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Safety and Emissions Control
Introduction
Manufacturing requires very 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 is over-pressure relief. 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.
Manufacturing requires very 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 is over-pressure relief. 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.
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 rupture discs (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, rupture discs (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 rupture discs (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 rupture disc (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.
pressure relief valves (PRVs) or rupture discs (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, rupture discs (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 rupture discs (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 rupture disc (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.
Eliminating fugitive emissions
An important feature of rupture discs (bursting discs) is that they can significantly reduce the problem of fugitive emissions. The American Institute of Chemical Engineers 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 be significant contributors to a plant's overall fugitive emissions. This not only has
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 prevent these emissions from occurring.
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 be significant contributors to a plant's overall fugitive emissions. This not only has
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 prevent these emissions from occurring.
What type of rupture disc (bursting disc)?
The quality of bursting discs has improved dramatically in recent years, with the last decade seeing significant research and development going into discs for hygienic and aseptic applications in the pharmaceutical
and biotechnology industries. Up until about ten years ago, most discs used in pharmaceutical plants were made from graphite, as they have very low burst pressures and excellent corrosion resistance.
Consideration, however, should be given to the fact that they shatter, rather than burst, when operating. As with any fragmenting device, care therefore needs to be taken with system design to avoid downstream problems.
The current trend is now to select non-fragmenting discs, which give the user obvious benefits by 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 more unusual materials. These materials facilitate installation in multi-purpose vessels, a practice many chemical and 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 core 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 (clean-in-place) procedures.
The quality of bursting discs has improved dramatically in recent years, with the last decade seeing significant research and development going into discs for hygienic and aseptic applications in the pharmaceutical
and biotechnology industries. Up until about ten years ago, most discs used in pharmaceutical plants were made from graphite, as they have very low burst pressures and excellent corrosion resistance.
Consideration, however, should be given to the fact that they shatter, rather than burst, when operating. As with any fragmenting device, care therefore needs to be taken with system design to avoid downstream problems.
The current trend is now to select non-fragmenting discs, which give the user obvious benefits by 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 more unusual materials. These materials facilitate installation in multi-purpose vessels, a practice many chemical and 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 core 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 (clean-in-place) procedures.
Disc life
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, will offer improved performance and reliability compared to earlier models operating under similar 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 only be achieved by matching this to specific process conditions. Disc types available on the market today, if
handled and installed correctly, will offer improved performance and reliability compared to earlier models operating under similar conditions.
In conclusion
Rupture discs (bursting discs) are an essential safety component and thanks to recent technological advances in design 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.
Rupture discs (bursting discs) are an essential safety component and thanks to recent technological advances in design 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.