Pumps theory www.chemicallibrary.blogspot.comDocument Transcript
There are various means and ways for conveying fluids (both compressible and non-compressible) and two phase suspensions. Most widely used equipments can be classified asfollows. Pumps Fans Blowers CompressorsOut the four machines mentioned above, pumps are mostly used for transporting incompressiblefluids. Pumps Positive Displacement Centrifugal Reciprocating Rotary Before starting detailed discussion on pumps, there are some terms that are commonlyencountered while dealing with pumps, especially centrifugal pumps. These terms will be brieflydiscussed below.HEAD:- The mechanical energy equation is given below. Pa/ρ + gZa/gc + αaVa2/2gc + ηWp = Pb/ρ + gZb/gc+ αbVb2/2gc + hfVarious terms encountered here are given below.Pa/ρ & Pb/ρ Pressure HeadsgZa/gc & gZb/gc Height HeadsαaVa2/2gc & αbVb2/2gc Velocity HeadsηWp Pump Work
hf Friction Losses During Flow From Section a to Section b The pressure at any point in a liquid can be thought of as being caused by a verticalcolumn of the liquid which, due to its weight, exerts a pressure equal to the pressure at the pointin question. The height of this column is called the static head and is expressed in terms of feet ofliquid. The static head corresponding to any specific pressure is dependent upon the weight ofthe liquid according to the following formula. We can predict the approximate head of any centrifugal pump by calculating theperipheral velocity of the impeller and substituting into the above formula. A handy formula forperipheral velocity is: The head developed is approximately equal to the velocity energy at the periphery of theimpeller. Where H = Total head developed in feet. v = Velocity at periphery of impeller in feet per sec. 2 g = 32.2 Feet/Sec D = Impeller diameter in inches V = Velocity in ft./secSUCTION LIFT:- It exists when the source of supply is below thecenter line of the pump. Thus the STATIC SUCTIONLIFT is the vertical distance in feet from the centerline of the pump to thefree level of the liquid to be pumped.Suction Lift – Showing Static Heads in aPumping System
Where the Pump is Located Above the Suction Tank.(Static Suction Head)SUCTION HEAD:- It exists when the source of supply is above the centerline of the pump. Thus the STATICSUCTION HEAD is the vertical distance in feet from the centerline of the pump to the free level of theliquid to be pumped. Suction Head – Showing Static Heads in a Pumping System Where the Pump is Located Below the Suction Tank. (Static Suction Head)SPECIFIC SPEED:- Specific speed (Ns) is a non-dimensional design index used to classify pump impellers asto their type and proportions. It is defined as the speed in revolutions per minute at which ageometrically similar impeller would operate if it were of such a size as to deliver one gallon perminute against one foot head. The understanding of this definition is of design engineering significance only, however,and specific speed should be thought of only as an index used to predict certain pumpcharacteristics. The following formula is used to determine specific speed:
WhereN = Pump speed in RPMQ = Capacity in gpm at the best efficiency pointH = Total head per stage at the best efficiency pointCAVITATION:- Consider that a pump transfers fluid from a section a to section b. The power calculatedfrom the mechanical energy balance equations dependent upon the inlet and outlet pressures.Consider a case where the suction or inlet pressure becomes almost equal to or just above thevapor pressure of the fluid. The fluid sucked in will have bubbles of air entrained in it and thiswill be a serious cause of pump capacity and thus efficiency. In other words, when the pressureof the liquid is reduced to a value equal to or below its vapor pressure the liquid begins to boiland small vapor bubbles or pockets begin to form. As these vapor bubbles move along theimpeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse. Thisphenomenon is termed as cavitation. The collapse or "implosion" is so rapid that it may be heardas a rumbling noise, as if you were pumping gravel. The accompanying noise is the easiest wayto recognize cavitation. Vibration and mechanical damage such as bearing failure can also occuras a result of operating in excessive cavitation, with high and very high suction energy pumps.On the other hand what will happen if the suction pressure falls below the vapor pressure of thefluid? The answer is that the pump will be unable to suck any fluid and thus no fluid will betransferred through the pump. In such a case the pump will be vapor locked. This generallyhappens when the pump is started to operate. The sure of such a problem is Priming whichmeans that the operating fluid be manually or mechanically circulated through the pump so thatthe pump becomes void of air or vapors and the pump is able to operate on the designedpressure.NET POSITIVE SUCTION HEAD (NPSH):- The Hydraulic Institute defines NPSH as the total suction head in feet absolute,determined at the suction nozzle and corrected to datum, less the vapor pressure of the liquid infeet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pumpto determine if the liquid will vaporize at the lowest pressure point in the pump. A liquid increases greatly in volume when it