Basic Theory Of Pump 61d2m
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6TH Week: Pump Design(8-Apr-2010)
1. Basic Theory of Pump Water, like electricity, will always flow along the path of least resistance. In order to lift water the pump must provide a path (area of low pressure or low resistance) to which the water will naturally seek to flow. It is critical then to recognize the role atmospheric pressure plays in creating such lift. At sea level the atmosphere exerts a force of 14.7 lb/in2 (PSI) on the earth's surface. The weight of the atmosphere on a body of water will prevent lift from occurring unless an area of low pressure is created. The below figure shows three hollow tubes, each with a surface area of 1 square inch, rising from sea level up into the atmosphere. In tube (A) atmospheric pressure is the same inside the tube as it is outside: 14.7 PSI. Since the weight of the atmosphere is being exerted equally across the surface, no change occurs in the water level inside the tube. In tube (B) a perfect vacuum is created making atmospheric pressure greater on the water outside the tube. The resulting differential causes water, flowing naturally to the area of lowest pressure to begin filling the tube until it reaches a height of 33.9 feet.
Why is 33.9 feet the highest water can be lifted in this example? Because at this point the weight of the water inside the tube exerts a pressure equal to the weight of the atmosphere
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pushing down on the ocean's surface. This height represents the maximum theoretical suction lift and can be verified using the following calculation. Divide atmospheric pressure at sea level by 0.0361 lb/in3 (the weight of one cubic inch of water) to obtain the theoretical suction lift. 14.7 (lb/in2) / 0.0361 (lb/in3) = 407.28 (inches)
2. Classification of Pump The simplest way to classify pumps is based on the method used to transmit power to the pumped liquid, mechanical principle behind this energy transfer, and mechanical device for moving fluid. These considerations result in two major pump classes: kinetic (dynamic) and positive displacement.
2.1 Kinetic pumps -- mainly centrifugal -- develop pressure by radial force, dynamic lift, or momentum change. Energy is continuously imparted to the liquid in centrifugal pumps, resulting in radial, mixed, or axial flow, depending on the impeller design. (1) Centrifugal -Mixed Flow, Radial Flow -Axial Flow (2) Special Effect -Vortex Pump -Jet Pump(Eductor) -Vacuum Pump -Ejector -Hydraulic Ram -Gas Lift -Electromagnetic
2.2 Positive displacement pumps discharge a given volume for each stroke of a reciprocating pump or revolution of a rotary pump. Energy is added in intermittent pulses by altering or displacing the confined volume of fluid in one or more cavities. Although it is difficult to compare the performance and limitations of various pump designs,
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common criteria are pressure and flow. Reciprocating pumps can generate enormous pressure, centrifugal pumps offer practically unlimited capacity, and rotary pumps are in between. (1) Reciprocating -Piston Pump, Plunger Pump, Diaphragm Pump (2) Rotary -Gear Pump, Vane Pump, Screw Pump, Cam Pump
2.3 Advantages of centrifugal pumps (1) Run at higher speeds to produce capacity, resulting in smaller sizes and lower costs (2) Deliver fluid free from pressure fluctuations (3) Operate at minimum flow without exceeding system pressure (4) Operate at maximum flow without excessive power (5) Designs match various system requirements (6) Use simple foundations due to absence of vibration (7) Handle various types of slurries consistently (8) Can be direct driven, easily adopted to direct motor, V-belt or other driver
2.4 Disadvantage of centrifugal pumps (1) Difficult to obtain very low flows at moderate to high pressures (2) Need to maintain minimum flow (3) Not suitable for high viscose, abrasive fluids
2.5 Advantages of reciprocating pumps (1) Operate with high efficiencies (2) Handle varying pressures at a constant speed (3) Abrasive-Slurry, High viscosity (4) Negligibly affected by corrosive fumes (5) Simple to maintain, long life (6) Produce momentary pressure spikes to clear pipeline obstructions (7) Are self-priming and not bothered by air entrainment
2.6 Advantages of rotary pumps (1) Provide continuous delivery with minor pressure fluctuations (2) Handle a wide variety of viscosities (3) Are self-priming
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2.7 Disadvantages of positive displacement pumps (1) Relatively operate with low capacity (2) Pulsating flow (3) Greater initial & maintenance cost (4) Short-life packing (5) Relatively wide ground area requirements 2.8 Pump selection chart
Fig 1. Approximate upper limit of pressure and capacity by pump class 3. Centrifugal pump fundamentals 3.1 Head The pressure at any point in a liquid can be thought of as being caused by a vertical column of the liquid which, due to its weight, exerts a pressure equal to the pressure at the point in question. The height of this column is called the static head and is expressed in of feet of liquid. The static head corresponding to any specific pressure is dependent upon the weight of the
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liquid according to the following formula.
