Many slurry pumping systems in the mining industry operate over a wide range of duties as processing requirements and material properties change. The pump selection must be able to achieve the maximum duty point, and often this is the only duty used for selection, which includes the application of industry standard guideline operating limits, such as those published by ANSI/HI (Hydraulic Institute). This can lead to selecting a pump that is not optimal for the full range of operating conditions. In order to prevent this, it is useful to assess the whole operating range and develop a map of where the pump operates and for what portion of the time.
Determining the Operating Range
There are a number of variables that result in the final output required by a pump system, which can include:
- Process flow rate
- Material properties (slurry density usually, but also solids density and rheology with different ore types)
- Pipeline length and elevation for systems with multiple discharge points.
A typical tailings system may have all these variations, and a pump equipped with a variable speed drive (VSD) will have a continuously varying duty point. The requirement and frequency of all these variables can be used to draw up an operating frequency map for a pump. A typical example is shown below:
Figure 1: Typical VSD Pump Operating Frequency Map
The chart includes the defined operating points that could be given to a vendor as the pump duty - a maximum and minimum pump head at the maximum, nominal and minimum flow rate duties. While the pump must be able to achieve the maximum output, it is clear to see that this system will spend very little of its life at the maximum duty point, but far more around the middle of its range.
Pump Size Selection for Wear
Pump selection can be based on the ANSI/HI guidelines as well as other criteria which are used to provide good wear life for wet end components under those operating condition. In this context the frequency map can be used to evaluate the best pump selection, as many duties overlap these criteria.
The example below shows the BEP limits and discharge velocity limit criteria1 for two different pump sizes. In this instance the BEP limits are between 50% and 110% of BEP (between the two lines on the chart is acceptable) and a maximum of 8 m/s discharge velocity, based on the pump type and slurry properties.
Figure 2: Pump Frequency Map with Pump Operating Limits
Pump 1 is smaller than Pump 2, but there is a reasonable overlap in operating area for the two pumps based on the BEP criteria. However, neither pump is ideal. For Pump 1 the discharge velocity limit is exceeded for the maximum flow rate duty points. For Pump 2 the high head duties at the nominal and minimum flow rates fall outside the BEP limits.
If the analysis considers only the six defined operating points and weighs them equally it may be that Pump 2 would be selected – Pump 1 exceeds the discharge velocity limit for the maximum points. However, if the frequency map is considered it shows that the selection focus should be around the red shaded area near the nominal flow rate, as the system will spend the majority of the time in the centre zone. This would result in the selection of Pump 1, where approximately 97% of the operation is within the BEP range. Even though the discharge velocity is exceeded under some conditions, this accounts for less than 10% of the operation.
Pumping systems in the mining industry often have wide ranging operating envelopes that overlap between pump sizes. Developing a frequency map for the pump operating range helps select the right pump for the application, resulting in improved performance from the system and a consequent overall reduction in the cost of ownership.
1 Wilson, K.C., G.R. Addie, A. Sellgren and R. Clift (1997) Slurry Transportation using Centrifugal Pumps, 2nd Edition
About The Author
Pr.Eng, BSc Eng (Mech)
Malcolm is a Senior Mechanical Engineer at Paterson & Cooke's Cape Town office. He has worked in the mining industry for over 18 years and his principal area of expertise is in the mechanical design of pumping and pipelines systems and in the development of control philosophies and functional specifications.