vaporizes. One cubic foot of water at roomtemperature becomes 1700 cu. ft. of vapor at the same temperature. It is obvious from thisexample that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH issimply a measure of the amount of suction head present to prevent this vaporization at the lowestpressure point in the pump. As the liquid passes from the pump suction to the eye of the impeller, the velocityincreases and the pressure decreases. There are also pressure losses due to shock and turbulenceas the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases thevelocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feetabsolute required at the pump suction to overcome these pressure drops in the pump andmaintain the majority of the liquid above its vapor pressure. The NPSH Required varies with
speed and capacity within any particular pump. Pump manufacturers curves normally providethis information. Net Positive Suction Head (NPSH) NPSH Available is a function of the system in whichthe pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressureas it arrives at the pump suction. Fig shows four typical suction systems with the NPSHAvailable formulas applicable to each. It is important to correct for the specific gravity of theliquid and to convert all terms to units of "feet absolute" in using the formulas. Various termsused are PB = Barometric pressure in feet absolute. VP = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute. P = Pressure on surface of liquid in closed suction tank, in feet absolute. Ls = Maximum static suction lift in feet. LH = Minimum static suction head in feet. hf = Friction loss in feet in suction pipe at required capacity
In an existing system, the NPSH Available can be determined by a gauge on the pumpsuction. The following formula applies: Where Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric, minus if below atmospheric) corrected to the pump centerline. hv = Velocity head in the suction pipe at the gauge connection, expressed in feet. SUCTION SPECIFIC SPEED:- In designing a pumping system, it is essential to provide adequate NPSH available forproper pump operation. Insufficient NPSH available may seriously restrict pump selection, oreven force an expensive system redesign. On the other hand, providing excessive NPSHavailable may needlessly increase system cost. Suction specific speed may provide help in thissituation. Suction specific speed (S) is defined as: Where N = Pump speed RPM GPM = Pump flow at best efficiency point at impeller inlet (for double suction impellers divide total pump flow by two). NPSHR = Pump NPSH required at best efficiency point.An example:Flow 2,000 GPM; head 600 ft. What NPSHA will be required?Assume: at 600 ft., 3500 RPM operation will be required.
SUCTION ENERGY:- The amount of energy in a pumped fluid, that flashes into vapor and then collapses backto a liquid in the higher pressure area of the impeller inlet, determines the extent of the noiseand/or damage from cavitation. Suction Energy is defined as: Suction Energy = De x N x S x Sg Where D e = Impeller eye diameter (inches) Sg = Specific gravity of liquid (Sg - 1.0 for cold water)AFFINITY LAWS:- The affinity laws express the mathematical relationship between the several variablesinvolved in pump performance. They apply to all types of centrifugal and axial flow pumps.They are as follows: 1. With impeller diameter D held constant: 2. With speed N held constantWhere:
Q = Capacity, GPM H = Total Head, Feet BHP = Brake Horsepower N = Pump Speed, RPMFUNDAMENTAL FORMULAE FOR CENTRIFUGAL PUMPS:- Where GPM = gallons per minute CFS = cubic feet per second Lb. = pounds Hr. = hours BBL = barrel (42 gallons)
Sp.Gr. = specific gravity H = head in feet psi = pounds per square inch In. Hg. = inches of mercury hv = velocity head in feet V = velocity in feet per second g = 32.16 ft/sec2 (acceleration of gravity) A = area in square inches I.D. = inside diameter in inches BHP = brake horsepower Eff. = pump efficiency expressed as a decimal Ns = specific speed N = speed in revolutions per minute v = peripheral velocity of an impeller in feet per second D = Impeller in inches Nc = critical speed f = shaft deflection in inches P = total force in pounds L = bearing span in inches m = constant usually between 48 and 75 for pump shafts E = modules of elasticity, psi - 27 to 30 million for steelCONSTRUCTION OF CENTRIFUGAL PUMPS:- As a rule, the casing for the liquid end of a pump with a single-suction impeller is madewith an end plate that can be removed for inspection and repair of the pump. A pump with adouble-suction impeller is generally made so one-half of the casing may be lifted withoutdisturbing the pump.Since an impeller rotates at high speed, it must be carefullymachined to minimize friction. An impeller must be balanced toavoid vibration. A close radial clearance must be maintainedbetween the outer hub of the impeller and that part of the pumpcasing in which the hub rotates. The purpose of this is to minimizeleakage from the discharge side of the pump casing to the suctionside.Centrifugal pump impellers.A. Single-suction.B. Double-suction
