Question: Petroleum Pipeline Vent Design? 2

[SciWriter]

I'm a science writer interested in the design of petroleum pipelines.

I'm curious why older (circa 1960) buried high-pressure jet fuel pipelines have regularly spaced "shephards' crook"-like vent pipes on the surface while newer pipelines do not.

Were these vents a safety mechanism for in-pipe pressure emergencies? Are their valve mechanisms pressure-triggered? (And if so, what pressure is necessary to cause them to vent?)

Any comments or thoughts would be mightily appreciated. Thank you.


Bryant

 

[butelja]

I believe these vents are typically used on the casement at road crossings. Where a pipeline crosses a road, river, or railroad, the pipeline is encased in a larger outer pipe to give it extra protection. The outer casing pipe is seal welded to the inner pipe, and the casing pipe is vented above ground. This gives warning of any leakage and directs the leakage to a somewhat safer location, where the vents are placed. The outer casing also facilitates easier replacement of the line in the event that leakage develops. I thought these were still in use even on new lines.

 

[SciWriter]

Thank you very much! So only a rupture of the inner casement would result in any venting, right?

I drove several pipeline routes recently; in the older reaches (like in Fallon, Nevada), there are vent pipes. In newer lengths of the line (like those in Concord, California), I saw none.

Thanks again.

 

[JeffH]

Butelja answered the question well as to the function of the vent pipes and casing pipe, however here is a minor correction:

The casing pipe is not seal welded to the carrier pipes. Indeed every effort is made to keep the two electrically isolated to avoid interference or conduction of possible electrical potentials present on the carrier pipe.
There is usually a (non-conducting) seal installed between the carrier pipe and the casing in a attempt to keep the casing from filling with water and to keep possible leaks from escaping but the effectiveness of these seals is questionable in practice.

The electrical potential is usually a purposely induced direct current for cathodic protection but could also be stray AC currents present due to nearby high voltage transmission lines.

The potential stray AC currents present a safety hazard if they are conducted from the carrier pipe to the casing pipe and then to the readily accessible above ground vents.

The shorting of the induced cathodic protection current (or potential) from the carrier pipe to the casing can result in decreased cathodic protection for carrier pipe and creates a local spot of high corrosion as the induced current flows from the carrier pipe to the casing.
Indeed, a study of the causes of pipeline leaks will find that an inordinate amount are caused by shorts between the carrier pipe and the road casing pipe and subsequent corrosion.

It is for this reason that pipeline companies, the USDOT Office of Pipeline Safety, and other related parties have taken a hard look at the use of casing pipes at road crossings and recommend their elimination whenever practicable.

Cased crossings are still used in new pipeline construction but usually only if it is a requirement imposed the entity that owns the ROW being crossed.

Jeff

And yes, only a rupture of carrier pipe would result in venting.

 

[SciWriter]

Jeff:

Splendid! Thanks for the detailed help.

--Bryant

 

[ivu]

Jeff,

If casing pipe is not recommended, which is the proposed alternative to protect the pipeline at the crossings?
IVU

 

[JeffH]

Ivu,
It depends upon what you are trying to protect the pipline from. The two biggest causes of pipeline leaks (according to USDOT statistics among others) are corrosion and outside (3rd party) damage.

As for corrosion mitigation the biggest factor is the integrity of the pipeline coating.
Obviously, installing a casing pipe first allows the carrier pipe to be inserted with out damaging the carrier pipe's coating but that method suffers from the pitfalls previously mentioned.
The alternative is to apply a tough outer coating that is able to protect innercoating from the abrasions that can be expected when jacking or pushing a carrier pipe under the crossing. A polymerized concrete coating with a thickness of around 80 mils is fairly common. Tradenames include 'Powercrete' or 'Kemacoat'.

http://www.powerlonestar.com/

http://www.pipelines.com/

As for outside damage, although I can't site the specific study at this time I believe it was the Gas Research Institute that performed a study about 5 years ago that showed that, at the depths typical for pipeline crossings, the loads from even the heaviest of railroad cars have negligable effect on the integrity of pipeline buried below, i.e. that a casing pipe is not needed for protection of transmitted surface loads, as has been assumed for many years.

Jeff

 

How do you calculate surge in a petroleum pipeline.
( 12" , API 5LX 46 )

 

[odem]

Sciwriter,
Typically in older lines casing was used at road and river crossings to protect the line and the vent pipes acted as a marker for the ends of the casing. The reason you do not see vent pipes on newer lines is because casing isnt used as often. Today, pipe with a heavier wall thickness is used at crossings. This simplifies construction and relieves some of the problems with the casing and the line pipe shorting each out when using cathodic protection.

ODEM

 

[CarlB]

SciWriter,
I have only a little experience on this topic but have seen a system as you describe, on a base in Mississippi. A likely reason for the regularly spaced vents is pipelines were designed with slope for draining purposes, and in flat ground they need to be sloped up and down, with low point drains alternating with high point vents. After a line is drained, vents allow the line to be recharged and air pockets removed. May still need to do that today, although cross country lines are usually designed so they can be pigged, thus don't need drains and vents.