The key point

A screw compressor, as a machine, is not subject to registration with the UDT simply because it is a compressor. However, its pressure vessel, or a separate vessel installed within the compressed air system, may be subject to technical supervision.

The assessment should not be based solely on the motor power, compressor capacity or model name. Four pieces of information are key: the tank capacity, the permissible overpressure, the installation method and the data on the nameplate.

If the unit has a tank, its capacity V in litres and the permissible overpressure P or PD in bars must be read from the nameplate. The following product is then calculated:

P × V

The result allows for a preliminary determination of whether the tank is eligible for simplified supervision or whether it requires notification to the UDT and the obtaining of an operating permit.

Legal basis in 2026

For pressure equipment, the following are of primary importance: the Act on Technical Supervision, the Regulation specifying the types of equipment subject to supervision, and the Regulation on the technical conditions for the supervision of certain pressure equipment. The regulations do not create a single, straightforward category labelled ‘screw compressor subject to UDT supervision’. Classification is determined by the parameters of the specific pressure equipment.

Therefore, two compressors with similar power and similar capacity may give rise to different obligations. It is sufficient for them to have tanks with different capacities, different pressures or different installation methods.

Tank in a compressor unit

A tank in a compressor unit is defined as a tank that forms part of a complete unit. This may be a set in which the compressor, tank, fittings and other equipment form a single assembly supplied by the manufacturer.

For such a tank, the following threshold applies:

P × V ≥ 800 bar·l

In addition, the overpressure must exceed the limit value specified in the requirements. In typical industrial installations operating at 8, 10 or 13 bar, the pressure condition is met. It is therefore essential to correctly calculate the product of the capacity and the overpressure.

Example calculations for a unit

If the tank has a capacity of 90 litres and operates at a gauge pressure of 8 bar, the calculation is as follows:

90 × 8 = 720 bar·l

The result is less than 800 bar·l. Such a tank in a power unit is subject to simplified supervision.

For a 100-litre tank at 10 bar, the result is:

100 × 10 = 1000 bar·l

The 800 bar·l threshold has been exceeded, so the unit must be notified to the UDT and a decision obtained before it is put into service.

A 270-litre tank at 10 bar gives:

270 × 10 = 2,700 bar·l

This falls within the range of 800 to 3,000 bar·l. Full supervision applies.

A 500-litre tank at 10 bar gives:

500 × 10 = 5,000 bar·l

The result exceeds 3,000 bar·l. Such a tank is still subject to full inspection, but the scope and frequency of inspections are more extensive than for a smaller P × V product.

A free-standing tank is a different category

A separate tank positioned next to the compressor should not automatically be treated as a tank within a compressed air system. Most often, this is a fixed, free-standing tank that forms part of the compressed air system.

A lower threshold applies to fixed tanks than to tanks within compressed air units. In a typical compressed air system, the following value should be checked:

PD × V > 300 bar·l

For example, a 50-litre tank at 10 bar gives:

50 × 10 = 500 bar·l

If it is a free-standing unit, it exceeds the 300 bar·l threshold and may require full supervision. The same result, when assessed for a tank forming part of a unit, would be lower than the 800 bar·l threshold. The method of installation is therefore of real significance.

How can you identify the type of tank?

The safest approach is to check the manufacturer’s documentation, the declaration of conformity, the instruction manual and the nameplates. You should not base your assessment solely on the appearance of the unit.

A compressor mounted on a cylinder may be sold as a single unit. In another configuration, the compressor may stand separately, whilst the cylinder may be connected by a hose as an independent unit. Differences in design and documentation affect the classification.

If the documentation does not specify this clearly, it is advisable to send the following to the relevant UDT branch:

• a photograph of the rating plate,
• a diagram of the installation,
• the manufacturer’s instructions,
• a declaration of conformity,
• a description of the installation procedure.

This enables an assessment to be made based on the actual configuration, rather than a general product description.

Is a compressor without a conventional tank exempt from UDT requirements?

The absence of a large tank beneath the compressor does not exempt the user from the obligation to check the documentation. A screw compressor may contain components operating under pressure. If the manufacturer identifies any of these as a separate pressure vessel, its parameters, design and intended use must be assessed.

However, one should not assume that every separator, filter or housing component automatically requires registration. The basis for this is the manufacturer’s data, markings and technical documentation. In case of doubt, the classification should be confirmed by the relevant UDT branch.