A Centrifugal pump imparts velocity to a liquid. This velocity energy is then transformed largely into pressure energy as the liquid leaves the pump. Therefore, the head developed is approximately equal to the velocity energy at the periphery of the impeller. This relationship is expressed by the following well-known formula:
Where H = Total head developed in feet. v = Velocity at periphery of impeller in feet per sec. g = 32.2 feet/sec2 We can predict the approximate head of any centrifugal pump by calculating the peripheral velocity of the impeller and substituting into the above formula. A handy formula for peripheral velocity is:
D = Impeller diameter in inches, V = Velocity in ft./sec
The above demonstrates why we must always think in of feet of liquid rather than pressure when working with centrifugal pumps. A given pump with a given impeller diameter and speed will raise a liquid to a certain height regardless of the weight of the liquid, as shown in Fig.2
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Fig. 2 Identical pumps handling liquids of different specific gravities. All of the forms of energy involved in a liquid flow system can be expressed in of feet of liquid. The total of these various heads determines the total system head or the work which a pump must perform in the system. The various forms of head are defined as follows. Suction Lift exists when the source of supply is below the center line of the pump. Thus the Static Suction Lift is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
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Fig. 3-a Suction lift showing static heads in a pumping system where the pump is located above the suction tank. (static suction head) Suction Head exists when the source of supply is above the centerline of the pump. Thus the Static Suction Head is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
Fig. 3-b Where the pump is located below the suction tank. (static suction head)
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Static Discharge Head is the vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank. Total Static Head is the vertical distance in feet between the free level of the source of supply and the point of free discharge or the free surface of the discharge liquid. Friction Head (hf) is the head required to overcome the resistance to flow in the pipe and fittings. It is dependent upon the size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. Frictional tables are included in Water Data. Velocity Head (hv) is the energy of a liquid as a result of its motion at some velocity V. It is the equivalent head in feet through which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate the water. Velocity head can be calculated from the following formula:
The velocity head is usually insignificant and can be ignored in most high head systems. However, it can be a large factor and must be considered in low head systems. Pressure Head must be considered when a pumping system either begins or terminates in a tank which is under some pressure other than atmospheric. The pressure in such a tank must first be converted to feet of liquid. A vacuum in the suction tank or a positive pressure in the discharge tank must be added to the system head, whereas a positive pressure in the suction tank or vacuum in the discharge tank would be subtracted. The following is a handy formula for converting inches of mercury vacuum into feet of liquid.
The above forms of head, namely static, friction, velocity, and pressure, are combined to make up the total system head at any particular flow rate. Following are definitions of these combined or "Dynamic" head as they apply to the pump. Total Dynamic Suction Lift (hs) is the static suction lift minus the velocity head at the pump suction flange plus the total friction head in the suction line. The total dynamic suction lift, as
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determined on pump test, is the reading of a gauge on the suction flange, converted to feet of liquid and corrected to the pump centerline*, minus the velocity head at the point of gauge attachment. Total Dynamic Suction Head (hs) is the static suction head plus the velocity head at the pump suction flange minus the total friction head in the suction line. The total dynamic suction head, as determined on pump test, is the reading of the gauge on the suction flange, converted to feet of liquid and corrected to the pump centerline*, plus the velocity head at the point of gauge attachment. Total Dynamic Discharge Head (hd) is the static discharge head plus the velocity head at the pump discharge flange plus the total friction head in the discharge line. The total dynamic discharge head, as determined on pump test, is the reading of a gauge at the discharge flange, converted to feet of liquid and corrected to the pump centerline*, plus the velocity head at the point of gauge attachment. Total Head (H) or Total Dynamic Head (TDH) is the total dynamic discharge head minus the total dynamic suction head or TDH = hd + hs (with a suction lift) TDH = hd - hs (with a suction head)
3.2 Capacity Capacity (Q) is normally expressed in gallons per minute (gpm). Since liquids are essentially incompressible, there is a direct relationship between the capacity in a pipe and the velocity of flow. This relationship is as follows:
Where A = area of pipe or conduit in square feet. V = velocity of flow in feet per second. Q = Capacity in gallons per minute NOTE: On vertical pumps the correction should be made to the eye of the suction or lowest impeller
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3.3 Power and Efficiency The work performed by a pump is a function of the total head and the weight of the liquid pumped in a given time period. The pump capacity in gpm and the liquid specific gravity are normally used in the formulas rather than the actual weight of the liquid pumped. Pump input or brake horsepower (bhp) is the actual horsepower delivered to the pump shaft. Pump output or hydraulic horsepower (whp) is the liquid horsepower delivered by the pump. These two are defined by the following formulas.
