In our previous article we addressed the first three of the following strategies to reduce helium consumption in production leak testing applications. In this article we will address the final two.
- Reducing helium waste from leaks and improper installation
- Reducing helium waste due to poor equipment design
- Reducing helium waste from unused helium left over in gas storage bottles
- Reducing helium consumption through dilution of helium gas
- Reclaiming used helium for re-use
HELIUM DILUTION STRATEGIES
One way to reduce helium consumption is to dilute it with air or a gas such as nitrogen. When implemented properly, this can be an effective method. If a test specification requires helium leak testing using 100% helium at a prescribed test pressure it is relatively straight forward to estimate the equivalent leak rate limit if a lower helium concentration is used. Assume a specification requires a part to be pressurized to 300 psig with 100% helium with a reject limit of 1 x 10-5 atmcc/sec. If a 10% helium in nitrogen mix were used, that would reduce helium consumption by 90% and require that the reject limit leak rate be changed to 1 x 10-6 atmcc/sec. A defect that would create a leak of 1 x 10-4 atmcc/sec helium with 100% helium tracer gas would flow approximately the same amount of total gas if the mixture was 10% helium in nitrogen. However, only 10% of the leaking gas would be comprised of helium. Thus, the helium leak detector must look for a leak 10 times lower to find an equivalent leak.
There are practical consequences when implementing the helium dilution strategy. First, one must ensure the gas mixing is done correctly and can be properly validated. Gas mixing equipment on the market is capable of effectively blending helium with nitrogen and providing accurate helium mixtures down to a few percent. The mixed gas is stored at the appropriate pressures and mixtures, ready to be used in the leak testing system. Helium can also be blended dynamically with nitrogen on the leak testing system to achieve the desired gas mixture. Finally, helium mixing can be achieved by alternately filling the test part with helium and nitrogen in steps until the desired gas mixture and pressure is achieved. The drawback of this final method is that the gas mixture inside the part will not instantaneously be homogeneous. This may result in less than desirable test repeatability and may require longer test cycle times to allow for the gas to properly mix.
The second consequence which must be weighed when implementing helium dilution is the increase in leak test sensitivity that is required. Helium mass spectrometer leak detectors are capable of detecting leaks in the vacuum test mode to below 1 x 10-9 atmcc/sec. However, the practical limit in a production environment might be influenced by the following factors of required test cycle time, characteristics of the test part, characteristics of the leak test tooling or fixture, and design of the leak testing system. All of these factors ultimately affect the helium background present during the test. For example, helium may permeate through a seal or the actual part creating a natural helium background during the testing. The helium background should normally be well below the reject limit. In some cases implementing longer test cycle times, or redesigning the test fixture can reduce helium backgrounds to acceptable levels. In other cases, more attention to the test part and test fixture cleanliness may be required.
RECLAIMING USED HELIUM
Finally, reclaiming used helium is a strategy that some users implement. Most helium recovery or reclaiming systems function on the following principles: receive the used helium into a low pressure receiver tank as it vents from the test part at the end of the test, re-pressurize the gas, filter and cool the gas, measure concentration and add new helium gas to compensate for losses, and return the gas to the leak test system. Recovery systems can be centralized to accommodate multiple test systems or be dedicated to a single leak test system. Many recovery systems have efficiency ratings in the low 90% range and can also maintain helium concentrations in that same range. Sizing and implementing a helium recovery system can be complex and requires a thorough analysis and understanding of the leak testing system. Often return on investment (ROI) calculations are run in order to justify their purchase. In recent years, in addition to ROI calculations, some users are justifying helium recovery systems using additional factors including, initiatives to practice corporate sustainability, mitigating risks associated with potential production line downtime due to helium supply interruptions, and pressure by gas suppliers to reduce helium usage year to year.
There are a wide range of strategies that can be implemented to reduce helium consumption in production leak testing applications. No one solution will work for all applications, but each strategy must be evaluated based on the unique requirements of the application.