Calculating stress at 4" riser were 4" Ethafoam 200 is installed?

[kadtwog]

Folks,

I am new to the pipeline game I have spent most of my time on the B31.3 side of the fence, Hoping someone here may be able to point me in the right direction.

Here is the situation we are installing a new sectionalization valve on a 30" pipeline, on either side of the valve we will be installing 4" risers for pressure taps. The 4" riser will be butt welded to a 30" X 4" extruded tee and the line wrapped with 4" of ethafoam 200. I am most concerned with any moment forces that would be exerted on the riser as a result of thermal expansion of the mainline. So here are the questions I have.

Because we have km's of buried line on both sides of this valve can we consider this line to be fully restrained (Practically speaking, I know 31.4 and Z662 say we can)
How do you guys look at the 30" slab Gate valve with respect to stress? It seems a lot like an anchor being restrained by a lot of earth on both sides any thoughts?
If the valve does function as an anchor, I would suspect any movement we see with respect to thermal growth to move away from the valve in both directions?
What affect if any would the freeze thaw cycle have on this line?


Regards,

 

[BigInch]

sectionalization valve = Mainline Block Valve, MLV
There is no reason that you've mentioned that a valve should act like an anchor. A 30" moving pipe will take the valve along with it. Have you seen what kind of anchor you need to stop a 30" pipeline, even with just a normal temperature variation?

Find something else to worry about, like where are you going to get a 30x4 reducing tee, or how you're going to pay for it IF you find one, and if you do find one, where are you going to find another one. Use 4" reinforcement saddles.

 

[kadtwog]

Biginch,

Thanks for the response let me try and answer a few of your questions.
Yes this is a Main Line Block Valve.
Yes I understand the magnitude of the forces created by even a small temperature delta, my question with respect to the valve as anchor was limited to this isolated location and the few feet up and down stream of the valve.
Again you are correct the reducing tee’s are indeed expensive and long lead item’s and yes reinforcement saddles would be an excellent option. However this is the preferred installation at our company at the moment.
So that being said I was still hoping to get an answer to the original question with respect to the actual amount of linear growth of the Main line one would expect to see at this location. The concern is with respect to the riser and any forces applied to it by the mainline movement, the temperature delta will be in the 50 deg C range.
Please forgive my ignorance to the pipeline business as I am still very green.


Regards,

 

[BigInch]

Thermal axial stress is [α] * [Δ]T/E

Axial stress between the bypass around the valve and the mainline would come from the temperature difference between the bypass and the mainline. As that axial force in the bypass is normally going to be at least 6 ft above the top of the mainline pipe, you can figure the moment at the beginning of the bypass considering it as a frame with a movement of one anchor. Since there is a reinforcing saddle saddle, divide the moment by 2 and put half at the saddle and half at the bypass.

Change the company standard. It is an error. Nobody in their right mind would use a custom fabbed 30x4 reducing T. That's rediculous.

 

[ColonelSanders83]

You have not stated whether the pipeling is staight at this location or if it contains any bends close enough to invaladate the rigid anchor lenghts. The rigid anchor lengths you mention are only valid for straight runs with no disconiuities and thier distances are very much dependant on soil properties.

A main block valve will be a disconinuety, depending on your soil properties this could make a big differance. If you pipline is burried in muskeg, the valve will sink more than the attached pipline in effect creating a kink, this will be amplified by any themal growths.

As BigInch has stated, whether or not you have any movments in the line you still have moments at the attachment point due to differential thermal growths between your main line and the bypass. If you main line moves at all you will have additional movments that your riser will need to handle. I can garuntee you that if your 30" main line moves, those four inch risers are going to go with it. You need to make sure they can handle it. Even ethafoam may be able to overstress you connection points depending on surrounding soil properties and line movments.


I also agree with BigInch regarding 30X4 reducing tees, there are so many more cost effective and technically acceptable options available.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

http://www.ap-dynamics.ab.ca/

 

[BigInch]

The valve needs to have a concrete slab, mat, or other type of support, perhaps even a concrete cap supported by piles in some cases, in order to equalize the soil bearing stress below the valve with the bearing stress below the adjacent pipe, or support the valve and all the adjacent pipe for a distance until the pipe can support itself once again.

If it looks like a 30" valve is going to do some moving, I would recommend a redesign of that segment adding anchoring and flexibility in specifically controlled places. You just can't allow a 30" valve to move around wherever IT wants to go.