Reliability Issues †Centrifugal Slurry Pumps Essay Example for Free

Reliability Issues Centrifugal Slurry Pumps EssayIntroductionPumps were probably the archetypical machine ever developed, and atomic calculate 18 now the second near general machine in mapping somewhat the world, out-numbered only by the electric move. The precise earliest type of pump is now known as a pee wheel, Persian wheel or noria, consisting of a wheel of buckets that rotates to pick up wet from a stream and dump it into a trough. other early pump was the Archimedean screw, similar to the modern screw conveyor except that the flights were much fixed to the tube so that the whole administration would whirl together. Both of these devices atomic number 18 still utilize, most commonly in grassroots agricultural applications. Pumps be now produced in an enormous range of types and sizings, for a in truth wide scope of applications, and this makes it difficult for all individual reference document or organisation to cover pumps and pumping as a general top ic. So the coarse field of pumping is classified into sub-divisions and then dealt with at that take.In the mining industry, the upper end of the pump scale includes impellers with diameters over 2.5m, slurry lines 10km long, particle coat up to 100mm, move regularises handling more than 7000tph, and motors over 10MW. Finer slurries of around 1mm particle size of it are pumped for hundreds of kilometres in just about operations. There are more ways to classify pumps. This just one of them.This document only addresses centrifugal pumps, with a focus on hotshot-stage radial-flow slurry pumps. Centrifugal pumps are capable of meeting duties of up to 1.4 m /s at 30MPa, and grittyer volumes at light 3 pressures. The maximum flow rate at low discharge pressure is around 180 m /s. Industrial applications requi bound noble delivery pressures for the most part use reciprocating fixed-displacement pumps, but they are limited in the amount of flow they brush aside gear up out p er unit. In general purpose applications, where different types of pumps could all deliver the doing sought, centrifugal pumps are usually the preferred choice repayable to dis whitethorn lifecycle costs.Basic Requirements for ReliabilityAssuming class pump manufacture and break ination, the basic requirements for real long-term operation of centrifugal pumps are 1. Continuous operation at best-efficiency point (BEP) 2. Adequate net positive suction well (NPSH) 3. Low velocity changeful flow at bottom the pump and doneout the system 4. process of legatos that are benign ie a) Chemically and physically stable b) At near-ambient temperatures c) Free of particles likely to political campaign put out or blockage Pumps of a basic end satisfying all these requirements hold up run for 50 years and more without study component replacement. The first three requirements are satisfied by matching pump murder to expected duty. Where stage 4 fag end non be addressed done pre -treatment of the changeful, the pump configuration, geometry and materials must be optimised to give best results. Obviously, item 4.c) is a dominating issue for slurry pumps as it can non be eliminated and must be managed.Centrifugal Pump ConstructionCentrifugal pumps bewilder two main sub-assemblies the rotating separate (impeller, scape, bea go), and the fixed separate ( shell, shrill connections, stand, foundations. Pumps of all types may be single stage or multi-stage. Multiple stages are use where it is not practical to induce the necessary discharge pressure using a single impeller. The simplest way to imagine a multi-stage pump is as one pump with its discharge feeding straight person into the suction of a second pump so that the overall discharge pressure is increased spell the flow rate stays the same. However, this system of rules is properly described as single stage pumps in series. A true multi-stage pump consists of multiple impellers attach on a single pricking, positioned in a single casing made up of multiple put ups. Multi-stage pumps of this type are not used with slurries, but sometimes slurry pumps are mount in series.CasingThere are two types of casing formulates volute and diffuser. A volute casing has a snails jaw shape, while a diffuser casing has interior vanes. Diffuser casings are rarely used on single-stage radial pumps, and are not commonly used for handling slurries due to the flow restriction and high drudge rates that would result. Slurry pumps have volute casings which house the impeller and have a spiral-shaped outer volume that extends 360 degrees and increases in cross-sectional area as it approaches the discharge flange. At full circle the volute overlaps itself, creating the cut-point, also known as cut-water point or tongue. The model shape is to have a steady linear increase in cross-sectional area for 360 degrees around the circumference starting from the cut-water point, but this can be difficu lt to manufacture. pard to a clear water pump, a slurry pump has a much larger radial gap amidst cut-water point and impeller, to digest insecurity of blockage. Where a pump is identified as oversize for its duty, and is suffering high recirculation strike, it may be possible to fit liners with an extended cut-water point that throttles the flow. In theory, when a pump operates at its best efficiency point (BEP), the pressure acting on the impeller and casing are uniform.However, in practice the pressure is rarely altogether uniform, and if a pump is direct away from its BEP the imbalanced in the radial forces acting on the impeller become significant. These forces are larger for bigger pumps operating at higher pressures. Running a large pump on a