Presentation on theme: "Centrifugal Pump Course Dec-01 to Dec"— Presentation transcript:
1Centrifugal Pump Course Dec-01 to Dec Centrifugal Pump Course Dec-01 to Dec.03, 2003, Jubail Intercontinental Hotel, Al-Jubail, K.S.A under the auspices of The Arabian Petrochemical Company, PETROKEMYA (SABIC Affiliate)Prepared & Presented By:Syed Ali Hussain,Maintenance Reliability Group,
2Contents Why we require pump? General Types of pumps Dynamic, Positive displacement types.What is Centrifugal?Working principle of Centrifugal pumpGeneral Components of centrifugal pumpStationary components Casings, seals, nozzles. Bearing housings etc.Rotating componentsImpellerSpecific Speed and Impeller profileShaft and shaft sleeveCoupling
3Why we require pump?Since ages man think of transferring the liquid from the lower surface level to Higher surface level. The need to transfer water from an area of lower level to area of Higher level give birth to develop some mechanical device to fulfill the requirement. Pump was one of the outcome of the thought processes of the old scientists and technologists. Present day pumps are used to transfer liquid from a lower level to higher level by consuming some energy provided by the driver.
4General Types of Pumps Generally there are two types of pumps. Positive Displacement.KineticPositive Displacement:The fluid is forced to move because it is displaced with some piston, vane, screw or roller. Positive Displacement pumps act to force water into a system
5regardless of the hindrance which may oppose the transfer Dynamic:The pump moves with some speed and adds energy to the liquid thus increasing the pressure of the liquid and consequently the fluid is transferred.We are discussing centrifugal pump which lies under this heading. The centrifugal pump is sub class of Dynamic pump
6What is Centrifugal ?Centrifugal is a term pertaining to direction of the force. This is a force which moves or tends to move away from the center of a circle. Centrifugal force is the force of spinning.In the case of centrifugal pumps the liquid is entered at the eye of an impeller and is passes through the periphery of the impeller. This increases both the pressure and velocity of theliquid being pumped. When the liquid leaves the outer periphery of impeller, there is continuous increasing area at the volute which converts the
7the velocity of the fluid into pressure energy and thus at the discharge nozzle of the pump, we get high pressure.There are energy conversions in Centrifugal pump operation. The mechanical energy from the driver (Electric motor, turbine, engine) is converted into Velocity or Kinetic energy; the velocity is converted into pressure (potential
8Energy). The pump is also known as Velocity machine.
9General Components of Centrifugal Pumps A centrifugal pump has two main components:A rotating component comprised of an impeller and a shaftA stationary component comprised of a casing, casing cover, and bearings.The general components, both stationary and rotary, are depicted in Figure. The main components are discussed in brief below. The next figure shows these parts on a photograph of a pump in the field.
12Stationary Components Casing Casings are generally of two types: volute and circular. The impellers are fitted inside the casings.1. Volute casings build a higher head; circular casings are used for low head and high capacity.A volute is a curved funnel increasing in area to the discharge port as shown in Figure. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid.
14One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturer's recommended capacity. Running volute-style pumps at a lower capacity than the manufacturer recommends can put lateral stress on the shaft of the pump, increasing wear-and-tear on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial thrusts become significant at reduced capacities.2. Circular casing have stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally, the diffusers are applied to multi-stage pumps.
16The casings can be designed either as solid casings or split casings The casings can be designed either as solid casings or split casings. Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. A split casing implies two or more parts are fastened together. When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.Suction and Discharge NozzleThe suction and discharge nozzles are part of the casings itself. They commonly have the following configurations.
171. End suction/Top discharge - The suction nozzle is located at the end of, and concentric to, the shaft while the discharge nozzle is located at the top of the case perpendicular to the shaft. This pump is always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the impeller eye.2. Top suction Top discharge nozzle -The suction and discharge nozzles are located at the top of the case perpendicular to the shaft. This pump can either be an overhung type or between-bearing type but is always a radially split case pump.
193. Side suction / Side discharge nozzles - The suction and discharge nozzles are located at the sides of the case perpendicular to the shaft. This pump can have either an axially or radially split case type.Seal Chamber and Stuffing Box Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing.
20When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control. Figure below depicts an externally mounted seal chamber and its parts.
21Gland: The gland is a very important part of the seal chamber or the stuffing box. It gives the packing or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports as per the standard codes like API 682.Throat Bushing: The bottom or inside end of the chamber is provided with a stationary device called throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller.Throttle bushing refers to a device that forms a restrictive close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland.
22Internal circulating device refers to device located in the seal chamber to circulate seal chamber fluid through a cooler or barrier/buffer fluid reservoir. Usually it is referred to as a pumping ring.Mechanical Seal: For sealing of the pump.Bearing housingThe bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.
23Single-suction: Liquid inlet on one side. Rotating ComponentsImpellerThe impeller is the main rotating part that provides the centrifugal acceleration to the fluid. They are often classified in many ways.Based on major direction of flow in reference to the axis of rotationRadial flowAxial flowMixed flowBased on suction typeSingle-suction: Liquid inlet on one side.Double-suction: Liquid inlet to the impeller symmetrically from both sides.
