Hot Buried Piping - Expansion Provision 1

[CSGWB]

I am after some confirmation advice on the expanision provision for a 500 metre long buried carbon steel pipeline containing a viscous liquid at 165C/392F (pressure <1000 kPa/150PSI).

We are looking at the installation of this pipeline and have been advised by the local statutory authority that we must bury it. We have currently allowed expansion provision in the form of expansion loops located every 120 meters with a pipeline anchor fitted between. The expansion loops will be located in concrete pits to allow free expansion. The pipeline will not be insulated.

I am having trouble finding any references for this type of pipeline. Can anyone give some assistance.

Regards

CSGWB.

 

[Zapster]

Expansion joints are typically not a first choice; however, in your case, have you considered them? They may be more economical than expansion loops for your application.

 

[CSGWB]

Zapster,

Agreed that expansion joints would solve the problem, however, the process fluid (bitumen/asphalt) does not have a good track record with expansion joints.

CSGWB.

 

[BigInch]

I don't know of any standards for hi temp underground piping. I can give you a few thoughts,

Your temperature is what I believe to be at or slightly past the limit for most (economical) coatings for buried pipelines. The real trick to burying this pipeline in contact with the soil may be to find a corrosion and mechanical protection coating system with proven reliability at the 160ºC temperature. At least the length is short, so that may allow consideration of some more expensive coating types. Hairline cracks in any coating will allow the eventual ingress of water and once that begins, disbondment coating loss and high corrosion rates are guaranteed. The most critical aspect of the pipeline project may indeed be the coating application quality control procedures.

Stress and Expansion. That's a very hot line, so practically speaking you can forget about anchoring unless the pipe has a very tiny x-sectional area or if you don't have quite a few expansion loops. Every time you try to anchor, you introduce stress, so absolute minimum anchoring should be the rule. Allowing as much expansion as possible, while directing expansion away from equipment is more effective than trying to stop it. You may have some success controlling it. Full anchor blocks will be rather large. The easiest way to connect pipe to a large concrete anchor block is by the use of anchor flange fittings (flanges, but no bolts) embedded directly into the anchor block.

Will you try to bury the expansion loops? If so, some semi-rigid material allowing expansion of the loops should be placed around the pipe in the expansive areas of the loops to allow expansion, rather than forcing growth into the trench wall which will tend to direct the pipe upwards. Anchoring via concrete blocks or by "virtual soil anchors" will introduce restraint of the pipe and will produce high axial loads which may tend to buckle the pipe, which if it does, will buckle upwards, so it is possible that controlling this buckling will require a burial depth greater than the customary 1 meter of cover. You may also want to investigate limiting bend (both horizontal or vertical) angles of the pipe (to 5º (?)or less) in any long stretch to minimize the initiation of buckling by creating eccentric axial loads meeting at the bends.


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[CSGWB]

BigInch,

I am also concerned about coating at this temperature. Most common coatings will not withstand this situation. I am led to believe that fusion bonded epoxy may be the way to go, but I haven't, as yet, looked into it.

With respect expansion loops, we were looking at running the pipeline through into a reinforced concrete pit, which would contain the expansion loop and allow free movement ithout soil resistance. Since we only have a relatively short length, we can get away with the small number of pits. This pit would be covered to prevent water from getting in.

Anchor blocks would be fitted directly between these loops to try and balance the axial loads. I expect that these will be concrete encase flange type, however I am not sure how the differential expansion will effect the concrete in the long term. We may end up without and anchor and plenty of rubble around the pipe.

Thanks for your comments.

CSGWB

 

[BigInch]

Its too high a temp for the usual FBE coatings I've seen. Most are topping out at 120 C, although I have seen one that claims to go to 160 C. That's high and it may be a claim only. They may not have the proven experience to back it up, but hopefully I'm wrong. At least if you have to go to an exotic coating, you only have 500 m to coat.

Watch the use of rubble, for coating scratch reasons. With a lot of movement possible, there is also a lot of potential for contact with sharp things. Use clean sand for trench backfill and bedding well around the pipe.

A pit is a good idea to keep expansion free, but allowing for movement at the entrances/exits will probably allow some water entry. Sooner or later, you'll get some water in there anyway, but with good coating .. n/p. Make enough removeable covers for inspection purposes with access to allow visual inspection under the pipe at supports.

Watch the potential to develop an explosive atmosphere in pits if you have a volitile product, especially if any instruments or valve operators will be contained inside. Allow for circulation and venting if necessary.

Why are they making you go underground? Crossing public domain? Well, the ground insulation effect may help with the heat loss once you manage to heat it up.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[CSGWB]

Biginch,

Rubble reference was more to do with my fear that the concrete will break up more than anything, but you are correct in nominating good clean sand. That one goes without saying with up to 100mm of movement.

Yes, underground piping is because we are running through government owned road easement.

Thanks again

CSGWB

 

[rconner]

I don't profess to have any experience with such a line/application, but I was just curious if a larger tunnel or in effect utility corridor might be feasible/cosntructed, that could allow this line though underground to be designed/supported anchored inside same much like an aboveground process line at such high temperatures etc. (and additionally allow for access if necessary etc?)

 

[CSGWB]

rconnor,

That is my preferred option. Unfortunately my client is apprehensive about running a casing pipe or culvert over this distance. It has a relatively large cost disadvantage, but does protect the line and permit controlled movement and more importantly allows the line to be insulated (if necessary).

CSGWB

 

[Zapster]

When I first read your post, I assumed that you were discussing a cased pipe.

What is the required life of the buried pipe? What is the consequence of a failure in the pipe? If there is a failure, due to direct burial, how much money was saved? How do you ensure that the bitumen/asphalt can never solidify in the line? Will this line be steam traced? What will be the depth of burial? How many thermal cycles will the line go through? What is the line diameter? What is the minimum flow rate and what is the maximum heat transfer that is acceptable? Is there a cost associated with the heat loss from the 500m line?

 

[BigInch]

Why would it have to be cased? If he can find a coating that works at that temperature, it shouldn't be a problem. I designed a hot heavy crude line 140 km long to operate at 190 F. It was buried in soil. Viscosity at 70 F = 1300 cp, 90 cp at 190 F and when mixed with a lighter diluent oil or naptha.

Solidification is prevented by having enough standby equipment and spares to exceed the MTTR with a safety factor. Even in case of a complete break, solidification in the pipeline would not occur for about a week, due to residual heat maintained in the soil. It would get a little thick, but still pumpable with all standby pumps on line and configured in series. When the flow gets going, the pumps can be dropped off series and rearranged to paralled flow for low pressure higher volumetric flow.

For long term shutdown for maintenance, the heavy crude was totally replaced with light oil pumped up from the coastal refinery in the parallel diluent pipeline, (normally mixed with the heavy to reduce viscosity) then the line could be shut down with only light oil contained inside.

There is a heat loss from the pipe contents to the soil, but that's just part of the operation cost of the pipeline. When you have too much oil in a far away place ready to burn so you can heat up more and flow a whole lot more to market, you just find the break-even point for flowrate, temperature, viscosity, mixture ratios and do it. Besides, insulation gets wet, gets compressed and when the volume of soil is high and the effective radius of the pipe-soil system is very large, it isn't of all that much value, since near the pipe the temperature is pretty close to the same on one side of the insulation as it is on the other. (And finding the $/(º-BOPD) breakeven point often isn't easy, when the ground temperature between winter/summer dry/wet seasons never really stabilizes to reach a constant heat capacity or transfer rate around the pipeline.) That particular line's capactiy should eventually reach 750,000 BOPD.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com