lower floor rated capacity can create unbalanced radial forces that may (over time) damage the bea sound or chap the shaft. If it is known that a pump may need to occasionally operate well away from its BEP, the manufacturer shoul d include an oversize shaft arrangement in the design, but with commercial competition driving purchase decisions this may have to be specializedally requested. Another option for reducing imbalanced radial forces is to use a twin-volute design, which consists of a wall splitting the volute in half for about half its circumference, ending after the cut-point but beforehand the discharge flange. This is not practical for most slurry applications.Casings must be designed to allow the impeller to be installed inside, and so are manufactured in at least two parts. strong casings have a removable cover, either on the suction side or shaft side or two, but the volute shape is a one-piece casting. Casings may also be split, either axilely or radially. Axially split housings make inspection easier because the upper piece can usually be removed without disturbing the shaft or piping too much. Split casings may tend to breathe at high pressures, resulting in leakage, ancestry entrainme nt, vibration, misalignment etc. Casings are normally sufferd with ribbing at the location of highest stresses, to minimise this. Open or semi-open impellers require close clearances against the casing to ensure pumping efficiency. The casings generally include a side-plate that can be adjusted for minimal clearance using jacking screws or shims, especially in saping applications eg slurries.ImpellerImpellers are classified consort to their design features ie Suction flow orientation o Single suction ie inlet on one side only o Double suction ie inlet on both sides The direction of exit flow relative to the shaft axis ie o radial tire flow o Axial flow o Mixed flow Vane shape ie o Single curvature vanes, also called straight vanes the impeller lifts that accelerate the smooth are straight and parallel to the axis of rotation o Francis or screw vane the surfaces that accelerate the changeable are trend in relation to the axis of rotation Mechanical construction o Enclos ed ie with side walls or shrouds o Open ie no shrouds o Semi-open ie shroud on one side only o Partially shrouded ie shroud not extending to impeller tips The open area through which the fluid flows into the impeller is called the suction nub. For a closed-shroud impeller, this is simply the hole in the shroud. The suction eye area is an important feature of the pump design. The area taken up by the shaft, if it protrudes through the eye, is deducted when calculating eye area. Impellers can be single suction or double suction.A single suction impeller has an inlet eye on one side only, with the shaft extending out the opposite side so the impeller overhangs. A double suction impeller can be thought of as two mirror-image single suction impellers mounted back-to-back. They accept fluid from both sides and usually have a shaft that extends straight through the impeller with sorts providing support on both sides. Double suction impellers are usually fed fluid from a single inlet flan ge, with the fluid flow being split into two streams by channelling inside the casing. Double suction units turn in advantages in minifyd fluid velocity at the impeller eye, and better balancing of axial hydraulic forces, while single suction units are simpler in design, manufacture and maintenance. Most if not all slurry pumps are single suction type. few pumps may have an inducer, which is an axial flow impeller with a few blades installed between the suction inlet and the main impeller, intended to improve the suction head seen by the main impeller. Impeller shrouds often consist thin pump-out vanes cast into the outside of the shrouds. Their purpose is to help clear any solids from the back hub of the impeller (opposite the inlet eye), reduce pressure at the lettre de cachet area, reduce axial lick, and discourage recirculation. Some impellers have similar vanes on the eye side as well as the shaft side in this case, those on the shaft side are usually called expeller vane s. In clear water pumps, a cylindrical ring is usually cast or machined into the outside surface of the shrouds, concurrent with a matching feature in the casing, to help seal off the discharge fluid from the suction fluid and prevent internal circulation.Clearances here are tight in put in to ensure pumping efficiency typically around 0.25mm on wheel spoke for most common sizes of industrial pumps. In larger pumps the casing (and sometimes also the impeller) is usually protected at this point by replaceable wear rings, which may be high-wear items, and need to be re lay before efficiencies fall too low. It is good practice to replace wear rings once the clearance r all(prenominal)es twice the original specification. Wear rings are provided in a wide range of designs and materials accord to the pressures, hurrys and fluids involved. The wear rings on impeller and casing are often made from differing materials that are not subject to galling, to reduce problems should progress to occur. Wear ring features may include labyrinths, water injection, inspection ports, adjustment mechanisms etc.Pumps handlinglight slurries may make use of wear rings, sometimes with water injection to reduce wear from the slurry. Pumps handling heavier slurries usually just use pump-out vanes. Slurry pump impellers must be designed to resist wear and tear, and this requires some pumping efficiency features to be sacrificed. For example, vane edges will be blunter, vanes and shrouds will be generally thicker, and the number of vanes will be