Stray currents from oil tank ICCP system to remote substation earthing (grounding) array.

[ScottyUK]

Wasn't sure whether to post this question here or in the electrical area, so I will probably post a link to this question from there.

We have a legacy installation of large crude oil storage tanks which are equipped with motor-driven mixers located low in the wall of the tank designed to keep the oil in gentle motion. Each tank is protected by a dedicated ICCP system with groundbeds local to the tank bund. In addition to the mixers, the tanks have the typical level instrumentation and electrically-actuated valves.

The motors are fed from a remote substation and the cable is a steel wire armoured type which is connected to the substation's earthing array. The cable armour provides a path for current to return to the source in the event of a fault. The motors are located in an ATEX Zone 2 which is loosely equivalent to a North American Div 2. There are fairly prescriptive rules about what can and can't be done with electrical equipment in this type of location.

The armour of the mixers provides a path for current from the ICCP system to enter the earth at the substation's copper earthing grid, bleeding current off the rectifiers and resulting in much higher groundbed currents to achieve adequate potential shift on the tank. It also provides a route for the ICCP systems on neighbouring tanks to interact. The mixers are particularly problematic because the cables are large and the cable armour resistance is low, so although there are some currents flowing the armour of the valve actuator and instrument cables the currents are much smaller than those in the mixer cables.

I'm interested in what other users have implemented in this scenario to meet the conflicting requirements of the ICCP system which would tend to introduce isolation flanges and joints to control stray currents, and the electrical and ATEX rules which at a simplistic level would tend to bond everything together. I'm not especially happy about heavy DC currents flowing in cable armour where disconnecting the cable could result in an incendive spark, but I'm not seeing any acceptable alternatives either. I'm inheriting this mess, so if someone has solved the problem before I'd appreciate some hints where to look.

 

[waross]

1. Install REF protection to trip on motor ground faults.
2. Isolate and insulate the motor end of the cable armour.
3. Bond the motor to the tank ground.
But, not knowing your ATEX rules, I suspect that this may not be allowable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

Have REF on the transformer at the source, and a core balance CT on each of the motor circuits. Typical E/F trip settings are 5A and 0.5s definite time, so we meet the required disconnection times for an earth fault.

The isolating gland gives me some cause for concern because of the potential difference between the gland body and the tank and the possibility of generating an incendive spark. I guess this could be mitigated by an insulating boot, although they do tend to accelerate corrosion by trapping contamination under the boot.

 

[LittleInch]

A similar thing happens on pipelines where they run A/G, but still connected to the Cp system. Earthing of the CP system through non isolated connections and instruments is a common problem. The other option is to electrically isolate the motors from the tank by means of an insulating gasket and insulating bolt sleeves. Don't forget to isolate / insulate the instruments as well, including any air lines.

There will always be some spark / touch potential issue, but that's the payback for having an impressed current system. Warning signs and notes are the best you can do and insulate any sections where people might create a spark potential.

You could always replace it with a sacrificial anode system...

What lightening protection does the tank have?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[waross]

Hi ScottyUK.
Given that we are working under different codes, I hope that I am not wasting your valuable time, but:
We had serious corrosion issue with gland connectors in a potash plant. Rather than use stainless steel connectors we used standard connectors and covered them with "Cold Shrinks". The first shrink was placed on the cable next to the gland connector to increase the diameter. The second shrink was placed over the gland connector and overlapped the shrink on the cable. RTV silicone was used to seal the end of the shrink at the junction box.
If your codes allow an insulating nipple between the gland connector and the pecker-head, then cold shrink tubing may be a good choice to insulate the gland connector.
If the cable armour is not insulated from the motor then you may also get an incendive spark when removing the motor. Isolating the armour prevents the flow of the protection current in the cable armour and with no cable flow there should be no sparking when the cable or motor is removed.
The shrink range would not accommodate both the cable diameter and the gland connector diameter so a smaller shrink was used to increase the cable diameter.
Cold shrinks:

http://www.cablejoints.co.uk/upload/3M_Cold_Shrink_Tubings.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Hello Scotty. I am interested in this problem. Any resolution?
Thanks.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

Sorry, I thought I'd replied. Oops.

I think we're going to use a core balance CT on the motor cable, which should allow us to meet our minimum disconnect times and allows a less-than-ideal earth return via the mass of earth to be employed, thus breaking the metallic path bath to the remote earthing grid. Over here it would be described as a T-T system where the equipment at either end is earthed independently without a metallic connection between them. An RCD (GFCI) or equivalent - e.g. the CBCT I'm proposing - is required for a T-T installation by our regulations.

The cable will be terminated at a non-metallic terminal box local to the motor, and a short pig-tail cable will make the final connection from box to motor. I can either use an armoured cable for the pig-tail and make sure that the glands are NOT bonded, and post a warning of possible PD between glands, or use a non-armoured cable which avoids the problem completely. The gland of the cable from the remote source will have a synthetic rubber shroud fitted which will reduce the likelihood of inadvertent contact being made to local steelwork.

I have three of these things to install within the next month or so, and I'll try to remember to get some photos. I'll add them to this thread.


I had hoped there might be a 'standard' solution from the CP world, but as usual it's the electrical guys tidying up the mess caused by everyone else. ;-)

 

[waross]

Thanks Scotty. That sounds like a good solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

"...metallic path back..." in case it's not obvious!

 

[waross]

I much prefer a local ground such as you are using. It reduces the touch potential hazard by keeping the motor housing at the same potential as the local earthed metal.
With long cables, the impedance of the supply and grounding conductors may limit the fault current and delay ground fault tripping. During this period, the supply conductor(s) and the grounding conductor will form a voltage divider that may easily impress 2/3 of the line to neutral voltage on exposed motor parts. 2/3 of 230V in the UK, 2/3 of 277V in the USA and 2/3 of 347V in Canada are all potentially lethal touch voltages.
The local ground does not eliminate the voltage rise, but it does create an equi-potential zone which reduces or eliminates the touch potential.
I know that you know all this, Scotty, but others will be reading this thread.

Bill
--------------------
"Why not the best?"
Jimmy Carter