24Based on mechanical construction (See Figure) Closed: Shrouds or sidewall enclosing the vanes.Open: No shrouds or wall to enclose the vanes.Semi-open or vortex type.
25“Specific Speed” is expressed as: NS= NQ1/2/H3/4Where N: rpm, Q: capacity (gpm), H: head (ft)Specific speed may be further defined as the revolutions per minute at which geometrically similar impellers would run if they were of such size as to discharge one gallon per minute against a one-foot head. The physical meaning of specific speed has no particular value, being a dimensionless number, largely used as “type” number. It is a constant for all similar pumps and does not change with the speed of the same pump. The specific speed determines the general shape or class of impeller.
26As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, D1, decreases. This ration becomes 1.0 for a true axial flow impeller.Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific speed develop head partly by centrifugal force and partly by axial force. A higher specific speed indicates a pump design with head generation more by axial forces and less by centrifugal forces. An axial flow or propeller pump generates its head exclusively through axial forces. Radial impellers are generally low-flow, high-head design, where axial flow impellers are high-flow, low-head designs.Increased speeds without proper suction conditions often cause serious problems from vibration, noise, and pitting.
28Closed impellers require wear rings and these wear rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent internal re-circulation. Vortex pump impellers are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. The number of impellers determines the number of stages of the pump. A single stage pump has one impeller only and is best for low head service. A two-stage pump has two impellers in series for medium head service. A multi-stage pump has three or more impellers in series for high head service.Wear rings: Wear ring provides an easily and economically renewable leakage joint between the impeller and the casing. clearance becomes too large the pump efficiency will be lowered causing heat and
29vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring clearance and replace the rings when this clearance doubles.ShaftThe basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts.Shaft Sleeve Pump shafts are usually protected from erosion, corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, corrosion,
30and erosion-resistant material shall be provided to protect the shaft and erosion-resistant material shall be provided to protect the shaft . The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate. (Leakage between the shaft and the sleeve should not be confused with leakage through the mechanical seal).
31Coupling: Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. Shaft couplings can be broadly classified into two groups: rigid and flexible. Rigid couplings are used in applications where there is absolutely no possibility or room for any misalignment. Flexible shaft couplings are more prone to selection, installation and maintenance errors. Flexible shaft couplings can be divided into two basic groups: elastomeric and non-elastomericElastomeric couplings use either rubber or polymer elements to achieve flexibility. These elements can either be in shear or in compression. Tire and rubber sleeve designs are elastomer in shear couplings; jaw and pin and bushing designs are elastomer in compression couplings.
32Non-elastomeric couplings use metallic elements to obtain flexibility Non-elastomeric couplings use metallic elements to obtain flexibility. These can be one of two types: lubricated or non-lubricated. Lubricated designs accommodate misalignment by the sliding action of their components, hence the need for lubrication. The non-lubricated designs accommodate misalignment through flexing. Gear, grid and chain couplings are examples of non-elastomeric, lubricated couplings. Disc and diaphragm couplings are non-elastomeric and non-lubricated.Auxiliary ComponentsAuxiliary components generally include the following piping systems for the following services:
33Seal flushing , cooling , quenching systems Seal drains and ventsBearing lubrication , cooling systemsSeal chamber or stuffing box cooling, heating systemsPump pedestal cooling systems Auxiliary piping systems include tubing, piping, isolating valves, control valves, relief valves, temperature gauges and thermocouples, pressure gauges, sight flow indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and all related vents and drains.All auxiliary components shall comply with the requirements as per standard codes like API 610 (refinery services), API 682 (shaft sealing systems) etc.
34Contents Pressure and Head Absolute pressure Specific Gravity Head and pressure conversion formulaSuction Head, Discharge Head, Total headVapor pressure and NPSHNPSH, NPSHA, NPSHRNPSH problems and cavitationNPSH Calculation.Cavitation characteristics
35Pressure and HeadPressure is force acting on a unit of area. It is expressed in Pounds/square inch or Newton/ square meter.Head is the height of the liquid.Atmospheric PressureLiquid Pressure10 psig
36The gauge is showing a pressure of 10 psig The gauge is showing a pressure of 10 psig. This is gauge pressure and is not including the Atmospheric pressure which is 14.7 psi.Absolute pressure:Absolute pressure=Gauge pressure + Atmospheric pressureAbsolute pressure of the tank shown in the previous slide is =24.7 psia.Absolute pressure will always be greater than Gauge pressure.
37A 10 feet of water exerts a pressure of 4. 33 psig A 10 feet of water exerts a pressure of 4.33 psig. A 100 feet of water column exerts a pressure of 43.3 psig. Dividing 4.33/10 and 43.3/100, we see that for each 1 foot of water psig pressure is exerted.43.3 psig4.33 psig100 feet10 feet
38Specific Gravity is the weight of the substance divided by the weight of the same volume of water. The specific gravity of water is 1. A liquid having a specific gravity of less than 1 will weigh less than water for an equal amount. If the weight is less then the force exerted will also be less and hence for an equivalent height the head of the liquid having a Sp. Gravity of less than 1 will be lower than that of water.The formula for converting Head into pressure isHead=Presssure/sp.gravity.0.433Suppose the gauge is showing a pressure of 30 psig for a liquid with Sp. Gravity of 0.7. The head will be 30/ =98.9 feet.