decreased in order to open up the channels between them. Passages through slurry pumps, including impeller vane spacing, are larger than for clear water pumps. Open impellers are sometimes used for very stringy materials, but tend to be weak and wear quickly, and so are not very common. Vane shape is obviously a major element of impeller design. Two critical factors are the blade entry angle (1) and blade exit angle (2), as measured between t he centre-line of the vane and a tangent to the privileged or outer diameter (respectively) drawn from their tips, in the opposite direction to rotation. Most modern pumps have impellers with 2 smaller than 90 degrees ie backward-curved blades.Theoretically, a forward-curved blade would give higher head, but at less efficiency. Some pumps have 2 at 90 degrees, and these are sometimes referred to as expellers. Many clear-water impeller designs rely on close running clearances between vane tips and casing to minimise recirculation from one vane chamber to the next, and maximise efficiency. even small amounts of vane tip wear can have an effect on head and overall efficiency. The outer and inner vane tips should be sharp, not rounded or chamfered.Replacing a pump which is too large for its duty can be a major exercise. It usually requires changes to the foundations, drive arrangement and piping, spares holdings, and so on. A model of the ideal size may be just not acquirable. As a n alternative, in some cases it may viable to install a reduced-diameter impeller without changing other components. If done correctly, trimming the impeller will move the pumps BEP to match the actual system operating point. The efficiency at the new BEP will be lower than the BEP with the original impeller, but higher than was being achieved in practice when operating well away from the original BEP. The performance variation can be estimated using the affinity laws which often apply to a specific impeller before and after machining bleed rate Pump head Motor power Q1 / Q2 = n1 D1 / n2 D2 H1 / H2 = (n1 D1 / n2 D2) P1 / P2 = (n1 D1 / n2 D2) 2So if running at the same speed, trimming an impeller by a certain proportion will result in a corresponding drop in flow rate, a greater decrease in head produced, and an even greater decrease in the motor power consumed. However, these equations are based on several assumptions and some caution is called for. Impellers are complex three-dime nsional objects and their effects on the liquid are due to other factors that are also affected by machining, beyond just the outside diameter eg open area, discharge blade angle and so on. The sideline settings should apply. Diameter reductions should not exceed 10%. Reductions beyond 20% are generally considered extreme. Some references state 30% as the maximum reduction advisable. Some overlap in the vanes should be retained. The angle between the vane centreline and the tangent to the outer diameter drawn at its tip should be restored to original by filing, with most filing occurring on the trailing side of the vane.The vanes will probably be thicker after cutting, and should be filed back to original shape, by filing on the traling side of the vane. Vane tips should be kept sharp, not rounded or chamfered. Outer tips should be sharpened by filing on the trailing side, and inner tips by filing mostly on the leading side. Inefficiencies will take the form of increased disc friction, increased flow path length within the casing, and more recirculation across vane tips. Impellers apply forces to the fluid and are subject to the equal and opposite forces themselves.The typical single-suction impeller engages with fluid entering the pump and at first accelerates it axially into the pump, before diverting it into the radial direction. The impeller pushes the fluid into the pump, and at the same time pushes itself axially back toward the inlet point. Another way of looking at this effect is to consider that the impeller is mostly exposed to pressurised fluid all over the shroud surfaces, but not at the eye on the suction side. The wedge on the impeller must be resisted by the shaft arrangement, which must always include bearings capable of serious thrust loading. Double-suction pumps typically have less axial loading, but can still experience axial thrust, especially if flow is restricted more on one side due to internal differences in the pump, or restri ctions in fluid supply on one side. Clean water pump designs may incorporate features to reduce this imbalance, such as having wear rings on both sides of the impeller, with the pressure within the volume they enclose largely equalised by balancing holes passing right through the impeller.Another manner is the use of a balancing disc. This is a disc mounted on the shaft in a separate chamber, with a geometry and clearances designed to counterbalance thrust effects. However, these are not practical for slurry pumps, which may use pump-out vanes instead, to lower the pressure toward the inner area of the non-suction shroud. Axial thrust make full usually consist of a steady state component plus dynamic fluctuations. Heavy axial loading is often associated with recirculation.Where failure occurs it is usually a result of overloading and over-heating of bearing components. Measures to correct excessive axial loading include Restoring BEP operating conditions (which may include selecti ng a more appropriate pump size or trimming the impeller) Ensuring internal clearances / wear are not excessive drifting correct bearing type and installation including clearances / pre-load To further complicate this issue of axial thrust, single-suction pumps handling fluids with a high suction head may experience