Users of sanitary stainless steel fittings, tubing, valves and various other types of sanitary process equipment are presented a wide variety of interior surface finish choices in today’s marketplace. What is considered clean, hygienic or sanitary in some markets may not be in others. Add to that the fact that the terminology of measuring surface finishes has changed over the years and you end up with a lot of confused users. We will try to bring some clarity to these issues in this post.
The world of sanitary process tubing, fittings and process equipment originated in the dairy industry. Over time these products have migrated into a multitude of other industries including food, biopharmaceutical, beverage, personal care and other industries concerned about cleaning their process systems. For years surface finishes were measured by the grit rating of the final abrasive used in the mechanical polishing process. What was acceptable in the food and dairy industry was 150 grit. Pharmaceutical users often specified 180 grit or higher.
As profilometers became commercially available, surface finish measurement started to be categorized by measuring the final surface finish itself. A profilometer is an instrument that measures the actual metal surface by dragging a stylus across the metal a predetermined length, measuring the distance between the peaks and valleys of the surface. Initially the values were stated using the RMS (Root Mean Square) method. Eventually the industry standardized on stating the values in RA, and arithmetic average. Ra values are generally 11% less than RMS values. In the US Ra is measured in micro-inches, in other
parts of the world in micro-meters.
Surface Finishes: Ra vs. Grit
The standard sanitary internal surface finish in the food, dairy and beverage markets is 32Ra. This has been determined by the 3A (the organization that sens standards for the dairy industry) to be the minimal standard for piping. The pharmaceutical industry uses a broader range of surface finishes as determined by the end user. Most biopharmaceutical company specifications now adhere to the ASME BPE standards. The ASME BPE categorizes 7 different surface finishes, 4 for mechanically polished fittings/tubing and 3 for mechanically polished plus electropolished.
Surface Designation Maximum Ra
SF0 No finish requirement
Surface Designation Maximum Ra
ASME BPE standards have additional standards for acceptability as well as standardized dimensions.
Today there are a broad variety of sanitary fittings and tubing on the market available with 32Ra interior surface finishes, included buttweld and many different types of sanitary clamp connections. ASME BPE fittings are commercially available in the SF0(no polish), SF1(20Ra) and SF4(15Ra+electropolish)as standard in both buttweld and sanitary clamp configurations. Other surface finishes would normally require a special order. Ironically, commercial sanitary tubing is no longer offered in 32Ra. The tubing manufacturers have honed their manufacturing processes to the point that the minimum surface finish they offer in 20Ra, considerable better than the minimal 3A standards.