Lecture # 8. Transportation & Metering of Fluids

Transcription

1 Lecture # 8 Transportation & Metering of Fluids

2 Fluid Transportation Fluids are transported through pipes or tubes: Pipes Heavy walled, large diameter, and moderate length 20 ft to 40 ft Pipes can be threaded Rough surface Pipes are joined by: Screwed Flanged Welded fittings Pipes are made by: Welding Casting Piercing a billet in a billet mil Tubes Thin walled and coils comes several hundred feet long Tubing cannot Smooth surface Tubes are joined by: Compression joints Flare fittings Soldered fittings These are extruded or cold drawn.

3 Sizing of pipes and tubes Pipe sizes: Pipes are specified by their diameter and wall thickness. Steel pipes nominal diameter ranges from 1/8 to 30 in. For pipes, more than 12 in diameter, nominal diameter is outside diameter. Nominal value close to actual inside diameter for 3 to 12 pipe. Appendix 5 is for steel pipe sizes. (IPS = Iron Pipe Size, NPS = Normal Pipe Size) Thus, 2 in. nickel IPS pipe means 2 in. nickel pipe having same dimensions as 2 in. steel pipe. Wall thickness is represented by Schedule #. 10 schedule # are given as 10, 20, 30, 40, 60, 80, 100, 120, 140, 160. For pipe, less than 8 in. diameter, only 40, 80, 120, & 160 are common. Tube Sizes: Tubes are sized by outside diameter. Wall thickness is given by BWG (Birmingham Wire Gauge) number, ranges from 24 (very light) to 7 (very heavy).

4 Selection of Pipe sizes For specific situation: the optimum size of of pipe depends on Relative cost of investment Power Maintenance Stocking pipe and fittings In small installations rule of Thumb are sufficient. Low velocities should ordinarily favored for gravity flow from overhead tanks For large complex systems the cost of piping may be substantial and also computer programs of optimizing pipe sizes are justified

5 Joints and Fittings Method to join the tubes and pipes depends not only on the properties of fluid but also on the thickness of wall. Thick Walled tubular structures are joined by : Screwed Fitting (higher schedule # pipe is required for threading.because of difficulty of threading and handling of large pipes they are rarely used in the field with pipe larger than 3in.) Flanges Welding Thin-walled tubing get attached by: Soldering Flare or Compression fittings Pipes made of brittle material ( like glass, carbon or cast iron) are joined by: Flanges (Flanges are matching disks or rings of metal bolted together and compressing a gasket between their faces. A flange with no opening used to close a pipe is called a blind flange or blank flange) Bell and Spigot Joints

6 Comparison of Joints and Fittings For Larger steel pipe in process piping and high pressure services welding has become the standard method. Welding makes the stronger joint than screwed or flanges. Welded joints are leak-proof whereas other types of joints are not. Environmental protection Legislation considers flanges and screwed joints to be the source of leakage and emission of Volatile matter. The only drawback of welded joint is that it cannot be opened without destroying it.

7 Allowance for Expansion Pipes has to face varying temperature and pressure and such changes cause the pipe to expand or contract. If the pipe is rigidly fixed to its support, it may tear loose, bend or even break. In large lines, fixed supports are not used instead the pipes rests loosely on rollers or is hung from above by chains or roads. For high temperature lines (for taking up expansion and to avoid the strain on the valves and fitting) the bends, bellows, packed Expansion joints, and flexible metal hose are employed.

8 Leakage Prevention around Moving Parts In Process Machinery sometimes one part has to move on another part without leakage like; Packed Expansion Joints Valve where the stem should be free to turn without allowing the fluid in the valve to escape. Shaft of Pump or Compressor Agitator Shaft passes through the wall of pressure vessel Common devices for minimizing the leakage while permitting relative motion are Stuffing Box and Mecahnical seals.

