Reduction of MDMT at Reduced Pressures

[EmmanuelTop]

Hello,

This question was originally posted in Metal and Metallurgy engineering forum, but I asked the Admin to move it here (thank you metengr for your reply). So here it goes - slightly modified:

An EPC contractor proposed Carbon Steel vessels in Propane refrigeration (mechanical refrigeration) service. The materials to be used are P275 NL1 for Propane accumulator (operating at ~16 bara and 47 degC), and P355 NL for Propane chiller (operating at -16 degC and the corresponding vapor pressure of C3).

The concern from the Operator side is that the vessels will not be able to sustain depressurization conditions, when temperature can get as low as -42 degC but this can happen only when the vessel pressure is essentially atmospheric. I see from the Propane vapor pressure curve that the temperature can go below design -29 degC only when the pressure inside the vessel goes below 1 barg (~2 bar abs).

The design follows PED and EN10028. When I look at ASME UCS-66, I see there is a significant margin for reducing MDMT of Carbon Steel if the coincident pressure (in this case 1 barg = less than 10% of Design pressure) is lower than Design pressure, but I don't know how this works from EN10028/PED perspective.

Can somebody throw some light on this subject? I don't have Mechanical/Materials background so please bear with me. The basic question is can P275 NL1 and P355 NL be used at -42 degC and 2 bar abs pressure (which occurs during depressurization only), and if yes where in the codes/standard is the evidence of that?


Dejan IVANOVIC
Process Engineer, MSChE

 

[metengr]

These two materials are not listed in ASME B&PV Code, Section VIII, Div 1 therefore UCS-66 does not apply. Impact testing may be required to ensure no brittle fracture behavior at -42 deg C.

 

[EmmanuelTop]

Which Carbon Steels the Figure UCS 66.1 (attached) applies to? Is it applicable to all Carbon and Low Alloy Steels?



Dejan IVANOVIC
Process Engineer, MSChE

 

[metengr]

The list of carbon and low alloy steel specifications permitted for use as pressure boundary material is listed in Table UCS-23. So, if one of the material specifications listed in Table UCS-23 is not listed under a material curve grouping in UCS-66, it would default to Curve A.

 

[SJones]

Yes, and when you moved it you lost my reply, so I'll start again.

1. Where do you get -29 deg C as a minimum design metal temperature from given the quoted process conditions and the specified Charpy test temperatures of the material specification (provided that they have been exercised correctly)?

2. EN 10028 is a material specification, and PED does not address the issue in prescriptive form. Pick the code and stick to it. If you wish to adhere to ASME VIII, Div 1, then review UG-10.

3. Confirm the material grade of the P355. There is no "NL."

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.

 

[EmmanuelTop]

Thank you for the reply, metengr.

1) Does that make sense in this specific case? P275 NH is in Group (Curve) D, and I see that e.g. NL1 has the same composition, CEV, tensile stress as NH, and even higher minimum impact energy value than NH (see attached) - but according to what you say it should be lumped under the Curve A, because it is not listed in Table UCS-23? To me is looks like the NL1 grade is actually made for lower temperatures than the NH grade.

2) Secondly, what confuses me further, is that the actual Propane vessel in question is made of P275 NL1 and is designed for 21 bar abs @ -29 degC / +65 degC. But if this material belongs to Curve A, doesn't the Curve A say that this material, if used for lower temperatures than -8 degC, must be impact-tested?

3) Is there any equivalent table/graph for MDMT reduction in PED/EN/ISO standards?





Dejan IVANOVIC
Process Engineer, MSChE

 

[EmmanuelTop]

Steve,

1) -29 degC is the minimum ambient temperature.
2) I am apparently clear on how ASME works, but this project does not folow ASME Sec VIII Div 1 - it follows PED and EN standards.
3) Grade is NL1 in all cases.

Dejan IVANOVIC
Process Engineer, MSChE

 

[metengr]

EmmanuelTop;
What I am stating is simply this, the material specifications listed in Table UCS-23 are what is permitted for pressure boundary material using ASME Section VIII, Div 1 rules.

If your specific material grade under an existing material specification is unlisted, it possibly can be used but it has to be re-certified to an existing material specification under one of the permitted material specifications already in Table UCS-23. Otherwise, you would need a code case by ASME Section VIII, Div 1 to obtain use and allowable stress values and low temperature application for UCS-66.

