Pharmaceutical manufacturing requires strict monitoring and control; product leakage or contamination can cost the manufacturer dearly, and stringent measures are in place to keep these occurrences to a minimum. An important element of process control — not just in the manufacture of pharmaceuticals but in chemical processing generally — is overpressure relief.
Although working pressures in pharmaceutical plants are generally low, they are still quite capable of causing catastrophic damage to lightly-built equipment items in the event of a line blockage or pressure surge. For this reason the use of pressure relief devices is obligatory. The EU’s Pressure Equipment Directive (PED), which became mandatory on 28 May 2002, makes it an offense to put into service any equipment that falls within the scope of the Directive without complete compliance. In the pharmaceutical sector, most applications for overpressure protection are found within manufacturing, and especially in reactor protection.
The choice of pressure relief device depends on the installation (aseptic or non-aseptic) and the equipment to be protected. Currently, process engineers have two choices: 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. At this point the PRV opens, vents the excess pressure safely and then re-closes.
For non-aseptic applications, this ability to re-close automatically often makes PRVs the first choice. But PRVs are never 100% leak-tight, so there is always a risk of contaminating the process. PRVs can also 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.
For aseptic and hygienic conditions, therefore, bursting discs are preferred. These non-reclosing devices, manufactured from metal or graphite, are designed to vent at a preset pressure, instantly alleviating potentially dangerous pressure build-up. Unlike a PRV, a bursting disc is leak-tight and has a smooth process face that eliminates the danger of product fouling and allows 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 little or no contamination. The damaged bursting disc can then be replaced at the end of the production run.
Eliminating fugitive emissions
An important feature of bursting discs is that they can significantly reduce the problem of fugitive emissions. The American Institute of Chemical Engineers (AIChE) states that in a medium-sized synthetic organic chemical plant there are over 3,500 possible leak points, with leaks accounting for over 30% of all routine organic emissions.
PRVs are one of the hidden culprits: although individually they may only leak tiny amounts of product, collectively they can contribute significantly to a plant’s overall fugitive emissions. This has not only environmental implications, but financial ones as well. For this reason, even if PRVs are not located in aseptic areas, bursting discs are often fitted below them to stop leaks.
What type of bursting disc?
The quality of bursting discs has improved dramatically in recent years, with the last decade seeing significant research and development on discs for hygienic and aseptic applications in the pharmaceutical and biotechnology industries. Until about ten years ago, most discs used in pharmaceutical plants were made from graphite, as this gives low burst pressures and excellent corrosion resistance. However, graphite discs shatter, rather than burst, when they operate. As with any fragmenting device, care therefore needs to be taken with system design to avoid problems downstream. The trend now is for non-fragmenting discs, which have the obvious benefits of avoiding batch contamination and downstream fragmentation.
Today’s metallic discs offer superior operating performance over their graphite counterparts, and previous drawbacks of non-availability at lower pressures and low corrosion resistance have been overcome through new designs and the ability to manufacture high-performance products in unusual materials. This facilitates installation in multi-purpose vessels, a practice favored by many chemical and pharmaceutical companies.
Most manufacturers have developed their own unique metal disc types, but they all share a number of core features: low burst pressures, non-fragmenting design, a smooth process side, and the ability to fit either directly into industry-standard clamp fittings, or in holders between flanges where appropriate. Most are also designed to withstand full vacuum and the rigors of cleaning in place (CIP).
It is all very well to alleviate overpressure, but how do you know that a disc has burst? This problem is solved by installing a sensor within, or adjacent to, the disc; this sends an instant signal to the plant’s control system 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 disc holders so that they do not need replacing after a disc has burst.
This eliminates the need to re-wire 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, which are typically Atex-compliant, can operate under very demanding service conditions, such as those found in common discharge systems, which feature superimposed back-pressures and corrosive media.
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 only be achieved by matching this to specific process conditions. Disc types available on the market today, if handled and installed correctly, offer improved performance and reliability compared to earlier models operating under similar conditions.
Bursting discs are an essential safety component in all pharmaceutical manufacturing facilities and, thanks to 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.
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