thrust on the impeller in the opposite direction, away from the inlet. And then there are pumps with highly multivariate duties and suction conditions that may experience impeller thrust in different directions at different times.ShaftThe shaft transmits windup(prenominal) power to the impeller from the driving motor or engine. It must also support the impeller and restrict its axial and radial movement. The loads on the shaft include self-weight of the rotating components, torque, and forces transmitted to / from the fluid. Design of a shaft requires consideration of maximum allowable divergence, the span or overhang, the location and direction of all loads, any te mperature variations, and the critical speed. Loads are normally at their maximum on start-up. All objects have a natural frequency at which they will vibrate after being struck. Machines made of several components with complex shapes normally have several natural frequencies, some of which dominate. In the case of pumps, if the rotational speed of the impeller matches a dominant natural frequency, small imbalances may be amplified to a level where they interfere with operation and/or reliability.These are known as critical speeds. Steady operating speeds between 75% and 120% of the first critical speed should be avoided. Pumps with longer overhang on the shafts have lower critical speeds. Shafts are referred to as rigid or flexible, according to whether the running speed is lower or higher than the first critical speed. Pumps with a flexible shaft must pass through a critical speed on each start-up. This is not usually a problem because frictional forces with the fluid and the bear ings act as dampers for a period sufficient for transition through the critical speed. Pumps with speeds below 1750rpm, which includes most slurry pumps, are usually of the rigid-shaft design. The shaft must be designed so that any deflection will not bring moving parts into contact, for example at wearing rings, or cause non-concentricity in critical areas such as the shaft seal. As a general rule, shaft deflection should not exceed 0.15mm even under the most extreme conditions.Deflection and critical speed are related stiffening a shaft to reduce deflection will also raise its critical speed. For pumps with overhung impellers, as is the case for most slurry pumps, this often results in the shaft diameter between bearings being preferably large. The fluid passing through a pump creates a hydrodynamic bearing effect, known as the Lomakin Effect. That is, to some extent, the impeller rotating in the casing with fluid present is like a shaft rotating in a journal bearing with oil pr esent. The result is that the shaft is better supported when running than when idle, so that the shaft deflection will be less, and the critical speed of the shaft assembly will be higher. However, the Lomakin Effect varies with pump head and internal clearances, both of which diminish with wear. Therefore the effective critical speed may be expected to decrease with time in operate. To allow assembly, shafts step up in diameter from coupling to bearing to impeller, so that any torque problems are very likely to appear first at the coupling kinda than the impeller, at least in single stage pumps.Shaft Seal and armThe shaft connects the drive to the impeller, and so must pass through the pressurised casing. Achieving a reliable seal between shaft and casing is one of the most problematic areas in pumping. Centrifugal pumps have two types of seals mechanic seals and back backpacking seals. Many designs of automatic seals have been attempted for slurry pumps, without comprehensi ve success, and the remainder of this discussion concentrates mainly on packing seals and stuffing nookes. Note, however, that packing is only suitable within pressure and temperature limitations. Depending on pump design and duty, the seal may need to prevent either air ingress into the casing, or fluid egress out of the casing or both of these at different times, if operation is variable. Many casings are designed with the seal area built into a compartment configured to improve sealing performance. For mechanical seals, this compartment is usually referred to as the seal chamber, while for packing seals, it is known as the stuffing disaster.Slurry pump seals usually consist of several rings of packing fitted in a stuffing box around the shaft, often with provision for grease lubrication or water injection to reduce friction and provide additional sealing (particularly for when the pump is stopped). There are many stuffing box design variations and many types and configurations of packing. Stuffing boxes will accept a number of rings of packing, with a packing ring or throat bush preventing extrusion into the casing, and a secreter (sometimes called a follower) used to adjust packing compression. A lantern ring may be substituted for one of the packing rings, to cater for injection of grease or sealing water, water being particularly unavoidable if air would otherwise be sucked into the fluid stream at this point.Sealing water (or an alternative strip liquid) is usually mandatory for Slurries Liquids for which leakage is not delicious Liquids that are not suitable for sealing purposes Suction lifts greater than 4.5m (air ingress may interfere with priming) Discharge pressures above 70kPa The packing must be placed under some compression and this tends to result in wear on the shaft, which is often branchd to avoid having to replace the entire shaft once wear is advanced.There are numerous designs of shaft sleeves. The shaft sleeve must be resistant t o friction and heat, and several different materials and surface treatments are