Surface Finish Affects Performance
The surface finish of process vessels, piping and related components can be a critical factor in their performance, maintenance costs, and service life. Until recently, specifying and measuring surface finish involved varying degrees of speculation. Today, it is more likely that this characteristic will be influenced by industry standards, which manufacturers and processors must satisfy.
Increasingly stringent specifications are creating greater demand for improved surface finish on most metal components that are part of process equipment. In particular higher purity requirements for pharmaceutical and biotechnology products are dictating the characteristics of surfaces in contact with process fluids. Increasingly, process equipment components must meet requirements in the ASME Bio-processing equipment standard, ASME-BPE-2009.
This standard provides specifications for the design, manufacture and acceptance of vessels, piping and related components for application in equipment used by the biotechnology, pharmaceutical, and personal care product industries. It includes aspects related to sterility and cleanability, materials, dimensions and tolerances, surface finish, material joining, and seals. Meeting the surface finish requirements of this standard is rapidly becoming a universal necessity in the manufacture of other fluid process equipment. As a result, suppliers of equipment and components are often required to quantify the surface roughness of their finished products.
Some additional standards and specifications that directly or indirectly affect surface finish requirements include:
- ASME B46.1-2002 - Surface Roughness, Waviness, and Lay
- ISO 4287 and 4288 - Geometrical Product Specifications (GPS)
- DIN ISO 1302, DIN 4768 - Comparison of Roughness Values
- ASME Y14.36M - Surface Texture Symbols
- ASME B16.5 - Pipe Flange Face Roughness
- DIN 7079 Standard for Fused-Glass Sight Glasses in Metal Frames
Such standards have come into play because process engineers realize that the surface finish of vessels, piping and related components can have profound effects on how well a fluid system performs. Typically, surface roughness is a critical parameter in the assessment of surface finish on fluid system components. This parameter can affect fluid flow resistance (friction), adsorption/desorption, the build-up of chemicals from a process fluid, corrosion formation, pressure drop, etc. Ultimately, surface finish can affect service life and maintenance costs.
Surface Finish Measurements and Charts
All manufactured components have some form of surface texture, which has elements of lay (the machining or forming pattern), surface roughness, and waviness. In addition, inherent material properties may contribute to surface porosity, inclusions, and residual elements. The parameters of texture are vertical amplitude variations, horizontal spacing variations, or some hybrid combination of these. Surface roughness is an expression of finely spaced vertical surface irregularities, as opposed to waviness, which is irregularities with spacing greater than surface roughness.
Depending on conventions in different countries, industries, applications, etc. the units used to express surface finish or roughness will vary. Likewise, various industry standards are used to specify the degree of roughness allowed or recommended in different applications. These standards include those published by ANSI, ASME, SAE, ISO, and other organizations.
Commonly used expressions of finish include:
- Standard grit reference - refers to the grit of a surface finishing medium or method, which does not provide a consistent measure of roughness, since results depend on a part's material, finishing method, lubricant used (if any), and applied work pressure.
- N - New ISO (Grade) Scale numbers. These are used on manufacturing drawings that specify surface finish in terms of an ISO standard. Each roughness grade number can be correlated to a specific Ra number that is expressed in microns.
- Ra - Roughness average, most commonly expressed in micrometers (microns). This is the most universally recognized and used international standard of roughness measurements. It is the arithmetic mean of the absolute departures of a roughness profile from the mean line of the measurement. Ra may also be expressed in microinches.
- CLA - Center Line Average in micro-inches. This is a conversion using Ra(μm) x 39.37.
- RMS - Root Mean Square in micro-meters or micro-inches; i.e., the average of peaks and valleys of a material's surface profile as calculated from a number (n) of measurements (x) along the sampling length:
- Rp - Maximum profile peak height.
- RSm - The mean spacing between profile peaks on the mean line, measured along the sampling length.
- Rt - The total height of a roughness profile, typically expressed in microns, is the maximum peak-to-valley height along the assessment length.
Most expressions of roughness can be converted from one form to another. For example, CLA (microinches) = Ra(μm) x 39.37(inches/meter)
Other conversions use factors that have been establish as generally acceptable over time. In the case of RMS, a range of factor values from 1.1 to 1.7 can be acceptable. A factor of 1.1 is probably use most often, i.e., RMS(μin.) = CLA(μin.) x 1.1. Table 1 lists conversions for some commonly used roughness expressions and values.
Table 1. Conversion chart for equivalent expressions of roughness.
1. A factor of 1.1 X CLA is used throughout this table to calculate RMS(μin.)
2. Typically, for values of Ra from 50μm to 3.2μm, the conversion factor for Rt (μm) is 4. As surface roughness decreases from 3.2μm, the conversion factor increases, reaching 12 at 0.025μm. This is reflected in the table above.
Primary manufacturing processes establish the initial surface characteristics of components and their roughness values. In the case of metallic components, additional finishing processes may be used to reduce the degree of roughness to fit a specific application. Table 2 lists typical Ra values for various metal finishing methods. In the case of fluid system components, the motivation to reduce surface roughness could be to reduce flow resistance and pressure drop, improve sealing, reduce build-up of process chemicals on the metal surface, improve corrosion resistance to increase life, etc. In sight glasses, for example, the surface roughness of both the glass and the metal mounting ring are critical for achieving a good seal in the installation.
Table 2. Typical range of Ra surface roughness values in various metal forming operations.
Source: Wikipedia: http://en.wikipedia.org/wiki/Surface_finish
Various types of polishing operations are commonly used to reduce the surface roughness of metals used in fluid vessels, piping and related components. These fall into two categories: mechanical polishing and electropolishing. As the name implies, mechanical polishing involves the application of physical force on abrasive media to remove surface irregularities. While it's theoretically possible to achieve low roughness values with certain mechanical polishing techniques, the time and cost involved usually makes this impractical. Generally, mechanical polishing is used when moderate roughness values are acceptable, which means numerous surface scratches and other irregularities remain. These can cause many of the problems mentioned earlier on this page.
Electropolishing is an electrolytic process (the reverse of plating) combining electric current and chemicals to remove metal. The peaks of burr, folds, inclusion and other anomalies of a metal surface are dissolved more quickly than valleys as a result of the greater concentration of current over the protuberances. This electrochemical action produces a smoothing and rounding of the surface profile, resulting in irregularities as small as 0.01 micrometer (0.04 micro-inch). It prevents or reduces most of the problems associated with rougher metal surfaces. The inherent benefits of electropolishing subsequent to mechanical polishing include:
- Removal of surface occlusions
- Removal of inclusions and entrapped contaminants such as lubricants and grit particles
- Cleaner surface of the â€œwet contactâ€ areas
- Reduced surface area/chemical reactivity for less absorption and adsorption
- Less contamination and build-up of process chemicals on a surface
- Superior surfaces for cleaning and sterilization
- Elimination of localized corrosive cells (galvanic differences) remaining after mechanical polishing
- Resultant passivated surfaces enhance corrosion resistance
- High luster reflective appearance
- Reduced surface friction