– S L Abhyankar, Designer, Trainer, Consultant for Pumps, Valves, Pumping Systems The author says that this is one of the most difficult exercises in pumping; the selection of the pumps and systems needs a good understanding of the solids and slurry to be pumped. Article provides important criteria to be considered.
Synopsis To pump solids effectively and efficiently, the pump selection depends on a good understanding of the characteristics of the solids and/or the slurry to be pumped. Conceptual aspects as well as some of the important criteria to be considered are covered alongwith the kind of pumps used for such operations.
Pumping solids is no easy job. Not only is it difficult, it is also complex. It is complex, because solids to be pumped are of various, shapes, sizes and characteristics. Mention of some typical examples will give a bird’s eye view of the complexity of pumping solids.
● One good example of complex solids is sewage. Sewage is complex and is totally heterogenous.
● Another example is paper pulp.
● Yet another is cement grout.
● Yet another is pumping carbamate, which has a melting point of about 150oC. So, it will tend to solidify at ambient temperature. That means to start a pump to pump carbamate, whatever carbamate would have settled in the pump from previous running needs to be first melted or liquefied by heating. The pump should have a heating jacket (See Fig. 8). Similar is the case with pumping tar or bitumen. Pump-manufacturers manufacturing pumps for pumping solids would be testing the pumps primarily with water. So, for selection of pumps for pumping solids the pumping duty i.e. rate of flow and head or pressure, has to be first translated into equivalent duty with water.
Different types of solids have different methods for translation of pumping duty to duty with water. These methods are based on some typical characteristics of the application viz.
1. Percentage content of solids by volume or by mass. It becomes useful to know the density of both: of the solids and of liquid with solids. Percentage content of solids is also called as consistency in applications like paper-pulp.
2. Maximum size of solid particles - This consideration very much influences the width of hydraulic passages in the design of the pump a. If the solids are like flakes, a grinder or pulveriser before the pump would make the pumping easier. An example is of recycling the flakes-laden cooling water in steel plants.
Pump-manufacturers manufacturing pumps for pumping solids would be testing the pumps primarily with water. So, for selection of pumps for pumping solids the pumping duty i.e. rate of flow and head or pressure, has to be first translated into equivalent duty with water.
b. If the solids are of the type of sticks or long fibres, a chopper before the pump would make pumping as much easier. See Fig. 3
3. Whether the solids are settling type, e.g. sand-laden water as handled by dredging pumps a. Settling types of solids at low concentrations may cause erosive wear. Resilient lining such as rubber-lining would have better life against erosive wear. See Fig. 4 b. Settling types of solids at high concentrations may cause abrasive wear. Surface hardness would provide better life against abrasive wear. c. Also operating speed of the pump should be
low. d. Smooth surfaces as obtainable in coatings can help make the particles to ‘glide’ instead of they causing abrasion. See Fig. 7 e. The coatings can also help providing certain amount of corrosion-resistance.
4. If the solids are of non-settling type, they would make the liquid turbid at low concentrations and viscous at high concentrations. For Concepts of Viscosity see article at http://www.columbia.edu/itc/ldeo/lackner/E4900/Themelis3.pdf
It would be useful to understand the difference between erosion and abrasion.
Resilient lining as of rubber would help to make the impinging particles to bounce away and in turn reduce the severity of erosive wear. Smooth surfaces also help to make the impinging particles to glide away.
Since methods for translation of pumping duty to duty with water are different according to the characteristics as detailed above, for viscous liquids, i.e. liquids with non-settling solids the method has been to use the chart (See Fig 9).
Actually the chart in Fig. 9 was based on similar chart available in earlier editions of standards of Hydraulic Institute (HI). However there is the standard ISO-TR-17766:2005, which is more recent, gives equations and formulas which can be used algorithmically.
For liquids with settling solids, one can realize that such solids need to be forced to rise. An interesting arrangement is Jet Lift Dredging. (See Fig 10) It can be noted that in Jet Lift Dredging the solids are raised only into the transportation pipe and do not get into the
pump at all. That eliminates wear and tear in pumps, due to solids. But the jet principle is not efficient in power-consumption.
When solids are of settling type, the correction of the total head is done by getting the correction chart (See Fig. 11) based on Cave’s equations.
That settling solids need to be forced to rise means that the correction is required only to the head of pumping and not to the flow-rate. Higher pumping head is also associated with depletion of pump-efficiency. If the pumping head needs to be corrected by 1.25 times, the pump-efficiency will be less by 1/1.25 = 0.8 times of efficiency when pumping clear liquid.