9 Stuffing Box

10 Stuffing Box (Cont..)

11 Stuffing Box (Cont..)

12 Stuffing Box (Cont..)

13 Stuffing Box (Cont..)

14 Mechanical Seal

15 Mechanical Seal (Cont..)

16 Mechanical Seal (Cont..)

17 VALVES A small obstruction can be placed in the path of the fluid that can be moved about as desired inside the pipe with little or no leakage of the fluid from the pipe to outside. That obstruction including its movement mechanism are boxed in one unit which is called Valve. Valves are used in the piping networks to meet the following purposes; To regulate the flow (i.e. to stop or slow down the flow) Control the temperature, pressure, liquid level or other properties of fluid at points remote from the valve itself Unidirectional Flow under certain conditions of temperature and pressure

18 Terminology for Valve body Parts

19 CLASSIFICATION OF VALVES CLASSIFICATION BASED ON MECHANICAL MOTION Linear motion valve Rotary motion valve Quarter turn valve CLASSIFICATION BASED ON VALUE SIZE Small valves (NPS 2 and smaller) Large valves (NPS 2 1/2 and larger) CLASSIFICATION BASED ON FUNCTION Isolation Gate valve, Ball valve, Butterfly valve, Diaphragm valve Control (flow/pressure) Globe valve, Ball valve, Butterfly valve, Diaphragm valve Prevention of flow reversal Check valve (swing, lift, piston, etc.) Flow diversion Ball valve, Plug valve, Angle valve (Three way, Four way, etc.)

20

21 ADVANTAGES & DISADVANTAGES OF GATE VALVE ADVANTAGES Pressure drop through the valve is minimal. Good shutoff characteristics. Operation torque is smaller than those of globe valves. DISADVANTAGES Cannot be throttled. Not suitable for frequent switch-on/off operation. Requires large space envelope for installation, operation and maintenance. Repair or machining of valve seats in place is difficult.

22 Typical Usage of Gate Valve Block valve for control valve Pump suction valve Pump discharge valve Block valve for level controller & level gauge Drain valve of equipment Drain valve of process & utility line First block valve of sampling nozzle Block valve for safety valve Block valve for equipment Block valve for steam trap By-pass valve for emergency shut-down valve Flow control valve for large size gas & city water line

23 Valve symbols for PID (Piping and Instrumentation Diagram)

24

25 Turbo machine A turbo machine is a device in which energy is transferred either to or from a continuously flowing fluid by the dynamic action of one or more moving blade rows The word turbo is a Latin origin and implies that which spins or whirls around

26 Classification of Turbo-machinery Major subdivisions A. Power classifications (power is added or extracted from the fluid) Pumps are power addition machines and include liquid pumps, fans, blowers and compressors. Fluids are water, fuels, air, steam, refrigerants. Turbines are power extraction devices and include windmills, water wheels, hydroelectric turbines, automotive engine turbochargers, gas turbines. Fluids; gases, liquids, mixtures.

27 Classification of Turbo-machinery (Cont..) B. The manner in which the fluid moves through and around a machine Open flow No casing or enclosure for the rotating devices Examples: propeller is an open flow pumping device. Windmill is an open flow turbine Enclosed or encased flow devices

28 Classification of Turbo-machinery (Cont..) C. Turbo-machines are further categorized according to the nature of the flow path through the passages of the rotor. When the path of the through-flow is wholly or mainly parallel to the axis of rotation, the device is termed an axial flow turbo-machine. When the path of the through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbo-machine. Mixed flow turbo-machines are widely used. The term mixed flow refers to the direction of the through-flow at rotor outlet when both radial and axial velocity components are present in significant amounts.

29 Classification : Flow Path

30 Classification of Turbo-machinery (Cont..) D. Compressibility of the fluid Incompressible The density is constant through the entire flow process; liquid pumps. Compressible; Gas flows: compressors, Fan and Blower E. Impulse or reaction machines Impulse: pressure changes are absent in the flow through the rotor. In an impulse machine, all the pressure change take place in nozzles Example: Pelton wheel Reaction: pressure changes in rotor are absent

31

32 Terminology (Mechanical) of Fluid Moving Machinery

33 Terminology (Cont.)

34 Flow Dynamics in Fluid Moving Machine

35 Sectional view of Impeller

36 Sectional view of Impeller (cont.)

37 Sectional view of Impeller (cont.)

38 Pump Classification

39

40 Basic Definitions Capacity: it is expressed in terms of volumetric flow rate. Head: it is the height of fluid column equivalent to the total pressure differential (under adiabatic conditions) measured immediately before and after device.

