Water Pumps. turbo-hydraulic pumps, positive-displacement pumps.

Transcription

1 WATER PUMPS

2 Water Pumps Water pumps are devices designed to convert mechanical energy to hydraulic energy. All forms of water pumps may be classified into two basic categories: turbo-hydraulic pumps, positive-displacement pumps.

3 Turbo-hydraulic pumps are: centrifugal pumps, Propeller pumps, and jet pumps.

4 Analysis of turbo-hydraulic pumps: is a problem involving fundamental principles of hydraulics. Positive-displacement pumps: move fluid strictly by precise machine displacements such as a gear system rotating within a closed housing (screw pumps) or a piston moving in a sealed cylinder (reciprocal pumps).

5 Centrifugal Pumps Modern centrifugal pumps basically consist of two parts:- 1. the rotating element ( commonly called the impeller); 2. the housing that encloses the rotating element and seals the pressurized liquid inside. The power is supplied by a motor to the shaft of the impeller.

6 Cross section of a centrifugal pump

7

8

9

10 Jet (Mixed-Flow) Pumps

11 Pumps and Pumping Stations Pumps add energy to fluids and therefore are accounted for in the energy equation Energy required by the pump depends on: Discharge rate Resistance to flow (head that the pump must overcome) Pump efficiency (ratio of power entering fluid to power supplied to the pump) Efficiency of the drive (usually an electric motor) 2 2 v1 p1 v2 p2 z1 H pump z2 H 2g 2g L 2 H v L h f h h K minor f i 2 g

12 Pump Definition (Total) Static head difference in head between suction and discharge sides of pump in the absence of flow; equals difference in elevation of free surfaces of the fluid source and destination Static suction head head on suction side of pump in absence of flow, if pressure at that point is >0 Static discharge head head on discharge side of pump in absence of flow Static discharge head Total static head Static suction head

13 Pump Definition (Total) Static head difference in head between suction and discharge sides of pump in the absence of flow; equals difference in elevation of free surfaces of the fluid source and destination Static suction lift negative head on suction side of pump in absence of flow, if pressure at that point is <0 Static discharge head head on discharge side of pump in absence of flow Total static head Static discharge head Static suction lift

14 Pump Definition Static discharge head Total static head (both) Static suction head Static discharge head Static suction lift Total static head Static discharge head Static suction head Static discharge head Static suction lift Note: Suction and discharge head/ lift measured from pump centerline

15 Pump Definition (Total) Dynamic head, dynamic suction head or lift, and dynamic discharge head same as corresponding static heads, but for a given pumping scenario; includes frictional and minor headlosses Energy Line Total dynamic head Dynamic discharge head Dynamic suction lift

16 Example. Determine the static head, total dynamic head (TDH), and total headloss in the system shown below. El = 640 ft p s =6 psig El = 730 ft El = 630 ft p d =48 psig Total static head 730 ft 630 ft 100 ft 2.31 ft TDH 48 6 psi ft psi H TDH Static head ft 24.7 ft L

17 Example. A booster pumping station is being designed to transport water from an aqueduct to a water supply reservoir, as shown below. The maximum design flow is 25 mgd (38.68 ft 3 /s). Determine the required TDH, given the following: H-W C values are 120 on suction side and 145 on discharge side Minor loss coefficients are 0.50 for pipe entrance 0.18 for 45 o bend in a 48-in pipe 0.30 for 90 o bend in a 36-in pipe 0.16 and 0.35 for 30-in and 36-in butterfly valves, respectively Minor loss for an expansion is 0.25(v 2 2 v 12 )/2g El = 6127 to 6132 ft 30 to 48 expansion 8500of 36 pipe w/one 90 o bend and eight butterfly valves El = 6349 to 6357 ft Short 30 pipe w/30 butterfly valve 4000of 48 pipe w/two 45 o bends

18 1. Determine pipeline velocities from v = Q/A.. v 30 = 7.88 ft/s, v 36 = 5.47 ft/s, v 48 = 3.08 ft/s 2. Minor losses, suction side: 2 v30 Entrance: hl ft 2g 2 v30 Butterfly valve: hl ft 2g 2 2 v30 v 48 Expansion: hl ft 2g 2 o v 48 Two 45 bends: hl 2* ft 2g h L,minor 0.91 ft