The constant 3960 is obtained by dividing the number or foot pounds for one horsepower (33,000) by the weight of one gallon of water (8.33 pounds.) The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump. Therefore the pump efficiency is the ratio of these two values.
3.4 Specific Speed and Pump Type Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions. It is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute 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 pump characteristics. The following formula is used to determine specific speed:
Where N = Pump speed in RPM
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Q = Capacity in gpm at the best efficiency point H = Total head per stage at the best efficiency point The specific speed determines the general shape or class of the impeller as depicted in Fig.4 As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, Di, decreases. This ratio becomes 1.0 for a true axial flow impeller. Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific speeds 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 with a specific speed of 10,000 or greater generates it's head exclusively through axial forces. Radial impellers are generally low flow high head designs whereas axial flow impellers are high flow low head designs. Values of Specific Speed, Ns
Fig. 4 Impeller Design vs Specific Speed 3.5 Net Positive Suction Head (NPSH) and Cavitation 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 in feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump. The pressure which a liquid exerts on its surroundings is dependent upon its temperature. This pressure, called vapor pressure, is a unique characteristic of every fluid and increased with increasing temperature. When the vapor pressure within the fluid reaches the pressure of the surrounding medium, the fluid begins to vaporize or boil. The temperature at which this vaporization occurs will decrease as the pressure of the surrounding medium decreases.
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A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room temperature becomes 1700 cu. ft. of vapor at the same temperature. It is obvious from the above that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH is simply a measure of the amount of suction head present to prevent this vaporization at the lowest pressure point in the pump. NPSH Required is a function of the pump design. As the liquid es from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feet absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. The NPSH Required varies with speed and capacity within any particular pump. Pump manufacturer's curves normally provide this information. Net Positive Suction Head (NPSH) NPSH Available is a function of the systetm in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction. Fig. 4 shows four typical suction systems with the NPSH Available formulas applicable to each. It is important to correct for the specific gravity of the liquid and to convert all to units of "feet absolute" in using the formulas.
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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 Fig. 4 Calculation of system Net Positive Suction Head Available for typical suction conditions. In an existing system, the NPSH Available can be determined by a gauge on the pump suction. 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. Cavitation is a term used to describe the phenomenon, which occurs in a pump when there is insufficient NPSH Available. When the pressure of the liquid is reduced to a value equal to or below its vapor pressure the liquid begins to boil and small vapor bubbles or pockets begin to form. As these vapor bubbles move along the impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse. The collapse, or "implosion" is so rapid that it may be heard as a rumbling noise, as if you were pumping gravel. In high suction energy pumps, the collapses are generally high enough to cause minute pockets of fatigue failure on the impeller vane surfaces. This action may be progressive, and under severe (very high suction energy) conditions can cause serious pitting damage to the impeller. The accompanying noise is the easiest way to recognize cavitation. Besides possible impeller damage, excessive cavitation results in reduced capacity due to the vapor present in the pump. Also, the head may be reduced and/or be unstable and the power consumption may be erratic. Vibration and mechanical damage such as bearing failure can also occur as a result of operating in excessive cavitation, with high and very high suction energy pumps. The way to prevent the undesirable effects of cavitation in standard low suction energy pumps is to insure that the NPSH Available in the system is greater than the NPSH Required by the pump. High suction energy pumps require an additional NPSH margin, above the NPSH Required. Hydraulic Institute Standard (ANSI/HI 9.6.1) suggests NPSH margin ratios
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of from 1.2 to 2.5 times the NPSH Required, for high and very high suction energy pumps, when operating in the allowable operating range. 3.6 NPSH and Suction Specific Speed In deg a pumping system, it is essential to provide adequate NPSH available for proper pump operation. Insufficient NPSH available may seriously restrict pump selection, or even force an expensive system redesign. On the other hand, providing excessive NPSH available may needlessly increase system cost. Suction specific speed may provide help in this situation. 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. For a given pump, the suction specific speed is generally a constant - it does not change when the pump speed is changed. Experience has shown that 9000 is a reasonable value of suction specific speed. Pumps with a minimum suction specific speed of 9000 are readily available, and are not normally subject to severe operating restrictions, unless the pump speed pushes the pump into high or very high suction energy. An example: Flow 2,000 GPM; head 600 ft. What NPSHA will be required? Assume: at 600 ft., 3500 RPM operation will be required.