41 Suction Head: it s a vertical distance from pump center-line to liquid supply line. The term suction lift would be used when the pump is placed above the liquid level. Discharge Head: it s a vertical distance from pump Centre line to point of free delivery of liquid. Total Static Head: it is the vertical distance between the discharge level and supply level of liquid. Velocity Head: it is vertical distance a body would have to fall to acquire the velocity V. it corresponds to the pressure head that would cause that velocity. Friction Head: it is the pressure head (in meters) required to overcome the resistance to flow in pipes

42

43

44 NPSH (Net Positive Suction Head) and Cavitation Net positive suction head is the term that is usually used to describe the absolute pressure of a fluid at the inlet to a pump minus the vapor pressure of the liquid. The resultant value is known as the Net Positive Suction Head. The Vapor pressure of a fluid is the pressure at which the fluid will boil at ambient temperature. If the pressure within a fluid falls below the vapor pressure of the fluid, gas bubbles will form within the fluid (local boiling of the fluid will occur). If a fluid which contains gas bubbles is allowed to move through a pump, it is likely that the pump will increase the pressure within the fluid so that the gas bubbles collapse. This will occur within the pump and reduce the flow of delivered fluid. The collapse of the gas bubbles may cause vibrations which could result in damage to the pipework system or the pump. This effect is known as cavitation. Formula for NPSH see the text book

45 Pump Types Positive Displacement Pump: Definite volume of the liquid is trapped in a chamber, which is alternately filled from inlet and emptied at higher pressure through the discharge. Reciprocating type Rotary type Centrifugal pumps: these are the type of machines where mechanical energy of liquid is increased by centrifugal action.

46 Double acting Piston Pump

47 Rotary Pumps: Discharge liquid by continuous scooping of liquid from pump chamber due to rotation of one or more members within a stationary casing Gear Pump

48 Screw Pump ( A helical Screw rotor revolves in a fixed casing)

49 Diaphragm Pumps (A flexible diaphragm fabricated of metal, rubber, or plastic material instead of piston or plunger, which reciprocate)

50 Lobe Pump (Pump delivers liquid by virtue of rotation of rotation of two, three or four lobes in a stationary casing)

51 Vane pumps ( rotary pumps which operate on a principle of creating a vacuum inside a pump due to rotation of rotor allowing the space to fill with a liquid and then forcing the liquid out of pump under pressure by diminishing the volume)

52 Centrifugal pumps ( It consist of impeller rotating within a casing. Liquid enters the pump near the center of the impeller and is thrown outward by virtue of centrifugal action)

53 Multistage Centrifugal Pumps (High energy Centrifugal pump can develop a head of more than 650ft in single stage but generally when a head is greater than about 100ft is needed two ore more impellers can be mounted in series on a single shaft.)

54 Axial Flow Pumps (Propeller Pumps) Used for very high capacity and low head duties

55 Turbine Pumps (These have mixed flow impellers with the effect that the flow is partly axial and partly centrifugal) usually casing submerged in the liquid to be pumped

56 Jet Pumps (It has no moving parts and works on the principle of momentum transfer of one fluid to another fluid being pumped

57 Electromagnetic Pumps (works on the principle as induction motor) A strong magnetic field is imposed perpendicularly to the liquid stream that carries electric current this results in a driving force, mutually perpendicular to the magnetic field and electric current, that causes the liquid to flow

58 Characteristic Curves The Plots of Actual Head, Total power consumption, and efficiency versus volumetric flowrate are called Characteristic curves Head-Capacity Curve The figure shows the head capacity relation. The theoretical head-capacity relation is straight line but the actual developed head is considerably less and drops precipitously to zero as the rate increases to certain value in any given pump. The theoretical Zero head flow rate corresponds to maximum flow the pump can deliver at any condition. The rated or optimum operating flowrate is of course less than the zero value.

59 Reasons of Difference in Actual Head to theoretical Head Circulatory Flow Fluid Friction in the passage and channels of the pump Shock losses from sudden changes in the direction of liquid leaving the impeller Joining the stream of liquid traveling circumferentially around the casing Important points Friction is Highest at maximum flow rate Shock losses are minimum at rated operating conditions of pump and becomes greater as the flow rate is increased or decreased from rated value.

