Leak Detection in High Pressure Oil Filled Cable?

[duh]

Hello,
Are there any technologies out there that enable one to detect a leak in buried high pressure, oil filled cable? The cable is located in an urbanized area, so a tracer-based method of locating is not feasible since there are roads, parking lots, sidewalks, etc in the way. I've heard things about ultrasound being used, but I have not been able to find any information on this. Does anybody have experience with this?
Thanks

 

[mc5w]

If there is a way to pressureize the space between the cable and conduit with say nitrogen, argon, or carbon dioxide you can keep water from leaking in at detect oil leaks by means of pressure rise if the oil pressure is high enough.

Gasoline filling station pipes are a double wall construction so that leaks will head back to the pit over the tank where the submersible pump goes into the tanks. Useful hint for next new cable is to put a flexible nonmetallic conduit or pipe into your underground duct and then put the oil filled cable inside. Detection of oil leaks and water leaks then very easy. Should also have a telephone cable and pressuring gas pipe in adjacent ducts.

Your existing contruction could make leak detection difficult but low power wireless that runs off of solat cells could be installed at each manhole. The gas companies are using this technology for their alarms as well as older dry cells.

Mike Cole, mc5w@earthlink.net

 

[duh]

Thanks Mike,
Each conductor in the pipe is stranded and wrapped with paper and a concentric neutral. 3 such conductors exist inside the pipe. The pipe itself is then filled with oil which is the insulating medium. It also adds cooling properties. The pipe is coated with a special substance to make it resiliant to corrosion. The pipe is then direct buried in special sand to further increase the heat dissipation. So, no ducts were used. We put a concrete cap on the pipe as well, to provide some mechanical protection.

Unfortunately, this was all done some 35+ years ago and nobody back then had thought about leak detection if the pipe should fail! I think you're right...I think the existing construction seems impossible to retrofit with some sort of leak detection system...

 

[mc5w]

You might also try a very accurate volume meter that is temperature compensated at your oil source to measure how much oil is getting in. Still subject to measurement errors but you will know that you have a leak if the system is eating a quart or more a month.

A method that alledgedly was invented by a radiologist who was having a plumber look for an undergound leak was to put in a short lived radioisotope that normally is used a tracer in the human body. Since the radioactivity is short lived and if you put extra pressure on the cable to open the leak the isotope will flow to the leak and stop around there and the hazard to the environment would be minimal.

You would need to use a scintillation detector rather than a geiger counter. A scintillation detector is more sensitive and would detect radioactivity a few feet down.

How long is this cable and how much juice does it carry? Is putting in a new cable alongside or on a different route reasible? If this cable is carrying a lot of power then a backup cable would make more sense because you will need it. Perhaps a reliability study would show that another cable is very well needed - 35 years is a lot of time for load growth.

 

[BJC]

duh
I put "pressure monitoring oil filled cables" in google and got a bunch of hits. The Brits seem to have a handle on this.

http://www.nationalgrid.com/uk/library/archive/performance_reports/report98/mn_oil_pressure.html

http://www.cableconsulting.net/images/adobe_flyer_monitoring_techniques.pdf

 

[duh]

Mike,
The high-pressure cable is 1.8km long and is at a voltage of 138 kV. Installing another cable would definitely help with maintaining the reliability of the system, but I am more concerned with the environmental impact of an oil leak. Because of its length and location, we can't just dig "willy-nilly" once we know we have a leak somewhere along its length.

The radioactivity idea is interesting. I have also heard of it being used for the body, but not for an application like this. I would be curious to know how a system like this would affect the insulating properties of the oil, the paper and the outer coating. Also, how quickly would the isotopes reach the leak spot? Thanks for the info!

BJC,
Thanks for the links...I'll try to contact the Brits and see what they have to say!

 

[mc5w]

Getting the radioactive tracer to the leak spot could be done by running a temporary hose form one end to the other so that you can circulate the oil. You could also do the equivalent with 2 tanker trailers one full at one end and the other empty at the other end. Some oil filled cables already have this because the oil has to be pumped through radiators or even chillers top get rid of heat.

If it is the case that you have a way to recirculate the oil or to pump enough from 1 tanker to the other to completely displace out the original oil that would allow the radioactive tracer to reach the leak and run into the soil. 1.8 KM is a rather short distance. If you had to do this with 20 KM you would be in a sticky wicket. Once the blob of radioactive oil gets past the leak the soil will remain radioactive. You might need 2 or 3 people each with a scintillation detector - the lead man follows the radioactive slug that is inside the pipe and the two followers look for any residual radioactivity that leaked out.

One way to get a radioactive tracer would be to grow some plants using radioactive carbon to produce radioactive vegetable oil that is compatible with your insulating oil. There are also some synthetic and semi synthetic oils out there that are a lot less flammable than your present oil.

Actually, for a cable this old you might want to consider an oil change anyways. You cable is just old enough to have PCBs in it. Even if it does not have PCBs it could have enough water and other contaminants that need to be cleaned up.

Speaking of PCBs, back in the 1970s a chemist at Goodyear patented a process for destroying PCBs and other halogenated hydrocarbons by mixing them with a dispersion (suspension) of sodium metal in kerosene. A good book on sodium is the ACS monograph on Sodium by Marshall Sittig. There is a chapter on making dispersions by mixing molten sodium with kerosene or light oil at just above the melting point, vigorously stirring (possibly ultrasonic nowdays) and then cooling the mixture down.

You did not mention which country where this was installed or what the power frequency was. I am curious as to what this short of a hop is connecting.

 

[duh]

Mike,
Country = Canada...therefore 60Hz freq. This 138kV line connects two substations located in our downtown core. The substations step the voltage down to a distribution voltage (15kV and 25kV). At one substation, we have a pumping station, which maintains the pressure throughout the high pressure, oil filled cable.

Thinking about it more, the radioactive isotopes may not be possible. It would likely work, but the perception that the public would have on such an action may be detrimental. Also, my supervisor is leery of adding anything to the insulating oil...since we have no way of knowing if the additional substance will harm the insulation or not, or what the long-term effects of the addition will have on the insulation. Best technology would be one involved with passive detection (if there's even such a thing!)...maybe something attached to the outside of the pipe that can identify the location of the leak????

I'm still looking into this...and will post my findings here, once I find something that meets my needs!

Thanks!

 

[sdlloyd]

duh,

The cable construction sounds like a 138kV HPPT system. The cables are normally wrapped with a skid wire (I think you called this a concentric neutral) which aids cable installation. There is only one cable manufacturer in North America for this cable, Okonite. They supply cable to installation contrators.

You would be well advised to seek some professional help as it is too easy to make a make a serious mistake. I believe there is an outfit in Canada called EHV Power who can help you, try

http://www.ehvpower.com,

if they cannot do it directly I am sure they will be able to recommed someone who can.

Hope this helps.

http://www.cableconsulting.net