I talk a lot about “pressure and leak testing” our piping systems before they are put into service or returned to service after breaching the pressure boundaries, but the same practice should apply to our process vessels/tanks. But I almost always get the “you must be on drugs” look when I ask about this in a PSSR or after a release from a vessel. In this article, I will speak to flammable liquids process safety and refer to RAGAGEPs specific to these processes.
Several codes and standards specifically address leak/tightness testing of tanks and vessels before they are placed into hazardous-material service. The most directly applicable depends on whether you’re talking about an atmospheric storage tank, a pressure vessel, or process equipment.
NFPA 30 — Flammable and Combustible Liquids Code
For tanks containing flammable liquids, NFPA 30 is probably the most directly relevant code.
NFPA 30 Section 21.5.2, Tightness Testing states that, in addition to the applicable structural/design testing, all tanks and connections are to be tested for tightness after installation and before being placed in service. For aboveground tanks, the test is generally conducted at operating pressure using air, inert gas, or water, subject to the specific tank configuration.
Importantly, NFPA 30 also says:
- Leaks or deformations must be corrected before the tank is initially placed in service.
- Field-erected tanks may have their required construction/hydrostatic tests count as the tightness test.
- Specific test pressures are provided for shop-fabricated aboveground tanks.
- Air can NOT be used to test a tank that contains flammable or combustible liquids or vapors.
That last point is particularly important for a hazardous-material service startup procedure.
API 650 — Welded Tanks for Oil Storage
If you’re dealing with a field-erected atmospheric storage tank, API 650 is another major standard. API 650 §7.3.7 requires a hydrostatic test of the tank. It also specifically addresses examination of welded joints for leakage after the hydrostatic test. API 650 is particularly useful because it gives you a formal construction/testing framework rather than simply saying “check for leaks.”
One important distinction:
API 650 hydrostatic testing is primarily a tank construction/integrity test, while NFPA 30’s tightness test is specifically tied to being installed and placed into service.
Depending on the tank, one test can sometimes satisfy the other requirement, but I would document that explicitly rather than assuming it.
NFPA 329 — Underground/Leak Detection
NFPA 329 is more focused on detection of releases of underground flammable and combustible liquids, rather than being the primary construction pressure/tightness test standard.
It does, however, contain useful guidance concerning the hazards of pneumatic testing. NFPA material specifically cautions against using air/non-inert gas pressure testing on tanks or piping containing flammable/combustible liquids because of rupture and liquid-expulsion hazards.
The important distinction: “pressure test” vs. “leak test”
This is where I think your question gets particularly interesting. There are actually three (3) different things that often get lumped together:
| Test | Primary purpose |
|---|---|
| Hydrostatic/pressure test | Demonstrate structural pressure integrity |
| Pneumatic pressure test | Demonstrate pressure integrity where hydro isn’t practical |
| Tightness/leak test | Demonstrate that the completed system doesn’t leak under operating conditions |
A tank can pass a hydrostatic pressure test and still have a leaking flange, valve, threaded connection, manway gasket, instrument connection, etc. after the final piping and equipment are installed.
That’s why NFPA 30’s wording is significant: it expressly calls for tightness testing after installation and before placing the tank into service.
For your type of application
Based on the hazardous-material systems you’ve been asking about—flammable liquids, stainless-steel process piping, tanker unloading, storage tanks, and hazardous-material service—I would not rely solely on the vessel manufacturer’s original hydrostatic test certificate.
I’d establish a documented pre-service leak/tightness test covering:
- Tank/vessel shell and heads
- Manways
- Nozzles
- Flanges
- Valves
- Instrument connections
- Tank-mounted piping
- Transfer/unloading connections
- Flexible hoses
- Tank outlet/inlet piping
- Drain connections
- Vents and vapor-return connections
- Secondary containment, where applicable
And we MUST distinguish the tank structural test from the completed-system tightness test (e.g., leak test).
For a flammable-liquid system, be very cautious about simply specifying a pneumatic test pressure. NFPA 30 specifically prohibits using air to test tanks containing flammable/combustible liquids or vapors.