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A related problem is in selecting a new pump, especially at higher flow, for an existing system. Suction specific speed will highlight applications where NPSHA may restrict pump selection. An example: Existing system: Flow 2000 GPM; head 600 ft.; NPSHA 30 ft.; Specific Gravity 1.0; Suction Nozzle 6 in. - What is the maximum speed at which a pump can be run without exceeding NPSH available? (NPSHMargin Ratio = 1.5 from above @ S.E. = 173 x 106)
Running a pump at this speed would require a gear and at this speed, the pump might not develop the required head. At a mini-mum, existing NPSH A is constraining pump selection. Same system as 1. Is a double suction pump practical? For a double suction pump De = .75 x 6" = 4.5 S.E. = 4.5 x 3550 x 9000 x 1.0 S.E. = 136 x 106 (High S.E.) For a double suction pump, flow is divided by two.
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Using a double suction pump is one way of meeting system NPSH and obtaining a higher head. The amount of energy in a pumped fluid, that flashes into vapor and then collapses back to a liquid in the higher pressure area of the impeller inlet, determines the extent of the noise and/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) High Suction Energy starts at 160 x 10 6 for end suction pumps and 120 x 10 6 for horizontal split case pumps. Very high suction energy starts at 1.5 times the High Suction Energy values. For estimating purposes you can normally assume that the impeller eye diameter is approximately 90% of the suction nozzle size, for an end suction pump, and 75% of the suction size for a double suction split case pump. According to the Hydraulic Institute, ans NPSH margin is required above the NPSHR of the pump to supress incipient cavitation. The amount of margin is a function of Suction Energy and the critical nature of the application as follows: Suction Energy NPSHMargin Ratio (NPSHA/NPSHR) Low
1.1 - 1.3
High
1.2 - 1.7
Very High
1.7 - 2.5
Suction specific speed 9,000, pump speed 3550 RPM, suction nozzle size 6 inch, specific gravity 1.0, and the pump type is end suction. De ~ .9 x 6" = 5.4" Suction Energy = De x N x S x Sg = 5.4 x 3550 x 9,000 x 1.0 = 173 x 106 Since 173 x 106 > 160 x 106 , this is a High Suction Energy pump.
3.7 Pump Characteristic Curves The performance of a centrifugal pump can be shown graphically on a characteristic curve. A typical characteristic curve shows the total dynamic head, brake horsepower, efficiency, and net positive Suction head all plotted over the capacity range of the pump.
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Figures 5, 6, & 7 are non-dimensional curves which indicate the general shape of the characteristic curves for the various types of pumps. They show the head, brake horsepower, and efficiency plotted as a percent of their values at the design or best efficiency point of the pump. Fig. 5 below shows that the head curve for a radial flow pump is relatively flat and that the head decreases gradually as the flow increases. Note that the brake horsepower increases gradually over the flow range with the maximum normally at the point of maximum flow.
Fig. 5 Radial Flow Pump Mixed flow centrifugal pumps and axial flow or propeller pumps have considerably different characteristics as shown in Figs. 6 and 7 below. The head curve for a mixed flow pump is steeper than for a radial flow pump. The shut-off head is usually 150% to 200% of the design head, The brake horsepower remains fairly constant over the flow range. For a typical axial flow pump, the head and brake horsepower both increase drastically near shutoff as shown in Fig. 7.
Fig. 6 Mixed Flow Pump
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Fig. 7 Axial Flow Pump The distinction between the above three classes is not absolute, and there are many pumps with characteristics falling somewhere between the three. For instance, the Francis vane impeller would have a characteristic between the radial and mixed flow classes. Most turbine pumps are also in this same range depending upon their specific speeds. Fig. 8 below shows a typical pump curve as furnished by a manufacturer. It is a composite curve which tells at a glance what the pump will do at a given speed with various impeller diameters from maximum to minimum. Constant horsepower, efficiency, and NPSHR lines are superimposed over the various head curves. It is made up from individual test curves at various diameters.