39Suction Head is the sum of the pressure changed to head, plus the velocity changed to head, at the inlet of the pump.Discharge Head is the pressure at the discharge changed to head.Total Head is the discharge head minus Suction head. It is the actual distance pump pushed the liquid at the expense of energy.Reference for height measurement is the pump centerline.Total head can be estimated by measuring the heights at the suction and discharge tanks and subtracting these heads or by reading the pressure gauges at the suction and discharge and converting them to head measurement and subtracting them.For a suction lift system Suction lift is added to the discharge head to calculate the Total Head.
40Discharge Head Suction Head Total Head Discharge Head Total Discharge HeadDischarge HeadSuction HeadTotal HeadDischarge HeadSuction LiftSuction Lift System
41Vapor Pressure and Net Positive Suction Head (NPSH) Vapor Pressure is the Pressure at which the liquid will transform into vapor at a given temperature. Vapor pressure is a function of temperature.Understanding the phenomenon, when the Absolute suction pressure is not high enough. Liquids vaporize or evaporate at the pump suction. In a liquid the vapors exert a pressure before they escape. Vapor pressure is the pressure of the vapor that is trapped in the liquid. Vapor pressure causes the liquid to vaporize or evaporate. Heating a liquid increases the its vapor pressure. To keep the liquid at the pump from vaporizing, the absolute suction pressure must be higher than the Vapor pressure at the given temperature.
42Net Positive Suction Head (NPSH) is a common term used in Centrifugal pumps. It can only be understand by understanding the terms NPSH available and NPSH required.NPSH (available) is the energy level expressed in terms of liquid height at the suction flange of the centrifugal pump, with the liquid vapor pressure as its datum. OrIt is the difference between Total suction head and vapor pressure of the liquid in feet of liquid, at the suction flange.It can be attained by subtracting all the losses like entrance losses, losses through fittings, elbows, strainers etc. and the vapor pressure from the Absolute Suction head.NPSH available is specified by the user of the pump.
43NPSH (required) is the minimum head required needed at the suction of a pump to enter the impeller without vaporizing. OrThe reduction in total head as the liquid enters thePump. NPSH required is determined by the Pump manufacturer.It is worth to know that why NPSH available is related to Vapor pressure. The impeller imparts energy to the liquid only in liquid form. From the model on the next slide shows that there will be some loss of energy when the liquid will travel from the suction flange to the impeller eye, where it meets the impeller vanes. Therefore head (pressure) at the vane tip is less than the head (pressure) available at the suction flange. As soon as the static head in the area between suction flange and the vane tip reaches vapor pressures, the liquid starts vaporizing. NPSH A is therefore related to Vapor pressure at its pumping temperature; indicates how much energy is available for the passage from suction flange to leading edge of the impeller vanes.
44E A B C D A B C D E Entrance loss Friction Turbulence friction, Entrance loss at vane tipIncreasing pressure due to impellerPoint of Lowest pressure where Vaporization startsINCREASING PRESSUREABCDERelative Pressures in the Entrance section of a pump
45As long as the actual energy loss, NPSH R remains less than NPSH A the total head will exceeds the vapor pressure , which guarantees that the pumpage will remain liquid during its travel.NPSH R varies as the square of the proportionate speed increase. In general standard centrifugal pump will not perform well if gas entrainment exceeds 1%.The mandatory margin between NPSH A and NPSH R is usually 1 foot.
46NPSH Problems and Cavitation If NPSH A is not greater than NPSH R, there will be a marked reduction in Head or Capacity or even a complete failure to operate. Excessive vibrations may take place when there is vapor passing through the impeller. Cavitation will take place which results in Pitting and erosion of the impeller.Cavitation is a phenomenon in which bubbles will form if, during liquid flow its static head becomes lower than its vapor pressure at the prevailing temperature. As soon as energy input causes the pressure to become greater than the vapor pressure the bubbles will suddenly collapse or implode. As the vapor collapse, the adjacent walls are subjected to a tremendous shock from the in rush of liquid into the cavity left by the bubble. This shock actually flakes off small bits of metal and the parts
47Take on the appearance of having been badly eroded Take on the appearance of having been badly eroded. This erosion shows up not at the point of lowest pressure where the bubble is formed, but further down stream where the bubble collapses. The most sensitive areas usually are the low pressure sides of the impeller vanes near the inlet edge and the front shroud where the curvature is greatest.A standard method to observe and judge cavitation on the test stand or in the field is to hold the speed constant and reduce the suction pressure at various capacities. A 3% drop in discharge pressure is considered evidence of cavitation. It is plotted in the Head capacity curve and it is determined as the NPSH R of the pump. More explanation is given in coming slides.
