Air compressor tank cathodic protection

[Bone Head]

Early on I was lucky enough (insert rolling eyes here) to work at Davy Compressor. Apparently Davy was well aware of rusted tank syndrome; their solutions were two depending on the air the compressor made.
The military high pressure units (3500-5000 psi making 15 cubic ft) had spun steel bottles. O.D. of 10".
The main commercial workhorse were formed tanks with 1" plus walls, with a standard practice of hooking up a fogging pump and running 2 to 4 quarts of light oil into the tanks before shipping.
Commercial models had the lid bolted on them. I guess one could unbolt it to inspect the inside. Never heard of anyone doing that.

 

[francist]

I think this is because the water is not changed very often, the Oxygen in the water must be depleted after a while and so there is no more rusting. Is this reasonable?

It’s very reasonable. One of the worst things you can do to an old-school hot water heating system is keep dumping the water and have to add new. Our system at work is 130 years old and still using the same pipes and cast iron radiators. When we changed out the boiler in 2012 we of course had to dump the entire system and start fresh. At the first year change the sidestream filter was filthy and black but now after ten-plus years it’s barely grey.

Oh, and if you ever have occasion to want to falsify age on a metal item, say a knife blade or something, a very common technique is to hang it in the toilet tank (no, NOT the bowl ). It’ll change must faster than hanging in a bucket of water that just sits there.

 

[taiwanluthiers]

It’s very reasonable. One of the worst things you can do to an old-school hot water heating system is keep dumping the water and have to add new. Our system at work is 130 years old and still using the same pipes and cast iron radiators. When we changed out the boiler in 2012 we of course had to dump the entire system and start fresh. At the first year change the sidestream filter was filthy and black but now after ten-plus years it’s barely grey.

So what happens if you keep running down the water heater because you use too much hot water, and what about tankless heaters?

 

[francist]

Okay different system — I’m referring to a hot water heating system for heating a building, not a domestic use system where that water is used for showers and things.

 

[taiwanluthiers]

Okay different system — I’m referring to a hot water heating system for heating a building, not a domestic use system where that water is used for showers and things.

Yea those one used for showers have sacrificial anodes. You also wouldn't want the water coming out rusty in the shower.

 

[matthewsx]

I think the main reason they don’t do this is cost. Consumer air compressors are commodity items and draining them frequently will make the tank last longer than the compressor head.

Also, oil from the pump will coat the tank.

John

 

[Eddyde]

Water heater tanks are lined with porcelain enamel. The anode is there to prevent galvanic corrosion, not rust. Thus the anode will not work to prevent rust in a compressor tank.
From what I understand, explosions from rusted compressor tanks are extremely rare. However, since the consequences of such an event are deadly, the prudent thing to do is have the tank pressure tested regularly, usually every 3-5 years.
Hydrostatic testing is done by filling the tank with water then pressurizing it to 50-100 PSI over its normal maximum pressure. Since water cannot be compressed it cannot store energy. The only stored energy is in the small bubble of highly compressed air or other gas at the top of the tank. if the tank were to fail, it won't explode, just rupture and the water would simply gush out. The testers I used to hire would use a nitrogen or CO2 tank as a pressure source.

 

[taiwanluthiers]

Water heater tanks are lined with porcelain enamel. The anode is there to prevent galvanic corrosion, not rust. Thus the anode will not work to prevent rust in a compressor tank.
From what I understand, explosions from rusted compressor tanks are extremely rare. However, since the consequences of such an event are deadly, the prudent thing to do is have the tank pressure tested regularly, usually every 3-5 years.
Hydrostatic testing is done by filling the tank with water then pressurizing it to 50-100 PSI over its normal maximum pressure. Since water cannot be compressed it cannot store energy. The only stored energy is in the small bubble of highly compressed air or other gas at the top of the tank. if the tank were to fail, it won't explode, just rupture and the water would simply gush out. The testers I used to hire would use a nitrogen or CO2 tank as a pressure source.

That seems overkill as I'm willing to bet exactly nobody does this, and there's probably compressors over 20 year old.... I know hydrostatic pressure test is required for stuff like compressed gas tanks and LPG cylinders, but I don't see it for air compressor tanks...

 

[ChazzC]

You could plate it... but again it would be a custom procedure.

Why is it screwing a piece of magnesium onto the bottom of the tanks not work to protect the tank? They do this for boat hulls and that's saltwater which is MUCH worse for rust.

I mean for water heaters I think the sacrificial anode is screwed through the top of the tank.

The anodes for boast/ships work because the entire hull is submerged and everything is in contact with the anode electrically (wooden/fiberglass hulls don't provide this connection, so only a steel keel and anything connected to it would be protected – until the anode is consumed and not replaced). As noted by others, while an anode in the bottom of a tank work protect the bottom, it won't protect the portions of the tank that are not in contact with the anode by both water & metal).

The anode in water heaters is a rod that runs from the top to near the bottom, so all of the interior of the tank is part of the circuit.

The problem with anodes is that they are consumed doing their job, so even sink plating and hot-dipped coatings eventually wear out.

Here's a This Old House video that discusses water heater construction & anodes.

 

[tq60]

Cost...

Our compressor was not much more than similar size from TSC, but a bit of looking and we found a good source.

7.5HP Single phase, 2 stage
80 gallon upright
Intercooler between pump and tank
Mag starter
Auto drain
Manual drain

$1950.00 in 2009, Champion, TSC was basic, no auto drain or intercooler and $2100.00

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