Definition
A TDT inspection of a silo tanker is a periodic technical inspection of the tanker’s pressure vessel, carried out by the Transport Technical Supervision authority (TDT, Polish: Transportowy Dozór Techniczny), which is the condition for legally authorising the vehicle to operate and to carry loose bulk material.
For a terminal user it comes down to a single rule: a silo tanker without a valid inspection is dead equipment. It may have a sound tractor unit, a clean vessel and a paid-up policy, but if the TDT decision has expired, the tanker has no right to travel with a load or — at our site — to enter the loading bay. From the perspective of running a fleet and a terminal in Chorula, I treat the supervision documentation just as seriously as the brakes: it is not bureaucracy, but the condition for working safely with equipment that operates under pressure.
Why TDT, not UDT
This is the first and most frequently confused distinction. A silo tanker’s vessel is a pressure device — during pneumatic discharge, compressed air fluidises the material and pushes it into the recipient’s silo. Any device working under pressure is subject to technical supervision. The only question is: which supervision authority.
In Poland, supervision is divided by sector. UDT (the Office of Technical Inspection, Urząd Dozoru Technicznego) looks after stationary and industrial equipment — vessels in plants, cranes, boilers. TDT (Transport Technical Supervision, Transportowy Dozór Techniczny) is responsible for pressure and tank equipment mounted on vehicles and used in transport. Since a silo tanker is a road tanker with a pressure vessel, the competent authority is TDT — never UDT. I say this plainly, because in the industry the word “UDT” is reflexively applied to every vessel inspection, and that is a mistake that can route documents to the wrong branch and delay the tanker’s authorisation by weeks.
The practical effect is that it is TDT that opens and maintains the vessel’s supervision documentation, sets the inspection dates and issues the decision authorising operation. The whole “life story” of the vessel — from the first authorisation, through successive inspections, to out-of-schedule inspections after repairs — lives in the inspection logbook on the TDT side.
Types of inspection in the supervision cycle
A TDT inspection is not a single test but a set of activities of differing scope and frequency. In terminal and service practice we encounter five basic ones:
- Internal inspection — assessment of the condition of the vessel shell from the inside: corrosion, signs of pitting, the state of the welds, the heads and the internal supports. It requires entering the vessel or an endoscopic examination, so the tanker must be empty and cleaned.
- Pressure test (hydraulic) — the vessel is filled with water and the pressure is raised to the test value, checking the tightness and strength of the shell. The test is carried out with water rather than air, because water is incompressible — in the event of a rupture there is no sudden decompression and no explosion hazard.
- External inspection — a visual examination of the vessel from the outside under working pressure or during operation: armature, valves, connections, the aeration system, markings. This inspection is more frequent than the internal inspection.
- Leak test — a check of whether the vessel and fittings leak at connections, valves and manholes.
- Inspection of fitting performance — safety valves, shut-off armature, pressure gauges, the aeration system and safety devices. A sound vessel with a faulty safety valve is still equipment that is not authorised.
External inspections are carried out more often, and the internal inspection and the pressure test at longer intervals — the exact schedule follows from the technical supervision regulations and the technical requirements for pressure equipment, and the specific dates are entered by TDT into the documentation of each vessel.
How the dates are set
Here comes the second thing worth understanding well. TDT gives the date of the next inspection as a month and year — e.g. “09/2027”. It is not counted from the date the tanker was manufactured or from its registration, but from the date of the decision authorising operation. It is that decision that starts the vessel’s supervision clock.
| Element | What it means |
|---|---|
| Decision authorising operation | The TDT document admitting the vessel to service; the starting point for the dates |
| Date format | Month + year of the next inspection (e.g. 06/2028) |
| External inspection | Shorter interval — more frequent inspection |
| Internal inspection + pressure test | Longer interval — less frequent inspection |
| Out-of-schedule inspection | Outside the cycle — after a repair, failure or upgrade of the vessel |
For a fleet operator this means that every vessel has its own individual calendar. With a fleet of several dozen tankers — in our case 31 silo tankers in the ~60 m³ class — keeping track of the dates is a discipline in itself. A missed month means not only that the tanker is grounded, but also that it drops out of the delivery schedule, so we plan inspections in advance, in windows of low utilisation, so that they do not clash with the peak of orders.
In my experience the greatest risk lies not in the inspection technique itself, but in the organisation. A tanker can run for months without a hitch, and then in a single week several things converge: the internal inspection date, the vehicle’s roadworthiness test and the tractor’s service window. That is why we keep the supervision dates in parallel with the fleet schedule, and not as a separate “to be dealt with someday” list. A tanker whose inspection falls due in the high-utilisation season goes for inspection earlier — it is better to give up the equipment for two days in March than to lose it for a week at the peak of deliveries.
What the inspection looks like in practice
Before the vessel goes under supervision, it must be empty, cleaned and degassed. An internal inspection cannot be done on a dirty shell — material residue masks pitting and signs of corrosion. With us, a tanker due for an internal inspection first goes through a full wash, and the cleanliness is confirmed by a document. Only then can the inspector assess the shell: the circumferential and longitudinal welds, the heads, the internal supports, the points of contact with the aeration system.
