How Leak Testing Works

How Leak Testing Works

    How Leak Testing Works

    Leak testing is a broad term that includes a multitude of technologies. For the purposes of this article, different leak test methods will be referenced but not described in full detail. This article defines the broad term leak test and takes a closer look at the pressure decay leak test method. Furthermore, this article will describe how a pressure decay test works, considerations for the pressure decay method, and how recent technological advancements have impacted manufacturing environments.

    What is Leak Testing?

    A fixed head leak tester is a procedure used to determine if an object, product, or system functions within a specified leak limit. A leak occurs when a gas or liquid flows through an object via an imperfection or manufacturing defect such as a hole, crack or weak seal. These imperfections create high- and low-pressure zones within a product, forcing the gas or liquid to flow from the high-pressure area to the low-pressure area. The primary leak test method discussed in this article uses pressurized air to identify leaks.

    Leak Testing Methods

    There are many different types of leak test methods, which have different detectable leak rate limits. This article will primarily discuss the pressure decay leak test method. A pressure decay test identifies if a part is leaking or not within a predetermined leak rate limit. The smallest detectable leak rate for the pressure decay method is 10-4 mbar*l/s or 0.0059 SCCM.

    The benefits of pressure decay leak testing include:

    Economical

    Fast (depending on the internal test volume of a part)

    Easy to set up

    Only requires compressed clean,

    dry air

    Quantitative

    Nondestructive

    No pre or post processing required

    Determining a leak rate is vital to selecting the best leak test method. 

    Typical industries for pressure decay leak test methods include:

    Automotive

    Sealed Electronics

    Medical Devices

    Packaging

    Consumer Goods

    How a pressure decay test works

    During a pressure decay test, a product is attached to a leak test system and filled with air. Once pressurized, the air source is closed off and the pressure is allowed to settle. During the test any decrease in air pressure over time signifies a leak.

    Variations of pressure decay methods

    Pressure Decay – Measures the pressure change of an object under positive pressure

    Vacuum Decay – Measures the pressure change of an object under negative pressure

    Occlusion – Checks for a blockage in the gas flow path of an object

    Burst – A destructive or nondestructive ramping pressure test that measures the point at which the device opens or has a catastrophic event (rupture).

    Crack – Typically performed on check valves to detect weeping prior to reaching the opening pressure. A downstream sensor monitors for weeping.

    Chamber – Finds leaks in sealed packaging or devices that do not include an opening for filling.

    Depending on the functional use of an object or part, any of the above tests may be required.

    Considerations for leak testing:

    What is the intended use of the part?

    What medium is being constrained inside or outside of a part? A medical device manufacturer designing an IV set may try to keep saline inside the IV set. An automotive manufacturer may have designed their manifold to prevent exhaust gases from escaping the exhaust manifold. An acceptable hole or porosity in these parts is contingent upon the application by which a leak limit will be determined. 

    Medium

    An oil molecule is larger than a water molecule and a water molecule is larger than an air molecule. If an eight micron hole is subjected to 45 psi of pressure, air will create a noticeable leak, whereas water will only create a droplet and oil may not leak at all.

    What pressure range is a part subjected to during use? What safety factor is required? Finding the appropriate pressure range for an application is vital. If the test pressure is too low, quality may be affected, and faulty parts may pass the leak test and go on to fail during use. Conversely, selecting a pressure range that is too high will extend the time it takes to complete the test and possibly damage the part.

    Internal Volume

    The test volume plays a significant role in creating a repeatable and sensitive test. Reducing the overall volume enables shorter test times with greater sensitivity. Parts that cannot have a volume reduction will benefit from pneumatic and sensor assemblies appropriately sized to meet test cycle expectations.

    Acceptable Leak Rate

    Everything leaks but what leak rate is acceptable for an application? Leak rates are most often specified by regulatory requirements specific to an industry. Often when testing a new product some trial and error is involved in finding what leak rate is acceptable for the product’s application.

    Material

    The material of the part under test affects test time. Compliance influences both the fill and settle steps in a leak test. If a malleable object is subjected to pressure and is not given enough time to settle before a test begins, the part may be expanding or contracting during the leak test, yielding inconsistent results.

    Design

    During the design and development of a product it is important to consider the test criteria to ensure a part has been manufactured correctly. Design criteria such as test pressures, access ports, shared walls or vessels, internal test volumes, and leak rate specifications all have an impact on the complexity and time required for a leak test.  How the object connects to a leak test system helps to determine what kind of leak test will be run on the object. A fully enclosed object with no port to fill the item, such as a waterproof enclosed electronic device, requires a chamber test which includes a sealing fixture. An object with one port or opening, such as a catheter, can be directly connected to the front port of a leak tester without a fixture.

