracture is a failure that occurs by
cleavage with little indication of plastic deformation. In contrast, a ductile fracture occurs
mainly by shear, usually preceded by considerable plastic deformation.
Design against brittle fracture requires selection of the proper grade of steel for the application
and avoiding notchlike defects in both design and fabrication. An awareness of the
phenomenon is important so that steps can be taken to minimize the possibility of this
undesirable, usually catastrophic failure mode.

A lot of information is in internet.

Regards
r6155

 

[nickypaliwal]

@Steve: Thanks for the comment/clarification. I did not see your comment since we may be commenting in parallel.

I believe r6155 has similar views.

Thanks both.

 

[nickypaliwal]

@Steve: Thanks for the comment/clarification. I did not see your comment since we may be commenting in parallel.

I believe r6155 has similar views.



Just like to add that the allowable stress of the material does not change with decreasing temperature as there is only acceptance or rejection of material at lower temperature. so tensile for sure should not be affected.

As r6155 says, I believe I need to revisit the literature of brittle fracture.

Thanks in any case to all your views.