49The formula includes the following abbreviations and signs which are as follow. Pa= Absolute pressure in atmosphere surrounding gage.Ps= Gage pressure at the suction may be negative or positive according to the arrangement.Pt= Absolute pressure on free surface of liquid in the closed tank connected to the pump suction.Pvp= Vapor pressure of the liquid being pumped.hf= Lost head due to friction.V = Average velocityZ = Vertical distances may be positive or negative depending upon the arrangement.W = Specific weight of liquid at pumping temperature.
50The formula is as follows. NPSH available = (Pa-Pvp) / w +Ps/w + Zps =V2 /2gOrNPSH available = (Pt-Pvp) /w +Z –hfCavitation characteristics of a centrifugal pump is to cause a break down in the normal Head Capacity curves. This is done by holding the speed and suction pressure constant and varying the capacity, or by holding the speed constant and reducing the suction pressure at various capacities. Either of these methods will produce a breakdown in the Head characteristics as shown. The Hydraulic Institute has permitted a drop in head of 3% to be accepted as evidence that cavitation is present under test conditions. It is recommended to operate the pump above the break away sigma if noise and vibrations are to be avoided.
51Normal Q-H curve% Head NormalBreak Away% Capacity Normal
53PUMP PERFORMANCE; CHARACTERISTIC CURVES The performance characteristics of a centrifugal pump are clearly defined by its performance curves. They are also known as Head Capacity curve or simply Q-H curves. Prior to discuss the characteristic curves, following terms are to be familiarized in order to get a clear idea.Capacity is the volume of liquid per unit time delivered by the pump. It is denoted by Q and the units are gallons/minute or gpm, m3 /sec.Head is sum of the gauge pressure head and the velocity head at the discharge flange minus the sum of corresponding heads at the suction flange equals the energy in foot-pounds added per pound of liquid pumped and is called Total head developed by the pump. It is expressed in feet or meters and is usually denoted by H.
54BHP is the Horse power required by the pump for any flow rate of the liquid with a specific gravity of 1.0. It is denoted asBHP= QH(Sp.gravity)/3960. In SI Power units are Watt and is given by P=9797QH(Sp.gravity) when Q is in m3 /sec and Head is in metersNPSH is the Net Positive suction head required at some particular flow rate. The explanation is given in previous slides. The unit is feet or meters.Efficiency is equal to the rate at which imparts energy to the liquid, divided by the rate at which the pump requires energy. It is denoted in percentage. The formula is as follow.Efficiency= QH. Specific Gravity/3960.hp X 100From the performance curves some general principles are set which help us in making the basic concept of
55Centrifugal pump performance. When the Total Head decreases, the pump capacity increases except at very low capacity.As the level of suction head is dropped Total Head will increase resulting in decreasing the capacity.Power requirement increases with increasing the flow rate.NPSH requirement increases with the flow rate.The efficiency is relatively low at high and low flow rates.Best Efficiency Point (B.E.P) is the point on the curve at which the pump operation is most efficient and therefore most economical.By pinching the Discharge valve of the pump, the capacity decreases and so the NPSH requirement and correspondingly the Head increases.
56Total Head in Efficiency Horse Power Capacity Q Shut off head B.E.PNPSHTotal Head inNPSHEfficiencyEfficiencyHorse PowerHorse PowerCapacity Q
57The head produced by the pump is limited to that which is developed at shut off (zero capacity). Even if there is blockage in the discharge line the pressure will not exceed the shut off value. Correspondingly the pump driver will not be overloaded as the horsepower required will be decreasing as the flow is throttled back.
58AFFINITY LAWSAny machine, which imparts velocity and converts velocity to pressure, can be categorized by a set of relationships, which apply to any dynamic conditions. These relationships are referred to as the “Affinity Laws”. They can be described as similarity processes, which follow the following rules:Capacity varies as the rotating speed – that is, the peripheral velocity of the impeller.Head varies as the square of the rotating speedBHP varies as the cube of the rotating speed.The affinity laws apply to centrifugal gas compressors as well as to centrifugal pumps, but are most distinctly
59useful for estimating pumps performance at different rotating speeds, or impeller diameters starting with pumps with known characteristics.The basic variations can be analyzed by these relationships:1 - By changing speed and maintaining constant impeller diameter, pump efficiency will remain the same, but head, capacity, and BHP will vary according to the Laws.2 - By changing impeller diameter, but maintaining constant speed the pump efficiency for a diffuser pumpwill not be affected if the impeller diameter is not changed by more than five percent. Note that the change in efficiency will occur if the impeller size is reduced sufficiently to affect the clearances between the casing and the periphery of the impeller.
