Leak testing in mass production lines for automotive and gas equipment components has traditionally been one of the bottleneck processes.
The method of visually checking for air bubbles by submerging the work in water after sealing is commonly used, but it has drawbacks such as possible oversights, the need for a drying process, and difficulty of automation.
Many alternative testers have been used, but they have shortcomings in terms of sensitivity, pressure resistance, running cost, handling, etc.
The differential pressure Air Leak Testers have been improved in place of the conventional methods and used as the most effective automatic leak testers.
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To introduce Air Leak Tester, it is requiredl to set a quantified leak specification. It is necessary to determine what range of leak is acceptable by comparing with the conventional water-dunk test or by verifying under actual conditions.
It can also be determined with reference to past cases or specifications for similar works. The leak detection capability of leak testers depends on test conditions such as test time and test pressure. By using several non-leaking works and a reference leak (Leak Master) and performing some tests under different conditions, the actual performance of detection can be known from the data.
&#; Leak represents the volume at atmospheric pressure
Pressurized air is sealed inside the work, and the work is then immersed in water or coated with soapy water to visually check air bubbles.
A gas (hydrogen, helium gas, organic halogen gas or chlorofluorocarbon gas) not present in the air is mixed with compressed air and sealed inside the work as a tracer gas to detect the gas leaking into the sealed container (chamber) containing the work with a gas detector (hydrogen leak detector, helium leak detector, etc. ).
(Applicable: Helium/hydrogen/battery leak test)
Since the leak rate is equal to the flow rate, the leak can be measured by feeding air into the work through a micro flow meter.
(Applicable: Air Flow Tester AF-R221&#;
Positive or vacuum pressure is applied. The pressure drop over time due to leak after sealing with a valve is then detected by a pressure gauge, pressure sensor, or pressure switch.
(Applicable: Digital Pressure Gauge DP-340BA&#;
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This method uses a differential pressure gauge (U-tube, differential pressure sensor, etc.) instead of a pressure gauge to detect the pressure drop due to leak as a differential pressure with the master tank with high sensitivity.
(Applicable: Air Leak Tester&#;
This method is used for packaged goods such as confectionery and medicines (e.g., pillow packaging).
When a package containing air is placed in a capsule and the inside of the capsule is evacuated, the package expands, but if the package has a leak, it does not expand.
The expansion is detected as distortion.
A soapy solution is typically used for both air and N2 leak detection. Once the system is pressurized to the test pressure, the flanges being tested are coated with the solution. If the solution begins to bubble, a leak is present at the flange and a repair is required. If a leak is located, the system must be depressurized before any repair on the system may commence.
An alternative method of leak detection for N2 testing utilizes an oxygen detector. Once the system is pressurized to the test pressure, the flanges are wrapped with plastic wrap. The tester then punctures the wrap, inserts the oxygen detector probe and measures the oxygen content within the flange. Generally, less than 20% oxygen content confirms the flange is leaking.
When using N2/He mixture media, a mass spectrometer (calibrated for He) is used to measure the quantifiable leak rate. Similar to using an oxygen detector, the system is pressurized to the test pressure and flanges are wrapped in plastic wrap. The tester then punctures the wrap with the He mass spectrometer and records the leak rate by measuring the He detected. Acceptable leak rates are generally less than or equal to 150 scf/yr per flange in open areas, while enclosed areas are less than or equal to 20 scf/yr per flange with an average system leak rate of less than or equal to 5 scf/yr over the entire system.
Within each test package, it is beneficial to utilize the system pressure safety valves (PSVs) when available. Temporary PSVs must be used if no PSVs are available within testing limits, or if the Operator does not want them used during testing. It is important to size temporary PSVs according to each systems normal operating pressure, meaning multiple PSVs may be required. If incorrectly sized PSVs are used, proper system pressurization cannot be guaranteed. General industry practice shows that systems should be leak tested to 85-95% of the PSV (permanent or temporary) set point. This provides an adequate test pressure, while minimizing the possibility of over-pressurizing and having to recertify the PSVs.
There are two primary isolation methods used during leak testing, process and mechanical. Process isolation utilizes valves within the system, while mechanical isolation utilizes torqued blind flanges, spectacle blinds or skillets. If using process isolation, closure of a single ball valve is acceptable for systems with a pressure class rating equal or below ASME 300#, for higher pressure classes, double block and bleed isolation (or equivalent) is required. Double block and bleed isolation may consist of two ball valves with a bleed valve, a single bodied valve with twin balls and a bleed, or a single ball valve with twin seals.
It is recommended that secondary isolation be used on systems with pressure class ratings greater than ASME 300#. If the secondary system, which may or may not see pressure depending on the primary isolation, has a lower pressure rating than the primary, a vent path is needed to provide over-pressure protection in case of primary isolation failure.
Leak test packages should be developed to provide personnel with knowledge of the system(s) to be tested, identify equipment that may be needed and address any concerns associated with testing. Test packs shall include the following information:
High level procedure describing the system(s) to be pressurized; test pressure; secondary system(s) adjacent to the pressurized system(s); test medium utilized and how it shall be delivered into the system(s); flange management section to provide tracking of bolt torqueing and witness joints.
&#;Test System&#; P&IDs marked up to highlight lines/equipment to be pressurized; witness joints; test pressure; primary and secondary isolations; testing media injection point(s); vent point(s) for depressurization; PSV(s) and/or temporary PSV(s) (if required); connection points for additional pressure gauges; check valves and dead legs downstream of them.
&#;Secondary System&#; P&IDs marked up to highlight lines and equipment that could be pressurized as a result of leakage from the &#;Test System&#; and vent point(s) shown locked open in the event of &#;Test System&#; leakage.
By following these guidelines, it is expected that leak testing will be undertaken safely and effectively, whether it is completed onshore or offshore.
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