    Conveyor has been a staple in the materials handling industry for decades. As the demand for reduced cost, increased throughput and integrated automation grows, so does the need for conveyor systems of all types.

    Conveyor is available in many styles and is used in countless applications. In this Equipment 101 article, Modern spotlights the basics of some of the most common conveyor types:
? Non-powered skatewheel and roller conveyor
? Powered, or live, belt and roller conveyor used in handling packages and other small products
? Powered chain and roller conveyor used in handling pallet loads of goods

    NON-POWERED CONVEYOR

    Non-powered conveyor, the simplest form of conveyor, uses the natural forces of inertia and/or gravity to keep products moving.
   
Portable sections of non-powered conveyor are often used for loading packages onto the back of an over-the-road truck. Non-powered conveyor can also serve as takeaway conveyor for cartons coming out of an automated sorter, and it’s often used in workstations and pick modules where employees complete their tasks and then push their work along to the next zone or station. The two most common types of non-powered conveyor are skatewheel and roller.

Skatewheel conveyor
Only a little energy is needed to turn the small wheels of a skatewheel conveyor, and that makes skatewheel good at maintaining the speed of a product. Because each wheel turns independently, skatewheel conveyor is also a good choice for the curved sections of a conveyor line.

    Roller conveyor
Non-powered roller conveyor is commonly used for workstations and pick modules because it provides a better working surface and is often less expensive than skatewheel conveyor. It’s also good at slowing the inertia of products coming out of a high-speed sorter.

    Whether it’s skatewheel or roller, non-powered conveyor that relies on gravity to move product is still used a lot in the market, says Russ Devilbiss, sales manager for Carter Controls and chair of the Material Handling Industry of America’s (MHIA) Conveyor & Sortation Systems industry group. The limitation with non-powered conveyor, however, is control. “With gravity, you can’t control the force, and you always want to be in control of your product,” says Devilbiss.

    POWERED PACKAGE-HANDLING CONVEYOR
When it comes to moving packages and other relatively small items, two styles of conveyor dominate the market: belt conveyor and powered roller conveyor.

    Traditionally, belt conveyor has been used for transporting products, while roller conveyor has been used for accumulating products.

    Another factor taken into consideration when choosing between belt or roller conveyor depends on the size of the product being moved.


    Belt conveyor
In traditional belt conveyor, an AC motor drives a pulley that then turns a long, looped belt. Underneath the belt sits either a bed of non-powered rollers or a sheet of metal known as a slider bed. The belt can be made of a variety of materials with a variety of surfaces, depending on the items it is intended to convey.

    For example, according to Ken Ruehrdanz, warehousing and distribution market manager for Dematic, a belt surface can be smooth where you need to slide an item off a conveyor easily or ribbed to give it more gripping power, like on inclines and declines.

    And, belted incline or decline conveyor can be used to move product from one level of a facility to another. Spiral conveyor is another way to move product from one level to another. If floor space is limited, spiral conveyors are often a good solution. 

    Regardless, traditional belt conveyor is a simple, time-tested technology. It’s less expensive than powered buffer conveyor, it provides a more stable surface, and it can convey a variety of products. Poly bags, envelopes and electronics are examples of products handled easily by belt conveyor that are often too small or too light to be conveyed directly on rollers.

    Roller conveyor
Despite the advantages of belt conveyor, many of today’s distribution centers are filled with roller conveyor because it allows accumulation of products. Accumulation is a way to make the conveyor store product for a determined amount of time then released into an automated sorter or palletizer, for example.

    Zero pressure accumulation means products on the conveyor do not touch each other. Minimum pressure accumulation, however, allows the products to make contact, but with a determined degree of impact that will not cause damage.

    Powered roller conveyor falls into a number of different categories, depending on the way the rollers are driven. Three common categories are line-shaft, belt-driven and motorized roller.

    Line-shaft conveyor: In a section of line-shaft conveyor, a long metal shaft runs below the bed of rollers. Rubber o-rings connect the rollers to the shaft so that when the motor turns the shaft, the shaft turns the rollers.

    Line-shaft conveyor is the least expensive type of roller conveyor. It has been in the market for a long time, but it has limitations. Even though line-shaft flexible conveyor costs less, explains Don Erickson, director engineering for Automotion, it is parts-intensive, which leads to high maintenance requirements.