[SI-LIST] Re: impedance for 10-mile pipe at 120Hz, skin effect
- From: Glen Walpert <gwalpert@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 13 Dec 2003 09:03:12 -0500
At 05:24 PM 12/12/2003 -0500, Ingraham, Andrew wrote:
>Aballe73 wrote:
>
> > I am trying to use PSPICE to model the behaviour of an electrified
> > and buried pipeline (for natural gas or oil). ...
> > ... There is a rectifier having one of
> > these leads connected to the pipe, and the other lead connected to a
> > grounding system: so a 120Hz rectified current is going through the
> > ground into different points of the pipe and coming back to the
> > rectifier through the pipe.
>
>Yikes! I get scared when someone talks about mixing electrical current with
>potentially explosive chemicals. I hope that sufficient safeguards will be
>provided, but still, it scares me (even if the scenario I envision of a
>spark igniting the gas could "never" happen).
Presumably this is for a cathodic protection system intended to protect the
pipeline from corrosion. I think that essentially all large buried
pipelines have cathodic protection or one sort or another, usually using
active power supplies rather than zinc or magnesium anodes. The 120 Hz is
presumably the ripple from an unfiltered rectifier. There is no reason a
filtered DC supply without significant AC components could not be used,
except perhaps reliability in a harsh environment.
The ASM Metals Handbook volume 13 - Corrosion has some references for the
design of cathodic protection systems which anyone designing such a system
for the first time should read. Most such systems are designed by
companies specializing in cathodic protection, there are lot of details to
be considered.
> > I understand that I should not considered the system as a
> > transmission line because is too short (10 miles or so) compared to
> > the current frequency (120Hz).
>
>This is not correct. You can always treat it as a transmission line even
>when it is very short.
>
>It's the opposite that you need to watch out for ... using a lumped circuit
>approximation when it is electrically long. But there is nothing really
>wrong about using transmission line theory when it is electrically short.
>The transmission line theory still holds! It's just that transmission line
>theory can be mathematically difficult, so people try to find alternatives
>(like lumped equivalent circuit models) when the full transmission line
>treatment isn't absolutely necessary. Plus, in time-domain simulations,
>skin effect resistance has traditionally been easier to do using lumped
>models rather than transmission line models.
>
>Regards,
>Andy
>
>
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