available eg seriouslyened high-chrome stainless steel, ceramic, plasma spray or tungsten carbide coating etc. To prevent chipping, coatings should not extend to the edges of the sleeve. The sleeve does not contribute to specialisation, so the shaft itself must be large enough to carry all the loads, and this means that including a sleeve in the design enlarges the seal diameter. For small pumps, this may decrease pumping efficiency and raise the purchase cost to the point that sleeves may be abandoned and a stainless steel shaft used instead. Glands may be solid, or split to allow replacement without disassembly of pump or bearing assembly. They are usually made of tan, cast iron or steel. Special designs are used to improve safety if the fluid is angry.The leakage of fluid past the packing is controlled by tightening the gland, compressing the packing axially and expands it radially so that leakag e paths along the shaft sleeve are constrained. However, some fluid flow between packing and sleeve is usually necessary to avoid overheating the packing and damaging the sleeve surface. Once the sleeve surface is damaged, the sealing efficiency decreases and more tightening is ask, further damaging the sleeve, and so on. The secret is to provide a configuration of packing and seal water injection that suits the application, and then avoid over-adjustment.To further reduce the pressure at the shaft seal area, where the rear pump-out vanes are not sufficient, some slurry pumps are fitted with a second smaller open-faced impeller, usually called an expeller. Many different designs have been tried. If sealing water is used, there will be a design intention regarding the ratio of water to pass in to the volute compared to out past the gland follower. This can be controlled using the number of packing rings on each side of the lantern ring, but the lantern ring must be installed at th e injection point. For low-cal water pumps, this seal water is sometimes provided from the pump discharge.Clean water must be used to avoid contaminating the packing with grit filtration or cycloning may be necessary if the water contains some grit. When managing sealing arrangements, thought must be given to what happens when the pump is stopped. The pressure in the stuffing box changes to static conditions, which may result in slurry leaking into the packing and contaminating it, causing rapid sleeve wear on re-starting. But if sealing water continues to be applied, the slurry may be diluted, and eventually a sump can be filled with sealing water if left idle for a long time. For lengthened stoppages, sumps may be best dropped, for various reasons. On restarting, sealing water supply should start before the pump starts.Stuffing boxes in extreme applications may be provided with galleries through which cooling water can pass to prevent excessive temperatures around the packing. I n applications where leakage must be more hardly controlled, or where elevated temperatures in the seal area must be avoided (for example where the fluid is volatile), mechanical seals may be suitable, provided that the fluid is not damaging to the seal components. A comparison between mechanical seals and packing seals is given below. Packing seals o Low initial cost o Tend to dribble gradually o Easily replaced when necessary o Can handle large axial shaft movements o Always some leakage required o Require regular adjustment o Not suitable for hazardous / volatile fluids o Often cause progressive shaft sleeve wear o Can result in significant shaft power losses o Limited to low pressures and speeds Mechanical seals o marginal or zero leakage o No adjustments required o Suitable for hazardous / volatile fluids o No shaft wear o Do not consume significant shaft power o Can handle high pressures and speedso Tend to fail choppyly o Replacement requires pump disassembly o High ini tial cost Packing seals work as a result of axial compression, so that the packing rings extrude outward and apply radial pressure to the adjacent components, these being the static surface of the stuffing box, and the rotating shaft sleeve. A dynamic seal is formed between the packing rings and the sleeve surface, with some fluid flow between the two being necessary for lubrication and cooling. For clean water pumps, this fluid may be supplied from the inner end of the stuffing box, or from the discharge pipe via small diameter piping. In the case of slurries, grit in the fluid would add to friction and wear, so the lubricating and cooling fluid is usually injected from a separate clean water supply.The injection pressure should be 10 to 25psi greater than that at the inside end of the stuffing box, and this figure should be available from the pump designer. A rule of thumb is to set the gland feed water pressure to between 35 and 70kPa above pump discharge pressure. Pressure regul ation is often helpful. In theory, some slurry pumps should operate with a pressure at the inside of the stuffing box which is below atmospheric pressure, so that the packing is required only to prevent air ingress into the pump. However, when the pump is turned off, or in abnormal operating conditions, slurry can pass back into the seal and contaminate the packing with grit, so these situations still call for water injection. Grease or oil may be used instead of water in some applications.Packing material must be able to withstand the operating environment and remain resilient to perform satisfactorily despite minor shaft misalignment, run-out, wear and thermal expansion / contraction. Packing is available in a huge range of materials (lubricant, binder and fibre / matrix) and in many sizes, shapes, and constructions, to suit different