In most centrifugal pumps, the shaft is fitted with a replaceable sleeve. The advantage ofusing a sleeve is that it can be replaced more economically than the entire shaft. Seal piping (liquid seal) is installed to cool the mechanical seal. Most pumps in saltwaterservice with total head of 30 psi or more are also fitted with cyclone separators. These separatorsuse centrifugal force to prevent abrasive material (such as sand in the seawater) from passingbetween the sealing surfaces of the mechanical seal. There is an opening at each end of theseparator. The opening at the top is for "clean" water, which is directed though tubing to themechanical seals in the pump. The high-velocity "dirty" water is directed through the bottom ofthe separator, back to the inlet piping for the pump.OPERATION OF CENTRIFUGAL PUMP:- Liquid enters the rotating impeller on the suction side of the casing and enters the eye ofthe impeller. Liquid is thrown out through the opening around the edge of the impeller andagainst the side of the casing by centrifugal force. This is where the pump got its name. Whenliquid is thrown out to the edge of the casing, a region of low pressure (below atmospheric) iscreated around the center of the impeller; more liquid moves into the eye to replace the liquidthat was thrown out. Liquid moves into the center of the impeller with a high velocity (speed).Therefore, liquid in the center of the impeller has a low pressure, but it is moving at a highvelocity. Liquid moving between the blades of the impeller spreads out, which causes the liquid toslow down. (Its velocity decreases.) At the same time, as the liquid moves closer to the edge ofthe casing, the pressure of the liquid increases. This change (from low pressure and high velocityat the center to high pressure and low velocity at the edge) is caused by the shape of the openingbetween the impeller blades. This space has the shape of a diffuser, a device that causes thevelocity-pressure relationship of any fluid that moves through it to change. A centrifugal pump is considered to be a nonpositive-displacement pump because thevolume of liquid discharged from the pump changes whenever the pressure head changes. Thepressure head is the combined effect of liquid weight, fluid friction, and obstruction to flow. In acentrifugal pump, the force of the discharge pressure of the pump must be able to overcome theforce of the pressure head; otherwise, the pump could not deliver any liquid to a piping system.The pressure head and the discharge pressure of a centrifugal pump oppose each other. When thepressure head increases, the discharge pressure of the pump must also increase. Since no energy
can be lost, when the discharge pressure of the pump increases, the velocity of flow mustdecrease. On the other hand, when the pressure head decreases, the volume of liquid dischargedfrom the pump increases. As a general rule, a centrifugal pump is usually located below theliquid being pumped. (NOTE: This discussion assumes a constant impeller speed.). When a centrifugal pump is started, the vent line must be opened to release entrained air.The open passage through the impeller of a centrifugal pump also causes another problem. Itspossible for liquid to flow backwards (reverse flow) through the pump. A reverse flow, from thedischarge back to the suction, can happen when the pressure head overcomes the dischargepressure of the pump. A reverse flow can also occur when the pump isnt running and anotherpump is delivering liquid to the same piping system. To prevent a reverse flow of liquid througha centrifugal pump, a check valve is usually installed in the discharge line. With a check valve in the discharge line, whenever the pressure above the disk risesabove the pressure below it, the check valve shuts. This prevents liquid from flowing backwardsthrough the pump.ROTARY PUMPS:- A number of types are included in this classification, among which are the gear pump, thescrew pump, and the moving vane pump. Unlike the centrifugal pump, which we have discussed,the rotary pump is a positive displacement pump. This means that for each revolution of thepump, a fixed volume of fluid is moved regardless of the resistance against which the pump ispushing. As you can see, any blockage in the system could quickly cause damage to the pump ora rupture of the system. You, as a pump operator, must always be sure that the system is properlyaligned so a complete flow path exists for fluid flow. Also, because of their positivedisplacement feature, rotary pumps require a relief valve to protect the pump and piping system.The relief valve lifts at a preset pressure and returns the system liquid either to the suction side ofthe pump or back to the supply tank or sump. Rotary pumps are also different from centrifugal pumps in that they are essentially self-priming. As we saw in our discussion of centrifugal pumps, the pump is located below the liquidbeing pumped; gravity creates a static pressure head which keeps the pump primed. A rotarypump operates within limits with the pump located above the source of supply. A good example of the principle that makes rotary pumps self-priming is the simpledrinking straw. As you suck on the straw, you lower the air pressure inside the straw.Atmospheric pressure on the surface of the liquid surrounding the straw is therefore greater andforces the liquid up the straw. The same conditions basically exist for the gear and screw pumpto prime itself.