60Parameter Variation 1 Variation 2 However, the head, capacity and BHP will vary as follows:ParameterVariation 1Variation 21. With impeller diameter, D, held constant2. With speed, N, held constantCapacityA. Q1/Q2 = N1/N2.A. QI/Q2 = D1/D2HeadB. H1/H2= (N1/N2)2B. H1/H2 =(DI/D2)2
62These relationships may be manipulated in any mathematically valid way These relationships may be manipulated in any mathematically valid way. The common denominator being a speed change, the relationships may be complied as follows:Q2 = (N2/N1).Q1 (4)H = (N2/N1)2 H1 (5)P = (N2/N1)3 P1 (6)
63Contents Pump operation Discharge recirculation Minimum flow control Effect of viscosity on pump performanceParallel and Series operationCapacity regulation
64PUMP OPERATION.We will discuss different Terms which are frequently used in Today's Industry as far as Centrifugal pump jargon is concerned.Internal Recirculation.Effect of Pump performance due to Viscosity.Minimum FlowCapacity RegulationParallel and Series Operation.Internal RecirculationThere is a small flow from Impeller Discharge to Suction
65through the wearing rings through the wearing rings. This takes place at all the capacities but does not usually contribute in raising the temperature of the liquid being pumped unless operation is near shut off.When the capacity is reduced by throttling a secondary flow reversal at the suction or discharge tip of the impeller vanes occur. Suction recirculation is the reversal of flow at the impeller eye and travel upstream with a rotational velocity approaching the peripheral velocity of the diameter. A rotating annulus is formed and hence this annulus moves upstream from the impeller inlet and through the core of the annulus an axial flow is produced. Shearing between annulus and the axial flow through the core produces vortices which form and collapse, producing noise and cavitation at the suction side of the pump.
66Guide vanes may show cavitation damage from the impingement of the back flow from the impeller eye in Suction recirculation.Discharge Recirculation is the reversal of flow at the Discharge tip of the impeller vanes. The high shear rate between the inward and outward relative velocities produces vortices that cavitate and usually attack the pressure side of the vanes.The tongue or diffuser vanes may show cavitation damage on the impeller side from operation in discharge recirculation.
68Viscosity and effect on pump design and operation There are some significant differences in pump behavior, and system response, when pumping fluid other than water. One of the major factors is viscosity. This is a characteristic of all fluids, including water, different for each fluid. It is of course obvious that fluids differ from solids; it is not so obvious that fluids can approach solids so that a distinction may be blurred.Effect on Pump InstallationThe viscosity of the liquid is a very important factor in the selection of a pump. It is the determining factor in frictional head, motor size required and speed reduction necessary. Frequently, for high viscosity liquids, it is more economical to use a large pump operating at a reduced speed since the original higher total installation cost is more that offset by reduced maintenance and subsequent longer life of the unit.
69Effect of viscosity on performance Liquids of high viscosity offer more resistance to flow than water. This increased friction occurs both in the piping and in the pump itself. More energy is required to force the liquid through the flow passages in the pump casing and through the pump impeller. The disc friction of the impeller; that is, the resistance of the liquid to the rotation of the impeller, is greater when pumping a high viscosity liquid. These friction losses within the pumpresult in the pump generating a lower head and reduce the volume of liquid handled by the pump. These losses also result in a lowering of the pump efficiency and an increase in the brake horsepower requirements.
70The reduction in head, capacity, and efficiency and the increase in horsepower requirements of a pump handling a viscous liquid depend to a large extent on the type pump, the design of the passages inside the pump, the design of the impeller, the size of the pump, the capacity of the pump and the pump speed.The following equation are used for determining the viscous performance when the water performance of the pump is known:Qvis = CQ X QwHvis = CH X HwE vis = CE X Ewbhp vis = QviS X Hvis X sp. gr. / 3960 X E visCQ, CH and CE are determined from graphs, which are based water performances
71The following equations are used for approximating the water performance when the desired viscous capacity and head are given and the values of CQ and CH must be estimated form graph using QviS and Hvis, as:Qs (approx.) = Qvis / CQHw (approx.) = Hvis/ CH
72MINIMUM FLOW CONTROLAlthough a constant – speed centrifugal pump will operate over a wide range of capacities, it will encounter difficulty at low flow. In general, low-flow problems are worse for large high – energy pumps, for pumps handling hot or abrasives – laden liquids, for pumps designed for high efficiency at best efficiency point (BEP), and for pumps for low net positive suction head (NPSH).Source of trouble at reduced flowThe sources of the pump’s distress are fourfold – thermal, hydraulic, mechanical, and abrasive wear.ThermalInescapable energy conversion loss in the pump warns the liquid.HydraulicWhen flow decreases far enough, the impeller encounters suction or discharge recirculation, or perhaps both. Flashing,
73cavitations, and shock occur, often with vibration and serious damage. MechanicalBoth constant and fluctuating loads in the radial and axial directions increase as pump capacity falls. Bearing damage, shaft and impeller breakage, and rubbing wear on casing, impeller, and wear rings can occur.Axial-flow and mixed-flow pumps with high specific speed give comparatively higher head and take comparatively more power at low flow. A bypass system may be necessary not only to reduce stress but also to prevent motor overload.Abrasive wearLiquids containing a large amount of abrasive particles, such as sand or ash, must flow continuously through the pump. If flow decreases, the particles can circulate inside
74FACTORS IN DESIGN OF A MINIMUM FLOW CONTROL SYSTEM Pump size the pump passage and quickly erode the impeller, casing, and even wear rings and shaft.FACTORS IN DESIGN OF A MINIMUM FLOW CONTROL SYSTEMPump sizeHigher capacity, brake power, specific speed, and suction specific speed all tend to make a minimum flow control system difficult to design and costly to build and operate.Discharge PressureHigh discharge pressures mean high head loss in bypass lines. Liquids that can flash and cavitate demand special precautions in valving, orifices, and piping.
