A key action that is taken to prevent corrosion or damage in stainless steel systems is also the most basic – a physical, visual examination. Depending on the component, its location as well as Current Good Manufacturing Practices (cGMP) regulations, the examination can be done with unaided eyes and a light source, or it can require mirrors, magnification and borescopes. It may also require the inspector to physically enter the system through an access manway to determine the condition of the overall inner surface. Regardless of the exact procedure, examinations should be a regularly scheduled part of a preventative maintenance plan.
Broadly speaking, there are two different examination methods; direct and indirect (or remote). Guidelines for both methods are outlined in ASME BPVC 2011a, Section V, Article 9.
For direct visual examinations, the two most important factors are the distance from the surface being inspected and the light source being used. The distance should be less than 24” (61cm) and the surface should be viewed at less than a 30° angle. That angle also applies to the positioning of the light source as well. For example, a patch of micro pitting would appear to be a white area on the surface when the light comes from a low angle. That same area would appear to be normal if the light source is perpendicular to the surface.
The type of light source used is itself important. Superbright LED lights can give the surface a blue tint and throw off a visual examination. In fact, the switch in light sources has led to false positives in examinations as more surface imperfections have been visible leading to the rejection of components which meet the criteria under incandescent light sources. The important lesson from that is that the light source used for examinations should be consistent.
The color of the light emitted should be in the 5000-6500K range and bright enough for things to be seen clearly. To provide an easy reference, daylight lightbulbs are 5000-6500K with Bright White bulbs being below that range. In more technical terms, the light source should put out 1000 lumens and positioned so that the light coming off the surface measures between 500 and 1000 lux. The size and shape of the inspected object will affect viewing angle, distance and other factors, as will the contrast and the cleanliness of the surface.
The same illumination factors apply to indirect visual examinations. Although being done through the use of mirrors, or electrooptic devices, proper lighting is just as critical. Borescope manufacturers are only now becoming aware that “simply putting the brightest light” on the end of a borescope is not the best solution because of how it can change what is seen through them.
What Are You Looking For?
Once the method for observation is determined, the next question is what the inspector is looking for. The inspector is attempting to identify a number of issues. Any one of them could lead to a compromise of the component or the larger system, or at the very least lead to a rejected batch of product.
The inspector is primarily looking for:
- Residual product, media or buffer
- Foreign debris
- Damaged to the surface such as corrosion, scratches, wear or pitting
- Discoloration or rouge
- Level lines not resolved through clean in place (CIP)
- Heat tint from welds
Document, Document, Document
Many industry standards require examinations on a set schedule or after certain milestones, such as after a production run. Depending on the industry, the examinations can range from a general gross examination to looking through sight glass, or more thorough examinations where the system is opened up. The former are more often used to verify the success of CIP procedures. The latter, more intrusive examinations, are generally scheduled. Quarterly or biannual examinations would be more robust procedures involving lowering mirrors or using borescopes to inspect deeper into the system. Annually scheduled examinations would, of course, be even more detailed and could include sending inspectors into confined spaced to inspect the surface of a tank from only 24” away – something that cannot be done through a sight glass.
Regardless of the type of examination, those same industry standards typically require a level of documentation that includes some or all of these aspects:
- Equipment used for examination
- Date and time
- Examination method used
- Surface conditions
- Specific locations examined
- Location examinations were made from
- Recording of observations
- Photos and location of where photos were taken
As the maxim goes, “the job is not done until you’ve finished the paperwork.” In this case, having detailed documentation not only shows proof of compliance, but when looked at over time, it can reveal trends or recurring situations that can be addressed to prevent problems in the future.
The inspectors themselves need to be highly knowledgeable about where they’re looking and what they’re looking for and to do so in a way that meets the requirements of the ASME or other applicable standards. But beyond that, they must also know how to properly document the examinations so that they meet the relevant standards to show that the system is operating in compliance with CGMP and the latest guidelines.