Simplified supervision does not mean a lack of obligations

Simplified supervision means that the UDT does not issue an authorisation for operation and does not carry out the standard periodic inspections required under full supervision. However, this does not mean that the vessel can be used without technical inspection.

The operator must comply with the instructions, the permissible pressure and safety regulations. Condensate must be drained regularly, connections must be checked for leaks and the condition of the safety valve must be monitored.

Under no circumstances should the valve setting be adjusted, the tank welded, or structural modifications made without following the appropriate procedure. Internal corrosion is a particular concern in systems where compressed air is not properly dried. Condensate accumulating at the bottom of the tank can accelerate material degradation.

How do I register the tank with the UDT?

If the equipment is subject to full supervision, the operator must submit an application for an inspection of the pressure equipment. The documents must be submitted to the relevant local UDT branch or office.

The application must be accompanied by documentation in Polish for each piece of equipment being registered. After checking the documents, the inspector carries out an acceptance inspection whilst the equipment is in operational condition.

Among other things, the following may be checked: the vessel’s markings, technical condition, safety equipment, correctness of installation and compliance of the configuration with the documentation. Following a successful inspection, the UDT issues a decision authorising operation.

What documents should be prepared?

The scope of the documentation depends on the specific piece of equipment. The following are most commonly required:

• an operating manual in Polish,
• a declaration of conformity,
• tank documentation,
• installation drawings or diagrams,
• safety valve specifications,
• a legible nameplate.

The nameplate must, above all, show the manufacturer, serial number, year of manufacture, capacity and permissible pressure. The details in the application must match the equipment’s documentation.

A problem may arise when purchasing a second-hand compressor without documentation. The absence of an instruction manual, declaration or legible nameplate makes assessment and registration difficult. Before purchasing a second-hand unit, it is worth checking whether it has an inspection logbook, a valid approval certificate and complete documentation.

CE marking does not replace the UDT decision

The CE marking and declaration of conformity confirm that the manufacturer has carried out a conformity assessment in relation to the relevant requirements. However, they do not replace the UDT decision required for equipment subject to full supervision.

The seller may provide the documentation and assist with preparing the necessary details. However, the obligation to submit an application and obtain the UDT decision rests with the operator, i.e. the entity that will be using the equipment.

UDT and compressor capacity selection

Eligibility for technical supervision does not indicate whether the compressor has been correctly selected to meet the plant’s current demand. These are two separate issues.

A screw compressor should be selected on the basis of the total air consumption, the required pressure, the simultaneous operation of consumers, planned plant expansion and the nature of changes in demand. It is the effective capacity that matters, not just the motor power or theoretical capacity.

A compressor that is too small may run continuously under heavy load and fail to maintain the required pressure. A unit that is too large is not always advantageous either. Frequent cycling and operation outside the optimum range can increase energy consumption.

How can energy losses be minimised?

The cost of producing compressed air depends not only on the compressor. The tightness of the system, pressure drops, the condition of the filters, the operation of the dryer and the set working pressure are all of great importance.

Leaks cause energy losses because the compressor produces air that does not perform any useful work. Clogged filters and incorrectly sized pipework increase flow resistance. Users often try to compensate for pressure drops by increasing the pressure, which can further increase energy consumption.

Regular inspection of the system, rectifying leaks, ensuring proper air drying and adjusting the operating pressure to match the actual load improve the efficiency of the entire compressed air system. These measures do not affect UDT certification, but they can reduce operating costs.

The most common user errors

• Assessing requirements solely on the basis of tank capacity.
• Applying the 800 bar·l threshold to every tank, including free-standing ones.
• Commissioning a device subject to full supervision before obtaining the relevant decision.
• Treating the CE marking as a substitute for the UDT decision.
• Purchasing a second-hand compressor without documentation or a legible nameplate.
• Failing to drain condensate regularly and to inspect the safety valve.

Summary

A screw compressor is not automatically subject to registration as a complete machine. The obligation applies primarily to the pressure vessel and depends on its parameters and method of installation.

For a pressure vessel within a compressor unit, the key threshold is P × V equal to at least 800 bar·l. For a typical free-standing pressure vessel, the 300 bar·l threshold and the pressure condition must be checked. Before commissioning, it is necessary to read the data from the nameplate, calculate P × V and verify the documentation.

The final qualification should relate to the specific configuration. In case of any uncertainty, it is safest to submit the documents to the relevant UDT branch. This helps to avoid misinterpretation, downtime and the use of the tank without the required authorisation.