19 3. Minor losses, discharge side: 2 v 36 8 Butterfly valves: hl 8* ft 2g 2 o v36 One 90 bend: hl ft 2g h L,minor 1.90 ft

20 4. Pipe friction losses: S h 1.85 f Q 2.63 L 0.43CD h f 1.85 Q L CD h f, suction ft /12 h f, discharge ft /12

21 5. Loss of velocity head at exit: Exit: h L v g 0.46 ft 6. Total static head under worst-case scenario (lowest water level in aqueduct, highest in reservoir): Static head ft 230 ft 7. Total dynamic head required: TDH H h h h static L, minor f L, exit ft ft

22 Pump Power P Q TDH CF E p P = Power supplied to the pump from the shaft; also called brake power (kw or hp) Q = Flow (m 3 /s or ft 3 /s) TDH = Total dynamic head = Specific wt. of fluid (9800 N/m 3 or 62.4 lb/ft 3 at 20 o C) CF = conversion factor: 1000 W/kW for SI, 550 (ft-lb/s)/hp for US E p = pump efficiency, dimensionless; accounts only for pump, not the drive unit (electric motor) Useful conversion: kw/hp

23 Example. Water is pumped 10 miles from a lake at elevation 100 ft to a reservoir at 230 ft. What is the monthly power cost at $0.08/kW-hr, assuming continuous pumping and given the following info: Diameter D = 48 in; Roughness e = ft, Efficiency P e =80% Flow = 25 mgd = ft 3 /s T = 60 o F Ignore minor losses El = 230 ft 2 El = 100 ft 1 10 mi of 48 pipe, e =0.003 ft 2 v1 2g p 1 2 v2 p2 z1 H pump 2g z2 HL H TDH z z H H h pump 2 1 stat L f TDH Hstat hf

24 El = 230 ft 2 El = 100 ft 1 10 mi of 48 pipe, e = ft TDH Hstat hf H ft 130 ft stat 2 L v hf f D 2 g v Q / A 3.08 ft/s Find f from Moody diagram 3.08 ft/s4 ft 6 vd Re 1.01x10 e D x10 ft /s ft 4 ft 7.5x10 4

25 El = 230 ft 2 10 mi of 48 pipe, El = 100 ft 1 e = ft 6 Re 1.01x10 e D 7.5x10 4 f

26 h f 2 10*5280 ft 3.08 ft/s ft 2 4 ft ft/s TDH H h ft ft stat f P ft /s ft 62.4 lb/ft Q TDH CF Ep ft-lb/s hp 918 hp kw $0.08 hr Daily cost 918 hp $1315 / d hp kw-hr d

27 Pump Selection System curve indicates TDH required as a function of Q for the given system For a given static head, TDH depends only on H L, which is approximately proportional to v 2 /2g Q is proportion to v, so H L is approximately proportional to Q 2 (or Q 1.85 if H-W eqn is used to model h f ) System curve is therefore approximately parabolic

28 Example. Generate the system curve for the pumping scenario shown below. The pump is close enough to the source reservoir that suction pipe friction can be ignored, but valves, fittings, and other sources of minor losses should be considered. On the discharge side, the 1000 ft of 16-in pipe connects the pump to the receiving reservoir. The flow is fully turbulent with D-W friction factor of Coefficients for minor losses are shown below. K values 40 ft 6 ft Suction Discharge

29 The sum of the K values for minor losses is 2.52 on the suction side and 5.52 on the discharge side. The total of minor headlosses is therefore 8.04 v 2 /2g. An additional 1.0 v 2 /2g of velocity head is lost when the water enters the receiving reservoir L v 1000 ft v v The frictional headloss is: hf f D 2 g 1.33 ft 2 g 2 g Total headloss is therefore ( )v 2 /2g = v 2 /2g. v can be written as Q/A, and A = pd 2 / 4 = 1.40 ft 2. The static head is 34 ft. So: 2 v TDH Hstat HL 34 ft g ft Q /1.40 ft 34 ft 0.19 s Q 2 ft ft/s 2

30 s TDH 34ft 0.19 Q ft 5 2 TDH (ft) System curve Static head Discharge, Q (ft 3 /s)

