Transcript
Pump Pump has been defined differently by different investigators; the different definitions are, however, all similar and nearly equivalent.
y y y
A
device which raises or transfers liquids at the expense of power input.
A
machine designed to elevate, deliver and move various liquids
A
unit that transfers the mechanical energy of a motor or an engine into potential and kinetic
energy of a liquid. (2) Pumps are used for rising liquid from lower level to higher level. (3) create low pressure at suction side, high pressure at delivery side
Classification of pumps: According
to design and principle of operation, pumps may be divided into two general categories. The
choice is based on the liquid to be pumped and the de sired head and capacity. (a)
(b)
Positive displacement pump (PDP) (1) Reciprocating: piston and plunger (2) Rotating: gear, lobe Roto-Dynamic Pump (RDP) (1) Centrifugal pump (2) Axial pump (3) Mixed flow pump
Centrifugal Pump: A
centrifugal pump increases liquid pressure by increasing its velocity by means of a rotating impeller .
Liquid enters at the center of the impeller, is accelerated by the impeller vanes, and leaves through the side of the pump casing.
Difference between PDP and RDP RDP PDP _________________________________________________________ High Flow rate Low Much steady
pressure rise
High
Insufficient
high viscous liquid
OK
Generally needed
Priming
Self
_________________________________________________________
Classification of centrifugal pumps: Based on their utility, design and constructional features, centrifugal pumps can be classified with respect to follow characters: 1.
Type
of casing
a. Volute pump b. Turbine pump or diffusion pump 2.
Working head
a. Low lift centrifugal pump pump they work against the the heads up to 15 m b. Medium lift centrifugal pumps . Used to build up heads as high as 40 m c. 3.
High lift centrifugal pump employed to deliver liquids at heads above 40 m
Liquid handled
a. Closed impeller pump b. Semi-open impeller pump c. 4.
Open impeller pump
Number
a.
of impeller per shaft
Single stage centrifugal pump has one impeller, usually a low lift pump.
b. Multistage centrifugal pump
Has two or more impellers and pressure is built in steps, used usually for high working head and the number of stages depends upon the head required. 5.
Number
a.
of entrances to the impeller
Single entry or single suction pump : Water is admitted on one side of the impeller
b. Double entry or double suction pump: Water is admitted from both side of impeller; axial thrust is neutralized. Employed for pumping large quantity of fluid. 6. Relative direction of flow through impeller
a. Radial flow pump: Normally radial flow impellers are used in all centrifugal pumps b.
Axial
flow pump: Designed to deliver huge quantities of water at comparatively low
heads; ideally suited for irrigation purpose. c.
Mixed flow pump: mostly employed for irrigation purpose.
Centrifugal Pumps classification by flow Centrifugal pumps can be classified bases on the manner in which fluid flows through the pump. The manner in which fluid flows through the pump is determined by the design of the pump casing and the impeller. Three types of flow through the centrifugal p umps are 1. Radial flow 2.
Axial
flow
3. Mixed flow
Radial Flow Pump: Here head is developed principally by the action of centrifugal force. The liquid enters the impeller at the hub and flows radially to the periphery. The flow from the impeller is radial or pressure increase within its rotor rotor is due to centrifugal action. It is opposite to the Francis Turbine (radial flow turbine). The impeller blades are usually backward- curved, but there are also radial and forward-curved blade design, which slightly change the output output pressure. The impeller blades may generally be open, i.e. separated from the front casing only by a narrow clearance.
Shaft
Axial Flow In axial flow pump, the impeller pushes the liquid in the direction parallel to the pump shaft. Axial flow pump are sometimes called propeller pumps because they operate essentially the same as the propeller of the pump. The impeller of a typical axial flow pump and the flow through a radial flow pump are shown in fig below.
Mixed Flow A
pump in which the head is developed partly by centrifugal force and partly by the lift of the vanes on
the liquid. This type of pump has a single inlet impeller with the flow entering axially and discharging in an axial/radial direction.
O PERATING A
PRINCIPLES
centrifugal pump has the following characteristics: -
it is made up of a set of rotating vanes that are enclosed within a housing. These vanes are utilized to impart energy to a liquid through centrifugal force.
