Using of ltcs for short period of time for process less than minus 50 degrees to 100 degrees

[plasticnoob]

Hi all,

I am a mechanical engineer from the petrochemical industries and was task to design some temporary piping for the decon of my plant.


My query comes where I was asked to use SS304 of ss316 for a certain service that is between -50 degrees to -100 degrees. I have done my research and understand that ltcs (low temp carbon steel) can only take up to -45 degrees. However if I am just using for short time (3-7 days) is there any concern of using ltcs instead. The cost of materials and welding ss is much higher than ltcs and I would like to reduce the cost

 

[Dhurjati Sen]

Dear plasticnoob,

You may go ahead. If all goes well, do share the good news.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[XL83NL]

Doesnt souund like a good idea to me. Apart from what George said below, you should take into account that the applicable Code requirements which in this case most likely would be B31.3 para 323.2.2(d)(e), referring to fig 323.2.2B note (b); the stress ratio may max. be 0.3.

 

[georgeverghese]

If your plant manager is willing to take on some risk, talk to your materials selection engineer and a senior process engineer( who should both be familiar with ways of mapping out an operating window with constraints imposed in these circumstances to handle these low temp brittle failure risks), and the process safety engineer to see if this is possible. A mini hazop audit may also be necessary.

 

[LittleInch]

I loath the word "temporary" in the context of piping or operations when dealing with hydrocarbons as it leads to these exact issues where people try to reduce cost, reduce inspections and essentially operate much closer to the wind than they would do if this was "permanent".

I always fight that in any discussions and unless the only issue is fatigue, then I don't believe in doing something a certain way just because it is "temporary". Althought this migth not be the case here, I've seen "temporary" design in use 5 years later.

Now this may be a decommissioning operation, but it's still containing pressurized hydrocarbons.

There are many many failures in process plants as a result of inadvertent low temperatures causing damage and failure of components - the Longford plant disaster in Australia comes to mind.

If I was you I would simply ignore the word temporary and ignore the duration the piping is being used and then design it like you would any other piece of pipe with those temperature limitations. Then construct it and test it and support it like you would any other pipe and then sleep well at night. Now that may cost more than another design using materials which are not certified for the duty and you might get away with it. But always think - if this item which you have designed goes bang and results in injury or death, can you justify your design to those who investigate and prosecute afterwards?? The finger will be firmly being pointed at you and it's not somewhere I ever want to be.... Your choice.

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