When an imported reactor goes wrong, the failure is rarely a surprise. It was usually a decision that nobody checked at the time. These five checks are cheap, and each one has caught real problems.
The pattern repeats across glass-lined, stainless steel and PTFE-lined vessels: the vessel that causes trouble was not usually built badly on purpose. It was built to a different understanding of the order than the buyer had. One party meant a lining of a stated thickness; the other shipped to nominal thickness over a rougher blast profile. One party meant a mechanical seal; the other quoted a packed gland because that is what the price assumed. Nobody lied. Nobody checked.
The five checks below are in the order they should be applied. Licence first, because it decides whether you have a manufacturer at all. Medium second, because it decides whether the material choice is even valid in your process. Tests and inspection third and fourth, because one must be agreed before fabrication starts and the other must be witnessed before the crate is closed. Licensing last, because it is the only one that can stop a finished, paid-for vessel from leaving the country.
Check 1: the supplier's manufacturing licence
A workshop making pressure vessels should hold a special equipment manufacturing licence. Ask for a scan with the licence number visible, and check the scope covers pressure vessels and the material involved. A supplier who cannot produce this is not a manufacturer.
How to verify it, not just collect it
- Ask for the full certificate, not the cover page. The scope page is the one that matters — it names the categories of vessel and the materials the workshop is permitted to weld.
- Check that the licence is valid on the date of your order, not merely on the date of the quotation. Licences lapse and scopes are amended.
- Check that the licence holder's name is the party on your contract and the party on your invoice. This is where most substitutions happen: a trading company shows a licence belonging to a factory it says it "works with", and your vessel is then subcontracted elsewhere.
- Check the material. A licence scoped for carbon steel pressure vessels does not automatically cover 316L, duplex stainless, titanium or clad plate. If your vessel is 316L, a lined vessel or a special alloy, the scope has to say so.
- Write the licence number into the purchase contract as a condition, not as a courtesy.
Common mistakes
- Accepting an ISO 9001 certificate as a substitute. That is a quality-management certificate, not a manufacturing authorisation. A workshop can hold one and still not be permitted to build your vessel.
- Accepting a business licence. It proves the company exists and may trade. It says nothing about whether it may build pressure equipment.
- Asking once, early, and never re-checking. Ask again at the point of contract signing.
Why it matters: an unlicensed workshop can weld, and it can weld well. What it cannot do is issue documentation that a third party — your insurer, your inspector, your own regulator — will accept. If the pressure boundary ever fails, the argument is fought on paperwork. No licence, no argument.
Check 2: the specification against your medium
Before comparing prices, confirm that every quotation describes the same thing: lining thickness, jacket type, seal type, agitator form, nozzle standard, design pressure and design temperature. It is common for a cheap quotation to be a different vessel wearing the same description.
Do not compare quotations against each other
Compare them against one sheet that you wrote. Force a number or a named option into every field. A supplier who leaves a field blank has already told you something useful.
- Lining thickness. Glass lining is normally specified between 0.8 and 2.0 mm. Thickness is not a proxy for quality — a very thick layer is not automatically better, because it chips more readily at edges and nozzle necks. What matters is that the figure is stated, measured and recorded, not merely implied.
- Design pressure. 0.6 MPa is a common specification for both the inner vessel and the jacket. Ask for the two separately, and ask whether the figure quoted is design pressure or working pressure. These are not the same number.
- Design temperature and operating temperature. Glass-lined equipment commonly serves from -19 °C to 200 °C. A quotation that gives one figure without saying whether it is the lining limit or the steel limit is incomplete.
- Vacuum. If your process pulls vacuum, the inner vessel must be designed for it — typically down to -0.1 MPa. A vessel designed only for positive pressure can collapse inward on the first vacuum distillation. This failure is sudden and unrecoverable.
- Jacket type. Plain jacket, inner coil, immersion tube or half-pipe coil. Different heat-transfer behaviour, very different cost.
- Seal type. Packed gland or mechanical. This is not a detail. It decides whether your operator can run the vessel in a room with people in it.
- Agitator and drive. Paddle, anchor/frame, disc turbine, hydrofoil, impeller or dispersion disc — each suits a different mixing duty. The agitator and the seal and the drive must be specified as one system, because that is how they behave.
- Nozzle standard, orientation and count.
- Shell material. Carbon steel, 304, 316L, titanium, duplex stainless, Hastelloy or clad plate. If chlorides are present, remember that 316L is more resistant than 304 but is not immune to chloride stress corrosion cracking — temperature and concentration decide that, not the grade name alone.
The medium is a hard boundary, not a preference
Glass lining handles inorganic and organic acids, organic solvents and weak bases across a range of concentrations. It does not handle everything. The limits that catch people out:
- Hydrofluoric acid and any fluoride-bearing medium. Fluoride attacks the glass phase itself. There is no concentration or temperature at which this becomes acceptable.
- Phosphoric acid at 30 % or above, above 180 °C.
- Strong alkali above pH 12 and above 100 °C.
If your process touches any of these, the answer is not a thicker lining or a different supplier. The answer is a different material family: stainless steel, or a PTFE-lined vessel where the wetted surface is a fluoropolymer rather than a glass. Settling this before the order is the entire point of the check. Settling it after the vessel arrives means a new vessel.
Check 3: the test records, agreed in advance
Agree in the contract what will be tested and what you will receive: main seam radiography, hydrostatic test, dimensional inspection and the 20 kV spark test record for the lining. Records that are promised afterwards are frequently not produced.
Name the items explicitly
- Main seam radiography — and say which seams. Name the longitudinal and circumferential seams of the shell and of the jacket as separate items.
- Hydrostatic test of the inner vessel and of the jacket, as two separate tests, not one.
- Dimensional inspection against the approved drawing.
- The 20 kV spark test record for the lining, over the full wetted surface.
- Lining thickness measurement, taken point by point, with the instrument identified.
- Material certificates for plate and nozzle forgings, traceable to the heat number.
- Weld maps and welder qualification records.
- Agitator run-out and balance check, plus a no-load run of the drive.
- Seal leakage test.
- Nozzle orientation check against the drawing.
What a real record looks like
- It carries a date, an inspector's name and a signature.
- It identifies the instrument, and for the spark test states the voltage actually applied.
- It is supported by photographs or video of the test in progress, with the vessel identifiable in the frame.
A spark test report that says "passed" with no voltage, no coverage description and no date is not a record. It is a sentence. Records assembled after the fact tend to have round numbers, identical handwriting and no images.
Where linings actually fail
Pinholes and thin spots concentrate where the glass has to flow around a shape: nozzle necks, the manway flange, baffle welds and the underside of the top head. A spark test that sweeps only the barrel has tested the easy part. Insist that the necks and the baffle welds are covered, and ask which electrode was used.
In the field, a damaged lining usually announces itself twice. First as a bright metallic spot or a chalky white bloom where the glass has chipped — visible under a torch. Second, much later, as discoloured product and a corroded steel surface beneath the coating, because the medium reached the base metal through a path that was a fraction of a millimetre wide when the vessel left the works.
Check 4: inspection before loading
Arrange for the vessel to be photographed and, ideally, physically inspected before it is crated. Once it is inside a container and on the water, the cost of discovering a defect is a return shipment.
A shot list, agreed before packing
- The vessel outdoors, full length, before crating.
- The nameplate, legible, with the serial number in the frame.
- Every nozzle and flange face, photographed straight on.
- The manway seal surface, lit from the side so a score line shows.
Send us your duty. We'll send you a specification.
No obligation and no sales sequence. Tell us what you are trying to make, and we will tell you what it takes — vessel, material, limits and price.