applications particularly size, shaft speed, temperature, pressure, and chemical resistance. The number of packing rings varies between application s, the most common arrangement being throat bush or ring, three inner packing rings, lantern ring, two more packing rings, and gland follower. The lantern ring may be placed further in, to reduce slurry ingress. Packing size is usually proportional to shaft / sleeve outer diameter, as follows Shaft / Sleeve OD (mm) 15 to 30 30 to 50 50 to 75 75 to 120 120 to 305 Packing Size (mm) 6 8 10 12.5 16Shaft sleeve finish needs to be at least 0.4micron CLA to avoid excessive rotational friction, and the finish in the stuffing box bore needs to be at least 1.65 micron CLA to allow even compression during adjustment. The sleeve must be harder than the packing, and chemically resistant to the fluid pumped and the injection fluid. Any coating on the sleeve must have a good thermal shock resistance. The lantern ring allows for entry and scattering of the lubricant or flushing fluid. Lantern rings are usually split to allow installation and removal without pump disassembly. They were traditionall y made from metal such as stainless steel, but lubricant-impregnated plastics are now common. Gland followers are also usually split to allow easy replacement. They are usually bronze but may be steel or cast iron. Special purpose gland followers are used with volatile or hazardous materials, including capacity for diluting and safely flushing away leakage. The axial compression on the packing must be occasionally adjusted to control leakage. The correct leakage rate is one drip per second.Over-tightening should be avoided as it will result in over-heating and shaft wear. Most packing is supplied with impregnated lubricant, and over-tightening will press the lubricant out. Pumps need wasted sealing purvey if pressure at the inner end of the stuffing box is greater than 75psi. The use of harder packing material on the inner rings may help. The procedure for replacing packing is 1. Read the instructions provided by the pump manufacturer and packing supplier. 2. Loosen and remove gla nd follower. Inspect gland follower for wear, corrosion, warping etc. 3. Remove old packing rings using a packing puller, and the lantern ring. 4. Inspect shaft sleeve surface for deterioration, and clean up where possible. Replace if necessary. 5. Inspect bore of stuffing box for corrosion, wear, scaling etc, and clean up where possible. 6. Verify correct packing size to be used. 7. tightly wrap the correct number of packing coils around a mandrel of equal diameter to the shaft sleeve. 8. Cut each ring at an oblique case angle. 9. Install each ring, staggering the joins 90 degrees on subsequent rings.Suction / Intake DesignCentrifugal pumps operate most efficiently when the liquid to be pumped flows into the inlet nozzle in a smooth, uniform manner with minimal turbulence. Suction systems need to be designed to ensure that this happens. The most common problems are Uneven / turbulent flow Vapour collection Vortex formation Suction piping should be as short and straight as possi ble to minimise friction, and if unavoidably long, should be of large diameter. The suction line will normally be at least one pipe size larger than the pump inlet flange, requiring fitment of a reducer. A reducer should not change the pipe bore by more than 100mm. Fluid flow should be as uniform as possible right up to the pump inlet flange. There should not be any fittings likely to cause turbulence, sudden changes in flow direction or spin within ten pipe diameters of the pump inlet flange. There should be no short radius elbows at all, and no long radius elbows within three pipe diameters.All suction line connections need thorough sealing to prevent air being drawn in. For suction manifolds serving multiple pumps, all the above points apply, and branches should be angled at 30 or 45 degrees, rather than ninety degrees, and sized so that fluid flow is constant throughout. Flow should not exceed 0.9m/s. Improper suction conditions or designs can result in the fluid swirling as it approaches the pump through the suction pipe. This is called pre-rotation. It causes a drop in pumping efficiency because the pump is designed to process fluid that is entering without rotation, and can cause additional suction pipe wear.Sometimes a radial fin is fitted to the suction pipe or casing to reduce pre-rotation. The suction pipe design should cater for elimination of air from the suction line, and prevention of vapour pockets, in the simplest manner, meaning that For pumps with the feed being drawn from a level below (eg a dam pump), o Suction pipe should have a slightly upward slope toward the pump o The eccentric reducer should have the bland side on top For pumps with the feed being drawn from a level above (eg a thickener underflow pump), o Suction pipe should have a slightly downward slope toward the pump Vortexing in feed tanks needs to be avoided to prevent air being drawn down into the pump. Baffles may need to be fitted to tank walls. The tank fluid level need s to be kept well above the suction inlet.BearingsBearings provide axial and squint-eyed restraint to the pump shaft and attached components, while allowing free rotation. Axial loading on pump shafts may be significant as discussed separately, and the bearing arrangement always includes some thrust capability. The bearings most commonly used are deep-groove single course of study ball bearings, and single or double row angular contact ball bearings. Pumps may be in overhung configuration, where the shaft is supported by bearings on one side only, or have a shaft that passes right throught the