High Purity Valve Leak Testing
Leaking is a common issue for fluid systems and their components. Technically, it’s impossible for systems or products to be completely leak-free. However, in high purity applications, even a minor leak can have a detrimental effect on the smooth running of systems, thus raising high requirements for the leak rate.
Types of Leaks
Leaks are divided into internal leakage and external leakage by way of leaking.
Internal leakage, also known as across-seat leakage, refers to the leakage that occurs inside a piping component when there is a barrier in the internal flow path of the piping component and the closing function of the barrier completely prevents the flow of the medium, such as medium leaking through the seat of a closed valve.
External leakage is a leak between a piping system or piping components and the external environment and can be categorized into inboard leakage and outboard leakage. Inboard leakage refers to a leak from the external environment to the internal when the internal pressure of the piping system or piping components is lower than that of the external environment. Outboard leakage, on the other hand, refers to a leak from the internal to the external environment when the internal pressure of the piping system or piping components is greater than that of the external environment. Below is a figure of the FITOK high purity pressure regulator for reference.
When selecting high purity products, it’s important to consider the different requirements for internal and external leakage. For instance, the FITOK Diaphragm valves in the ALD, DQ, DP, DI, and other series have maximum internal and external leakage rates of 1×10-9 std cm3/s. The RTG series high purity tied-diaphragm pressure regulators have a maximum internal leakage rate of 5×10-8 std cm3/s and a maximum external leakage rate of 1×10-9 std cm3/s. In our catalog, the external leakage test is the inboard leakage test by default, but if customers require outboard leakage data, FITOK can provide it upon request.
To determine if a valve has internal leakage, observe if there is helium leakage at the valve’s outlet when it is in a closed state. For external leakage, introduce tracer gas inside or outside the valve and detect it on the opposite side to determine if there is a leakage. It’s important to note that the internal leakage test should only last 15 seconds due to helium’s high permeability to non-metallic materials, particularly common seat materials such as PTFE and PCTFE. If the test duration exceeds this time, some helium may penetrate the non-metallic valve seat, making it impossible to test the real sealing performance.
FITOK can perform internal leakage, inboard leakage, and outboard leakage tests in an ISO 4 clean room. A comparison table of the three tests performed on the valve is provided below.
|Test||Leak Direction||Test Pressure||Valve State||Locating Leak Point|
|Internal leakage||Across seat①||Assignable||Closed||Valve seat|
|External leakage||Inboard Leakage||Outside to inside||1 atm||Open||Yes|
|Outboard Leakage||Inside to outside||Assignable||Open||No|
① Across seat or other equivalent seals.
Helium is the most common test medium. This is due to a few reasons: Firstly, helium is an inert gas and is considered to be a relatively safe option to use. Secondly, helium has a small atomic diameter, which allows helium atoms to move freely. Lastly, the amount of helium present in the air is limited to only about 5 ppm, making helium leak testing highly accurate. Helium, when used in combination with mass spectrometer leak detectors, is widely implemented to detect leaks that are less than 1×10-6 std cm3/s with high reliability. However, due to the diameter of helium atoms, the minimum leak rate that can be detected is only 1×10-12 std cm3/s. Currently, some companies are using hydrogen mixtures as the medium for leak testing, which costs less than helium leak tests.