Second-Hand Blast Pots: Twelve Checks That Separate a Bargain From a Liability

The second-hand plant market is where a great many UK blasting contractors find their first pressure vessel, and there is nothing inherently suspect about that. Well-maintained pots come out of hire fleets and closing workshops in genuinely good order, and a careful buyer can pick one up for a fraction of the cost of new.

There is, however, a second category of used pot: the one that has stood behind a shed for four years, still carries the dust of its last job, and is being sold with a photograph and a single sentence of description. The two look identical in an online listing. Telling them apart takes an afternoon, a torch, an ultrasonic thickness gauge and a list of questions asked in the right order.

The arithmetic the ticket price conceals

A new pot of mid-range capacity, with a proper remote control system, metering valve, relief valve and full documentation, is a serious capital commitment for a small firm. The used alternative frequently appears at a fifth of that figure. The gap is real, but it is not the whole sum.

Set against the saving: the cost of a fresh examination and any remedial work; hire charges for a loan unit while yours is out of service; abrasive wasted by a metering valve that will not seat cleanly; and the position you occupy when a principal contractor's site team, an underwriter or a CDM adviser asks for the pressure systems paperwork and you cannot produce it.

Read the paperwork before you read the paint

A blast pot is a pressure vessel, and in the UK it falls squarely within the Pressure Systems Safety Regulations 2000. In practice that means the user must hold a written scheme of examination, drawn up by a competent person, and must have thorough examinations carried out at the intervals that scheme specifies — commonly twelve months for pots in regular service, alongside whatever in-service checks the scheme requires.

Ask for four things before anything else:

• the manufacturer's data plate, showing serial number, year of manufacture, design pressure and CE or UKCA marking;
• the current written scheme of examination;
• the most recent thorough examination report from a competent person, in date;
• records of any repair or modification, naming who carried it out.

A pot without these is not automatically scrap, but it is uncertificated, and its value should be adjusted accordingly. A fresh coat of paint costs very little and conceals a great deal.

The shell: wear happens in specific places

Abrasive does not wear a pot evenly. It attacks the cone, and it attacks the point where the outlet turns the flow towards the metering valve. Wall loss in those areas is localised, progressive and entirely invisible from the outside.

Ultrasonic thickness readings, taken at several points around the cone and along the straight shell and compared with the manufacturer's stated minimum, are the only defensible measure. Many steel pots are built from plate in the region of 6 mm; readings edging towards 4 mm on a working vessel warrant a conversation with a competent person, and a vessel that has lost a substantial proportion of its original wall has reached the end of its life however well it polishes up.

Look inside as well. A pot that has held damp media will show pitting; one that has run a different abrasive will carry contamination in the cone that emptying alone will not shift.

Remote control is not an optional extra

A pot operated by a manual valve at the vessel, with the nozzleman working at the end of a live hose, is not a system any UK site should be accepting. Remote deadman control — release the handle and both abrasive and air stop — is the baseline expectation under PUWER and under any sensible client's method statement.

Check the handle and its mechanism, the control line, the response time, and the type of control valve. A normally-closed design is what you want; anything relying on a spring returning against a live air supply deserves scrutiny. Then ask whether spares — handles, diaphragms, control valves — remain available from the manufacturer or a factor. A superb pot with an orphaned control system becomes a paperweight the first time a seal fails.

Valves, seals and the small leaks that cost money

The metering (pop-up) valve, the choke valve and the seat beneath them decide whether the pot performs consistently or surges. Inspect them, or have them inspected, and price replacements into the deal so that you know what you are genuinely paying. The moisture trap and inlet filtration are equally easy to overlook and equally expensive to live without, particularly through a British winter when damp air finds every weakness in a blast system.

The twelve-point sequence

1. Data plate, serial number and marking confirmed.
2. Written scheme of examination present and current.
3. Thorough examination report in date.
4. Ultrasonic wall thickness at cone and shell against the manufacturer's minimum.
5. Interior inspection for pitting and cross-contamination.
6. Relief valve condition, set pressure and evidence of certification.
7. Remote deadman function and response tested.
8. Metering and choke valves dismantled, or at minimum cycled and inspected.
9. Moisture trap and inlet filter condition.
10. Hoses checked for date marking, cover damage, bulging and coupling wear, with whip checks present.
11. Frame, wheels, lifting points and handles sound, with no welding of unclear provenance.
12. Spares availability and manufacturer support confirmed in writing.

Refurbished rather than merely used

There is a third option that often gets overlooked in the hunt for a bargain: a refurbished unit from a supplier who will strip, measure, replace worn valves, fit a new remote system where required and hand over the documentation with the sale. It costs more than the auction lot and considerably less than new, and it arrives with something a private sale cannot offer — a route back if the vessel turns out to be tired.

For a contractor whose pot is the difference between a working week and an idle one, that route is frequently the better arithmetic.