If you have a gas grill, and it isn’t hooked up to your home’s gas line, then you’re familiar with the 5-gallon propane tank or bottle. In fact, to summer grillers, this tank is as ubiquitous a tool as a spatula for flipping burgers. It’s so commonplace, in fact, that we rarely think about the engineering that goes into it, any more than we think about how our TV’s work. But the humble propane tank is actually an impressive piece of engineering:
• The four gallons of propane actually in a five gallon tank (they’re never filled 100-percent, to allow room for the propane to expand if temperature changes) contain almost 350 megajoules of energy — which is the equivalent of the kinetic energy in nearly 350 one-ton cars moving at 100 miles per hour! That’s also enough energy to heat 9,000 gallons of water by 10° F.
• The tank is usually pressurized to between 100 and 200 PSI (pounds per square inch). That means every inch of the tank could hold back the weight of an adult!
So that tank sitting under your BBQ or grill contains enough canned heat to warm an entire swimming pool, held under 10 or more times average atmospheric pressure. Pretty impressive for something you never think about!
How is this everyday marvel constructed? The TV show “How It’s Made” has a good video on the subject, but in case you’re not interested in spending the next 6 minutes watching industrial tools, here’s the short version:
- 1. The propane bottle begins with steel. A powerful hydraulic press punches out 19-inch steel disks from a coil of steel.
- 2. A second press clamps the disks in place while a hydraulic or motorized die presses upward, forming the disks into half-cylinders.
- 3. A trimmer trims (what else would it do?) the edges of the half-cylinders, leaving them smooth.
- 4. Half-cylinders, destined to become the top of completed bottles, have a flange inserted into a pre-punched hole and are welded in place. Meantime, the half-cylinders destined to become the bottom of a bottle have a steel ring (formed from a strip of steel that’s been rolled into a circle) attached to their base, to provide a “pedestal” for the bottle to rest on when upright.
- 5. The tops halves and bottom halves are pressed tightly together, then welded.
- 6. Because all the heat and pressure has made the steel hard but brittle — which is not good, in something that must expand and contract as the internal pressure changes — the bottles are now tempered. That is, they are heated, then cooled, in a controlled fashion that makes the metal more elastic and forgiving.
- 7. The tanks are painted.
- 8. The valve assembly is welded in place, to the flanges on the top.
- 9. Everything is thoroughly tested — pressure tests (internal and external pressure), mechanical tests of weld strength, and leak tests — before the hardware is certified for use.
The result? A massively strong, very safe container. It is a bit on the heavy side — the average bottle weighs around 30 pounds when full, of which about half is the propane. Since the tank, while rugged and dependable, is heavier than some backyard cooks may want to deal with, there are also lighter tanks made of fiberglass. The fiberglass tanks weigh around 30-percent less than steel tanks but are still very strong. (Remember: some sports cars, high-performance boats, and airplane components are made of fiberglass.) They also offer a window inside the tank, to see how much propane is left.
How are these tanks made? That is a tale for another blog posting.