An unfired pressure vessel is any closed container that holds gas or liquid above atmospheric pressure without a flame or other direct heat source of its own: the air receiver behind a compressor, a bladder expansion tank on a heating loop, a filter housing, a separator, the shell of a heat exchanger. Most fail slowly and visibly before they fail suddenly — rust at the base, a weeping weld, a relief valve painted over or wired shut. This checklist is a walk-around from the outside, in the order you would look: nameplate, relief device, gauges, shell and heads, supports, drains, piping and paperwork. Fill it in here and download a PDF without signing up, or speak while you walk, and the document writes itself.
What this inspection covers — and what it does not
In the US, most new pressure vessels are built to Section VIII of the ASME Boiler and Pressure Vessel Code, and many states and cities adopt the National Board Inspection Code (NBIC) for in-service inspection, repair and alteration. Depending on the jurisdiction, size, pressure and contents, a vessel may need periodic inspection by a commissioned inspector — often the insurer's — and a posted operating certificate.
This form does not replace that inspection, and it does not look inside the vessel; an internal inspection needs the vessel isolated, opened and entered under a confined space procedure by someone qualified to judge what they see. What it does is give owners, operators and maintenance staff a consistent record of the vessel's outside condition between those visits, so a problem is caught early and the next inspector arrives to a vessel with a history.
Start with the nameplate
Everything else on the form is judged against the nameplate. The maximum allowable working pressure (MAWP) and the temperature it applies at tell you the ceiling; the minimum design metal temperature (MDMT) tells you how cold it may get; the year built and the National Board number let anyone trace the vessel's records. Type them in as stamped. If the plate is missing or unreadable, that is a finding in itself, because nobody can then say what pressure the vessel is good for.
The relief valve comes first among the checks
A pressure relief valve is what stops a stuck regulator or a failed control from turning a vessel into a bomb. The checks are quick: the set pressure on the tag is no higher than the MAWP, the seal is intact, nothing can isolate the valve from the vessel, the outlet is open and aimed somewhere safe, and the tag date is inside the interval your site or insurer uses. Any failure among those is flagged as a safety finding.
Lifting the test lever is a decision for your site procedure, guided by the valve maker and the insurer; it is not something to improvise. Resetting, adjusting or repairing a relief valve belongs to an organization qualified to do it, which issues a new tag afterward.
Reading the shell
General rust that wipes off is a maintenance item. Pitting, rust scale that flakes in layers, and corrosion concentrated at the bottom of a receiver or along a waterline are signs of metal loss that may need thickness readings. Bulges, dents and flat spots on a vessel under pressure are more serious still, and so are cracks at welds and nozzles: depressurize, and have a qualified inspector look before it runs again.
On insulated vessels the danger is out of sight. Rain that gets past damaged jacketing soaks the insulation and corrodes the steel beneath it, so the form asks for the condition of the jacketing and whether damaged areas have been opened up.
Supports, drains and piping
Vessels rarely fail at their supports, but supports that rot or settle put loads on nozzles and welds that were never designed to carry them. The same goes for piping that hangs off a nozzle instead of its own hangers. Drains matter most on air receivers, where condensate collects at the bottom and quietly eats through the shell; a drain that has not been opened in months is one of the most common findings on any walk-around.
Common questions
What is an unfired pressure vessel?
A closed container designed to hold a gas or liquid above atmospheric pressure, heated (if at all) by something other than a direct flame — compressed air, steam from elsewhere, or a process. Air receivers, expansion tanks, separators and heat exchanger shells are common examples. Fired vessels, such as boilers, are covered by separate rules.
How often should a pressure vessel be inspected?
It depends on the jurisdiction, the vessel's size, pressure and contents, and the insurer. Many jurisdictions set an external inspection interval for vessels in scope and leave internal inspections to the inspector's judgment, while some small vessels are exempt. Owners commonly add their own monthly or quarterly visual check, which is what this form is for.
What does MAWP mean?
Maximum allowable working pressure: the highest pressure the vessel may be subjected to at the temperature stamped beside it, as set when it was designed and built. The relief valve should be set to open at or below it.
Can I test the relief valve myself?
Visual checks, yes, on every inspection. Lifting the test lever is often done by operators where the maker and site procedure allow it and the vessel is above a set fraction of the valve's set pressure. Bench testing, resetting or repairing the valve should be done by a qualified repair organization, and a new tag issued.
Does every pressure vessel need a posted certificate?
No. Whether a certificate is required depends on where the vessel is and on its size, pressure and service. The form records what you found — posted and in date, expired, not posted, or not required — and leaves the question of what your jurisdiction requires to your inspector or the jurisdiction itself.
Does this checklist replace the jurisdictional inspection?
No. It is the owner's or operator's own record between inspections. Any inspection the law requires is made by an inspector the jurisdiction has commissioned, who decides on the certificate.