-
it consists of two main parts: a rotating e lement including an impeller and a shaft; and a stationary element made up of a casing, stuffing box, and bearings.
-
it transfers the energy provided by a pri me mover, such as an electric motor, steam turbine, or gasoline engine to energy within the liquid being pumped. This energy within the liquid is present as a velocity energy, pressure energy, or a combination of both.
The method by which this energy conversion is accomplished is unique. The rotating element of a centrifugal pump, which is motivated by the prime mover, is the impeller. The liquid being pumped surrounds the impeller, and as the impeller r otates, the rotating motion of the impeller imparts a rotating motion to the liquid. There are two components to the motion imparted to the liquid by the impeller: one motion is in the radial direction outward from the center of the impe ller. This motion is caused by the centrifugal force, due to the rotation of the liquid, which acts in a direction outward fr om the centre of the rotating impeller. Also, as the liquid leaves the impeller, it tends to move in a direction tangential to the outside diameter of the impeller. The actual liquid direction is a result of the two flow directions (Figure 25). The amount of energy being added to the liquid by the rotating impeller is related to the velocity with which the liquid moves. The energy ex pressed as pressure energy will be proportional to the square of the resultant exit velocity:
H = energy (meter of liquid) V = velocity (meter/sec) g = acceleration due to gravity (meter/sec^2) From these facts, two things can be predicted -
any increase in the impeller tip velocity will increase the energy imparted to the liquid.
-
any change in the vane tip velocity will result in a change in the energy imparted to the liquid that is proportional to the square of the change in tip velocity.
For example: Doubling the rotative speed of the impeller would double the tip speed, which in turn would quadruple the energy imparted to the liquid.
-
doubling the impeller diameter would double the tip speed, which again would quadruple the energy imparted to the liquid.
Points to note about the liquid that is being discharged from the tip of the impeller art that -
the liquid is being discharged from all points around the impeller periphery.
-
the liquid is moving in a direction that is general ly outward from and around the impeller.
-
the function of the casing is to gather and direct the liquid to the discharge nozzle of the pump. The casing is designed so that, at one point, the wall of the casing is very close to the impeller periphery. This point is called the tongue or shear wat er of the casing. Fig ure 26 illustrates
-
a typical casing design.
At a point just before the tongue, all the liquid discharged by the impeller has been collected and is ready to be lead into the discharge pipe. In most cases, this liquid possesses a higher ve locity than would be feasible to handle because high velocity means a high frictional loss in the discharge piping. The velocity in the discharge nozzle is decreased by increasing the area for flow (volute chamber). Note: As the area increases, the velocity ve locity decreases. This velocity can be converted into pressure energy by either of the following: a volute (Figure 27), or a set of diffusion vanes surrounding the impeller periphery (Figure 28).
Diffuser:
Some centrifugal pump contains diffusers. A diffuser is a set of stationary vanes that surround the impeller. The purpose of diffuser is to increase the efficiency of the centrifugal pump by allowing a more gradual expansion and less turbulent area for the the liquid to reduce in velocity. The diffuser vanes are designed in a manner that the liquid exiting the impeller will encounter an ever increasing flow area as it passes through diffuser. This increase in flow area causes reduction in flow velocity, converting kinetic energy into flow pressure.
Classification: Centrifugal pumps are divided into two general classes 1. Volute pumps 2. Diffuser or turbine pumps (1) Volute type:
Here impeller is surrounded by a spiral case. The outer boundary of which may be curve called volute. The absolute velocity of fluid living the impeller is reduced in this casing.
(2) Vane type or diffuser type or turbine type pump: In diffuser pump the impeller is surrounded by diffuser vanes which provide gradually enlarging passages to effect a gradual reduction in velocity. Because of superficial resemblance to a reaction turbine, this type is often called turbine pump. High efficiency is obtained by fitting a set of fixed / moving diffuser vanes round the the outside of the impeller. The vanes provide more opportunity for for the gradual reduction of the velocity of the fluid so that le ss energy is wasted in eddies. e ddies.
