Virtually every aerosol manufacturer and filling company knows the downsides of water bath pressure testing and leak detection in today’s high-speed manufacturing world. Testing in the hot water bath is the industry standard methodology for both pressure testing and leak detection to ensure that aerosol products placed on the market are safe both for consumers and within the supply chain, and it has been industry practice for decades since it was introduced in the 1940s. What many fillers do not realize, however, is that there exists a viable alternative to water bath for both pressure testing and leak detection. This alternative method allows filling companies to provide true automated leak detection of every can while reclaiming the energy and personnel costs and excessive real estate required for water bath.
Water baths are expensive to buy and run and the increasing focus on environmental issues mean they are not sustainable in the long term. In addition, line speeds have increased dramatically, with the largest running at 600 cans per minute (cpm) or faster. Faster line speeds mean bigger baths are required to achieve the required 3-minute residence time; larger baths take up more space in the factory and increase costs significantly by needing more energy to heat more water that ultimately needs to be disposed of. Annual costs for running a water bath can run upwards of £35K ($45K US) per year, not to mention the additional cost of employing personnel to sit “spotting” leakers.
Also, when considering these “spotters,” it is generally accepted in the industry that often this is a futile waste of human resource as cans pass so quickly that operators cannot spot leaking cans with the naked eye. The line speeds of 10-20 cans per minute from the 1940s, when spotters could look for bubbles in a water bath to identify a leaker, are long gone. But many still hold onto this outdated test procedure and have spotters trying to identify leaks at speeds in excess of 60 cpm, even though studies have shown this is the limit of the human eye.
While production techniques and line speeds have dramatically changed since the introduction of the water bath, the use of spotters to identify leaking cans has not changed in many facilities. In today’s increasing move towards automation for production efficiencies as well as safety and brand quality demands, automated systems such as microleak detection systems are now available that remove the necessity to rely on operators for critical leak testing and ensure that 100% of cans are tested, in real time at speeds the human eye simply cannot achieve.
Because of the improved can manufacturing process, it is now also very rare to find a can that bursts in the water bath.
All the above factors have generated a business need to replace the water bath with an alternative system of ensuring cans are safe to put on the market. Obviously, any alternative system must be at least as good as the system it is replacing.
The primary technology being employed for the automated leak detection portion of the staged approach is chirped quantum cascade laser (QCL) technology, which can instantly detect, identify and reject a faulty can at speeds up to 600 cpm. Before we look at the alternative method in detail, it is worth discussing the practicalities of implementation.
A Risk-Based Approach
The use of the alternative approach requires the filler to move from a philosophy of inspection to a validated process when it comes to quality control and safety.
Even given the very good safety profile of aerosols, where defects are few, some fillers are adopting a staged approach to implementation. This involves adding an automated leak detector set at the reject level for post-water bath, which is typically 8.0 x 10-3 (mbar . l/s-1). This can be upgraded to the water bath alternative specification of 3.3 x 10-3 mbar . l/s-1. Pressure tested cans can then be run, with the added security of the water bath, and the control processes for clinching and filling can be validated with a statistically significant quantity of cans. Once confidence in the system has been established, the removal of the bath can proceed.
The chirped quantum cascade laser technology automatically detects and rejects any leaking can from the production line. When a can passes through the sample arch, the system draws in the air around the can and directs it through to the measurement cell. In the measurement cell the laser beam passes through the sample gas and some of the laser light is absorbed. The amount of light absorbed equates to the leak rate. If the absorbance is above the thresholds stated above, then the can is automatically rejected.
Because the QCL approach is so fast, effective, reliable, and easy to introduce into the filling line, the technology is being rapidly adopted by both brand owners and filling companies. It is currently used most frequently in combination with water baths. Once the water bath is eliminated as a leak detection system, however, companies begin to question the cost, large physical and environmental footprint, and inefficiency of using the water bath only for pressure testing, which in turn is driving interest in the alternative method.
The “Alternative” Method
An alternative method for aerosol leak detection is prescribed in the 2007 version of the European Agreement concerning the International Carriage of Dangerous Goods by Road, or ADR 2007, section 18.104.22.168.2.2. It has four key elements:
- A quality system that ensures all aerosols that leak or are deformed are rejected and not offered for sale.
