Cryogenic Tank Rehab

Ice or frost forming on the outside while it vents more often than it should, especially with little gas being drawn, is the clearest sign. Product disappearing faster than normal is another. If you suspect a vacuum problem, stop using it and have it checked.

Sources: LBNL dewar troubleshooting · Chart liquid cylinder manual

Every cryogenic cylinder loses some product. Manufacturers publish a normal evaporation rate (NER) as the measure of insulation performance. For example, MVE lists a 230 L MP Dura-Cyl at about 2.0% for nitrogen and 1.4% for oxygen or argon. Real numbers vary with conditions, so losses well above the rated NER are a reason to have it checked.

Sources: MVE liquid cylinder manual · Chart manual

No. Federal rules list DOT-4L as "test not required", so there's no periodic retest interval. They still must be inspected and repaired whenever they're damaged or not performing properly.

Sources: 49 CFR 180.209 · Chart manual

Repairs must be done by a facility holding DOT approval (49 CFR 107.805) or a registered facility in Canada. Allowed work includes opening the jacket, replacing the neck tube, and replacing insulation and piping in the vacuum space. Afterward the cylinder is pressure tested to its original spec, mass-spectrometer leak tested before and after the jacket is closed, and tested for heat conductivity.

Source: 49 CFR 180.211

Not always. Microbulk tanks span a wide size range, roughly 230 liters up to 7,000 liters (about 1,840 gallons) in common manufacturer lines. Smaller units are usually still built to the DOT-4L code, so the DOT-4L rules above apply the same way they do to a liquid cylinder. Larger microbulk tanks are typically built to ASME Section VIII, Division 1 instead — a different code for stationary pressure vessels — and the DOT-4L retest and repair-facility rules don't apply to those at all. Check the nameplate for which code your tank is built to; inspection and repair requirements follow from that, and can vary by state for ASME-coded vessels.

Source: Chart Perma-Cyl microbulk brochure

Cryogenic tank rehab covers liquid cylinders, dewars and microbulk tanks alike, and the goal is to bring the vessel back to like-new, manufacturer-spec condition. A typical scope covers teardown and inspection, helium leak checks, re-evacuating the vacuum space with a heated bake-out, servicing or replacing valves, regulators and relief devices, refinishing, and a liquid nitrogen hold test. Exact scope, timing and any warranty are set by the shop, in the quote.

Sources: Chart repairs FAQ · cylinder repair service page

Not every job needs the same amount of work. A refinish-only job is cosmetic — cleaning and repainting a tank that's still holding vacuum and performing fine. Swapping a valve, regulator or relief device fixes one failed part without opening the vacuum space at all. A full rehab goes further: breaking the vacuum space open, baking it out, leak testing it, and rebuilding it from the inside out. Which one fits depends on what's actually wrong, which is why a quote starts with telling us the symptoms rather than picking a service type up front.

Liquid cylinders carry a pressure relief valve plus a rupture disc as backup, set by pressure class (for example, Chart's standard relief settings are 22 psig LP, 230 psig MP and 350 psig HP). A burst rupture disc is a sign of damage and the cylinder should come out of service. Replacement devices must match the cylinder's pressure class.

Sources: Chart manual · LBNL · MVE manual

It depends on the manufacturer and the cylinder — check with the manufacturer before changing service. Sometimes it can, not in every direction. A cylinder going into new service should be purged with the new gas first, and oxygen service needs oxygen-compatible, oxygen-cleaned parts. Chart says a cylinder used for CO2 can't move to other gases, and an oxygen cylinder must not be converted to CO2.

Sources: MVE manual · Chart manual

Not until it's checked. Accidents, water immersion, fire or extreme heat, and severe weather all call for the cylinder to be tested, evaluated and repaired before reuse. Don't refill a damaged dewar — take it out of service.

Sources: Chart manual · LBNL