The pressure test is the moment when hidden defects most often come to light. The vessel is filled with water, vented and the pressure is raised to the test value — higher than the working value. It is held for a defined time and the inspector watches for any pressure drop, “sweating” of the welds or deformation. Using water instead of air is not a whim: water is practically incompressible, so even in the event of a loss of integrity the energy stored in the system is minimal. A pneumatic test of such a volume would be dangerous.
On the fittings side, the inspector checks the safety valves, the shut-off armature, the pressure gauges and the completeness of the markings. It is worth stressing here something I see with new drivers: a sound vessel is not enough. A vessel without a valid safety valve or with a faulty aeration system is still equipment that TDT will not authorise. Supervision looks at the device as a complete pressure system, not just at the shell.
The operating permit and what takes the tanker out of service
A positive conclusion of the inspection cycle ends with a decision authorising operation and the entry of a new date. From that moment the vessel is authorised — until the next date or until an event that invalidates the authorisation.
A tanker drops out of service in a few typical situations:
- lapse of the inspection date — the simplest and most common case; after the date there is no valid authorisation;
- a negative result of an inspection or pressure test — corrosion, pitting, cracked welds, a leak or deformation of the shell is detected;
- damage to the vessel — e.g. after a collision or overload that compromises the integrity of the pressure shell;
- a welding repair of the vessel without an out-of-schedule inspection — any welding work on the shell requires a fresh inspection and authorisation.
This last point is crucial in combination with servicing. Refurbishments, accident repairs and work on the vessels of silo tankers — the Spitzer SF/SK and Feldbinder EUT/KIP series — must be carried out by workshops with qualifications agreed with the technical supervision authority and with a documented aluminium welding procedure (the shell is usually an EN AW-5083 alloy, welded with ER5183/ER5356 wire). After such a repair the tanker does not return to service “straight away” — it returns only after an out-of-schedule inspection and a new authorisation.
Legal basis
The supervision of silo tanker pressure vessels is based on the Act on Technical Supervision and on the regulations setting out the technical requirements for the technical supervision of pressure and tank equipment in transport. These define which equipment is subject to supervision, which inspections fall within scope, who carries them out and how the dates are set. The Transport Technical Supervision authority acts as a specialised state body for transport equipment — and it has the final say on authorising a specific vessel. I do not quote act numbers or year-by-year intervals here, because the regulations are amended from time to time; the applicable dates are always confirmed by the TDT entry in a given tanker’s documentation.
The link with the terminal and servicing
From the perspective of the transloading terminal, a valid TDT inspection is not a detail but a condition of entry. The material is fed into the silo tanker directly from big-bags (FIBC), by gravity, without pneumatics, so as not to damage the granulate — but before the tanker pulls up to the bay, we check that its vessel has a valid authorisation. Equipment with an expired inspection is turned away. It is a simple rule that protects both us and the recipient: only equipment with a valid inspection enters the silos of our regular customers.
The same chain works the other way around in servicing. When a tanker returns from a vessel repair, its return to silo tanker transport is only possible after an out-of-schedule inspection and an authorisation. As a result, the whole loop — transloading, transport, servicing — speaks the language of a single document: a valid TDT decision or a standstill.
It is worth remembering that the materials we handle at the terminal (PE, PP, ABS, PS, PA, PVC, PET and other plastics that flow well and are not ADR goods) do not require a separate regime like dangerous goods — but the pressure vessel itself is subject to supervision regardless of what it carries. A TDT inspection concerns the construction of the tanker, not the classification of the load.
The most common operator mistakes
Over years of working with a fleet and a terminal I have gathered a list of mistakes that recur regardless of the company. They are worth knowing, because each of them ends in downtime or — worse — in equipment that is only seemingly authorised.
- Confusing TDT with UDT — directing documents or questions to the wrong authority, which delays authorisation. For a silo tanker it is always TDT.
- Counting the date from the manufacturing date — the date runs from the decision authorising operation, not from the tanker’s model year. These are two different dates.
- Returning to service after a welding repair without an out-of-schedule inspection — the most dangerous mistake, because it concerns the integrity of the shell under pressure.
- Overlooking the fittings — watching only the vessel date when it is a safety valve or the armature that fails the inspection.
- No cleanliness document before the internal inspection — the tanker arrives unwashed and loses a day to being re-scheduled.
Each of these mistakes is trivial to describe and costly in its consequences. That is why, with us, the supervision date of each of the 31 tankers is an item in the schedule, not a note in the fleet manager’s head. For the terminal, equipment with a valid inspection is the same as fuel in the tank: without it, the tanker simply does not move.
Related topics
- Silo tanker ~60 m³ — the construction of the pressure vessel under TDT supervision.
- Silo tanker transport — carrying bulk material in a tanker with a valid inspection.
- Transloading without pneumatics — gravity loading of a silo tanker at the terminal.
- Big-bag (FIBC) — the packaging from which the material is fed into the vessel.
The full scope of the terminal’s services is described on the SMIALA services pages, and a broader context of the transport and transloading of bulk material can be found in the hub at magnumchorula.pl/przeladunek.
Sources
- The Act on Technical Supervision and the regulations on the technical requirements for the technical supervision of pressure and tank equipment in transport.
- Transport Technical Supervision (TDT) — the scope of supervision over the pressure vessels of road vehicles.
- Operating practice of the SMIALA / PHS Magnum terminal in Chorula and of silo tanker servicing (a fleet of 31 silo tankers ~60 m³). Expert commentary: Aleksy Pasternak, a terminal and silo tanker service practitioner.
