Conventional Vacuum Tank Designs Stand Test of Time

A straight-side oval tank would look cool, but it’s more practical to stick with the tried-and-true vacuum tank design.

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QUESTION:

I've seen a lot of vacuum trucks over the years and they are pretty much the same. Round tank, trays, etc. I've been on a lot of job sites that also had water trucks that were shaped more into an oval or even an oval with straight sides. These water trucks are sharper looking in my opinion. How do I get one of these manufacturers to build me a tank similar to a water truck?

Vince Jamison Albuquerque, N.M.

ANSWER:

In the brief history of vacuum tanks, you'll find tanks were based on round cylinders, just as you described. This is because vacuum is a powerful force that evenly and constantly pulls on the inside of these tanks. I have seen tanks that have totally collapsed under the force of vacuum. I usually say the top of the tank has now kissed the bottom of the tank. If the tank itself is not round, vacuum will find the weakest points and cause it to collapse.

The rectangular tank has only been developed recently. Many portable sanitation contractors wanted a tank that could also serve as a truck bed, so the rectangular vacuum tank was born. First question: Why doesn't it collapse?

A lot of engineering and solid bracing is required in building a rectangular vacuum tank. Manufacturers keep their engineering secrets to themselves since they paid for and calculated what type of bracing is required inside the tank. If you take an ordinary rectangular tank (six sides) and add vacuum, it will collapse in minutes. At some point in the vacuum process, the flat sides of the tank will be sucked in.

This is also why you don't see round vacuum tanks with flat heads (the ends of the tank). The round cylinder can handle the force of vacuum, but the flat heads will be sucked in. I've seen round tanks after they've suffered minor damage in a traffic crash. The tank collapsed in its next work session because a weak point, a dent in the shell (the cylinder), did not have the tensile strength to resist the vacuum. It is often said that a chain is only as good as its weakest link. The same is true of vacuum tanks. If there is a weak spot in the tank, the force of vacuum will suck it in and collapse it.

YOU COULD DO THE MATH

So why not the water truck shape, the straight-side oval? You're right, these tanks have an impressive appearance. A standard water truck tank like this absolutely won't work for the reasons cited above. But we used to say that rectangular vacuum tanks were impossible. So what needs to happen? If you can find someone, a capable design and/or structural engineer, then the straight-side oval could be built.

Such a tank would require the proper bracing throughout. The mathematical calculations behind this bracing could be developed by an engineer with a computer program for such things, but most likely, this tank will be too heavy for its purpose. Even the heads, front and back would need a large amount of bracing.

The straight-side oval water tank is relatively simple to build. Roll the shell, put in some baffles to limit the amount of water slosh, install the tank heads and throw on some skids for mounting. Most water truck tanks are made from 3/16-inch plate and some are even made from 10-gauge material (slightly over 1/8-inch thick). Manufacturers have to build them to withstand the pounding of rough, job site roads and the shifting of the water back and forth as the truck stops and starts.

On the other hand, vacuum truck manufacturers have a different dilemma. To get started on a job, the force of vacuum is quickly introduced inside the tank. Most vacuum tanks are made from 1/4-inch plate because it has the tensile strength to withstand vacuum force. The concept is to not let the tank collapse, but to open a valve and the power of vacuum will suck in whatever that hose is put in.

So it's possible to have a straight-side oval tank serve as a vacuum tank, but it's not practical. The tank weight would likely be incredible, the costs would likely be too high, and the design raises the question of where do you hide your hoses and accessories.



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