How To Get the Most Out of Your Sanitary PD Pump? Improved Surface Technology to Fight Abrasion

November 4, 2013
Waukesha PD Pumps are offered with a variety of surface treatments to enhance their wear characteristics

Waukesha PD Pumps are offered with a variety of surface treatments to enhance their wear characteristics

A sanitary positive displacement pump is a significant investment and successful application is defined by being able to achieve the desired performance over an extended period of time. While we’ve focused in previous posts on seal types and material selections for abrasive applications, here we will focus on surface finish options that will improve pump life and maximize your investment.

By design sanitary PD pumps rely on tight tolerances to maintain their efficiency.  In applications were highly abrasive or corrosive products are being pumped, those tolerances tend to increase over time, causing the pump to lose efficiency.  There are now several different treatments available for sanitary stainless steel PD pumps that can negate the deleterious effects of abrasion and significant increase the life and efficiency of the pump.

To begin, sanitary positive displacement pumps, such as the Waukesha Cherry Burrell Universal I and II use all 316L stainless steel product contact surfaces (with the exception of the rotors, which are usually made from Waukesha’s high nickel, non-galling Alloy 88. 300 series stainless is an Austenitic steel. In general, 300 series steels are not very “hard”. A “hard” surface that does not gall is critical to improving performance in abrasive applications. To compound matters, most surface hardening techniques employ high temperatures in order to increase the carbon and nitrogen content, thus hardening the steel. The problem with high temperature hardening techniques is that chromium precipitation occurs at common nitriding temperatures. Chromium, specifically the chrome oxide passive layer, is what gives stainless it’s corrosion resistant properties and precipitation destroys the steel’s passive layer and leaves vulnerable.

So when we’re pumping a product like liquid sugar, spices, or other products with fine particulates, what can we do to prolong the life of our sanitary stainless steel pumps?

 The Armoloy process is a low temperature, multistate, chromium alloy process of electro coating based on a modified chromium plating technology. However, instead of the customary chromium plating solutions, the Armoloy process uses a proprietary chemical solution. The result is a satin finish chromium coating that is very hard, thin, and dense and has absolute adhesive qualities. Armoloy provides and enhances 3 characteristics of steel: increased wear/hardness, added lubricity, and increased corrosion resistance. Armoloy is one of the hardest chromium surfaces available, measuring 70 to 72 R c, as applied.
High hardness indicates good wear resistance, but the nodular, “orange peel”, surface created during the Armoloy process also improves the lubricity of the basis metal.  Armoloy’s low friction factor is invaluable in applications of extreme temperature.

Continually, as this is a chromium plating procedure, it only makes sense that by increasing the basis metal’s chromium content, we increase it’s resistance to corrosion. Armoloy resists attack by most organic and inorganic compounds (except sulfuric and hydrochloricacids). The Armoloy coating is typically more noble than the substrate; therefore, it protects against corrosion by being free of pores, cracks, and discontinuities, and by providing a uniform structure andc hemical composition. Porosity, hardness, and imperfect surface finishes of basis metals will affect the corrosion-resistant properties of Armoloy; however, all basis metals that are Armoloy coated will have enhanced corrosion-resistant characteristics.

Expounding upon their success with thin, dense, chromium coatings, Armoloy has introduced XADC, which attains even harder surface finishes through the addition of diamond particles.

Drawbacks to Armoloy include introducing a foreign material and coating to the fluid contact parts as well as effecting surface finish

CDC- Composite Diamond Coating
Composite diamond coating is a unique, patented coating with ultra-fine diamond particles contained within hard electroless nickel metal. CDC is made up of two materials, electroless nickel and diamond, that can provide surface harnesses in excess of those that have been chromium plated. CDC increases hardness and corrosion resistance, which greatly extends pump service life in heavy wear applications.

In addition, CDC is FDA approved, however, application to metals with surface finishes of 32 Ra often result in a final product with a surface finish in excess of 42 Ra, disqualifying the 3A stamp the Universal pumps carry. CDC is generally more costly than Armoloy

Unlike our previous two examples, Kolsterizing is not a coating technique, it is a low temperature diffusion technique. Kolsterizing is done at low temperatures (below 500 C) which allows carbon to remain in its solid interstitial lattice and prevents the precipitation of chromium carbides. Because of this, the Kolsterizing process allows the low temperature diffusion of large quantities of carbon and nitrogen into the surface of the steel without the precipitation of chromium. This leaves the corrosion resistance of the steel unchanged (if not slightly improved), while increasing the steel’s surfaced hardness (albeit not to the values of Armoloy and CDC).

The primary advantage of Kolsterizing is that no foreign alloying elements are added to the steel. There are no quantitative changes to the stainless steel. Quantitatively, only the levels are carbon are changed. This increases surface hardness and provides the steel with anti galling characteristics. The lack of addition of foreign alloys is critical in high purity applications. The drawback is that this process is not as robust as other plating technologies.

To conclude, successful application is defined by being able to achieve the desired performance over an extended period of time. While no sanitary pump will last forever, by considering different surface finish options during system design, we can maximize the life span of our sanitary PD pumps and achieve significant cost savings by preventing down time and costly pump repairs. Holland Applied Technologies is able to offer all of these services to our customers. Please contact a Holland Sales Engineer today with any additional questions you may have.