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Fig. 8 Composite Performance Curve
1. Basic Theory of Pump Water, like electricity, will always flow along the path of least resistance. In order to lift water the pump must provide a path (area of low pressure or low resistance) to which the water will naturally seek to flow. It is critical then to recognize the role atmospheric pressure plays in creating such lift. At sea level the atmosphere exerts a force of 14.7 lb/in2 (PSI) on the earth's surface. The weight of the atmosphere on a body of water will prevent lift from occurring unless an area of low pressure is created. The below figure shows three hollow tubes, each with a surface area of 1 square inch, rising from sea level up into the atmosphere. In tube (A) atmospheric pressure is the same inside the tube as it is outside: 14.7 PSI. Since the weight of the atmosphere is being exerted equally across the surface, no change occurs in the water level inside the tube. In tube (B) a perfect vacuum is created making atmospheric pressure greater on the water outside the tube. The resulting differential causes water, flowing naturally to the area of lowest pressure to begin filling the tube until it reaches a height of 33.9 feet.
Why is 33.9 feet the highest water can be lifted in this example? Because at this point the weight of the water inside the tube exerts a pressure equal to the weight of the atmosphere
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pushing down on the ocean's surface. This height represents the maximum theoretical suction lift and can be verified using the following calculation. Divide atmospheric pressure at sea level by 0.0361 lb/in3 (the weight of one cubic inch of water) to obtain the theoretical suction lift. 14.7 (lb/in2) / 0.0361 (lb/in3) = 407.28 (inches)
2. Classification of Pump The simplest way to classify pumps is based on the method used to transmit power to the pumped liquid, mechanical principle behind this energy transfer, and mechanical device for moving fluid. These considerations result in two major pump classes: kinetic (dynamic) and positive displacement.
2.1 Kinetic pumps -- mainly centrifugal -- develop pressure by radial force, dynamic lift, or momentum change. Energy is continuously imparted to the liquid in centrifugal pumps, resulting in radial, mixed, or axial flow, depending on the impeller design. (1) Centrifugal -Mixed Flow, Radial Flow -Axial Flow (2) Special Effect -Vortex Pump -Jet Pump(Eductor) -Vacuum Pump -Ejector -Hydraulic Ram -Gas Lift -Electromagnetic
2.2 Positive displacement pumps discharge a given volume for each stroke of a reciprocating pump or revolution of a rotary pump. Energy is added in intermittent pulses by altering or displacing the confined volume of fluid in one or more cavities. Although it is difficult to compare the performance and limitations of various pump designs,
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common criteria are pressure and flow. Reciprocating pumps can generate enormous pressure, centrifugal pumps offer practically unlimited capacity, and rotary pumps are in between. (1) Reciprocating -Piston Pump, Plunger Pump, Diaphragm Pump (2) Rotary -Gear Pump, Vane Pump, Screw Pump, Cam Pump
2.3 Advantages of centrifugal pumps (1) Run at higher speeds to produce capacity, resulting in smaller sizes and lower costs (2) Deliver fluid free from pressure fluctuations (3) Operate at minimum flow without exceeding system pressure (4) Operate at maximum flow without excessive power (5) Designs match various system requirements (6) Use simple foundations due to absence of vibration (7) Handle various types of slurries consistently (8) Can be direct driven, easily adopted to direct motor, V-belt or other driver
2.4 Disadvantage of centrifugal pumps (1) Difficult to obtain very low flows at moderate to high pressures (2) Need to maintain minimum flow (3) Not suitable for high viscose, abrasive fluids
2.5 Advantages of reciprocating pumps (1) Operate with high efficiencies (2) Handle varying pressures at a constant speed (3) Abrasive-Slurry, High viscosity (4) Negligibly affected by corrosive fumes (5) Simple to maintain, long life (6) Produce momentary pressure spikes to clear pipeline obstructions (7) Are self-priming and not bothered by air entrainment
2.6 Advantages of rotary pumps (1) Provide continuous delivery with minor pressure fluctuations (2) Handle a wide variety of viscosities (3) Are self-priming
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2.7 Disadvantages of positive displacement pumps (1) Relatively operate with low capacity (2) Pulsating flow (3) Greater initial & maintenance cost (4) Short-life packing (5) Relatively wide ground area requirements 2.8 Pump selection chart
Fig 1. Approximate upper limit of pressure and capacity by pump class 3. Centrifugal pump fundamentals 3.1 Head The pressure at any point in a liquid can be thought of as being caused by a vertical column of the liquid which, due to its weight, exerts a pressure equal to the pressure at the point in question. The height of this column is called the static head and is expressed in of feet of liquid. The static head corresponding to any specific pressure is dependent upon the weight of the
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liquid according to the following formula.