75Continuous bypass system Available heat sinkBy pass flow must go far enough upstream to prevent progressive temperature buildup or flow disturbance in the pump suction. This may mean a simple discharge back to an un insulated inlet line or discharge to a receiving tank or cooler with enough area and enough inflow of cool liquid to handle the thermal load. Bypass flow can discharge into a de-aerator storage tank, a condenser, a flash tank, or a cooling pond. Altitude, distance, and pressure inside the receiving tank are also factors, as is the fact that the interior must be available for inspection and for repair of spray or distribution pipes and orifices.Continuous bypass systemFigure illustrates a simple system, with a bypass line branching off the pump discharge upstream of the main line check valve and containing a fixed orifice dimensioned by analysis to provide minimum required pump flow.
76CONTINUOUS BY PASS OPERATION RECEIVING TANKORIFICEBY PASS LINESHUT OFF VALVECHECK VALVESHUT OFF VALVESHUT OFF VALVEPUMPCONTINUOUS BY PASS OPERATION
77The bypass receiving tank must be at a lower pressure than pump discharge and may be the pump suction source or some other vessel than will return the liquid to the pump (if it is not to be wasted). Locating the bypass branch off before the discharge check valve as shown keeps backflow from process or flow from a parallel operating pump from going back to the receiving tank or back through the pump during a pump shutdown.The choice of size for the bypass pipe depends on flow, equivalent line length, and how close the liquid temperature is to the boiling point. If the pressure breakdown after the orifice results in flashing flow, the orifice should discharge into the larger receiving tank. In that case, eliminate the shutoff valve downstream of the bypass.
78On – off bypass of minimum flow In figure a bypass line runs to a below – surface spray pipe in an overhead pressurized receiver. For low bypass flows and low heads of cool water, the on-off valve maybe single-stage. Control is simple, with a dead band of 2 to 5 % of design – point delivery. The meter, downstream of the bypass branch off, serves system control functions. Bypass piping is full of water, eliminating the danger of a sudden impact of a water slug on the orifice.Should the bypass piping discharge above the water level in the receiving tank, a check valve in this line is advisable. Without a check valve, the water in the bypass piping will drain back through an idle pump to the level in the receiving tank, if this is the suction source. When the pump is started, water slugging could occur in the empty portion of the pipe and fittings.
79ON OFF BY PASS OPERATION RECEIVING TANKORIFICEON OFF CONTROL VALVECONTROLSHUT OFF VALVESPRAY PIPECHECK VALVESHUT OFF VALVESHUT OFF VALVEPUMPMETERING ORIFICEON OFF BY PASS OPERATION
80Automatic Bypass Control System A single element that combines the pump check valves, pipe tee, pressure – reducing orifice, flow meter, and control valve can be an on-off or modulating bypass control, as in figure lift check acts as a flow meter. This opens and closes a pilot valve, which operates, and integral – control pressure – reducing valve. The element is self-powered, requiring no external source of electricity or air, and is designed to be fail-safe. The highest-pressure drops can be handled with commercially available systems. They system design must suit the pump characteristics.As in other bypass system, protection is necessary to prevent flashing in the piping down steam of the control element. A fixed orifice for on-off control or a variable orifice for modulating control is needed at the end of the bypass line and at the receiving tank wall. No anti flash orifice is required if the automatic control is at the end of the bypass line.
81AUTOMATIC BY PASS OPERATION RECEIVING TANKAUTOMATIC BYPASS ASSEMBLYSHUT OFF VALVESHUT OFF VALVESHUT OFF VALVEPUMPAUTOMATIC BY PASS OPERATION
82CAPACITY REGULATION CAPACITY REGULATION Capacity variation ordinarily is accomplished by a change in pump head, speed, or both simultaneously. The capacity and power input of pumps with specific speeds up to about 4000 increase with deceasing head, so that the drivers of such pumps may be overloaded if the head falls below a safe minimum value. Increasing the head of high – specific speed pumps decreases the capacity but increases the power input. The drivers of these pumps should either be able to meet possible load increases or be equipped with suitable over load protection. Capacity regulation by the various methods given below may be manual or automatic.
