Question:
Why is the aluminum coupling atop my tank — where the pressure relief valve is located — continually being eaten away and corroded after only a few months of operation?
Robert Kerr
Denver, Colo.
Answer:
The pressure relief valve is made of brass. The aluminum coupling in the tank is, obviously, made of aluminum. What happens is the basic process of electrolysis. The two metals are not compatible in this application. The process of electrolysis eats away at the aluminum because it is the weaker of the two alloys. This has happened to many aluminum tanks when brass is installed into aluminum fittings. The answer to the problem is a simple one.
1. When the aluminum coupling is corroded, it will need to be replaced. It is likely a 2-inch coupling.
2. It will need to be replaced with a coupling that is 1/2-inch larger than what is currently being used.
3. And here is the key: A plastic insert (PVC) needs to be put into the aluminum fitting to insulate it from the brass fitting or valve. These plastic bushings are readily available from any hardware store.
4. Then install the brass valve into the plastic fitting. The plastic fitting serves as insulation between the two alloys and puts an end to the process of electrolysis and corrosion of the aluminum fittings.
Check Your Baffles For Solid Welds
Question:
After three or four years, why are my baffles falling out? They literally fall down inside the tank? I thought baffles were supposed to be there forever to help reinforce the strength of the tank.
Jack McGhee
Boise, Idaho
Answer:
There are several reasons baffles can fall out inside the tank.
1. Corrosion of the welds, which leads to collapsing baffles, could be caused by the product you are hauling in the tank. The payload may include chemicals high in pH, which could lead to electrolysis and destruction of the welds. As time goes by, welds are attacked by the chemicals and they give way. You may not be aware you’re hauling highly acidic liquids or someone may be dumping chemicals in your portable restrooms that you are not aware of. Strange liquids and chemicals sometimes end up in portable restrooms.
2. Oftentimes baffles are simply stitch-welded inside the tank. Stitch welding is a way manufacturers can save time and money when they install the baffles. Stitch welding is pretty much what the name implies; baffles are secured with staggered welds rather than a continuous weld. So if one were looking at the finished welding job, it would appear “stitched.” This method minimizes the attachment of the baffle to the shell of the tank. It also aids in the corrosion caused by the electrolysis. A tank built like this will last a few years without noticeable change, but then the baffles may start tumbling. A way to avoid this is seal-welding both sides of the baffles. This means a solid weld around the circumference of the baffle. This is the more expensive method, but ensures the best longevity.
3. Another potential reason for baffle failure is the welder isn’t making a solid weld with full penetration from the baffle to the tank. The welder may have been careless or didn’t know how to attach a baffle the best way.
4. It’s a good idea to ask the tank manufacturer how he installs baffles. Are they stitch-welded or seal-welded? And when the tank is done, look inside to make sure you got what was promised. The rule of thumb is that stitch-welded baffles have a much shorter life.
When you get a chance, grab a flashlight and take a look at your baffles. It also wouldn’t hurt to rinse your tank out every now and again. If you’ve got manways, open them up and rinse out the tank. Then you can inspect the condition of the inside of your tank, baffles and all.