A Centrifugal pump imparts velocity to a liquid. This velocity energy is then transformed largely into pressure energy as the liquid leaves the pump. Therefore, the head developed is approximately equal to the velocity energy at the periphery of the impeller. This relationship is expressed by the following well-known formula:
Where H = Total head developed in feet. v = Velocity at periphery of impeller in feet per sec. g = 32.2 feet/sec2 We can predict the approximate head of any centrifugal pump by calculating the peripheral velocity of the impeller and substituting into the above formula. A handy formula for peripheral velocity is:
D = Impeller diameter in inches, V = Velocity in ft./sec
The above demonstrates why we must always think in of feet of liquid rather than pressure when working with centrifugal pumps. A given pump with a given impeller diameter and speed will raise a liquid to a certain height regardless of the weight of the liquid, as shown in Fig.2
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Fig. 2 Identical pumps handling liquids of different specific gravities. All of the forms of energy involved in a liquid flow system can be expressed in of feet of liquid. The total of these various heads determines the total system head or the work which a pump must perform in the system. The various forms of head are defined as follows. Suction Lift exists when the source of supply is below the center line of the pump. Thus the Static Suction Lift is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
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Fig. 3-a Suction lift showing static heads in a pumping system where the pump is located above the suction tank. (static suction head) Suction Head exists when the source of supply is above the centerline of the pump. Thus the Static Suction Head is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
Fig. 3-b Where the pump is located below the suction tank. (static suction head)
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Static Discharge Head is the vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank. Total Static Head is the vertical distance in feet between the free level of the source of supply and the point of free discharge or the free surface of the discharge liquid. Friction Head (hf) is the head required to overcome the resistance to flow in the pipe and fittings. It is dependent upon the size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. Frictional tables are included in Water Data. Velocity Head (hv) is the energy of a liquid as a result of its motion at some velocity V. It is the equivalent head in feet through which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate the water. Velocity head can be calculated from the following formula:
The velocity head is usually insignificant and can be ignored in most high head systems. However, it can be a large factor and must be considered in low head systems. Pressure Head must be considered when a pumping system either begins or terminates in a tank which is under some pressure other than atmospheric. The pressure in such a tank must first be converted to feet of liquid. A vacuum in the suction tank or a positive pressure in the discharge tank must be added to the system head, whereas a positive pressure in the suction tank or vacuum in the discharge tank would be subtracted. The following is a handy formula for converting inches of mercury vacuum into feet of liquid.
The above forms of head, namely static, friction, velocity, and pressure, are combined to make up the total system head at any particular flow rate. Following are definitions of these combined or "Dynamic" head as they apply to the pump. Total Dynamic Suction Lift (hs) is the static suction lift minus the velocity head at the pump suction flange plus the total friction head in the suction line. The total dynamic suction lift, as
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determined on pump test, is the reading of a gauge on the suction flange, converted to feet of liquid and corrected to the pump centerline*, minus the velocity head at the point of gauge attachment. Total Dynamic Suction Head (hs) is the static suction head plus the velocity head at the pump suction flange minus the total friction head in the suction line. The total dynamic suction head, as determined on pump test, is the reading of the gauge on the suction flange, converted to feet of liquid and corrected to the pump centerline*, plus the velocity head at the point of gauge attachment. Total Dynamic Discharge Head (hd) is the static discharge head plus the velocity head at the pump discharge flange plus the total friction head in the discharge line. The total dynamic discharge head, as determined on pump test, is the reading of a gauge at the discharge flange, converted to feet of liquid and corrected to the pump centerline*, plus the velocity head at the point of gauge attachment. Total Head (H) or Total Dynamic Head (TDH) is the total dynamic discharge head minus the total dynamic suction head or TDH = hd + hs (with a suction lift) TDH = hd - hs (with a suction head)
3.2 Capacity Capacity (Q) is normally expressed in gallons per minute (gpm). Since liquids are essentially incompressible, there is a direct relationship between the capacity in a pipe and the velocity of flow. This relationship is as follows:
Where A = area of pipe or conduit in square feet. V = velocity of flow in feet per second. Q = Capacity in gallons per minute NOTE: On vertical pumps the correction should be made to the eye of the suction or lowest impeller
6TH Week: Pump Design(8-Apr-2010)
3.3 Power and Efficiency The work performed by a pump is a function of the total head and the weight of the liquid pumped in a given time period. The pump capacity in gpm and the liquid specific gravity are normally used in the formulas rather than the actual weight of the liquid pumped. Pump input or brake horsepower (bhp) is the actual horsepower delivered to the pump shaft. Pump output or hydraulic horsepower (whp) is the liquid horsepower delivered by the pump. These two are defined by the following formulas.
