Use of Low Temp Carbon Steel piping material for 4500 kPa Super Heater Steam 1

[Meck91]

I need help on this issue, please

For 14" 4500 kPa(g) super heated steam system, based on design conditions, the wall thicnkess required is 0.75" for NPS 14. Piping Material is A-106 Gr B
MDMT is -29 Degree C

Based on ASME B31.3 Figure 323.2.2A & combination of nominal thickness 0.75” (19.05 mm) & MDMT of -29 C comes below the Curve B, therefore, impact testing is required for A-106 Gr. B Material or i have to use low temp carbon steel A 333 Gr. 6 as per client specification.

I never heard of use of LTCS material for super heated steam line. At the same time the pipe will never be subjected to -29 C in service. This line is also insulated.
Yes if the line is out of service for a duration of time during extreme cold of -29 C, the pipe material can see this temperature but the line will not be pressurized at that point.

For impact testing, do we need to consider pressure as well extreme cold temperature of -29 C at the same time? How can i jsutify that the use of LTCS for superheated line is overkill.

Woudld highly appreciate your feedback.

Thanks,

 

[metengr]

First, I would use B31.1 Power Piping Code. Second, I would stay with the SA 106 Gr B pipe material as intended, and not worry about MDMT falling below -20 deg C.

 

[SJones]

If you have looked at Curve B, what is the lowest temperature it achieves? This is exactly what your process or mechanical engineer has done because they do not understand the difference between design minimum temperature and minimum temperature without impact testing. Now they have been caught out by the thickness. Show them the definition of 301.3.1 and ask them to have another think about it.

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.

 

[moltenmetal]

How on earth could steam piping possibly be simultaneously exposed to 4500 kPa steam and be at -29 C temperature? If this is a realistic temperature/pressure pair that could actually be encountered in service, then you need to design for it, but think about it for a minute- is this not akin to designing a water line for -29 C, requiring low temperature steels, which will of course STILL rupture when the water line freezes SOLID! This is a common misapplication of the concept of minimum design temperature.

 

[weldstan]

Could not agree more with the above responses. There is so much engineering ignorance involving MDMT.

 

[sanshu1111]

This practice of considering MDMT as -29 Deg C for CS and -46 Deg C for LTCS is commom. Some time it is due to ignorance and many times as engineering of convenience in order to limit and also standardize piping classes. This does not mean that we can blindly follow this practice, such cases to be discussed and resolved appropriately.

In the original post, if you can ensure that startup procedure will ensure that pressurizing is in a controlled manner with metal temperature simultaneously taken care of, you can definetely defend your decision for avoiding impact test.

 

[SJones]

It's a game of numbers, and when the numbers are wrong, there is either greater expense from over specification, or a process safety issue from under specifying. The moral of the story is: set the design parameters as they are defined in the code. That way, everyone sings from the same song sheet. Sure, it's convenient to simply specify -29 deg C, -46 deg C, 20 bar, 50 bar, 100 bar etc, but it's lazy engineering and creates the difficulties that started this thread.

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.