- Pressure testing of all empty cans to ensure that they do not deform when filled and leak at a rate less than 3.3 x 10-3 mbar l/s-1.
- Leak testing of all filled aerosols so that they do not leak at a rate greater than 2.0 x 10-3 mbar l/s-1.
- A checkweigher to reject over- or under-filled canisters.
Currently the only alternative method that has been demonstrated to comply with the requirements set out in ADR 2007 is the one developed by the European Aerosol Federation (FEA). The FEA alternative method is an integrated quality assurance and testing package which aims to ensure substandard aerosols are identified and rejected before they enter transport and distribution.
The FEA Alternative Method
Quality assurance procedures are used by can makers to ensure that only cans that are pressure-stable and leak-tight are supplied to fillers. Central to this is that all empty aerosol cans are leak- and pressure-tested to a pressure equal to or more than the maximum expected in the filled aerosols at 50°C.
Quality assurance procedures are used by the valve supplier to ensure that only valves that have all their components in place and that will be pressure-stable and leak-tight once crimped on a can are supplied to the filler.
Quality assurance procedures are also used during handling and filling to check that only high-quality aerosols are produced. Procedures include:
- Checks on the crimping equipment settings to maintain the correct valve crimp dimensions.
-An in-line checkweigher system to ensure overfilled aerosols are rejected.
- A micro-leak detector on the filling line to test the valve and valve crimp of all filled aerosols for leaks.
The key directive in these requirements is that the canister must be leak-tested at a pressure equal to or more than the maximum expected in the filled aerosols at 50°C, or 2/3 of the rated deformation pressure of the canister, i.e., for an 18 bar can it must be 12 bar. The FEA requirements make no reference as to how this can be achieved1.
The FEA alternative method requires that the cans are pre-tested before filling to ensure burst resistance and checked for can defects that may lead to leakage. This is currently technically feasible for tinplate cans, but not for aluminum monobloc cans due to potential damage to the cans during testing.
A Viable Solution
Aerosols are pressurized containers and often use both flammable propellants and solvents, as well as potentially harmful substances, such as pesticides. It is paramount that the operator and facility are protected from the effects of leaking and bursting canisters. The likelihood of an aerosol bursting is low, provided the requirements of the British Aerosol Manufacturers’ Association’s standards are met. Of the 12 million cans tested during validation of the FEA alternative to the water bath only 100 leakers were detected, and zero cans burst.2 However, the consequences of a burst aerosol are significant and must be addressed in any system design.
It is important that any proposed system has an effective checkweigher. Overfilled canisters may present a safety risk due to hydraulic bursting if heated in the supply chain. Underfilled canisters are likely to have a different product-to-propellant ratio than initially intended. Since many formulations rely on the ingredients in the product to reduce the overall pressure by acting as a solvent for the propellant, without this pressure drop, the internal pressure may exceed the pressure rating of the can and burst.
If the underfill is due to an issue with the product filler, then canisters with higher levels of propellant, even pure propellant, could be filled. If these are exposed to heat in the supply chain, they could deform or burst due to higher pressures than the can is designed to withstand. Cans that are over or underfilled must be rejected from the line and disposed of safely.
The alternative to the water bath is a credible solution for ensuring the safety of aerosols placed on the market, but this is reliant on good quality components, a robust quality system, and an efficient, verified leak detector. As more and more aerosol manufacturers move away from water bath for leak detection and embrace chirped quantum cascade laser, the interest in adoption of this technology as a complete alternative to water bath grows. But whether as an addendum or alternative to water bath, the use of laser technology as a reliable method of improving safety and brand quality while enhancing efficiency and throughput is here to stay.
About the Author
Peter Watmough is global product manager, leak detection for Emerson Automation Solutions in the UK. More information on Emerson’s leak detection solutions can be found at www.Emerson.com/Cascade.
1 FEA Guide to Water bath and Its Alternative, 2017
2 “Guide on Hot Water Testing and Its Alternatives,” European Aerosol Federation, March 2009.