The constant 3960 is obtained by dividing the number or foot pounds for one horsepower (33,000) by the weight of one gallon of water (8.33 pounds.) The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump. Therefore the pump efficiency is the ratio of these two values.
3.4 Specific Speed and Pump Type Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions. It is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute 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 pump characteristics. The following formula is used to determine specific speed:
Where N = Pump speed in RPM
6TH Week: Pump Design(8-Apr-2010)
Q = Capacity in gpm at the best efficiency point H = Total head per stage at the best efficiency point The specific speed determines the general shape or class of the impeller as depicted in Fig.4 As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, Di, decreases. This ratio becomes 1.0 for a true axial flow impeller. Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific speeds 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 with a specific speed of 10,000 or greater generates it's head exclusively through axial forces. Radial impellers are generally low flow high head designs whereas axial flow impellers are high flow low head designs. Values of Specific Speed, Ns
Fig. 4 Impeller Design vs Specific Speed 3.5 Net Positive Suction Head (NPSH) and Cavitation 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 in feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump. The pressure which a liquid exerts on its surroundings is dependent upon its temperature. This pressure, called vapor pressure, is a unique characteristic of every fluid and increased with increasing temperature. When the vapor pressure within the fluid reaches the pressure of the surrounding medium, the fluid begins to vaporize or boil. The temperature at which this vaporization occurs will decrease as the pressure of the surrounding medium decreases.
6TH Week: Pump Design(8-Apr-2010)
A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room temperature becomes 1700 cu. ft. of vapor at the same temperature. It is obvious from the above that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH is simply a measure of the amount of suction head present to prevent this vaporization at the lowest pressure point in the pump. NPSH Required is a function of the pump design. As the liquid es from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feet absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. The NPSH Required varies with speed and capacity within any particular pump. Pump manufacturer's curves normally provide this information. Net Positive Suction Head (NPSH) NPSH Available is a function of the systetm in which the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction. Fig. 4 shows four typical suction systems with the NPSH Available formulas applicable to each. It is important to correct for the specific gravity of the liquid and to convert all to units of "feet absolute" in using the formulas.
6TH Week: Pump Design(8-Apr-2010)
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 Fig. 4 Calculation of system Net Positive Suction Head Available for typical suction conditions. In an existing system, the NPSH Available can be determined by a gauge on the pump suction. 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. Cavitation is a term used to describe the phenomenon, which occurs in a pump when there is insufficient NPSH Available. When the pressure of the liquid is reduced to a value equal to or below its vapor pressure the liquid begins to boil and small vapor bubbles or pockets begin to form. As these vapor bubbles move along the impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse. The collapse, or "implosion" is so rapid that it may be heard as a rumbling noise, as if you were pumping gravel. In high suction energy pumps, the collapses are generally high enough to cause minute pockets of fatigue failure on the impeller vane surfaces. This action may be progressive, and under severe (very high suction energy) conditions can cause serious pitting damage to the impeller. The accompanying noise is the easiest way to recognize cavitation. Besides possible impeller damage, excessive cavitation results in reduced capacity due to the vapor present in the pump. Also, the head may be reduced and/or be unstable and the power consumption may be erratic. Vibration and mechanical damage such as bearing failure can also occur as a result of operating in excessive cavitation, with high and very high suction energy pumps. The way to prevent the undesirable effects of cavitation in standard low suction energy pumps is to insure that the NPSH Available in the system is greater than the NPSH Required by the pump. High suction energy pumps require an additional NPSH margin, above the NPSH Required. Hydraulic Institute Standard (ANSI/HI 9.6.1) suggests NPSH margin ratios
6TH Week: Pump Design(8-Apr-2010)
of from 1.2 to 2.5 times the NPSH Required, for high and very high suction energy pumps, when operating in the allowable operating range. 3.6 NPSH and Suction Specific Speed In deg a pumping system, it is essential to provide adequate NPSH available for proper pump operation. Insufficient NPSH available may seriously restrict pump selection, or even force an expensive system redesign. On the other hand, providing excessive NPSH available may needlessly increase system cost. Suction specific speed may provide help in this situation. 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. For a given pump, the suction specific speed is generally a constant - it does not change when the pump speed is changed. Experience has shown that 9000 is a reasonable value of suction specific speed. Pumps with a minimum suction specific speed of 9000 are readily available, and are not normally subject to severe operating restrictions, unless the pump speed pushes the pump into high or very high suction energy. An example: Flow 2,000 GPM; head 600 ft. What NPSHA will be required? Assume: at 600 ft., 3500 RPM operation will be required.
