Request: Materials and Construction Type for LNG Piping

[saplanti]

Dear Experts,

I am partially piping and pipe stress engineer, and trying to get into LNG business. I am wondering the type of materials used for LNG piping and the construction types.

I understand that stainless steel and aluminium (alloys) are used at low temperatures. But what is the industry use the most for piping is my interest. Probably, we can use some impact tested CS pipe and flanges with toughness properties above -60 deg C (I am just guessing, I do not have the codes with me). I remember -29 deg C for A106B in acc. with B31.3.

Do you use double skin piping with vacuum insulation for LNG? Or any different?

In case the piping is double skin, do you use expansion joint on the second skin to take differantial thermal expansion? Or any other methods used?

LNG is for only storage and transportation as far as I understand. I assume LNG piping can run between storage vessels if necessary, from cooling process to the storage vessels, and from storage vessels to the LNG terminal for shipment or vice versa. Additionally, LNG terminals are always on the coast line and therefore the piping is short.
I guess the major piping for LNG plants is for natural gas to LNG plant and for the gas after heating and pressurising to send it to users. Therefore they can be carbon or low alloy steel depending on the temperature, pressure, corrosion/erosion rate and diameter.

Can you name any book and standard (other than NFPA 59A) or company practice to explain how to construct LNG piping?

Thanks in advance and kind regards.

Ibrahim Demir

 

[DSB123]

Easiest and most effective way to construct LNG pipework is to place the ends of the pipework together (with a small gap) and weld ends together all the way round.

 

[saplanti]

I was after more glabal explonation about the construction methods rather than this kind of details.

You are saying that to use pretensioning during the final connection if I understand correctly. However, this is not a good implementation. LNG is the refrigerated liquified natural gas, and the piping will shrink after the liquid runs in it due to the large negative temperature difference against the installation ambient temperature. Shrinking will introduce more tension in the system in addition to the pretension you referred. This will increase stresses on the piping system. I guess this is not wanted.

As far as NFPA 59A concern, the double skin piping with vacuum insulation is to be used, useage of aluminium in piping is restricted.

Kind regards,

Ibrahim Demir

 

[stanweld]

49CFR Part 193 governs LNG installation in the USA which mandates NFPA 59A which in turn mandates design and construction per ASME B31.3 with NFPA 59A's more stringent inspection criteria. Vacuum insulated pipe may or may not be used. There are many pitfalls to vacuum insulated pipe (from personal experinec) but it can readily be buried. Cold insulated pipe is used for above ground installations. Allowance in design, of course, must be made for expansion/contraction. Materials are principally either 304/304L or 316/316L.

 

[saplanti]

Stanweld,

Thank for the reply. From the emission perspective the abovegroung piping will probably give more access to the pipe for checking and noticing leakage.

Cold insulation is probably adequate by accepting some heat transfer from the environment. In this case, I guess, the LNG shall not be trapped in the pipe due to heating, and should be directly connected to a refrigerated tank unless is is going into the vaporiser. We can say the same thing for the buried pipe as well.

Vacuum insulation seems to be very difficult to maintain, consumes additional energy, and difficult to inspect. Is there a necessity for certain areas in the piping to use vacuum insulation, nothing else?

When we use double skin construction and stainless steel is used for the inner pipe, do you use stainless steel for outer pipe as well against the corrosion or for other reasons?

When double skin construction is buried, do you use coating on the external pipe regardless what pipe material is?

Do you limit use of flanges in the LNG piping with double skin? Or use more flanges to be able to construct the piping in spools and connect them at site?

Kind regards,

Ibrahim Demir

 

[DSB123]

saplanti,
You need to be careful how you read people's post's. Nowhere in my post did I mention pretensioning. You never pre-tension pipework during installation unless it's for "Cold-Pull" purposes which is not used in LNG systems. I said "place the ends of the pipework together NOT pull.

 

[saplanti]

DSB123,

Thanks for coming back. I put my statement conditionally, please see my statement in my second sentence " .... if I understand correctly.", I apoligize if I offended you.

If what you are saying is not a "Cold-pull", so can you please explain what you are trying to say? I am sorry I can not see very clearly what you are saying. Every welded piping is welded circumferantially with a small gap within the code acceptance. Some of them at certain location are flanged, and you need to tight them again in accordance with the code.
Are you saying that welded construction is preferred in LNG piping?

Regards,

Ibrahim Demir

 

[stanweld]

Whether to use VPI pipe, is an engineering/economic decision.

Welded construction is the norm for all LNG transfer lines.

In buried applications, coating of the external pipe is required if the outer pipe is carbon steel and is highly recommended if the outer pipe is stainless steel.

 

[saplanti]

Stanweld,

Thanks for all the clarifications. Everything is making sense now. What do you mean with VPI pipe? I am not familiar with the notation.

In case we have double skin piping, how and where do you implement supports, and what type supports do you use?

Are the supports connected on the outer pipe only?

How do you use guides and line stopers? I guess line stopers are the difficult ones to use.

Kind regards,

Ibrahim Demir

 

[stanweld]

Should have used VIP, vacuum insulated pipe.

 

[saplanti]

Stanweld,

Thanks for the clarification.

I wish I can get some enlightment on the supports from the users or experts.

Kind regards,

Ibrahim Demir