When applying a centrifugal pump on abrasive service, there are a few things you should know about the application to pass on to the pump supplier.
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Figure 1. Viscosity versus centrifugal pump performance
Basics of Abrasive Service
The obvious things are the hydraulic requirements of the application: head and flow. Know the make-up and physical properties of the liquid. Understand everything about it: Is it viscous? What is the temperature of the liquid? What is the solids percentage? Are the solids sharp? It is corrosive? What are the solids? How big are they? What are the conditions of service? Does the pump run 24/7? Is it intermittent? If so, how long does it run each day?
It is important for an owner to realize that abrasiveness and corrosiveness go hand in hand in pumping applications. Even water is corrosive. Most metals will develop a layer of protective oxide on their surface when exposed to a corrosive environment. When abrasive particles are added to the liquid, they tend to strip off this "protective layer" of oxidation and expose the bare metal to the corrosive and abrasive liquid. This interaction must be understood to obtain the most appropriate choice of material and hydraulic suitability for the life of the pumping unit.
Figure 2. Centrifugal Pump Performance Curve
[[{"type":"media","view_mode":"media_large","fid":"369","attributes":{"alt":"Centrifugal pump performance curve","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]These and any other relevant pieces of information need to be known, understood and clearly communicated to the pump professional. Never assume and never be vague ("sandy water" will not work). If you do not know, say you do not. Your experience will be better in the end.
Materials of Construction
Materials of construction cover a wide spectrum depending on the application and conditions. The vast majority of centrifugal pumps are constructed of Gray Iron 30. This material is adequate in "dirty water" service. (Dirty water is just off-color pond water or lazy river water that is not from close to the bottom.) Other materials that provide more abrasive resistance include ductile iron, ADI (Austempered Ductile Iron), Ni-Hard, high chrome and elastomeric lined pumps.
Simple cast iron (Gray Iron 30) has a Brinell hardness of around 170. It is usually fine for dirty water service if other factors are favorable. Cast iron is the least expensive to manufacture and buy, but would be a poor choice for truly abrasive service and cost the owner more during the normal life cycle.
Ductile iron and ADI are essentially the same basic material. ADI has a more closely controlled chemical and physical make-up. ADI grade ductile iron provides no better protection against abrasion than standard ductile iron, unless it goes through a heat treating process to raise its hardness level. With several different grades of hardened ADI available, be careful when specifying to ensure you receive the material for which you have paid. Some companies list ADI as a material, but do not perform the heat treating process as it tends to warp the metal and makes providing a quality part more difficult. ADI will have an unhardened Brinell of 150 to 225. Depending on the grade, the hardness can exceed 400.
Different pump manufacturers may use a trade name for ADI, so simply ask exactly what it is. Have the manufacturer provide chemical and physical properties or "Certs" for the heat from which the castings were poured as well as certified hardness testing after heat treatment.
Ductile iron is sometimes referred to as nodular iron due to its granular structure of graphite nodules through the matrix of the metal. Ductile iron has a Brinell hardness of 150 to 225 depending on the grade; it is not much different than gray iron in corrosive resistance.
Ni-Hard and Hi-Chrome iron share the same ASTM specification and have almost exactly the same properties. The two materials are interchangeable, and manufacturers often choose one based on the current cost of nickel versus chromium. Either material usually provides the same service life, which is why they have the same ASTM number and specification. The Hi-Chrome version has a slightly higher corrosion resistance due to the large volume of chromium (26 to 28 percent) in the formula.
Like ADI, these versions of hardened iron can vary in final hardness depending on the process and manufacturing technique used. The high end of Brinell hardness is 700+. Companies in the business of handling mineral slurries have perfected the processes required to provide quality hardened castings that will resist abrasive wear better than any other metals. It is usually a good idea to ask about the manufacturer's level of experience in this area to understand the processes they use. Properly treated Ni-Hard or Hi-Chrome will have a hardness level exceeding that of silica sand. For pumping abrasive-laden liquid, consider a manufacturer who has these materials available.
Speed Kills
Speed kills on highways and inside a pump. When you increase the speed of a pump, the wear rate changes by a factor of four times.
If a given pump yields 1,000 hours of life and you reduce the speed to half the original, the pump life will increase to 4,000 hours. Not every application allows the speed to be cut in half; when it is possible, it can significantly increase the life. An abrasive handling pump should generally have a top speed of no more than 1750-rpm. Some large slurry handling pumps have just a few hundred RPM but compensate with a large impeller; it is not uncommon to find impellers in the 7- to 8-ft range in pumps specifically designed for slurry applications.
Cut the speed down as far as possible to extend the life of the pump. Your pump professional can help you understand how to accomplish this.
Figure 3. Friction loss from sludge
[[{"type":"media","view_mode":"media_large","fid":"371","attributes":{"alt":"Friction loss from sludge","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]Harder Is Not Always Better
When the question arises about how hard metal should be to resist tough abrasive service, I always ask: What do the people who etch and engrave tombstones use as a template for the intricate designs and names? The answer is rubber. Sometimes absorbing the impact of the abrasive particles is better than resisting them.
Many large slurry pumps are rubber lined for life and for renewal. This is not by accident, but by design. The abrasive particles and nature of the liquid must be known so the selection of metal versus rubber can be made. If it is rubber, it has to be the correct composition and hardness to provide the longest life.
Many people do not understand what slurry actually is, and some manufacturers advertise they have slurry handling pumps when they do not. Almost every centrifugal solids handling pump has an upper limit of solids content of 4 to 6 percent. True slurry pumps can handle solids content of upwards of 70 percent.
Summary
Figure 4. Brinell Hardness Chart
[[{"type":"media","view_mode":"media_large","fid":"372","attributes":{"alt":"Brinell hardness chart","class":"media-image","id":"1","style":"float: left;","typeof":"foaf:Image"}}]]Always know the liquid and solids you are pumping. Do not guess or mislead the pump professional, or it will result in poor pump selection.
Select the right material for the job. Seek the advice of knowledgeable, experienced people and follow it. While harder materials generally give a longer life, it is in no way an absolute. Sometimes you want to go soft.
Slow your pump down. It saves money to select a larger pump and run it slower when pumping abrasive-laden liquid.
Pumps & Systems, April 2009