Thief hatches: what they do, why they fail, when to replace
A thief hatch is a combined access port and pressure-relief device on the top of a storage tank. It lets you gauge the tank (the "thief" part — from the old practice of "thieving" a sample) and provides pressure and vacuum relief so the tank doesn't buckle on a hot day or get crushed on a cold one.
How they fail
- Spring fatigue.Springs lose their set over time. The hatch relieves at a different pressure than it's supposed to.
- Gasket failure.Heat, sun, and chemical exposure degrade the seal. Result: emissions you didn't plan on.
- Corrosion.The body and pivot corrode, hatch sticks open or won't seal.
- Damage from gauging. Bent or damaged hatches from rough handling during sampling.
When to replace versus repair
Spring replacement and gasket replacement are cheap routine repairs. Corroded body, damaged pivot, or any sign that the relief mechanism isn't working — replace the whole hatch. The cost of a replacement is trivial compared to a single environmental violation.
Service cadence
Annual inspection minimum. Quarterly in high-corrosion environments or where regulatory requirements demand it. We document pressure readings and condition at every service visit so you have an audit trail.
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