Every polyethylene tank carries a specific gravity rating, and it is the number buyers skip. Capacity gets checked. Fitting locations get checked. Then a 1.5 rated tank shows up on site and gets filled with something far heavier than it was built for.
What the rating means
Specific gravity compares a liquid to water. Water is 1.0. A liquid at 1.5 weighs half again as much as the same volume of water.
That matters because a tank wall carries hydrostatic load, and hydrostatic load is a function of liquid height and density. ASTM D1998 sets wall thickness from that calculation. Increase the density and you increase the stress the wall sees at every point below the surface.
The specific gravity rating is the maximum density that wall was engineered for. It is not a suggestion and it is not spare margin.
Where common chemicals land
- Water and most dilute solutions: 1.0
- 12.5% sodium hypochlorite: roughly 1.2
- Ferric chloride: roughly 1.4
- 50% caustic soda: roughly 1.53
- 93% sulfuric acid: roughly 1.84
Sulfuric is the one that catches people. At 93% it is nearly twice the weight of water, and a standard 1.5 tank is not built for it. That service normally calls for a 1.9 rated tank.
What an over-rated tank looks like
It rarely fails on day one, which is exactly why it keeps happening. Polyethylene under sustained stress creeps. The first sign is usually a bulge low on the sidewall where head pressure is highest. Then the tank goes slightly out of round. Fittings near the bottom begin to weep because the flange face is no longer flat against its gasket.
By the time it is obvious, the tank is finished. There is no repair for a crept wall.
Specifying it correctly
- Start with the actual chemical and concentration. "Acid" is not a specification.
- Look up density at the temperature the tank will actually see, not at lab conditions.
- Round up. If your liquid is 1.42, buy the 1.5, not the 1.35.
- Think about the next service. If there is any chance the tank gets repurposed for something heavier, buy the higher rating now. The price difference up front is small. A replacement is not.
Bottom line
Tell us the chemical, the concentration, the temperature, and the operating height, and we will tell you the rating the application needs. That conversation takes about five minutes and it prevents the most expensive specification mistake we see.
Have a project on the board?
Tell us the chemistry, capacity, and ship-to ZIP — we'll have a real quote back the same business day.