6TH Week: Pump Design(8-Apr-2010)
A related problem is in selecting a new pump, especially at higher flow, for an existing system. Suction specific speed will highlight applications where NPSHA may restrict pump selection. An example: Existing system: Flow 2000 GPM; head 600 ft.; NPSHA 30 ft.; Specific Gravity 1.0; Suction Nozzle 6 in. - What is the maximum speed at which a pump can be run without exceeding NPSH available? (NPSHMargin Ratio = 1.5 from above @ S.E. = 173 x 106)
Running a pump at this speed would require a gear and at this speed, the pump might not develop the required head. At a mini-mum, existing NPSH A is constraining pump selection. Same system as 1. Is a double suction pump practical? For a double suction pump De = .75 x 6" = 4.5 S.E. = 4.5 x 3550 x 9000 x 1.0 S.E. = 136 x 106 (High S.E.) For a double suction pump, flow is divided by two.
6TH Week: Pump Design(8-Apr-2010)
Using a double suction pump is one way of meeting system NPSH and obtaining a higher head. The amount of energy in a pumped fluid, that flashes into vapor and then collapses back to a liquid in the higher pressure area of the impeller inlet, determines the extent of the noise and/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) High Suction Energy starts at 160 x 10 6 for end suction pumps and 120 x 10 6 for horizontal split case pumps. Very high suction energy starts at 1.5 times the High Suction Energy values. For estimating purposes you can normally assume that the impeller eye diameter is approximately 90% of the suction nozzle size, for an end suction pump, and 75% of the suction size for a double suction split case pump. According to the Hydraulic Institute, ans NPSH margin is required above the NPSHR of the pump to supress incipient cavitation. The amount of margin is a function of Suction Energy and the critical nature of the application as follows: Suction Energy NPSHMargin Ratio (NPSHA/NPSHR) Low
1.1 - 1.3
High
1.2 - 1.7
Very High
1.7 - 2.5
Suction specific speed 9,000, pump speed 3550 RPM, suction nozzle size 6 inch, specific gravity 1.0, and the pump type is end suction. De ~ .9 x 6" = 5.4" Suction Energy = De x N x S x Sg = 5.4 x 3550 x 9,000 x 1.0 = 173 x 106 Since 173 x 106 > 160 x 106 , this is a High Suction Energy pump.
3.7 Pump Characteristic Curves The performance of a centrifugal pump can be shown graphically on a characteristic curve. A typical characteristic curve shows the total dynamic head, brake horsepower, efficiency, and net positive Suction head all plotted over the capacity range of the pump.
6TH Week: Pump Design(8-Apr-2010)
Figures 5, 6, & 7 are non-dimensional curves which indicate the general shape of the characteristic curves for the various types of pumps. They show the head, brake horsepower, and efficiency plotted as a percent of their values at the design or best efficiency point of the pump. Fig. 5 below shows that the head curve for a radial flow pump is relatively flat and that the head decreases gradually as the flow increases. Note that the brake horsepower increases gradually over the flow range with the maximum normally at the point of maximum flow.
Fig. 5 Radial Flow Pump Mixed flow centrifugal pumps and axial flow or propeller pumps have considerably different characteristics as shown in Figs. 6 and 7 below. The head curve for a mixed flow pump is steeper than for a radial flow pump. The shut-off head is usually 150% to 200% of the design head, The brake horsepower remains fairly constant over the flow range. For a typical axial flow pump, the head and brake horsepower both increase drastically near shutoff as shown in Fig. 7.
Fig. 6 Mixed Flow Pump
6TH Week: Pump Design(8-Apr-2010)
Fig. 7 Axial Flow Pump The distinction between the above three classes is not absolute, and there are many pumps with characteristics falling somewhere between the three. For instance, the Francis vane impeller would have a characteristic between the radial and mixed flow classes. Most turbine pumps are also in this same range depending upon their specific speeds. Fig. 8 below shows a typical pump curve as furnished by a manufacturer. It is a composite curve which tells at a glance what the pump will do at a given speed with various impeller diameters from maximum to minimum. Constant horsepower, efficiency, and NPSHR lines are superimposed over the various head curves. It is made up from individual test curves at various diameters.
6TH Week: Pump Design(8-Apr-2010)
Fig. 8 Composite Performance Curve
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