Atexing problem on the horizon
The ATEX directive becomes law in July. Mark Brady of BOC Edwards outlines the requirements and looks at steps that users can take to mitigate the financial and process-disrupting effects by safely reconsidering their zone classification, allowing them to select cost effective ATEX vacuum equipment
The ATEX directive becomes law in July. Mark Brady of BOC Edwards outlines the requirements and looks at steps that users can take to mitigate the financial and process-disrupting effects by safely reconsidering their zone classification, allowing them to select cost effective ATEX vacuum equipment
At the end of June 2003 the EU's ATEX directives - 1999/92/EC ATEX 137: 'Minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres' and 94/9/EC ATEX 100a: 'Equipment and protective systems intended for use in potentially explosive atmospheres' - become mandatory.
ATEX 137 requires pharmaceutical and fine chemical manufacturers to produce an Explosion Protection Document (EPD) that demonstrates that their plant is safe, and to purchase equipment that is certified for the environment in which it is to be used. ATEX 100a applies to the manufacturers of equipment and protective systems used in potentially flammable atmospheres, requiring them to produce equipment that conforms to 'essential health and safety requirements'.
zone classification
The aim of the new directives is to create a safer working environment and to unify the approach to potentially explosive atmospheres throughout Europe. At the end of June 2006, users of equipment will also have to demonstrate the safety of the workplace.
The new ATEX legislation affects every pharmaceutical and fine chemical manufacturing plant in Europe, particularly those using mechanical equipment (such as a vacuum pumping system). Implementation will undoubtedly have time and cost implications, but there are steps manufacturers can take to mitigate the financial and process-disrupting effects of the directives.
The concept of zone classification according to the risk of explosion (figure 1) is familiar to most manufacturers. ATEX uses this concept in a similar way.
Zoning is central to the implementation of the directive, the aim being to provide a statement of risk for any given area based on the probability that a potentially explosive atmosphere exists. Importantly, under the rules, designation of zones is the responsibility of the equipment user: in this case the pharmaceutical manufacturer.
Zones define the risk associated with the process and equipment. A generally accepted approach to zoning is based on the time per annum that the chemical process is in the flammable zone. This 'time per annum' approach applies to equipment that is running constantly. The example in box 1 may help.
ATEX extends the zone concept to cover not just the external atmosphere around the plant, but any area where a potentially explosive atmosphere may be found. This requires the equipment user to consider zoning the following areas:
• the external environment around the equipment or process vessels;
• the internal environment within the equipment or process vessels;
• the inside of pipe work connected to the equipment or process vessels
Clearly the biggest change is the requirement to consider zoning the inside of vessels and equipment. This has a significant implication where vacuum equipment is concerned and has the potential to increase the cost of the equipment significantly. But by carefully considering the way that vacuum equipment is operated, it is possible to reconsider the zoning and minimise the cost.
available options
BOC Edwards works with customers in the pharmaceutical and fine chemical industries looking at ATEX zoning issues, and has identified a number of strategies to avoid the flammable range (Figure 2) and assist reclassification:
• Low pressure operation, <60mbar;
• Controlling the leak tightness of the process system;
• Inerting;
• O2 monitoring
These measures are already used to varying extents by manufacturers for safety reasons, but they can also be used to support the zoning case.
BOC Edwards has commissioned independent work to establish that at pressures below 60mbar it is not possible to sustain a burn in a potentially explosive atmosphere. This can be used in the zoning considerations.
It is good practice to run a pressure rise test on a process vessel before operation. This is normally conducted to ensure air leakage does not affect the quality of the product. It can also be used to show that the air leakage is below the calculated limit for flammability.
It is good practice to inert the process vacuum chamber back to atmospheric pressure at the end of the cycle. This ensures that the subsequent evacuation for the pressure rise test is conducted out of the flammable range. This inerting policy can also be applied at the end of a process to flush out flammable residues.
House vacuum systems have not traditionally had problems with handling potentially explosive atmospheres because they rarely occur. This unpredictability has led to a trend to classify them as Zone 0 which can have a significant effect on the cost of the vacuum system. Oxygen monitoring offers an alternative approach.
If the concentration of oxygen can be kept below 5%, the atmosphere will be out of the flammable range and the need for a more expensive vacuum system is removed. Oxygen monitors have traditionally had a poor reputation for reliability, but over the last few years there have been significant improvements in this technology, allowing continuous monitoring linked to an inerting system should the oxygen concentration exceed the limits.
If the concentration of oxygen is kept below 5%, the atmosphere is out of the flammable range and the need for a more expensive vacuum system is removed. Recent improvements in the reliability of oxygen monitoring technology offer the option of continuous monitoring, linked to an inerting system.
It is therefore possible to pump a flammable vapour or gas continuously without ever being in the flammable zone. Most processes do not allow for this, but through the use of inerting, monitoring, dilution and suppression it is possible to reconsider the zoning.
The supplier addresses the prevention or avoidance elements in product design and operation but must also consider protection from, or limiting the effect of, an incident. The supplier must certify the suitability of the equipment and CE mark it appropriately and for Zone 0/Category 1 installations. This certification must be underwritten by a notified body. For Zone 0, the equipment supplier is also required to address 'rare malfunctions', consider concurrent modes of failure and be safe with two faults.
As a leading vacuum and pressure equipment supplier BOC Edwards has had to come to terms with ATEX 100a on new and existing products and systems. The key issues are:
• Avoiding sources of ignition;
• Controlling internal and external surface temperatures;
• Preventing propagation of any flame that results from an explosion or sustained burn
The company has a policy of optimising the mechanical design to control temperatures and allow lower rotational speeds. This removes the need for additional accessories that may themselves bring additional failure modes. ATEX-certified claw pumps (Figure 3) and systems have been available since 2000. The new CDX1000 (Figure 4), a dry screw pump, epitomises this design philosophy, and offers innovations such as integral flame arrestors.
ATEX brings quantifiable improvements in health and safety. It looks at internal atmospheres for the first time, and need not involve huge capital investment or flight to lower cost regions of the globe. From the user's perspective ATEX requires an understating of the risks associated with operating processes in order to set a coherent zoning strategy and produce an EPD. If processes are at times within the flammable range then there is a risk of explosion and the avoidance options should be considered.
Expert consultation is usually the most effective method of interpreting and understanding the implications of zoning in a timely and cost-effective manner. How zoning strategies are set could have profound and immediate affects on process design, monitoring and control systems, working practices (including training) and, of course, costs. Equipment suppliers have had to address ATEX 100a.
As a designer and manufacturer of vacuum pumps, modules and bespoke systems, BOC Edwards can provide ATEX-certified equipment to suit individual zoning strategies. Certified systems have been in the field since 2000 and recommendations are in place to support the protection element of the EPD.