Paul Magès's argumentation for a derogation from the 1943 decree on pressurized gases (January 1964)

In this document

The regulatory problem

Paul Magès drafts an argumentation with a view to requesting derogations from the interpretation of the decree of 18 January 1943 (amended in 1961) on gas pressure appliances. This decree subjects to inspection reservoirs whose pressure × volume product ("PV") exceeds 80 p. 39.

Magès notes that Citroën hydro-pneumatic accumulators, from 0.5 to 1.5 litres, are not explicitly targeted by the texts, except by questionable assimilation to air-pressurized water distribution reservoirs p. 39, an assimilation he deems inappropriate given the volumes involved.

Technical description and manufacturing history

The accumulators are described as a metal shell containing, separated by a deformable membrane, nitrogen and a liquid (mineral, vegetable or synthetic oil), with a regulatory safety margin (operating pressure ≤ 2/3 of the test pressure, rupture ≥ 3× the operating pressure) p. 40.

Magès recalls that "since 1953, the year the 15 CV hydro-pneumatic was released", nearly 3 million accumulators have been fitted, at a rate of 2,500 per day at the time of writing (p. 40, p. 43).

The energy argument

He demonstrates, with figures to support it, that at constant PV (=80), high pressures store more energy than low pressures:

A graph dated February 1960 illustrates the accumulated energy as a function of pressure for several PV values p. 46.

The pneumatic / hydro-pneumatic suspension comparison

Magès shows that at equal volume and pressure, a 100% pneumatic suspension would benefit from double the flexibility compared to the hydro-pneumatic suspension, solely due to the current regulatory interpretation (the oil volume being counted in the total PV) p. 42. He therefore requests authorization to accumulate up to 3,700 Kg/m in a reservoir containing oil, subject to compliance with the usual safety coefficients p. 42.

The comparative safety argument

He puts the danger into perspective: the maximum energy targeted ("at the very most 4,000 Kg/m") is much lower than the kinetic energy of a 1,400 kg car at 140 km/h (more than 100,000 kg/m), or that of a burst truck tyre (≈ 7,000 kg/m for a PV of 390) p. 43-44. He states that he has "never" been aware of a rupture by bursting with metal projection or any bodily injury linked to these accumulators, and highlights the consistency of their rupture pressure even on recovered old parts p. 44.

A copy of the ministerial circular of 29 June 1950, specifying the interpretation of the 1943 decree (pressure and volume criteria), is attached in the appendix p. 45, dated "dac. GG 20.1.64".

Context

The decree of 18 January 1943 and its 1950 application circular regulated gas pressure appliances in France for the sake of industrial safety, at a time when automotive hydraulics were in their infancy.

The hydro-pneumatic suspension, launched on the Traction 15 CV in 1954-1955 and then generalized with the DS (1955), constituted a major innovation for Citroën, combining gas and liquid under pressure — hence the question of its attachment to regulations designed for other industrial uses.

Archivist's perspective

This January 1964 file testifies to a lesser-known aspect of Paul Magès's work: beyond technical design, it fell to him to defend the specificity of his accumulators to the administration in the face of generic regulations, relying on rigorous energy calculations and a safety assessment based on ten years of industrial experience.

The February 1960 graph p. 46 seems to have been reused here, four years prior to the writing of the text, which suggests that the question of the interpretation of the decree had already been concerning Magès for several years — consistent with his 1963 notes on the AP suspension, a period when new hydro-pneumatic applications were under study.

This document also sheds light on Magès's constant concern for the operational safety of hydraulics, already expressed in his 1961 BEA 43.027 note.