LNG Valves 1

[BraxtonL]

hi, i'm currently researching LNG cryogenic service valve...may i ask the following questions....

1. why the industry practice is using ball and butterfly valves in LNG ??

2. why most of the LNG cryo valves are soft seated ball valve and metal seat triple offset butterfly valve??

3. when we are going to use butterfly / ball valves under this LNG service?? which is better??


your comments would be much appreciated.

Thanks.

 

[JimCasey]

Metal-seat Triple-offset Butterfly Valves (TOBV) are well-suited to LNG service.
They are very inexpensive for inch of diameter.(Small mass, simple construction)
It is easy to fit them with an extension bonnet so that they can be used on cryo services and the stuffing box (stem seal) is outside the ice-ball.
The metal seat makes them inherently fire-safe while providing tight shutoff.

At 3 inches (75mm) and below, the ball valves work better. Small TOBVs have very limited capacity because even with the disk in the full open position there is still a lot of metal hanging in the flow stream. Remember, disk and stem are in the flow path with a TOBV. ANd in the higher pressure classes the shaft diameter increases to handle the additional torque, firther restricting flow. You can barely see daylight thru a small, opened TOBV. Ball valves have virtually unobstructed flow paths, and in these sizes there is not that much more metal to have to pay for. Firesafe/Cryogenic seats are available as are cryo stem seals and/or extended bonnets.

 

[BraxtonL]

Jim, many thanks for your valuable info...

 

[BraxtonL]

May i ask how to calculate the length of extended bonnet for cryo valves??

Per BS 6364 & MSS 134:
The length of the extension shall be sufficient to maintain the stem packing at temperature high enough to permit operation within the normal temperatre range if the packing material.

These Standards just shown the standard length for each sizes of valve but they did not mention how to calculate the length..

Your comments would be much appreciated.

Thanks.

 

[Albator]

As far as I know, the standards are effectively not that clear.

In fact, in my industry, we use rules of thumb to calculate this length in cryogenic applications. Some basic calculations were done in R&D some years ago, and then in function of fluid used, size of the valve and materials, we have only to look in a table to know which length we should apply.

It is not so easy to calculate exactly the minimum length required, it only has to be "sufficient".

Maybe some of your collegues or other people working on the same project have some experience in this subject?

 

[JimCasey]

Calculations are not really fun.
Lots of parameters to evaluate: Cross sectional area of the bonnet, material/thermal conductivity, annular void volume, outer shape/surface area, specific cryogen and its thermodynamic properties. Ambient temperature, windspeed. Expected thickness of insulation. Orientation of the valve. Most of these as a function of distance from the pipeline surface.

Cryo companies such as Linde and Praxair have valves that live down inside a chamber called a cold box. Those valves may need bonnets several feet long.


Finite element analysis programs can usually look at the heat transfer and give you a pretty good idea.

If it's just a garden variety cryo application. 7 inches is a reasonably typical extension for the bonnet.

 

[NGiLuzzu]

BraxtonL,
for general information about triple-offset valves, you may taka a look at:

- thread408-145006 within this Forum;

and

-

http://www.valveexperts.com/articles/index.cfm?action=article_story.cfm&article=8.

Hope this helps, 'NGL