In the tank must be vented to allow air into the tank to provide atmospheric pressure onthe surface of the liquid. To lower the pressure on the suction side of the pump, the clearancesbetween the pump parts must be close enough to pump air. When the pump starts, the air ispumped through the discharge side of the pump and creates the low-pressure area on the suctionside, which allows the atmospheric pressure to force the liquid up the pipe to the pump. Tooperate properly, the piping leading to the pump must have no leaks or it will draw in air and canlose its prime. Rotary pumps are useful for pumping oil and other heavy viscous liquids. In the engineroom, rotary pumps are used for handling lube oil and fuel oil and are suitable for handlingliquids over a wide range of viscosities. Rotary pumps are designed with very small clearances between rotating parts andstationary parts to minimize leakage (slippage) from the discharge side back to the suction side.Rotary pumps are designed to operate at relatively slow speeds to maintain these clearances;operation at higher speeds causes erosion and excessive wear, which result in increasedclearances with a subsequent decrease in pumping capacity. Classification of rotary pumps is generally based on the types of rotating element. Sometypes have been briefly mentioned below.1- GEAR PUMPS:- The simple gear pump has two spur gears that mesh together and revolve in oppositedirections. One is the driving gear, and the other is the driven gear. Clearances between the gearteeth (outside diameter of the gear) and the casing and between the end face and the casing areonly a few thousandths of an inch. As the gears turn, the gears unmesh and liquid flows into thepockets that are vacated by the meshing gear teeth. This creates the suction that draws the liquidinto the pump. The liquid is then carried along in the pockets formed by the gear teeth and thecasing. On the discharge side, the liquid is displaced by the meshing of the gears and forced outthrough the discharge side of the pump.
Simple gear pump2- SCREW PUMPS:- Several different types of screw pumps exist. The differences between the various typesare the number of intermeshing screws and the pitch of the screws. Figure shows a double-screw,low-pitch pump; and figure 9-11 shows a triple-screw, high-pitch pump. Screw pumps are usedaboard ship to pump fuel and lube oil and to supply pressure to the hydraulic system. In thedouble-screw pump, one rotor is driven by the drive shaft and the other by a set of timing gears.In the triple-screw pump, a central rotor meshes with two idler rotors. In the screw pump, liquid is trapped and forced through the pump by the action ofrotating screws. As the rotor turns, the liquid flows in between the threads at the outer end ofeach pair of screws. The threads carry the liquid along within the housing to the center of thepump where it is discharged. Most screw pumps are now equipped with mechanical seals. If the mechanical seal fails,the stuffing box has the capability of accepting two rings of conventional packing for emergencyuse.
3- SLIDING VANE PUMPS:- The sliding-vane pump figure below has a cylindrically bored housing with a suctioninlet on one side and a discharge outlet on the other side. A rotor (smaller in diameter than thecylinder) is driven about an axis that is so placed above the center line of the cylinder as toprovide minimum clearance between the rotor and cylinder at the top and maximum clearance atthe bottom. The rotor carries vanes (which move in and out as the rotor rotates) to maintain sealedspaces between the rotor and the cylinder wall. The vanes trap liquid on the suction side andcarry it to the discharge side, where contraction of the space expels liquid through the dischargeline. The vanes slide on slots in the rotor. Vane pumps are used for lube oil service and transfer,tank stripping, bilge, aircraft fueling and defueling and, in general, for handling lighter viscousliquids.