60 Power Curve Power Curve is drawn between Fluid power, total power versus flow rate The difference of two powers (ideal power and fluid power) represents the power lost. Power losses occur due to fluid friction and shock losses those converts the mechanical energy into heat. Leakage (it is unavoidable reverse flow from impeller discharge past the wearing ring to the suction eye, and this reduces the volume of actual discharge from pump per unit of power expended) Disk friction ( it is the friction between outer surface of the impeller and the liquid in the space between impeller and the inside of casing Bearing losses ( it constitute the power required to overcome the mechanical friction in the bearing and stuffing boxes or seals of the pump

61 Efficiency Curve Pump Efficiency: It is the ratio of Fluid power to the total power input. The efficiency rises rapidly with flow rate at low rates, reaches to maximum in the region of rated capacity, then falls as the flowrate approaches the zero-head value

62 Pump Priming (A centrifugal pump trying to operate on air can neither draw a liquid upward from an initially empty suction line nor force the liquid along a full discharge line. A pump with air in its casing is air bound and can accomplish nothing until the air has been replaced by a liquid) The theoretical Head developed in a centrifugal pump depends on Impeller speed Radius of impeller Velocity of fluid leaving impeller (If all factors are constant then developed head would be same for all fluids of all densities and is same for liquids and gases.) Important point The increase in pressure in the pump, however, is the product of developed head and fluid density. If the pump develops a head of 100ft and is full of water, the increase in pressure is 100x62.3/144 = 43psi (2.9 atm). If the pump full of air at ordinary density, the pressure increase is about 0.1psi (0.007atm). Positive displacement pumps can compress a gas to a required discharge pressure and are not usually subject to air binding

63 Turbomachinery for compressible fluids Following devices can be utilized to transport the compressible fluids; Fan It discharge large volume of gas ( usually air) into open spaces or large ducts These are classed as low speed rotary machines Generates pressure of order of a few inches of water Density of fluid does not change appreciably (incompressible flow theory is adequate to discuss the phenomena) Blower These are high speed rotary devices (either positive displacement or centrifugal) develop a maximum pressure of about 2 atm Density changes should incorporate in the analysis Compressor Discharge at pressure from 2 atm to thousands of atmospheres Density changes should incorporate in the analysis

64 Fans Large fans are usually centrifugal (operating principle is exactly same as centrifugal pump) Impeller blades are curved forward Fan impellers are mounted inside light steel metal casing Clearances are large and discharge heads low from 5 to 60in. (130 to 1500mm) H 2 O In ventilating fan all the added energy is converted to into velocity energy and almost none into pressure head. Due to negligible change in density, equations relating to centrifugal pump are adequate to use. Fans are rated in Standard Cubic Feet Volume in standard cubic feet is that measured at specified temperature and pressure regardless of actual temperature and pressure. Common standard temperature and pressure are 60 F and 30 in.hg, corresponding molal volume is 378 ft 3 /Ib-mol

65 Blowers Positive displacement blower shown in figure. Similar to gear pump except the design of teeth and clearance is only few thousands of an inch. Relative position of impellers is maintained by heavy external gears Single stage blower can discharge at 0.4 to 1atm gauge and 2 stage blower at 2 atm

66 Positive Displacement Blowers (cont.)

67 Centrifugal Blowers (cont.) It resembles with centrifugal pump, except casing is narrower and diameter of casing and discharge scroll are relatively larger than pump. Operating speed is 3600 r/min or higher Reason of high speed and larger diameters is that very high heads (measured in meters and of low density fluid) are needed to generate moderate pressure ratios. So the velocity in the vector diagram for centrifugal blower is approximately tenfold those of centrifugal pump.

68 Centrifugal Blowers (cont.)fan, blower and compressor\3-d-blower- Animation.flv

69 Compressors

70 Centrifugal Compressor (Cont.)Fan, blower and compressor\centrifugal- COMPRESSORNPOSAVI.flv These are multistage units consisting of series of impellers on a single shaft rotating at high speed in a massive casing These can work on enormous amount of air or process gas up to ft 3 /min at inlet to an outlet pressure of 20atm. Smaller capacity machines can deliver up to several hundred atmosphere Interstage cooling is required on high pressure units

71 Axial flow compressors (cont.)fan, blower and compressor\how-axial- Compressors-Worksflv-by-PRAVIN-TATHOD.flv In these units rotor vanes propel the gas axially from one set of vanes directly to the next. Axial flow machines handle even larger volumes of gas ft3/min, but at the lower discharge pressure of 2 to 10 or 12atm Interstage cooling is usually not required

72 Positive displacement Compressors (cont.)fan, blower and compressor\reciprocating-compressor-working-animation---maintenance.flv These machines are operated in the same way as the reciprocating pumps The important difference lies in the prevention of leakage and rise in temperature during compression Most compressors operating at discharge pressure above 3 atm are reciprocating positive displacement machines Reciprocating compressors are usually motor driven and nearly always double acting Sometimes, high compression ratios are required which is achieved by providing interstage cooling.