General formula for power (P) required for pumping is P = r* Q*H/102/ h Where r = specific density of liquid (for water r = 1) Q = flow-rate in litres per second
H = Total head in meters h = efficiency of pump
Since total head is Hstatic + Hf, the frictional head Hf is to be calculated by using charts. Charts for frictional head are most commonly available for pumping water and not for every other liquid. Moody’s chart should then be a handy tool (See Fig. 12) Method of using Moody’s chart is well explained in IS:2951 (Part 1). Friction in valves and fittings can be further calculated as explained in IS:2951 (Part 2).
Since liquids whether with non-settling solids (having some viscosity) or with settling solids do not flow easily, pumping installations for pumping liquids with solids should rather be installed with positive head above suction.
Since liquids whether with non-settling solids (having some viscosity) or with settling solids do not flow easily, pumping installations for pumping liquids with solids should rather be installed with positive head above suction. See Figs. 13, 14, 15, 16 and 17.
Most of the discussions above focused on centrifugal pumps. But liquids with high concentration of sol- ids and of high viscosity are pumped by Positive displacement pumps. Pumping of cement grout employs positive displacement pumps only. Another eminent example is of mud pumps used in oil wells. What the mud pumps pump is not actually mud. What they pump is drilling fluid, which is called as mud, because it very much looks like mud.
Viscosity Correction factors in Fig. 9 are to be applied only when pumping by centrifugal pumps. In case of positive displacement pumps, higher the viscosity, there would be less ‘slip flow’ i.e. the flow slipping backwards towards suction. Less the slip flow, better is the volumetric efficiency of the pump. Whereas this sounds charming, most positive displacement pumps have suction and delivery valves. One can realize that solids getting deposited at valve seats would render pumping difficult and inefficient. Yet for pumping liquids with high concentration of solids and of high viscosity positive displacement pumps would mostly be the only option.
Recommendations for further reading
1. For Concepts of Viscosity see article at http://www.columbia.edu/itc/ldeo/lackner/E4900/Themelis3.pdf
2. Slurry Pump Basics - publication by M/s. Metso Minerals, can be downloaded at http://www.metso.com/miningandconstruction/MaTobox7.nsf/DocsByID/119A6A698 EE232B0C2257E7A00457903/$File/Slurry%20Pump%20 Basic_13.pdf
3. Slurry Pumping Manual - publication by slurry pumps division of M/s. Weir Group plc, can be downloaded at https://www.scribd.com/document/338766642/WeirSlurryPumpingHandbook-pdf
4. “The Effects of Slurries on Centrifugal Pump Performance” - An article by Mr. George Wilson can be downloaded at http://turbolab.tamu.edu/proc/pumpproc/P4/P419-25.pdf
5. ISO-TR-17766:2005 - Specification for pumps handling viscous liquids
6. IS:5120 Technical Requirements of pumps for rotodynamic special purpose pumps
7. IS:5600 Specifications for pumps for sewage and drainage
8. IS:6596 Specification for pumps for handling paper stock
9. IS:9201 Pumps for handling slurry
10. IS:2951 (Part 1) - Recommendation for Estimation of Flow of Liquids in Closed Conduits, Part I: Head Loss in Straight Pipes Due to Frictional Resistance
11. IS:2951 (Part 2) - Recommendation for estimation of flow of liquids in closed conduits, Part 2: Head loss in valves and fittings.
Designer, Trainer, Consultant for Pumps, Valves, Pumping Systems.
Considered as the ‘Pump Man of India’, he is a leading consultant with over 50 years of experience in the pumps field. He was former Technical Advisor to the Indian Pump Manufacturers’ Association.
Fig. 4 Exploded view of pump with rubber-lining
Fig. 3 Chopper before pump
Fig. 2 S-Type Semi-open impeller
Fig. 1 Open, semi-open and closed impellers
Fig. 9 Viscosity Correction Chart (Ref. IS-5120-1977)
Fig. 5 In abrasion, the load is perpendicular to the surface and movement is parallel to the surface i.e. with zero angle of incidence Fig. 6 In erosion, particles impinge onto the surface with a distinct angle of incidence
Fig. 7 Pumps coated to have smooth surfaces would help to make solids to ‘glide’
Fig. 8 Pump with heating jacket
Fig. 11 Correction for settling solids
Fig. 10 Jet Lift Dredging
Fig. 12 Moody Diagram
Fig. 13 Dry pit sewage pump with vertical motor
Fig. 14 Installation with horizontal motor
Fig. 16 Wet Pit pump with horizontal submersible motor
Fig. 15 Wet Pit Pump with vertical submersible motor
Fig. 17 Wet Pit pump with side discharge, extension shaft and motor