casing with bearings on both sides. Most slurry pumps are of the overhung design.The bearings are usually rolling-element, but plain journal bearings are sometimes used on larger pump sizes. The bearings must be lubricated by grease injection or oil bath and may need provisions for cooling as well. This may be by having a cooling water jacket integral with the bearing housing, or by pumping the lubricating oil through a heat exchanger and filter. Oil lubrication is usually recommended rather than grease, if speed exceeds 5000rpm (which is very rare in a slurry pump). Grease-packed bearings should have one third of the chamber filled with grease. Oil baths should be filled to the centre point of the lowest rolling element. Inadequate loading of bearings can result in the rolling elements glide over the race instead of rolling, and this can cause heating and failure. To avoid this, bearing assemblies are usually designed with an assembly configuration, including preload, that ensures all bearings carry some load.Frame and FoundationsFor large pumps that are directly connected (ie no vee-belt drive), the motor and pump are usually mounted on the same bed-plate, which is fixed to the foundations in a way sufficient for eliminating looseness and distortion. This eliminates some misalignment issues at the source. Foundations including bed-plates should be checked occasional ly for deterioration (corrosion, ground subsidence, concrete cracking, loose fasteners, missing secure etc), and the alignment between pump and motor should also be checked if there is any cause for concern.The framework should have provisions for drainage of any spillage and seal leakage etc, so that this does not become trapped and contribute to corrosion etc. Where pumps operate at high temperature (ie above around 100C) the pump casing should be supported at its axial centre-line, to help reduce thermal stresses. It is generally preferred that all suction and discharge piping have its own supports, so that the pump casing and foundations do not carry any significant static or dynamic piping loads, and so that pump components can be independently removed and replaced. Where this is not the case, extra pump and foundation attention may be needed at the design stage.Drive ArrangementMany drive arrangements are possible to suit the circumstances. Electric motor drive is the most po pular, followed by internal combustion engines. unsettled speed drives are sometimes necessary and often convenient, but always more expensive and less reliable. In minerals handling plants, slurry pumps are most often electric motor driven, with belt drives. Belt drives allow speeds to be changed through minor modifications ie pulley changes. Short, low head slurry system designs usually provide motors that are 10 to 20% oversized, to cater for any under-estimates in slurry or system characteristics such as viscosity and friction, and to allow for minor system modifications during the service life.InstrumentationPumps may be controlled to allow Variation of flow rate, pressure, liquid level Protection against damaging operating conditions Flexibility in matching pumping performance to duty For centrifugal pumps, control is usually accomplished by speed setting (including turning off/on), or valve setting. This may be manual or automatic. For slurries, control by throttling val ve is rare due to the wear rates that usually result. Typical instrumentation includes Tank / sump level switches Pressure sensors Flow sensors Density sensors In each case, protection from damage by the slurry is critical. This is commonly achieved by using sensors that do not need to contact the slurry eg nucleonic density sensors mounted outside the pipe, with source on one side and detector on the other. Ideally, it is good to have instrumentation available, either permanently mounted or portable, toVerify operation at BEP, by measuring the difference between suction and discharge pressureDetermine flowEnsure that NPSH is sufficient to prevent cavitationCompare flow to motor amperage, to identify when impeller adjustment is neededNeed to search more on valves for slurry applications.Notes on Material SelectionWhere there is some line up of parts coming into contact during pump operation, thought should be given towards minimising the damage that may result. An example of th is is at the wear-ring / impeller interface. Studies have shown that damage can be minimised by manufacturing adjacent components from materials that Are dissimilar, except where known to be resistant to adhesive wear and galling Have a difference in austereness of at least 10Rc, if either has hardness less than 45Rc Because it may be difficult to always prevent cavitation from occurring, impellers are usually made of cavitationresistant materials such as chrome-manganese austenitic stainless steel, carburised 12% chrome stainless steel, cast nickel-aluminium bronze, etc. Obviously corrosion resistance is another key choice factor that these materials satisfy. Slurry pumps are subject to heavy wear in the form of abrasion and erosion. The aggressiveness of the slurry is determined by the hardness of the particles in the slurry, their shape (rounded or sharp), the pulp density, and the size distribution.Slurries can become less aggressive as they travel through a minerals processin g plant as the sharp edges become rounded off. hurrying and angle of impingment are also very important factors affecting the resultant wear rates, with wear rate being proportional to velocity squared according to some references. The impingement angle associated with maximum wear rate seems to be dependent on the hardness and brittleness of the material being