Volute type and vane type
Pw = Power developed to the fluid = rgHQ _____________________________________ w = shaft angular velocity = 2pN/60 N = shaft Rev/min t = shaft torque bhp = power required to drive the pump = break horse power bhp = wt _________________________________________________ h = Pw / bhp = rgQH / wt
V1= absolute value of water at inlet Vw1 = velocity of whirl at inlet Vr1 = relative velocity of liquid liquid at inlet Vf1 = velocity of flow at inlet = angle made by absolute velocity (V1) at inlet with the direction of motion of vane = angle made by relative velocity( Vr1) at inlet with the direction of motion of vane. V2, Vr2, Vf2, , are the corresponding value of outlet.
Pump Head (head deliver delivered) ed) The pump characteristics curve shows the relation between the head developed by the pump and its rate of discharge when the pump is operating at a given rotating speed. If the pump is delivering fluid through a piping system with the static lift z , the he ad that the pump must develop is equal to the 2
static lift plus the total head loss (proportional approx. to Q ). The system characteristics curve shows that the relation between the required pumping head and the flow rate in the pipeline.
The actual pump-operati p ump-operating ng head and t he flow rate are d etermined etermined by t he
intersection of t he two curves. The
head that the pump must develop = static lift + total hea d loss
The particular values of h and Q determined by this intersection may or may not be those for the
maximum efficiency of the particular pump. If they are not, this means that the pump is not exactly suited to the specific conditions. If the discharge and the suction side of the pumps are the same size, size , the velocity heads cancel out , but frequently the intake pipe is larger than the discharge pipe. The official test code provides that the head on a pump be the difference between the total energy heads at the intake and discharge flanges. However, flow condition at the discharge side are usually too
irregular for accurate pressure measurement.
Shutoff head of pumps: When a pump is filled with the fluid to be operated at normal speed with discharge closed, the head developed is called shutoff head. Ideally, this would appear to be a case of a force vortex create with pressure head difference between 2
2
eye and impeller periphery is (u 2 u1 )/2g. Although
there is no flow delivered, there is great deal of circulation within the impeller which cause
rotation of fluid in the eye of the impeller and for a distance of several pipe diameters in the intake pipe. 2
So actual shutoff head is approximately u2 / 2g. Shutoff head depends on impeller vane at inlet and exit and nature of intake. Deep
well turbine pumps:
1. 2 or more impellers are arranged in series and vertical shaft 2. Q is same: H = nvH 3. Small dia of well casing
small dia of impeller H developed in one stage is small need no. of stages 1. Casing not concentric but volute volute Water discharge to from one impeller to eye of other and need Diffuser vanes
Specific speed of pumps For pumps the commonly used definition of specific speed is
ns= [neQ] / h
3/4
where ne is the rotative speed for maximum efficiency.
Basic
output parameters p arameters
Assume:
Applying
steady flow Neglect viscous work and heat transfer Velocity in inlet and outlet are uniform Incompressible flow Bernoulli head, H=(p/rg
2
2
+ v /2g +z)2 - (p/rg + v /2g +z)1 = hs - hf------------------------------(1.1 1.1)
H = Head output generated by pump hs = Head supplied hf = Head loss v1 = v2, and z1 = z2, usually H=(p2+p1)/rg
Characteristics Character istics at constant speed Though some centrifugal pumps are driven by variable speed motors, the usual mode of operation of a pump is at constant speed its typical characteristics for such operation is give in fig. 17.9 Head versus-discharge curve may be transformed into that for some other speed(N) by means of similarity laws (Q w n and h w n ); however efficiency of the pump drops of as the rotative speed is 2
moved away from the optimum speed. The important feature shown if fig. 17.9 is that, if a pump is not operating near the optimum point, its efficiency drops off, depending on how far the mode of operation is from optimum.
By different impeller and casing design it is possible to vary the characteristics as show in fig.17.11. Thus a flat characteristic permits a considerable variation in the rate of discharge with but very little change in head, while a steep characteristics gives only a small variation in the flow for a relatively large change in head. The axial-flow pump has much steeper head-capacity curve than does any centrifugal pump, and instead of the break power at shutoff being a minimum, it is not only is a maximum but is very much larger than the power required at the point of maximum efficiency. This is a disadvantage both in starting up and in continued operation at low capacity.
