Grounding Steel Tanks

[PW2006]

We use steel tanks (approx. 14~20ft diameter x 20ft~30ft high) to store the water based or oil based liquid outside on a concrete slab. The water based liquid is not burnable. The oil based liquid is burnable but is not very easy to be ignited like gas or diesel. The steel tank is either painted or galvanized. There are two situations: (a) the steel tank bottom sits directly on the concrete slab; (b) we put a 1/2" thick asphalt filled fiber board pad between the steel tank bottom and the concrete slab.

I wonder if we need to install grounding rod and grounding cable for the tank under the situation (a) and (b). If we do, how many grounding connections do we need to make for each tank?

Thanks.

 

[rybrk]

You have to provide grounding two connections for ss storage tank to discharge static electricity or any other surges or leakage currents from nearby electrical sources.

 

[PW2006]

But our tanks are not stainless steel tanks. They are just plain carbon steel tanks. They are located outdoor without any covers. Besides, is there any difference for case (a) and (b)?

Thanks.

 

[rybrk]

Why are u putting fibre pad for bottom tank? are u protecting bottom from rust or mechanical damage?

thanks

 

[VEBill]

Presumably the main purpose of adding a grounding system (and probably lightning rods) is so that when lightning does eventually hit the tanks, it will be diverted mostly to local ground.

Diverted to ground as opposed to running along any conductive pipes or signal wires into the control room and eventually going to ground via an operator. Or, setting fire to something along the way. Or damaging ALL the electronic equipment in the control room instead of just some.

I don't think that the pad (or not) makes much difference to this consideration.


Disclaimer - I'm not an expert in lightning protection(*).

(* Then again, I've seen many so-called lightning experts loose on the Internet that don't have a clue. Not here on Eng-Tips, I mean out there in

http://www crazy-ville.)

 

[Warpspeed]

Another problem can be stray earth leakage currents causing accelerated corrosion.

The tank should be insulated from the ground as you have already done with the asphalt filled fiber board. And it should then be earthed in one location only with an efficient ground stake.

Wandering ground currents cannot then find an easier path through the tank base, because it is effectively insulated from the ground. A single ground stake may eventually be eaten away over time if current is flowing through it, but the tank itself should remain corrosion free.

The tank needs to be grounded for electrical safety reasons, but the ground connection should be at one point only for each tank.

 

[PW2006]

Thank you, guys.

I don't really understand the stray earth leakage and how it will accelerate corrosion to the tank bottom if the tank bottom is not electrically insulated from the ground. There is no any electrical powered equipment sit on the tank, or connected to the tank. There is just steel piping hooked up to this steel tank. Are you talking about cathodic protection or something similar?

My reason of connecting grounding cable and rod to the tank originally was only for the lighting safety. The reasons of adding pad under the tank bottom originally were only for protecting the paint and preventing water/moisture from getting in.

Another question, is grounding this type of outdoor tank mandatory or very common practice? I have seen many similar tall outside tanks that did not have any grounding protection.

There is a circle of 3~4ft high hand rails on the top of each tank, and people can walk and work on the tank roof. But the chance of doing this in the rain and thunders is very slim.

At what situation, do I need to install the lighting rods? And how tall should it be? If the tank is 20ft diameter, how many lighting rods do I need on each tank? If I put many tanks close to each other in a tank farm, do I need to install lighting rod on each tank or only 1~2 lighting rods for all tanks but connecting all tank roofs together with electrical cables? Understand me, I don't want to overkill this lighting safety issue.

 

[rybrk]

Lighting arrestors should be installed at 6ft above height of Tank(If you can walk on above tank platform) other wise 3ft is sufficient .It will cover distance at 45degrees rad angle. Based on that you have to arrange number of lightining arresters.

thanks

 

[resqcapt19]

Most steel tank structures do not use lighting rods. The tank itself is both the rod and down conductor.
Don

 

[Warpspeed]

Cathodic protection may be one potential problem, where a small voltage difference can be generated between tank and ground, due to excess acidity or alkalinity in moist soil. A very weak constant dc current may then flow, causing ion migration and accelerated corrosion.

Another different but related problem are currents that flow through the earth itself, caused by man made power distribution systems. One cause is the three phase multiple earth neutral system, where the grounded neutral at one location is at a slightly different potential to the grounded neutral at another location.

A similar thing can occur with heavy traction current used in railway systems. While most of the current returns via the rails as intended, some small proportion can find an alternative path through the ground.

These ground currents always seek the path of least resistance, and buried pipework, or large buried uninsulated steel structures, are fair game to these currents.

These problems may or may not exist, depending on your location. But it is common practice, and always prudent to insulate large metal structures from the ground if at all possible, and then deliberately ground the structure at one point only.