Sizing safety valves — Liquefied petroleum gas sites

Pressure equipment is designed to contain fluids in an enclosure, the safety of which is ensured by the application of :

• rules for placing products on the market, specified in the "essential safety requirements" annex to the Pressure Directive 97/23/EC, as well as specific norms and standards;

• rules for installation, maintenance and inspection (under the responsibility of the operator); these rules have a regulatory component (decree of March 15, 2000), and a "technical reference framework" component to support the operator's organization.

One of these rules concerns the prevention of the risk of exceeding admissible limits, and in particular the pressure for which the enclosures are designed. In service, stresses in these enclosures must not exceed values that ensure a minimum safety coefficient specified in European regulations and technical reference codes. The most commonly used safety equipment are safety valves.

It covers the following main topics:

• regulations applicable to overpressure protection of pressure equipment ;

• The main factors that can lead to an increase in the internal pressure of storage tanks and piping at these sites;

• valve installation components affecting valve sizing ;

• types of overpressure, by vaporization of the liquid phase, or by thermal expansion of a liquid monophase trapped between two closed shut-off devices;

• valve sizing in both cases, using examples.

It is designed to provide a basis for :

• send prerequisites to a valve supplier, who will have to justify that the characteristics of his offer allow the evacuation of a flow rate at least equal to that calculated;

• to place an order.

It's important to remember, however, that satisfactory sizing of a safety device requires the involvement of all the players concerned: operator, pressure equipment...