Our vessels were chosen because their Ra measure (“average roughness”) passed the customer’s in-house testing, and that is a testimony to American Alloy Fabricators.
Three Categories of Surface Roughness
The roughness of a surface can be measured in these basic ways:
• Statistical descriptors – the average behavior of the surface height
• Extreme value descriptors — measures of isolated events, such as the maximum peak height (Rp), the maximum valley height (Rv) and the maximum peak-to-valley height (Rmax).
The Ra measure is defined as the average value of the departures from its centerline through a prescribed sampling length. It is widely considered to give a good general description of the height variations in the surface.
A profilometer is typically used to measure surface roughness — the small surface variations in vertical stylus displacement as a function of position. A typical profilometer can measure small vertical feature anywhere from 10 nanometers to 1.0 millimeter. The height position of the profilometer’s diamond stylus produces an analog signal that ranges from 20 nanometers to 25 µm. The horizontal resolution is controlled by the scan speed and the data signal sampling rate.
Surface Finish
Until recently, measuring and specifying surface finish has been somewhat speculative. The differing standards among equipment manufacturers have created a great deal of confusion and misunderstanding throughout the pharmaceutical and biotechnology industries. Increasingly stringent specifications for surface finish are now being demanded on all items of process equipment, and many suppliers of valves and pipe work are quantifying the surface roughness of their product. The specification of grit reference cannot be equated to a consistent surface finish. Variables affecting the final finish are grit size, tool load, condition of tool, feed rate, traverse rate condition of metal to be polished and lubricant used, if any.
For precise and consistent results, surface finish should be specified in a range or maximum level of roughness average (Ra, which is defined above). Ra is normally expressed in micrometers (microns) or microinches. Micron or microinch values that express surface finish
as Ra are interchangeable with those values identified as centerline average (CLA), or arithmetic average. Measured values expressed as root mean square (RMS) will read approximately 11 percent higher than values expressed in Ra (microinches x 1.11 = RMS). American Alloy Fabricators, Inc., uses a profilometer to measure surface roughness. Surface finish can then be described by using Ra.
The increasing purity requirements in the pharmaceutical and biotechnology industries require surface finishes that meet BPE standards. With the advent of ASME/BPE, the pharmaceutical and biotechnology industries finally have a standard that can be universally applied.
Our client received the vessels from us in 14 weeks, and they have delivered excellent results. For more information on mechanical polishing for your biotech or pharmaceutical company, contact American Alloy Fabricators at 610-635-0205 or via the Web at
http://www.americanalloyfab.com/contact.html.
