Fit to burst

An important element of process control – not just in the manufacture of pharmaceuticals but in chemical processing generally – is over-pressure relief. Stephen Bracegirdle, of Elfab, assesses the possible options

Pharmaceutical manufacturing requires very strict monitoring and control – product leakage or contamination can cost the manufacturer dear, and stringent measures are in place to keep these occurrences to a minimum.

Although working pressures within pharmaceutical plants are often low or even atmospheric, blockages or surges can cause internal pressure to build up, with potentially catastrophic consequences. For this reason the use of pressure relief devices is obligatory.

The EU's Pressure Equipment Directive, which became mandatory on 28 May 2002, makes it an offence to put into service any equipment that falls within the scope of the Directive without complete compliance. Within pharmaceutical manufacturing, most applications for over-pressure protection are generally found in the manufacturing processes, with protection of reactors being the primary application.

opening safely

The choice of pressure relief device is generally dependent on whether or not the installation is aseptic, and on 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 operate when the pressure in the vessel or pipework they protect reaches a certain level – they then open briefly to vent the excess pressure safely before reclosing.

PRVs are often the first choice for non-aseptic applications, whereas for aseptic and hygienic conditions, bursting discs are preferred. The reason 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 chance of contamination. In some industries this may be acceptable, but in the pharmaceutical and biotechnology industries, contamination 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 contamination (if any). The burst disc can then be replaced at the end of the production run.

An important feature of bursting discs is that they can significantly reduce the problem of fugitive emissions. A recent report by the American Institute of Chemical Engineers states that in a medium-sized synthetic organic chemical plant there are 3,500 possible leak points, with leaks accounting for at least 30% of all routine organic emissions.

financial reasons

PRVs are one of the hidden culprits: although individually they may leak only tiny amounts of product, collectively they can be significant contributors to a plant's overall fugitive emissions. This has not only environmental implications, but also financial ones. For this reason, even if PRVs are not located in aseptic areas, bursting discs are often fitted below them to prevent these emissions from occurring.

The quality of bursting discs has improved dramatically in recent years, and over the last decade there has been significant r&d carried out on discs for hygienic and aseptic applications in the pharmaceutical and biotechnology industries. Until about 10 years ago, most discs used in pharmaceutical plants were made from graphite, as they have very low burst pressures and excellent corrosion resistance. Consideration should be given, however, to the fact that they shatter, rather than burst. As with any fragmenting device, care should therefore be taken with system design to avoid downstream problems.

high performance

The current trend is to select non-fragmenting, metallic discs, which avoid potential batch contamination and downstream fragmentation. Previous drawbacks of non-availability at lower pressures and corrosion resistance have been overcome through new designs and the ability to manufacture high performance products in more unusual materials. These materials facilitate installation in multi-purpose vessels, a practice many pharmaceutical companies are considering or have already implemented as new plant is installed or existing plant upgraded.

Most manufacturers have developed their own unique metal disc types, but they all share a number of key features: low burst pressures, non-fragmenting design, a smooth process side, and all fit directly into industry-standard clamp fittings or in holders between flanges where appropriate. Most are also designed to withstand full vacuum and the rigours of CIP procedures.

But how do process controllers know a disc has burst? The answer is a sensor installed within or adjacent to the disc, which sends an instant signal to computer control systems that an over-pressure event has occurred and a disc needs replacing.

Sensor technology is now reaching new levels of ingenuity, with integral non-invasive devices fitted into the disc holders that do not need replacing after a disc has burst. This eliminates the need to rewire the device after activation, and is particularly important in potentially explosive areas, as a 'fit for work' certificate is not required in order to re-start the plant. These devices can operate under very demanding service conditions, such as those found in common discharge systems, which feature superimposed back-pressures and corrosive media. With ATEX directives becoming mandatory in July 2003, non-invasive detection systems such as Elfab's Flo-Tel will not only give operating benefits as described above, but provide an easy-to-install, off-the-shelf solution already compliant with the new legislation.

operating conditions

The question of disc service life should always be addressed at the system design stage, since most premature disc failures result from incorrect initial specification. It is not widely appreciated that each type of disc presents a different matrix of performance parameters and the optimum service life will be achieved only by matching this to specific process conditions. Disc types available on the market today will, if handled and installed correctly, offer improved performance and reliability compared with earlier models in similar operating conditions.

Bursting discs are an essential safety component in all pharmaceutical manufacturing facilities and, with recent technological advances in design and associated detection, provide a high-specification and versatile solution. Fail-safe, they eliminate the danger of potentially catastrophic pressure build-up, and can also play an important role in keeping fugitive emissions to a minimum.