struck. For very hard / brittle materials it is between 65 and 90 degrees, while for more ductile materials it may be around 25 degrees. Pumps handling slurries with greater than 6mm particle size are usually lined with rubber. However, if impeller tip speed exceeds 28m/s, rubber becomes subject to thermal degradation, and this usually restricts the use of rubber to a maximum head of 30m per stage. metallic element lined pumps may be used up to 55m head per stage.For wet end components, materials that may be used to resist wear include Ni-resist, carburised and pugnacious 12% chromium steel, etc. White iron slurry pump comp onents, which includes Ni-Hard, are restricted to impeller tip speeds of about 36m/s to avoid maximum disc stresses. make components are softer but can run at higher speeds, up to a tip speed of 45m/s. Centrifugal pumps are subject to cyclic loads due to such things as imbalance, unbalanced radial forces, fluctuating axial thrust, the vibration induced as each vane passes the cut-point, and variations in upstream and downstream fluid pressure and flow. This sets the scene for fatigue loading, which becomes more of an issue if the slurry is corrosive. Fretting may occur between assembled components where looseness is allowed to develop.This is best avoided through the use of correct manufacturing dimensions and surface finishes, good fitting practice etc. The materials commonly used for pump components include Impellers (require castability, weldability, and resistance to corrosion, abrasion, and cavitation) o Bronze, for non-corrosive liquids below 120C o Nickel-aluminium bronze, f or higher speed and mildly corrosive applications o Cast iron, for small low-cost applications o Martensitic stainless steel, where added resistance to cavitation, wear, corrosion (other than salt water) or high temperatures may be required o Austenitic stainless steel (mostly cast 316 grade), where a higher level of corrosion resistance is needed. Austenitic stainless steel with 6% minute is often used for salt water pumping.Casings (require strength, castability and machinability, weldability, and resistance to corrosion and wear) o Cast iron o Cast steel, where extra strength is required ie for pressures above 6000kPa (1000psi) and temperatures above 175C. o Austenitic cast irons with 15 to 20% nickel (Ni-Resist) may be used where abrasion and corrosion are issues. o Bronze, for water applications o Stainless steel, where corrosion is a major issue martensitic for higher pressures in mildly corrosive fluids, austenitic for more aggressively corrosive fluids. Shafts (require resistance to fatigue and corrosion) o Mild steel, where corrosion and fatigue are minor issuesLow alloy steel such as 4140 for added strength Martensitic stainless steel, where added strength and corrosion resistance are needed Shafts are usually chrome-plated, and care is needed to avoid this adding to the fatigue susceptibility through microcracking and hydrogen embrittlement. Shafts can be shot-peened prior to plating, and heat-treated afterward to reduce these effects. Wear rings (require castability and machinability, and resistance to corrosion, abrasion and galling) o Bronze for clean liquids and temperatures up to 120C o Stainless steel for applications with abrasion, corrosion and high temperatures but steps must be taken to avoid galling should the rings come into contact eg increased clearances, hardness differences etc.o o oImpellers other than those made from martensitic stainless steel can usually be repaired by welding, although in some cases this needs to be follow ed by specific heat treatment processes. In all cases, more exotic (and expensive) materials may be used for specific applications. Material selection is often a balancing act between optimising purchase cost and maintenance / operations performance. Where high temperatures are involved, material selection must take into account differences in expansion rates. Unlined slurry pump impellers and casings are often made from abrasion-resistant cast irons as per ASTM A532, which includes Ni-Hard. These materials consist of a martensitic matrix with secondary hard phases of chrome and iron carbides that increase wear resistance.They cannot be machined or welded, and tend to be prone to corrosion, and breakage through mechanical impact and thermal shock. Brittleness may be reduced by annealing, but this reduces wear resistance. Slurry pump impellers and casings may be lined with softer materials like rubber, where high temperatures can be avoided. These can reduce wear rates by absorbing t he impact energy of the particles, while resisting corrosion. Problems may arise in attach of the rubber at the cut water point, and on the impeller. The lining reduces the thickness of the metal section of the component, so stronger materials are usually used eg steel rather than cast iron.Manufacturers develop their own specifications for ideal liner thicknesses based on experience, but one reference suggests a volute liner thickness of 4% to 6% of impeller diameter. Natural rubbers seem well suited for wear liners for use with slurries with less than 6mm particle size for the impeller, and 15mm particle size for the volute. Provided the base materials are suitable, patches of high wear on wet end parts can sometimes be repaired by welding / hard-facing. However, this increases the likelihood of cracking. Also if the welding results in unsteady surfaces in critical points, the added turbulence can accelerate further wear. Many types and styles of surface coating have been tried, with some success. These include thermal spray coatings, airing surface treatments, spraying and trowelling of epoxies, etc.