31 Pump Selection Pump curve indicates TDH provided by the pump as a function of Q; Depends on particular pump; info usually provided by manufacturer TDH at zero flow is called the shutoff head Pump efficiency Can be plotted as fcn(q), along with pump curve, on a single graph Typically drops fairly rapidly on either side of an optimum; flow at optimum efficiency known as normal or rated capacity Ideally, pump should be chosen so that operating point corresponds to nearly peak pump efficiency ( BEP, best efficiency point)

32 Pump Performance and Efficiency Curves Shutoff head Rated hp Rated capacity

33 Pump Efficiency Pump curves depend on pump geometry (impeller D) and speed

34 Pump Selection At any instant, a system has a single Q and a single TDH, so both curves must pass through that point; operating point is intersection of system and pump curves

35 Pump System Curve System curve may change over time, due to fluctuating reservoir levels, gradual changes in friction coefficients, or changed valve settings.

36 Pump Selection: Multiple Pumps Pumps often used in series or parallel to achieve desired pumping scenario In most cases, a backup pump must be provided to meet maximum flow conditions if one of the operating ( duty ) pumps is out of service. Pumps in series have the same Q, so if they are identical, they each impart the same TDH, and the total TDH is additive Pumps in parallel must operate against the same TDH, so if they are identical, they contribute equal Q, and the total Q is additive Adding a second pump moves the operating point up the system curve, but in different ways for series and parallel operation

37 Efficiency, % TDH (ft) Example. A pump station is to be designed for an ultimate Q of 1200 gpm at a TDH of 80 ft. At present, it must deliver 750 gpm at 60 ft. Two types of pump are available, with pump curves as shown. Select appropriate pumps and describe the operating strategy. How will the system operate under an interim condition when the requirement is for 600 gpm and 80-ft TDH? Pump B Pump A Flow rate (gpm)

38 Efficiency, % TDH (ft) Either type of pump can meet current needs (750 gpm at 60 ft); pump B will supply slightly more flow and head than needed, so a valve could be partially closed. Pump B has higher efficiency under these conditions, and so would be preferred Pump B Pump A Flow rate (gpm)

39 Efficiency, % TDH (ft) The pump characteristic curve for two type-b pumps in parallel can be drawn by taking the curve for one type-b pump, and doubling Q at each value of TDH. Such a scenario would meet the ultimate need (1200 gpm at 80 ft), as shown below Pump B Pump A Flow rate (gpm)

40 Efficiency, % TDH (ft) A pump characteristic curve for one type-a and one type-b pump in parallel can be drawn in the same way. This arrangement would also meet the ultimate demand. Note that the type-b pump provides no flow at TDH>113 ft, so at higher TDH, the composite curve is identical to that for just one type-a pump. (A check valve would prevent reverse flow through pump B.) Again, since type B is more efficient, two type-b pumps would be preferred over one type-a and one type-b One A and one B in parallel Pump B Pump A Flow rate (gpm)

41 Efficiency, % TDH (ft) At the interim conditions, a single type B pump would suffice. A third type B pump would be required as backup One A and one B in parallel Pump B Pump A Flow rate (gpm)

42 External Considerations External Conditions Hydraulic Operating Conditions Installation Practices Drive Train Flow Rate Piping Base Coupling Driver Pressure Configuration Size and Style Size and Style Size Turbulent Flow Alignment Grouting Alignment Speed Support

43 Cavitation in Water Pumps One of the important considerations in pump installation design is the relative elevation between the pump and the water surface in the supply reservoir. Whenever a pump is positioned above the supply reservoir, the water in the suction line is under pressure lower than atmospheric. The phenomenon of cavitation becomes a potential danger whenever the water pressure at any location in the pumping system drops substantially below atmospheric pressure. To make matters worse, water enters into the suction line through a strainer that is designed to keep out trash. This additional energy loss at the entrance reduces pressure even further.