73 From Text Book Equations for Blowers and Compressors

74 Vacuum Pumps A compressor that takes suction at a pressure below atmospheric and discharges against atmospheric pressure is called a vacuum pump. The compression ratio used in the vacuum pumps is higher than in the compressors

75 Jet Ejectors An important type of vacuum pump that does not use moving parts is the jet ejector. Multistage ejectors can also be used to create more vacuum, as many as five stages are used in industrial processing. Jet ejectors needs very little attention and maintenance and are especially valuable with corrosive gases that would damage mechanical pumps. They are rarely used to produce absolute pressure below 1mmHg.

76 Flow Measuring Devices It is essential to measure the amount of material for the control purposes Selection of Meter depends on many factors: The applicability of the instrument to the specific problem Its installed cost and costs of operation The range of flowrates it can accommodate (Its range ability) The accuracy of the measurement

77 Flow meter Types Few types of flowmeters measures the mass flowrate directly, but majority measures the volumetric flowrate Volumetric flow meters 1. Differential Head type A. Orifice plates B. Venturi meters 2. Differential Area type (Rotameters) 3. Electromagnetic flowmeter 4. Vortex flowmeter 5. Ultrasonic flowmeter 6. Turbine flowmeter 7. Positive displacement flowmeter Mass flow meters 1. Coriolis Mass flowmeter 2. Thermal Mass flowmeters

78 Venturi meter A venturi tube also measures flow rates by constricting fluids and measuring a differential pressure drop. In the upstream cone of the Venturimeter, velocity is increased, pressure is decreased. Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument

79 Basic Equations of Venturi meter From Text book Pressure Recovery If the flow through venturi meter were frictionless, the pressure of the fluid leaving the meter would be exactly equal to that of the fluid entering the meter and the presence of the meter in the line would not cause a permanent loss in pressure. In properly designed meter, the permanent loss is about 10% of the venturi differential and approximately 90% of the differential is recovered.

80 Venturi meter (cont..) Disadvantages Highly expensive Larger and heavier to handle. Ratio of throat diameter to pipe diameter cannot be changed Although t he Venturi meters can be applied to the measurement of gas, they are most commonly used for liquids, especially water. For a given meter and manometer system, the maximum measureable flow rate is fixed. So if the flow range is changed the throat diameter is too large to give an accurate flow rate or too small to accommodate the larger flow rate The orifice meter meets these objections and to the venturi but at the price of larger power consumption

81 Orifice meter (The reduction of cross-section of the flowing stream in passing through the orifice increases the velocity head at the expense of pressure head, and reduction in pressure between the taps is measured by manometer)

82 Orifice meter (Cont.)

83

84 V-Element Meters The segmental wedge element is a proprietary device designed for use in slurry, corrosive, erosive, viscous, or high-temperature applications. It is relatively expensive and is used mostly on difficult fluids, where the dramatic savings in maintenance can justify the initial cost. Flow Co-efficient is constant at low flow rates. The minimum Reynolds number is only 500, and the meter requires only five diameters of upstream straight pipe run. The V-shaped restriction characterized by the H/D ratio, where H is the height of the opening below the restriction and D is the diameter. The H/D ratio can be varied to match the flow range and to produce the desired d/p.

85 Target meters A sharp edge Disk is set at right angles to the direction of flow as shown below Drag force exerted on the disk by the fluid is measured. The flow rate is proportional to the square root of this force and to the fluid density.

86 Turbine Meters Consists of a multi-bladed rotor mounted at right angles to the flow & suspended in the fluid stream on a free-running bearing. The diameter of the rotor is slightly less than the inside diameter of the flow metering chamber. Speed of rotation of rotor proportional to the volumetric flow rate. Not usable in dirty streams or with corrosive materials. Subject to fouling by foreign materials -fibers, tars etc.

87 Positive Displacement Meters This meter repeatedly entraps the fluid into a known quantity and than passes it out. The quantity of the fluid that has passed is based on the number of entrapments. The volume flow rate can be calculated from the revolution rate of the mechanical device. Can be used in viscous liquid flow Not suitable for fluids with suspended solids and for r low flow rate

88 ULTRASONIC FLOWMETERS A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream. The time for acoustic waves to travel from the upstream transducer to the downstream transducer td is shorter than the time it requires for the same waves to travel from the downstream to the upstream tu. The larger the difference, the higher the flow velocity. Only clean liquids and gases can be measured