Chapter 7: Slurry System Components Page 7.11

© Paterson & Cooke 2012 Revision 6.1

N = rotational speed of impeller (RPM) Di = impeller diameter (m). The maximum recommended impeller tip speed depends on the pump material. ANSI/HI Standard 12.1-12.6-2011 provides guidelines for material selection and pump operating limits based on the pump duty. The pump duty is classified as Class 1, 2, 3 or 4 depending upon the solids concentration and the size of the solids being pumped. For example, a Class 1 application is pumping solids finer than 75 μm at a density of 1.06 t/m3, whereas a Class 4 duty application is pumping solids coarser than 250 μm at a density of 1.40 t/m3. Figure 7.4 presents the ANSI/HI chart for determining the service duty class. The chart is based on water and silica (solids specific gravity of 2.65) slurries. ANSI/HI note the chart can be used to provide guidance for mineral slurries if the slurry relative density and average particle size are corrected as follows:

0.60.4ASS modactualmcorrectedm (7.2)

0.4modactual50corrected50 Add (7.3)

where Sm = slurry relative density = ρm / ρw 4°C ρw 4°C = 1 000 kg/m3 Amod = modified slurry abrasivity based on Miller Number. ANSI/HI provides Figure 7.5 as a guideline for estimating modified slurry abrasivity values based on Miller Number.

Figure 7.4: Service Duty Chart for Slurry Pumps, Ss = 2.65 (ANSI/HI 12.1-12.6-2011)

Page 7.12 Paterson & Cooke Slurry Pipeline Design Course

© Paterson & Cooke 2012 Revision 6.1

Figure 7.5: Slurry Abrasivity as a Function of Miller Number (ANSI/HI 12.1-12.6-2011)

The service duty class is used to establish the ANSI/HI recommended pump operating parameters as detailed in Table 7.1: The operating parameters presented in this chart are considered to be optimistic for rubber-

lined pumps using the older generation rubber compounds still widely used in South Africa for which impeller tip speeds of up to a maximum of 25 m/s are recommended. These older generation rubber compounds are in the process of being phased out by pump manufacturers. New generation rubber compounds can be used at tip speeds of up to 27 m/s. Tip speeds of 31 m/s, as indicted in Table 7.1 should only be used after consulting the pump manufacturer.

Polyurethane impellers can generally be operated at tip speeds of up to 27 m/s.

The recommended range of pump discharge flow rates depends on the pump geometrya.

Table 7.1: ANSI/HI Recommended Operating Limits for Centrifugal Slurry Pumps

Service Duty (Figure 7.4)

Class 1 Class 2 Class 3 Class 4

Maximum head per stageb (m) 123 66 52 40

Maximum impeller tip speed (m/s) All metal pumps Rubber-lined pumps

43 31

38 28

33 26

28 23

Range of discharge, Q/QBEP (%) Annular volute Semi-volute Near volute

20-120 30-130 50-140

30-110 40-120 60-130

40-100 50-110 70-120

50-90 60-100 80-110

a Annular casing pumps have a constant radial distance around the impeller. For near volute pumps the radial distance increases approximately linearly around the pump casing from the cutwater to the discharge. A near volute design is typical of water pumps while slurry pumps generally have semi-volute or annular design. b The maximum head for Class 1 duty will only be achieved using specially designed metal pumps.