SAFETY AND EMISSIONS CONTROL
Pressure Relief - The Options

The choice of pressure relief device is generally dependent on the installation (aseptic or non-aseptic) and the equipment to be protected. Currently, there are two choices available to process engineers: pressure relief valves (PRVs) or bursting discs. PRVs are designed to open when the pressure in the vessel or pipework they protect reaches a certain level - they then briefly open, venting the excess pressure safely before re-closing.

For non-aseptic applications, PRVs are often the first choice, while for aseptic and hygienic conditions, bursting discs are preferred. The reason for this is simple: PRVs are not 100% leak-tight, with the resulting risk of possible contamination of the process. In addition, PRVs can be subject to corrosion and product fouling, which also increases the chances of contamination. In some industries this may be acceptable, but in the pharmaceutical and biotechnology industries, contamination can be disastrous and must be avoided at all costs.

This is where bursting discs come in. Non-reclosing devices manufactured from metal or graphite, they are designed to vent at a preset pressure, instantly alleviating potentially dangerous pressure build-up. Their main advantages over PRVs are that they are leak-tight and have a smooth process face, eliminating the danger of leakage or product fouling and also allowing easy decontamination in situ. In some cases, a bursting disc may be used in conjunction with a PRV; in this instance, when the disc bursts and the pressure has been relieved, the PRV re-seats and then takes over as the primary safety device, allowing production to continue with no disruption and minimum (if any) contamination. The burst disc can then be replaced at the end of the production run.