 

[SJones]

The pressure equipment must be properly designed taking all relevant factors into account in order to ensure that the equipment will be safe throughout its intended life.
The design must incorporate appropriate safety coefficients using comprehensive methods which are known to incorporate adequate safety margins against all relevant failure modes in a consistent manner.

This is the framework under which the subject vessel is to be designed. Thus, it falls upon the responsible party to demonstrate compliance. One way of demonstrating compliance it to verify compliance with a nominated code in so far as such compliance also incorporates the meeting of modified, or additional, criteria stipulated in the Directive. The Directive does not stipulate a means of dealing with low ambient temperatures, but it has set the goal as quoted above.

Also note that H = elevated temperature quality
NL1 = low temperature quality

A further item to note is that EN 10028-3, for example, automatically requires Charpy testing. UCS-66 is for exemption of Charpy testing.

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.

 

[DBreyer]

You might want to consider the design code for the vessel. As you quoted european steel names and the PED I guess the vessel would be designed following a european code.
EN 13445 is the latest european code for pressure vessel design and also covers low temperature service.
It takes a somewhat similar approach (reduced tensile stress also reduces MDMT) to the ASME code but with curves fitting european materials.

Daniel Breyer
Inspection Engineer

http://lnkd.in/dFcQA-w

 

[SJones]

The vessel does not have to be designed to a European standard:

http://www.eng-tips.com/search.cfm?q="ASME+VIII"+++"PED"&action=search

Nothing has really changed with the switch to the new Directive 2014-68-EU, operative from July 2016.

The difficulty comes in trying to marry the two sets of requirements together, as the OP is finding when attempting to ram CEN materials standards into ASME VIII. Pick the wrong materials standard and it doesn't go too easily. The OP's EPC contractor has probably attempted to navigate the Directive requirements by the path of least resistance, i.e. by the specification of a harmonised material standard. What is not yet being made clear is the specific vessel design code being adopted, and we invite the OP to elaborate.





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.

 

[georgeverghese]

Would one of the operating scenarios for these 2 vessels be repressurisation to full operating pressure whilst metal temp is -40degC or so?

 

[EmmanuelTop]

Vessel design code is EN13445/PED. Materials as per EN 10028. There are no ASME requirements - it was just an attempt (rather confusing, I see now) to try to find analogies in terms of MDMT reduction when stresses are reduced, during transient conditions (depressurization in this case).

@ George - rapid pressurization is inhibited via administrative controls (operating procedures) and low-temperature permissive/interlock in DCS.


Dejan IVANOVIC
Process Engineer, MSChE

 

[SJones]

The analogy appears to be in EN 13445-2, Annex B, B.2.2.6

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.

 

[EmmanuelTop]

Thanks Steve, I had a similar thought after reading EN 13445-2.

In the example of P275 NL1 (normally rated for -40 degC), does the paragraph B.2.2.6 say that for cases when the stress ratio is less than 50%, the actual MDMT can be reduced for 25 degC, i.e. Tm = -40-25 = -65 degC, if the material is post-weld heat treated?



Dejan IVANOVIC
Process Engineer, MSChE

 

[SJones]

Not quite. The standard does not address the term 'MDMT.' By the definition of 3.1.3, Table B2-12 is used to derive the temperature at which the material is to achieve 27J impact energy. For your stated conditions = -42 + (70 - (80 x 0.5)) = -12 deg C. It achieves more or less the same thing in that instead of dropping the design temperature for a given Charpy test requirement (or exempting the test like ASME), the Charpy test requirements are altered. The same with B.2.3.

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.

 

[EmmanuelTop]

Thanks. I'm not sure I understand the equation - where do the numbers 70, 80, and 0.5 come from?

Looking at Method 2 (B.2.3) and the attached nomograms, what does the line for 20 mm thickness tell us? Can you give an example?

Many thanks
Regards

Dejan IVANOVIC
Process Engineer, MSChE

 

[SJones]

The numbers come from the data that you have supplied and Table B.2-12

The nomograms give the required maximum Charpy test temperature for a given design reference temperature, reference thickness, material condition, and strength.

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.