6 minute read · Published by Swhift Stainless Fabrication, building stainless tanks in Sandpoint, Idaho since 1980
This is the single question that most changes the price of a tank, and it is answered wrong surprisingly often — usually by assuming a tank plumbed into a water system must be a pressure tank, or that a storage tank cannot be one.
The actual difference
A non-pressure tank is vented to atmosphere. Whatever is inside sits at atmospheric pressure, and the only load on the walls is the hydrostatic head of the liquid — the weight of the column above any given point. That load is real and it is why tanks need stiffening, but it is modest and entirely predictable.
A pressure tank is sealed and its contents are held above atmospheric pressure. Now every square inch of the shell and both heads carries that pressure, and the shell is in tension. Wall thickness, head geometry and weld quality all become structural rather than merely functional.
| Non-pressure | Pressure | |
|---|---|---|
| Vented to atmosphere | Yes | No — sealed |
| Main load on the shell | Hydrostatic head | Internal pressure |
| Typical shape | Rectangular or cylindrical | Cylindrical with formed heads |
| Shell thickness | Set by stiffness | Set by pressure and diameter |
| Relief valve needed | No | Yes — always |
| Relative cost | Lower | Higher |
| Swhift warranty | 10 years | 5 years |
Why pressure tanks are round
A cylinder distributes internal pressure evenly through hoop stress in the shell — there is no corner for stress to concentrate at. A flat panel under pressure wants to bulge, and the corner where two flat panels meet is where it will fail. This is why every pressure vessel you have ever seen is cylindrical or spherical with formed, dished heads, and it is why a rectangular pressure tank is not a practical thing to build.
It also means shell thickness scales with diameter. A larger-diameter tank at the same pressure needs a heavier wall, which is exactly what our published size chart shows: gauge steps up as diameter increases, and higher test pressures on the larger diameters require a shell and head upgrade.
How Swhift tests pressure tanks
Our vertical pressure tanks are offered at a 150 psi factory test with a maximum operating pressure of 120 psi, or a 300 psi test with a maximum operating pressure of 180 psi. Horizontal tanks are offered at 135, 175 or 300 psi test, or a custom test pressure.
The third case: vacuum
Vacuum is not simply negative pressure, and a pressure tank is not automatically a vacuum tank. Under vacuum, atmospheric pressure pushes inward on the shell and there is nothing pushing back. The shell is in compression, and a thin shell buckles long before the material itself is overstressed.
This catches people out because vacuum can be pulled accidentally — draining a sealed tank without a vacuum breaker will do it. If there is any possibility of that in your installation, the tank needs to be built for it. Our warranty specifically excludes tanks subjected to vacuum that were not rated for vacuum.
Working out which one you need
- Is the tank open to atmosphere, or sealed? If it is vented, it is non-pressure — done.
- If sealed, what pressure does it actually see in service? Line pressure, pump discharge pressure, or thermal expansion pressure all count.
- Can the tank ever be subjected to vacuum, including by accident? If yes, say so.
- Does the tank have to fit a space that a cylinder will not? If so and it does not need to hold pressure, a rectangular non-pressure tank is almost certainly the answer.
- If you are not certain, say 'not sure' on the quote request. We would rather ask two questions than build the wrong vessel.