SLURRY PUMPS S062
Slurry pumps have some features that make them different than chemical pumps.
- The pumps are more massive
- They have looser tolerances.
- The clearances are more open.
- The parts have blunt rather than tapered edges.
- The metal parts are harder.
- They utilize “through bolt construction” because it is difficult to drill and tap the harder metal.
- Some designs are rubber lined to absorb the impact of abrasive fluids.
- They are less efficient than chemical pumps.
- Many slurries are dilatants. Their viscosity increases with agitation. You may have to convert to a positive displacement design if you have this problem. Kaoline or china clay is a good example of a dilatant. Some sugar syrups fall into this category also.
- 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.
- Some large slurry handling pumps run at 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
The speed of slurry pumps is important because of the wear rate of the pump volute, impeller and wear rings.
- The wear is generally proportional to relative velocity between the slurry and the pump elements to the power of two or three. In other words if you went from 1750 (1450) rpm to 3500 (2900) rpm. You would get four to eight times the wear.
Impeller tip speed is often used as the guide in selecting slurry pumps:
- For dirty water type applications limit the tip speed to 130 feet/sec (40 m/sec)
- For medium slurries up to 25% solids concentration by weight and solids size of 200 microns, limit the tip speed to 115 feet/sec (35 m/s)
- For slurries with higher concentration and larger solids, limit tip speed to 100 feet/sec (30 m/s)
- Pumps fitted with elastomeric impellers (rubber like) are commonly limited to 85 feet/sec (26 m/s) tip speed.
Materisls are important to consider when ordering your slurry pump:
- 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
- Cast iron and stainless steel generally do not require any special design modifications
- White irons and similar hard metals are difficult to machine and more brittle. This is the reason we usually clamp rather than bolt the pump pieces together.
- Elastomers are often used as liners that are clamped inside the pump casing and cover. In some cases the impeller is bonded with an elastomer material, although the duplex metals are becoming more popular.