Pipe Span limit

[Bobfromoh]

A 150' high vertical 10" SCH 80 line was supported by a spring can at the bottom near the ground (plus guided at intervals). The top of the line is connected to a tower. Due to equipment loads, the can was removed and an expansion loop was installed in the vertical run. The loop is 7' horizontal and 8' vertical. The customer said that the span was too much and that the loop should be supported. I didn't add any supports because the stresses were 60% of allowable and the deflections were OK. Is there guidance on when a loop in vertical pipe run should be supported?

 

[BigInch]

What do you mean by supported? An axial restraint?

There's no real guides I know of for support locations in vertical piping, other than have enough so as to not overstress the line, when you include thermal + external loads and surge forces, valve weights, construction and maintenance loads, etc. Did you include (if there is some necessity to do so) any hydrostatic test weights, for construction now or in the future? The client may have been more worried about wind vibrations due to the von Karmen effect or something, but in that situation providing guides would suffice. Its also true that sometimes clients simply worry about too many things they shouldn't. A laterally supported pipe can take quite a bit of axial load before buckling. You could always suggest that they prove the extra supports are needed, but you'd better know the answer to that ... before hand.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Bobfromoh]

Thanks for the response BigInch!
The customer was expecting a support on the horizontal pipe section of the loop. The customer could know something that I don't. I work off drawings and ISO's. I don't get to see the job site. I included hydrotest, wind and seismic loads. The stresses are low and the pipe is guided at reqular intervals up the tower. I just don't want to miss something that should have been obvious.

 

[pennpiper]

Bobfromoh
I cannot "see" your exact design and I cannot "see" your exact operating conditions, however I am somewhat familier with this type of situation. Over my more than 45 years in piping I have addressed this type of installation many times. it is not unusual, in fact it is as common as dirt around oil refineries and chemical plants.

All tall fractionation towers tend to have (side) nozzles near toe top. These nozzles (and the resultant piping) are often 10' and larger. The pipe is normally supported from the vessel with a cantalievered support (attached to the vessel) and a double trunnion (attached to the pipe) just below the top elbow. As the pipe travels down the vessel it will have guides also attached to the vessel. At the bottom of the drop the pipe is designed to have a horizontal run which is allowed to flex and compensate for the vertical expansion of the vessel.
If this line does not have a block valve at the nozzle, and it is properly supported just under the nozzle and it will always operate when the vessel operates then the pipe drop does not require spring cans or expansion loops.

Why was a spring can used in the first place?

Does this pipe line have a block valve at the nozzle and therefore operate or not operate on some cycle?

 

[Bobfromoh]

PENNPIPER,
The pipe is existing but smaller diameter. The cusomer is upgrading to larger pipe and components. The pipe comes down the tower to a platform 30' off the ground. the pipe runs horizontal for 7' and then down to ground level and runs about 20 feet to a heat exchanger. The pipe isn't as hot as the tower.

The tower picks up the pipe and for unknown reasons a spring can was added hear ground level to also pick up the pipe. When I analyzed this layout, the HX nozzle was WAY overstressed even with the smaller diameter pipe.

I took out the can and it was still overstressed. I then provided a vertical restraint near the HX and added the loop in the 30' section to reduce support loads and stresses. I don't see anything special here?

 

[BigInch]

(?) More supports would move some of the load from the top of the tower down to intermediate levels. (?)

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[PulseMaster]

Keep in mind that static stress may be ok, but the longer a pipe span, the more likely it will vibrate due to forces from two-phase flow, pulsation, etc.

 

[zdas04]

I've seen a few of these in oil fields (not gas fields, thank goodness) and the stress guy told me the reason for the supports under the vertical loops is tortion under wind loading. He said all the stresses work out fine without the supports until he adds a wind load and it wants to rotate the loop around the pipe at really high stresss.

David

 

[NozzleTwister]

Bobfromoh,

You’re revamping an existing system and no telling how many times it was revamped before. The Hx may not have been there when the spring was installed.

On your question about the self supporting loop, from the numbers you give, 7 ft. horz. 8 ft. vert. and 7 ft back to the slot, this loop sounds like it will self support fine for a 10" sch. 80 pipe. You must not have much differential movement if a loop this small works for you.

I too have many years of refinery stress experience and have installed loops on towers like this as well. I think you're on the right track and you may just need to educate your client. Don't be afraid to print out and show your dead weight deflections and stresses to the client to give him a warm fuzzy feeling about your system.

I do have some advice which you may have considered already.

You can reduce the weight your loop is supporting by reducing the 8 foot vert. section a little. Also you can reduce the guide loads near the loops by locating them further from the loop elbows.

Also be sure to check the guide designs for the loads that you have, thermal and wind loads circumferential to the tower being the most critical. Check the local stress at the tower to guide attachment location also.

At the base support where the spring used to be, that's probably a pretty heavy load so be sure the check the local stresses at that support.

On the top nozzle, are they increasing the nozzle size or just the pipe? In any case check the loads on this nozzle, especially if you're hanging a lot of pipe from it or your thermal loads are high.

Have you asked about tower steam-out? Will your line be cold and have you designed for that situation, also are threr any start-up or shut down cases that will give a different profile than the operating?

Good luck,


NozzleTwister
Houston, Texas

 

[Bobfromoh]

This is good information.
Thanks to everyone for the info!

BobfromOH