Energy losses in pumps: Hydraulic losses: The head loss has several components. First of all, as water enters the vanes of the impeller, there may be shock loss due to the turbulence because of an improper relative-velocity angle at vane inlet. This loss is relatively large at low and high flow rates; it grows smaller as optimum operating conditions are approached. The second loss is that of fluid friction in the passages between the vanes. It varies varie s 2
approximately as Q
Third loss is due to circulatory flow at discharge discharge from the impeller created by the difference in the velocity along the working face of the vane and an increase in i n relative velocity on the back face of the vane. The component of head loss changes very little with flow rate.
In addition to these hydraulic head losses the efficiency of a pump is reduced by bearing and packing friction and by disk friction as well as by the effect of leakage. A typical relationship among these various losses is show in fig.
Cavitation in pumps: Cavitation occurs when pressure in the suction line falls below vapor pressure, vapor is formed and moves along with the stream. These vapor bubbles or cavities collapse when they reach regions of higher pressure on their way through the pump. The most obvious effects of cavitation are noise and vibration.
An
important factor in satisfactory operation of a pump is the avoidance of cavitation both for
the sake of good efficiency and for the prevention of impeller damage.
Minimize this factor for satisfactory opt. of pump( good Land prevention of impeller damage). Cav. Increase is indicated by drastic change in L.
It depends on operating condition and design of the p ump.
It changes as change in H and Q.
Effect of Viscosity in pumps:
It is seen from the g raph that, as viscosity is increased head-capacity curve become steepe r and power required increases. The dash line indicates the max L for each viscosity curve. It is seen that both the head and capacity at point of maximum efficiency decreases with increase in viscosity. As these are accompanied by an increase in the brake power, there is a marked decrease in effieciency. If Lopt =0.85 for water then Lopt =0.47 when pumping a liquid of viscosity 116 times that of water.
Efficiency of Pump:
Fig shows the approximate optimum efficiencies of modern water pumps of large capacity. It is seen that a gradual merging of one type into another, and so that dashed line indicates the probable maximum values. Generally, the larger the pump, the higher the attainable efficiency. L decreases as pump size decreases. The efficiency of pump is largely determined by the conditions at exit from the impeller and in the casing and is practically unaffected by subdividing the inlet.
Calculation of Q and A for pumps:
The discharge from any type of impeller may be found by multiplying the outlet area by the velocity. discharge = outlet area v av. Velocity The area that is most readily computed is the circumferential area for radial-flow impellers and corresponding areas for other type of impelle rs as how in fig. The effective flow area is f TDB for radial and mixed-flow impellers and f TDe /4 for axial flow impellers 2
where f is a factor ( typically about 0.95) to allow for the space taken up by vanes or hub respectively). The effective flow area is to be multiplied by the component of velocity that is normal to it, which is radial component for the pure centrifugal impeller, or the axial component for the propeller type, orin general the meridional component.
System characteristics for pumps: Pump should be chosen so that under normal working condition, the speed and capacity are such that operation is occurring close to peak efficiency. If not energy will be wasted and operation will be uneconomic. Choice of pump for a particular situation is complicated by large number of alternatives that are possible. First of all, there are many different designs of pumps with variety of specific speeds. By changing the speed of operation of a particular pump its operating characteristics can be changed. Selecting from among different sized homologous pumps will provide variation in characteristics. In addition different speeds of operation can be used with various sizes of homologous pumps. Under certain conditions it may be advantageous to install pumps in series or in parallel. When pumps are installed in series or parallel it is very important that they have reasonably similar head-capacity characteristics throughout their range of operation, otherwise, one pump will carry most of the load and, under certain conditions, all of the loa d, with other pump acting as hindrance rather than a help. In parallel, if the operating characteristics of the pump are quite different, a condition of backflow can occur in one of the pumps. Finally one must always be sure that selected pump wi ll not encounter cavitation problems over the full range of operating conditions. The mode of operation is best determined by plotting the pumping characteristics and the pipe system characteristics on the same diagram. The point at whic h two curves intersect gives an indication of what will take place. Generally one can choose between changing the speed of a given pump or selecting a particular size of homologous pump in order to obtain proper characteristics. The later is usually preferable because pump efficiency tends to decrease rather rapidly as the speed is changed from the optimum.