44 A common site of cavitation is near the tips of the impeller vanes where the velocity is very high. In regions of high velocities much of the pressure energy is converted to kinetic energy. This is added to the elevation difference between the pump and the supply reservoir, hp, and to the inevitable energy loss in the pipeline between the reservoir and the pump, h L. Those three items all contribute to the total suction head, Hs, in a pumping installation

45 The value of Hs must be kept within a limit so that the pressure at every location in the pump is always above the vapor pressure of water; otherwise, the water will be vaporized and cavitation will occur. The vaporized water forms small vapor bubbles in the flow. These bubbles collapse when they reach the region of higher pressure in the pump. Violent vibrations may result from the collapse of vapor bubbles in water. Successive bubble breakup with considerable impact force may cause high local stresses on the metal surface of the vane blades and the housing. These stresses cause surface pitting and will rapidly damage the pump.

46 To prevent cavitation, the pump should be installed at an elevation so that the total suction head is less than the difference between the atmospheric head and the water vapor pressure head, or (Patm/ - Pvap/ ) > Hs

47

48 FAN AND BLOWER

49 Introduction Fan Components Provide air for ventilation and industrial processes that need air flow Outlet Diffusers Turning Vanes (typically used on short radius elbows) Baffles Heat Exchanger (US DOE, 1989) Filter Inlet Vanes Centrifugal Fan Belt Drive Motor Controller Variable Frequency Motor Drive 50 UNEP 2006

50 Introduction System Resistance Sum of static pressure losses in system Configuration of ducts, pickups, elbows Pressure drop across equipment Increases with square of air volume Long narrow ducts, many bends: more resistance Large ducts, few bends: less resistance 51 UNEP 2006

51 Introduction System Resistance System resistance curve for various flows Actual with system resistance calculated (US DOE, 1989) 52 UNEP 2006

52 Introduction Fan Curve Performance curve of fan under specific conditions Fan volume System static pressure Fan speed Brake horsepower (US DOE, 1989) 53 UNEP 2006

53 Introduction Operating Point Fan curve and system curve intersect Flow Q1 at pressure P1 and fan speed N1 Move to flow Q2 by closing damper (increase system resistance) (BEE India, Move to flow Q2 by reducing fan speed 54 UNEP 2006

54 Introduction Fan Laws (BEE India, 55 UNEP 2006

55 Types of Fans & Blowers Types of fans Centrifugal Axial Types of blowers Centrifugal Positive displacement 56 UNEP 2006

56 Types of Fans & Blowers Centrifugal Fans Rotating impeller increases air velocity Air speed is converted to pressure High pressures for harsh conditions High temperatures Moist/dirty air streams Material handling Categorized by blade shapes Radial Forward curved Backward inclined 57 UNEP 2006

57 Types of Fans & Blowers Centrifugal Fans Radial fans Advantages High pressure and temp Simple design High durability Efficiency up to 75% Large running clearances Disadvantages Suited for low/medium airflow rates only (Canadian Blower) 58 UNEP 2006

58 Types of Fans & Blowers Centrifugal Fans Forward curved Advantages Large air volumes against low pressure Relative small size Low noise level Disadvantages Not high pressure / harsh service Difficult to adjust fan output Careful driver selection Low energy efficiency 55-65% ( Canadian Blower) 59 UNEP 2006

59 Types of Fans & Blowers Centrifugal Fans - Backward-inclined Advantages Operates with changing static pressure Suited for high flow and forced draft services Efficiency >85% Disadvantages Not suited for dirty airstreams Instability and erosion risk ( Canadian Blower) 60 UNEP 2006

60 Types of Fans & Blowers Axial Fans Work like airplane propeller: Blades create aerodynamic lift Air is pressurized Air moves along fan axis Popular with industry: compact, low cost and light weight Applications Ventilation (requires reverse airflow) Exhausts (dust, smoke, steam) 61 UNEP 2006

61 Types of Fans & Blowers Axial Fans Propeller fans Advantages High airflow at low pressure Little ductwork Inexpensive Suited for rooftop ventilation Reverse flow Disadvantages Low energy efficiency Noisy (Fan air Company) 62 UNEP 2006

62 Types of Fans & Blowers Axial Fans Tube axial fans Advantages High pressures to overcome duct losses Suited for medium-pressure, high airflow rates Quick acceleration Space efficient Disadvantages Expensive Moderate noise Low energy efficiency 65% (Canadian Blower) 63 UNEP 2006

63 Types of Fans & Blowers Axial Fans Vane axial fans Advantages Suited for medium/high pressures Quick acceleration Suited for direct motor shaft connection Most energy efficient 85% Disadvantages Expensive (Canadian Blower) 64 UNEP 2006