83Discharge Throttling: This is the cheapest and most common method of capacity modulation for low and medium-specific speed pumps. Usually its use is restricted to such pumps. Partial closure of any type of valve in the discharge line will increase the system head so that thesystem-head curve will intersect the head capacity curve at a smaller capacity. Discharge throttling moves the operating point to one of lower efficiency, and power is lost at the throttle valve. This may be important in large installations, where more costly methods of modulation may be economically attractive. Throttling to the point of shutoff may cause excessive heating of the liquid in the pump. This may require a bypass to maintain the
84necessary minimum flow or use of different method of modulation necessary minimum flow or use of different method of modulation. This is particularly important with pumps handling hot water or volatile liquids, as previously mentioned.Suction ThrottlingIf sufficient NPSH is available, some power can be saved by throttling in the suction line. Jet engine fuel pumps frequently are suction throttled because discharge throttling may cause overheating and vaporization of the liquid. At very low capacity, the impellers of these pumps are only partly filled with liquid, so that the power input and temperature rise are about one-third the values for impellers running full with discharge throttling. The capacity of condensate pumps frequently is submergence –controlled, which is equivalent to
85suction throttling. Special design reduces cavitations damage of these pumps to negligible amount. Bypass RegulationAll or part of the pump capacity may be diverted from the discharge line to the pump suction or other suitable point through a bypass line. The bypass may contain one or more metering orifices and suitable control valves. Metered bypasses are commonly used with boiler-feed pumps for reduced capacity operation, mainly to prevent overheating. There is a considerable power saving if excess capacity of propeller pumps is bypassed instead of using discharge throttling.Speed RegulationThis can be used to minimize power requirements and eliminate overheating during capacity modulation.
86Steam turbines and internal combustion engines are readily adaptable to speed regulation at small extra cost. A wide variety of variable speed mechanical, magnetic, and hydraulic drives are available, as well as both ac and dc variable-speed motors. Usually variable – speed motors are so expensive that they can be justified only by an economic study of particular case.Regulation of Adjustable vanesAdjustable guide vanes ahead of the impeller have been investigated and found effective with a pump of specific speed Ns = The vanes produced a positive pre whirl which reduced the head, capacity, and efficiency. Relatively little regulation was obtained from the vanes with pumps having ns = 3920 and 1060.
87Adjustable outlet diffusion vanes have been used with good success on several large European storage pumps for hydroelectric developments. Propeller pumps with adjustable – pitch blades have been investigated with good success. Wide capacity variation was obtained at constant head and with relatively little loss in efficiency. These methods are so complicated and expensive that they probably will have very limited applicable in practice.Air AdmissionAdmitting air into the pump suction has been demonstrated as a means of capacity regulations, with some saving in power over discharge throttling. Usually air in the pumped liquid is undesirable, and there is always the danger that too much air will cause
88the pump to lose its prime the pump to lose its prime. The method has rarely been used in practice but might be applicable to isolated causes.Parallel Operation:Parallel operation of two or more pumps is a common method of meeting variable – capacity requirements. By starting only those pumps needed to meet the demand, operation near maximum efficiency can usually be obtained. The head – capacity characteristics of the pumps need not be identical, but pumps with unstable characteristics may give trouble unless operation only on the steep portion of the characteristic can be assured. Care should be taken to see that no one pump, when combined with pumps of different characteristics, is forced to operate at flows less than the minimum required preventing recirculation. Multiple pumps in a facility provide spares for emergency service for the down time needed for maintenance. Parallel pump operation is beneficial where the system resistance curve is flat.
89Series Operation for centrifugal pumps is carried out to increase the head. It enforces an identical flow through each pump where the discharge head is sum of the heads developed by each pump.As a general rule Series operation of pumps is beneficial where the system resistance curve is steep.See the curves on the next slide.
90H Q SERIES SERIES OPERATION SYSTEM RESISTANCE CURVES PARALLEL PARALLEL OPERATIONQ
91Centrifugal Pump Troubleshooting Guide Pump not reaching Design Flow rateA. Probable Cause:Insufficient NPSH (Noise may or may not present)Recommended Remedy:Recalculate NPSH available. It must be greater than NPSH required by pump at desired flow. If not:Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.
92B. Probable Cause:System head greater than anticipated.Recommended Remedy:Reduce system head by increasing pipe size and/or reducing number of fittings. Increase impeller diameter.C. Probable Cause:Plugged impeller, suction line, or casing: product fibrous or contains large size solids.For fibrous material:1. Reduce length of fiber when possible,Consider oversized pump.
93D. Probable cause:Entrained air-leak from atmosphere on suction side.Recommended Remedy:1. Check suction side gaskets and threads for tightness.2. Properly repack stuffing box.3. Install Vortex breakerE. Probable Cause:Entrained gas from process.Process generated gases may require a large pump.F. Probable Cause:Speed too low
94Recommended Remedy:Check motor speed against design speedG. Probable cause:Direction of rotation wrong.Reverse any two of three leads on a three phase motor.H. Probable cause.Impeller too small.Replace with proper diameter impeller.I. Probable cause:Impeller clearance too large
95Recommended Remedy:Reset impellerPump not reaching design Head (TDH)A. Probable Cause:Check all items under “Check all the items referred in Pumps not reaching design flow rate.”Refer to remedies listed under Problem No.1No-Discharge or Flow- Pump RunningNot properly primed
96Recommended Remedy:Repeat priming operation, recheck instructions.B. Probable Cause:Suction lift too high.1. Rearrange piping.2. Increase Suction head if possible.3. Determine if larger impeller would be better.4. Select new pump to handle higher suction lift.C.Probable CauseImpeller, Suction line, or casing is plugged..
