In one of our more recent posts, we touched on Waukesha Universal Pump rotor clearances, specifically how exceptionally tight Waukesha Positive Displacement pump rotor clearances are. Because of these tight tolerances, the volumetric efficiency of Waukesha Positive displacement pumps is almost 100% on fluids over about 200 CPS. In this post, we’re going to focus on what enables us to run at such tight tolerances- Waukesha Alloy 88. So what is Alloy 88? Let’s find out.
To begin, Waukesha Alloy 88 is the standard rotor material for Universal 1, Universal 2, Universal 3, Universal Twin Screw, Universal Lobe, Universal 420/520, and 5000 Series Rotary PD pumps. Alloy 88 was developed specifically to be the perfect balance of corrosion resistance and non-galling properties needed for high performance rotary positive displacement pumping. The solution was a nickel based, corrosion-resistant, non-galling or seizing material designated by the ASTM as A494 Grade CY5SnBiM or Unified Numbering System (UNS) #N26055, otherwise known as Waukesha Alloy 88. Waukesha Alloy 88 is listed in the 3-A Sanitary Standards as acceptable for product contact surfaces.
As mentioned above, Alloy 88 is a nickel-based alloy that provides excellent resistance to galling or rubbing wear. This is due to the formation of tin-bismuth rich phases in the alloy. Under a 200x-Unetch Micrograph, we can actually see the presence of globular tin and bismuth phases along the grain boundaries. Why is non-galling important? Well what it means is that when Alloy 88 contacts a stainless body, it far less prone to wear or seizing than 304 or 316 stainless, as well as Nitronic 60 or cast alloy CF10SMnN. That means our rotors last longer and don’t lock up.
The corrosion resistance of Alloy 88 is approximately equal to AISI 300 Series Stainless Steels, like 304 and 316. We do not, however, want to use Alloy 88 in contact with nitric acid. Occasionally, we’ll see nitric acid used to passivate new piping systems, though more commonly citric acid is used. If nitric acid is going to be used, remove the rotors and use a separate pump to circulate the passivation chemicals. And if you’re going to use a nitric acid-based CIP chemical, you should also pull the rotors during CIP and clean them in by hand in a mild detergent.
A few other notes about Alloy 88. Alloy 88 is very machinable, which makes it perfect for rotors or screws, but it is not weldable. It isn’t magnetic and we also cannot heat treat Alloy 88 and it is to be used in it’s as-cast condition.
In sum, Waukesha Alloy 88 is a nickel-based alloy that is approved by the 3A for dairy applications and is widely accepted throughout the industry as suitable for food grade use. Alloy 88 enables the ultra-tight clearances used in Waukesha positive displacement pumps that give us almost 100% volumetric efficiency with even relatively thin fluids. Its corrosion resistance is as good as 304 or 316 stainless, however we should take special care if the rotors may see nitric acid during CIP or passivation. If you have any additional questions about Waukesha Alloy 88, contact a Triplex Sales Engineer today!