64 Types of Fans & Blowers Blowers Difference with fans Much higher pressures <1.20 kg/cm2 Used to produce negative pressures for industrial vacuum systems Types Centrifugal blower Positive displacement 65 UNEP 2006

65 Types of Fans & Blowers Centrifugal Blowers Gear-driven impeller that accelerates air Single and multi-stage blowers Operate at kg/cm2 pressure Airflow drops if system pressure rises (Fan air Company) 66 UNEP 2006

66 Types of Fans & Blowers Positive Displacement Blowers Rotors trap air and push it through housing Constant air volume regardless of system pressure Suited for applications prone to clogging Turn slower than centrifugal blowers Belt-driven for speed changes 67 UNEP 2006

67 Assessment of fans and blowers Fan Efficiency and Performance Fan efficiency: Ratio of the power conveyed to air stream and power delivered by the motor to the fan Depends on type of fan and impeller Fan performance curve Graph of different pressures and corresponding required power Supplier by manufacturers 68 UNEP 2006

68 Efficiency Assessment of fans and blowers Peak efficiency or Best Efficiency Point (BEP) Backward Airfoil Type of Fan Peak Efficiency Range Radial Centrifugal fans: Airfoil, Backward curved/inclined Modified radial Tubular Radial Pressure blower Forward curved (BEE India, 2004) Forward Flow rate Axial fans: Vane axial Tube axial Propeller UNEP

69 Assessment of fans and blowers Methodology fan efficiency Before calculating fan efficiency Measure operating parameters Air velocity, pressure head, air stream temp, electrical motor input Ensure that Fan is operating at rated speed Operations are at stable condition 70 UNEP 2006

70 Assessment of fans and blowers Methodology fan efficiency Step 1: Calculate air/gas density t = Temperature of air/gas at site condition Step 2: Measure air velocity and calculate average Cp = Pitot tube constant, 0.85 (or) as given by the manufacturer p = Average differential pressure γ = Density of air or gas at test condition Step 3: Calculate the volumetric flow in the duct 71 UNEP 2006

71 Assessment of fans and blowers Methodology fan efficiency Step 4: Measure the power drive of the motor Step 5: Calculate fan efficiency Fan mechanical efficiency Fan static efficiency 72 UNEP 2006

72 Assessment of fans and blowers Difficulties in Performance Assessment Non-availability of fan specification data Difficulty in velocity measurement Improper calibration of instruments Variation of process parameters during tests 73 UNEP 2006

73 Energy Efficiency Opportunities 1. Choose the right fan 2. Reduce the system resistance 3. Operate close to BEP 4. Maintain fans regularly 5. Control the fan air flow 74 UNEP 2006

74 Energy Efficiency Opportunities 1. Choose the Right Fan Considerations for fan selection Noise Rotational speed Air stream characteristics Temperature range Variations in operating conditions Space constraints and system layout Purchase/operating costs and operating life Systems approach most important! 75 UNEP 2006

75 Energy Efficiency Opportunities 1. Choose the Right Fan Avoid buying oversized fans Do not operate at Best Efficiency Point Risk of unstable operation Excess flow energy High airflow noise Stress on fan and system 76 UNEP 2006

76 Energy Efficiency Opportunities 2. Reduce the System Resistance Increased system resistance reduces fan efficiency Check periodically Check after system modifications Reduce where possible (BEE India, 2004) 77 UNEP 2006

77 Energy Efficiency Opportunities 3. Operate Close to BEP Best Efficiency Point = maximum efficiency Normally close to rated fan capacity Deviation from BEP results in inefficiency and energy loss 78 UNEP 2006

78 Energy Efficiency Opportunities 4. Maintain Fans Regularly Periodic inspection of all system components Bearing lubrication and replacement Belt tightening and replacement Motor repair or replacement Fan cleaning 79 UNEP 2006

79 Energy Efficiency Opportunities 5. Control the Fan Air flow a) Pulley change b) Dampers c) Inlet guide vanes d) Variable pitch fans e) Variable speed drives (VSD) f) Multiple speed drive g) Disc throttle h) Operating fans in parallel i) Operating fans in series 80 UNEP 2006