97Recommended Remedy:For fibrous materials:1. Reduce length of fiber when possible.2. Consider oversized pumpD. Probable cause:Direction of rotation wrongReverse any two of three leads on a three phase motorE. Probable cause:Entrained air-leak from atmosphere on suction side.Recommended remedy:Check 1-D
98Recommended Remedy:For fibrous materials:1. Reduce length of fiber when possible.2. Consider oversized pumpD. Probable cause:Direction of rotation wrongReverse any two of three leads on a three phase motorE. Probable cause:Entrained air-leak from atmosphere on suction side.Recommended remedy:Check 1-D
99Pump operates for short period, then looses prime. A. Probable Cause:Insufficient NPSH.Recommended Remedy:Recalculate NPSH available. It must be greater than NPSH required by pump at desired flow. If not:Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.B. Probable cause:Entrained air-leak from atmosphere on suction side.
100Recommended remedy:Check 1-DExcessive Noise-Wet endA. Probable cause:Cavitation-Insufficient NPSH availableRefer to remedy under 1-A.B. Probable Cause:Abnormal fluid rotation due to complex suction piping.Recommended Remedy:Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.
101C. Impeller rubbingRecommended Remedy:Check and reset Impeller ClearanceCheck out board bearing assembly for axial end play.Excessive Noise –Power endProbable causeOverloaded bearing which is indicated by flaking or spalling of the bearing raceways.Check to be sure that actual operating conditions do not exceed the maximum allowable suction pressure and the maximum allowable specific gravity.
102B. Probable cause:Bearing contamination appearing on the raceways as a scoring, pitting, scratching, or rusting caused by adverse environment and entrance of abrasive waste materials from atmosphere.Recommended Remedy:Work with clean tools in clean surroundings.Remove all dirt from housing before exposing bearings.Handle with clean dry handsUse clean solvents and flushing oil.C. Probable cause:Brinelling of bearing identified by indentations on the ball races, caused by improper handling during fitting.
103Correct the source of vibration. Recommended Remedy:Be sure when mounting a bearing to apply the mounting pressure slowly and evenly. Press the inner ring while fitting the bearing on the shaft and press against outer ring when fitting in housing.D. Probable cause:False brinelling of bearing identified again by either axial or circumferential indentations usually caused by vibration of the balls between the races in a stationary bearing.Correct the source of vibration.Where bearings are oil lubricated and employed in units that may be out of service for extended periods, the shaft should be turned over periodically to re lubricate all bearing surfaces at intervals of one to three months.
104Probable Cause:Thrust overload on bearing identified by flaking ball path on one side of the outer race or in the case of maximum capacity bearings, may appear as a spalling of the races in the vicinity of the loading slot. These thrust failures are caused by improper mounting of the bearing or excessive thrust loads.Recommended Remedy:1. Follow correct mounting procedures for bearings.2. Check pump literature to be sure that actual conditions do not exceed the maximum allowable Sp. Gravity or suction pressure limitations.F. Probable cause:Misalignment identified by the fracture of ball retainer or a wide
105ball path on the inner race and a narrower cocked ball path on the outer race. Misalignment is caused by poor mounting practices or improperly machined shafts.Recommended remedy:Check Housing bore dimensionally to be sure that both bores are true with each other. Check shaft shoulders are squared with shaft center line. Run out of the shaft is within limits.G. Probable cause:Bearing damaged by electrical arcing identified as electro-etching of both inner and outer ring as pitting or cratering. Electrical arcing is caused by a static electric charge emanating from belt drives, electrical leakage or short circuiting.
106Recommended Remedy:1. Where current shunting through the bearing cannot be corrected, a shunt in the form of a slip ring assembly should be inorporated.Check all wiring, insulation and rotor windings to be sure that they are sound and all connections are properly made.Where pumps are belt driven, consider the elimination of static charges by proper grounding or consider belt material that is less generative.H. Probable Cause:Bearing damage due to improper lubrication, identified by one or more of the following.
107Abnormal bearing temperature rise. A stiff cracked grease appearance.A brown or bluish discoloration of the bearing race.Failure of the ball retainer.Recommended Remedy:Be sure that the lubricant is clean.Be sure that proper amount of lubricant is used.Be sure proper grade of lubricant is used.
108About the presenterSyed Ali Hussain is presently working as Pumps and Blowers Engineer in Maintenance Reliability Group. He handles Reliability improvements and trouble shootings, condition monitoring in Pumps, blowers and their drivers in Petrokemya. He is having eight years experience in Rotating equipment in Process industry. He was previously working with Pak Arab Fertilizers Pakistan as rotating equipment engineer. He is a graduate Engineer from UET Lahore Pakistan. He is Certified Vibration analyst I and life member of Pakistan Engineering Council, Pakistan.