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Creep testing of plastics for safe delivery systems

by Aug 13, 2019Weld Testing

The dispensing and transfer of gases and liquids in chemical manufacturing processes causes strain and stress on your production equipment. The creep test, a non-destructive testing (NDT) method, protects your systems from collapse prior to installation. At Axenics, we recommend creep testing of plastics and metals following the design, preparation and in-house welding stages.

What is creep in materials?

Creep a.k.a. cold flow is a slow deformation of materials due to these three Ts: time, temperature and tension. Creep takes place when a solid’s form distorts due to the exposure of persistent operational stresses, such as a piece of stainless steel pipe with high-temperature liquids consistently flowing through. The closer the metal gets to its melting point, the more extreme the creep damage becomes.

Creep follows Hooke’s Law of Physics, where the strain in a solid is directly proportional to an applied stress within the elastic limit of the solid. Therefore, even though the stress of the chemical transfer is below the designed strength of the material, creep still occurs when that material is in longtime operation. Dependent on the severity of the three Ts, creep can result in a permanent distortion of materials.

Metals or Plastics?

Axenics recommends creep-resistant materials based on the function of your critical systems. Often, we prefer using thermoplastic pipe for chemical delivery systems, including plastics welding, for a few reasons:

  • One, some thermoplastics actually have a greater resistance to creep than stainless steel.
  • Two, we use a proprietary method for welding thermoplastics and can perform tighter welds than many of our competitors, resulting in a smaller footprint for your production system.
  • Three, thermoplastic pipe systems are less expensive than metals – saving you money.

We regularly create end-to-end plastic kits made of Polytetrafluoroethylene (PTFE), a chemical-resistant plastic that is ideal for the transport of potentially harmful liquids or gases. We also provide systems constructed with polypropylene (PP), polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC).

The pipe kits use the same materials for filler joints in welding, and the welds are as strong – or stronger – as the base material.

What is creep testing?

Some of our customers have not yet switched their delivery systems from metals to plastics, so we perform creep testing on all welds for both stainless steel and thermoplastics.

Creep testing seeks any potential leaks in a component. After Axenics’ welders perform their own visual test of any weldments, the component moves to quality control. The creep test is the final examination in the production process before being delivered to the customer.

The metal tubing is blocked and sealed and then nitrogen is added to the interior of the tube. Nitrogen is our preferred gas since it prevents moisture from forming during the creep test, keeping the metal from oxidizing. Nitrogen is also a preferred shielding gas when creep testing on thermoplastics.

A gauge measures the pound per square inch (PSI) of the gas inside the component, dependent on the customer’s specifications. For example, a customer may request 150 PSI of pressure for five minutes, and during that duration, the PSI cannot drop by more than one pound, or else the material fails the test.

The creep test for thermoplastics generally occurs the day after welding, giving the weld 12 to 16 hours to cure completely. Then, a creep test for leaks takes place in the same format as for metals.

Testing every weldment on stainless steel and thermoplastics is key to ensuring you receive the highest-quality pipe or tube components for your gas or liquid delivery systems. Our customers rely on our 35-plus-year history of safe, reliable manufacturing, as they are working with potentially dangerous elements. The creep test is just one aspect of our in-depth weld testing options.

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