TESTING & INSPECTION

PMI (Positive Material Identification)

PMI (Positive Material Identification) testing is the analysis of materials to determine the chemical composition of a metal or alloy at particular (usually multiple) steps of alloy manufacturing or in-process alloy installation. Knowing the exact composition and grade of an alloy enables suppliers, plant workers, and other responsible parties in the chain of custody of components to match alloy specifications that are chosen for their specific properties such as heat resistance, corrosion resistance, durability, etc. Having the right alloy, because the right alloy with the right properties is often all that stands between a safe, efficient operation and lost time and revenue.Due to the inherent danger in analyzing materials that are in process, it is necessary to use instrumentation that is non-destructive. This testing qualifies the analysis as non-destructive testing, or NDT. Non-destructive testing (NDT) indicates that the material is not altered in any way and is essentially in the exact same condition it was prior to testing with no visible marks or structural damage left from the analysis.

It is important to check every alloy and component multiple times during every stage; this is known as a 100% PMI protocol. The material verification program/PMI process should begin at the incoming materials warehouse, where product may be tested via a PMI spot check method as part of the receiving process. As well as PMI testing, a material verification program should provide complete and accurate documentation indicating which materials have been tested and what the results were.

Coating Thickness Test

Coating thickness  measures dry film thickness. Dry film thickness measurements can be used to evaluate a coating’s expected life, the product’s appearance and performance, and ensure compliance with a host of International Standards.

The non-destructive coating thickness measurements can be taken on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium. Digital coating thickness gauges are ideal to measure coating thickness on metallic substrates. Electromagnetic induction is used for non-magnetic coatings on ferrous substrates such as steel, whilst the eddy current principle is used for non-conductive coatings on non-ferrous metal substrates.

Liquid Penetrant Testing

This is a method of detecting surface-breaking flaws such as cracks, laps, porosity, etc. 

Penetrant testing is one step up from visual inspection and offers many advantages, such as speed, large-area coverage and cheapness.

It is usually a six-stage process:

a) surface cleaning (degreasing etc)
b) application of a penetrant liquid (dipping, spray, brush)
c) removal of excess penetrant (solvent, water)
d) application of developer
e) inspection of test surface (visual, television camera)
f) post-inspection cleaning (anti-corrosion solutions).

The principle of liquid penetrant testing is that the liquid penetrant is drawn into the surface-breaking crack by capillary action and excess surface penetrant is then removed; a developer (typically a dry powder) is then applied to the surface, to draw out the penetrant in the crack and produce a surface indication. Cracks as narrow as 150 nanometres can be detected. The indications produced are much broader than the actual flaw and are therefore more easily visible. Liquid penetrant testing can be applied to any non-porous clean material, metallic or non-metallic.

Technical Tests

Technical Tests includes:

• Mechanical Properties, Pressure & Hydrotest
• Chemical Composition
• Dimensional Check

Valve leakage is tested using either a hydrostatic test (i.e., the test medium is a liquid, such as water) or a pneumatic test (the test medium is a gas, such as air). Zero leakage is rarely if ever possible, so the standards define the maximum allowable leakage (MAL) for valves under the specified testing conditions.

For both hydrostatic and pneumatic tests, the MAL is usually defined by valve size—a small amount of leakage through a valve with a small effective orifice poses much more risk than would the same amount of leakage through a valve with a large effective orifice. MAL may also be a function of the valve class and pressure category.

A typical hydrostatic valve test follows these basic steps:

1. The valve body is filled with the testing fluid at the specified temperature.
2. The specified pressure is applied for the specified length of time (usually at least 1 minute).
3. Leakage is measured across the valve element of interest (e.g., stem, seat, closure mechanism) using both measuring instruments and visual examination. Most valve standards specify that no visually detectable leakage is allowed.
4. A visual inspection is performed to ensure the valve has not been damaged during the testing procedure.

The tests that must be conducted depend on the type of valve and, therefore, the valve elements. Below are some of the most commonly performed tests of valve leakage.

• Valve seat leakage tests are required for pressure relief valves.
• Backseat tests are required for valves with a backseat element, including gate and globe valves.
• Closure tests are required to test the closure mechanism of several types of valves, including gate, globe, plug, check, and ball valves.
• Shell leakage tests are required for valves that are used in “full open” and “full closed” service, such as check, stop, and isolation valves.