What Are Pressure Equipment Category Calculators Based On?

28 August 2026 · GRC Teknoloji · 5 min read

Classifying a product under the Pressure Equipment Directive 2014/68/EU appears straightforward: select the equipment type, fluid group and state, then evaluate maximum allowable pressure (PS) together with volume (V) or nominal size (DN) on the relevant Annex II chart. The result affects the available conformity-assessment modules and the involvement of a notified body.

A Roman numeral on screen, however, does not explain how that decision was reached. The important question is: Which chart, comparison rule and source did the tool use?

Part I — What Annex II contains—and what it does not

Nine charts and two sets of axes

Annex II contains nine category charts. Vessels are evaluated on a PS–V plane, while piping is evaluated on a PS–DN plane. Article 4 states the entry conditions for the relevant chart in text. For a vessel containing a Group 1 gas, for example, the normal route requires both V > 1 litre and PS·V > 25 bar·L; PS > 200 bar is an alternative route.

The words and and or are therefore part of the calculation. Flattening them into a single list can move equipment below the threshold into a category incorrectly.

While the entry conditions can be read from the legal text, the internal category boundaries are published graphically. A numerical calculator must convert those lines into machine-readable rules. Unless the source and confidence of that conversion are disclosed, an apparently correct result is not auditable.

Which side applies at the exact boundary?

The entry thresholds use strict “greater than” comparisons. A point exactly on the threshold does not move into the upper region.

Consider a Group 2 gas vessel at 16 bar with a volume of 3.125 litres:

16 × 3.125 = 50 bar·L

Because the threshold is PS·V > 50, exactly 50 bar·L does not exceed it. Unless the alternative entry condition applies, the result is Article 4(3), not a category. At 3.2 litres, the product becomes 51.2 bar·L and category assessment begins.

A calculator should therefore be tested immediately below, exactly on and immediately above every threshold—not only with typical values.

Outside scope is not the same as Article 4(3)

Where PS does not exceed 0.5 bar, the equipment falls outside the pressure scope of the Directive. Where PS > 0.5 bar but the product threshold in Article 4(1) is not exceeded, Article 4(3) applies.

Equipment under Article 4(3) must be designed and manufactured in accordance with sound engineering practice, carry adequate instructions and must not bear CE marking under this Directive. Combining both outcomes under a single “exempt” label hides a material regulatory distinction.

Exceptions do not all work in the same way

The special rules in Annex II cannot be reduced to one generic category-upgrade command:

Portable fire extinguishers and breathing-apparatus cylinders are classified at least in Category III.
Certain vessels and piping containing unstable gas move from Category I or II to Category III.
Piping in Chart 7 that contains fluid above 350 °C moves from Category II to Category III.
Safety accessories are generally classified directly in Category IV.
For pressure cookers, the calculated category does not change; instead, the design is subject to a procedure equivalent to at least one Category III module.

The last case matters: raising the procedural requirement is not the same as changing the category.

Part II — What should a reliable calculator show?

A decision trace before a result

An output suitable for an engineering file should answer at least these questions:

Is the equipment outside scope, under Article 4(3), or within a category?
Which Annex II chart was selected, and why?
What is the resulting PS·V or PS·DN value?
Which entry condition and category boundary determined the result?
How was an exact boundary value treated?
Did a special rule change the result?
Which conformity-assessment modules are available?
What is the source confidence of the boundary data?

The GRC Pressure Equipment Category Calculator therefore shows more than the category. It exposes the selected chart, entered values, product, entry decision, applicable special rule and data confidence.

Why source confidence matters

The entry conditions for all nine charts can be verified from the legal text. The same level of confidence is not available for every digitised internal category boundary. The tool marks the data as high, medium, low or unverified accordingly.

Low confidence does not automatically mean that a result is wrong. It means that the boundary behind it needs additional technical confirmation. A tool should not manufacture certainty in areas such as Chart 5, whose internal lines could not be verified from publicly available textual sources.

Boundary tests, not only ordinary examples

The regression set for the calculator contains 58 cases. Forty-five verified expectations are asserted automatically after every change; thirteen source-uncertain cases are reported separately without breaking the build.

The suite concentrates on points such as PS = 0.5 bar, V = 1 litre, DN = 32, PS·V = 50 bar·L and PS·DN = 5000. This is where incorrect >/≥ operators and faulty combinations of conditions become visible.

A calculator does not replace the decision-maker

A category calculator is a useful preliminary tool for design, quotations and conformity-assessment planning. Fluid classification, actual operating conditions, assembly provisions and manufacturer responsibilities may still require engineering judgement outside the calculator.

A good tool does not merely state “Category II”. It explains why, shows which data it trusts and identifies where confirmation is required. Final classification remains the manufacturer’s responsibility; uncertain or critical applications should be reviewed with a notified body.