Lecture Sensors and analytical devices: Flow measurement - Nguyễn Công Phương

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Flow measurement is to quantify the rate of flow of materials. It is quite a common requirement in the process industries, The materials measured may be in a solid, liquid, or gaseous state,... In this chapter will introduce flow measurement, inviting you refer lecture for more details.

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Nội dung Text: Lecture Sensors and analytical devices: Flow measurement - Nguyễn Công Phương

Nguyễn Công Phương Sensors and Analytical Devices Some Basic Measurement Methods, Flow Measurement
Contents A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems sites.google.com/site/ncpdhbkhn 2
Some Basic Measurement Methods I. Sensor Technologies II. Temperature Measurement III. Pressure Measurement IV.Flow Measurement V. Level Measurement VI.Mass, Force, and Torque Measurement VII.Translational Motion, Vibration, and Shock Measurement VIII.Rotational Motion Transducers sites.google.com/site/ncpdhbkhn 3
Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 4
Introduction (1) • Flow measurement is to quantify the rate of flow of materials. • It is quite a common requirement in the process industries. • The materials measured may be in a solid, liquid, or gaseous state. • Solid state: flow can only be qualified as the mass flow rate. • Mass flow rate: the mass of material that flows in one unit of time. • Liquid or gaseous state: flow can be qualified as either the mass flow rate or the volume flow rate. • Volume flow rate: the volume of material that flows in one unit of time. • Note: mass is invariant, whereas volume is a variable quantity. sites.google.com/site/ncpdhbkhn 5
Introduction (2) • A particular complication in the measurement of flow rate of liquids & gases flowing in pipes: the need to consider whether the flow is laminar or turbulent. • Laminar flow: – A motion of the fluid being in a direction parallel to the sides of the pipe. – It occurs in straight lengths of pipe when the fluid is flowing at a low velocity. – Note that even laminar flow is not uniform across the cross section of the pipe: the velocity is greatest at the center of the pipe, & decreases to zero immediately next to the wall of the pipe. • Turbulent flow: – A complex pattern of flow that is not in a uniform direction. – Occurs in nonstraight sections of pipe & also occurs in straight sections when the fluid velocity exceeds a critical value. – Difficult to measure. • The usual practice is to restrict flow measurement to places where the flow is laminar, or at least approximately laminar – Measure the flow in the center of a long, straight length of pipe if the flow velocity is below the critical value for turbulent flow. Laminaris high, it is often possibleTurbulent – If the mean fluid velocity to find somewhere within the flow path where a larger diameter pipe exists & therefore the flow velocity is lower. http://blog.nialbarker.com/252/slow_is_faster sites.google.com/site/ncpdhbkhn 6
Flow Measurement 1. Introduction 2. Mass Flow Rate a) Conveyor-Based Methods b) Coriolis Flowmeter c) Thermal Mass Flow Measurement 3. Volume Flow Rate 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 7
Conveyor-Based Methods http://www.conveyorbeltguide.com/ ConveyorSafetyDevices.html • Appropriate for measuring the flow of solids in the form of powders or small granular particles. • A conveyor is a very suitable means of transporting materials in this form. • Transporting materials on a conveyor allows the mass flow rate to be calculated in terms of the mass of material: Q = Mv/L – Q: the mass flow rate – M: the mass of material distributed over a length, L, of the conveyor – v: the velocity of the conveyor sites.google.com/site/ncpdhbkhn 8
Coriolis Flowmeter • Dominates the mass flowmeter market. • Consists either of a pair of parallel vibrating tube or as a single vibrating tube that is formed into a configuration that has two parallel sections. • Tubes are commonly made of stainless steel. http://en.wikipedia.org/wiki/Mass_fl ow_meter#Operating_principle_of_ • The two vibrating tubes deflect according to a_coriolis_flow_meter the mass flow rate of the measured fluid that is flowing inside: d = kfR – d: the net deflection – k: a constant – f: the frequency of the tube vibration – R: the mass flow rate • Excellent accuracy: ±0.2% • Low maintenance requirements • Expensive • Failure may occur after a period of use because of mechanical fatigue in the tube. • Tubes are also subject to corrosion. sites.google.com/site/ncpdhbkhn 9
Thermal Mass Flow Measurement • Used primarily to measure the flow rate of gases. • The principle of operation is to direct the flowing material past a heated element. • The mass flow rate is inferred in two ways: – by measuring the temperature rise in the flowing material, or – by measuring the heater power required to achieve a constant set temperature in the flowing material. • Inaccuracy: ±2% • Can measure very small flow rates: http://www.azosensors.com/article.aspx? 10–9 liters per minute. ArticleID=277 sites.google.com/site/ncpdhbkhn 10
Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 11
Differential Pressure (Obstruction-Type) Meters (1) • Involve the insertion of some device into a fluid-carrying pipe. • This device causes an obstruction & creates a pressure difference on either side of the device. • Devices: orifice place, Venturi tube, flow nozzle, & Dall flow tube. • When such a restriction is placed in a pipe, the velocity of the fluid through the restriction increases & the pressure decreases. • The volume flow rate is proportional to the square root of the pressure difference across the obstruction. • The normal procedure is to use a differential pressure transducer, which is commonly a diaphragm-type device. • The pitot static tube: – Another device that measures flow by creating a pressure difference within a fluid-carrying pipe. – However, there is negligible obstruction of flow in the pipe. – The pitot tube is a very thin tube that obstructs only a small part of the flowing fluid & thus measures at a single point across the cross section of the pipe. sites.google.com/site/ncpdhbkhn 12
Differential Pressure (Obstruction-Type) Meters (2) http://www.piping- engineering.com/restriction-orifice-ro- flow-control-instrument.html Orifice plate Segmental Orifice Plate http://sageoilfield.com/page.php?page=292 sites.google.com/site/ncpdhbkhn 13
Differential Pressure (Obstruction-Type) Meters (3) • The orifice plate is a metal disc with a hole on it. • Inserted into the pipe carrying the flowing fluid. • Simple, inexpensive, available in a wide range of sizes. • Account for almost 50% of the instruments used in industry for measuring volume flow rate. • Problems: – Inaccuracy: at least ±2% – Permanent pressure loss: 50 to 90% of (P1 – P2) – A gradual change in the sharp edges of the hole: it wears away – Particles tend to stick behind the hole. • To minimize pressure loss: Venturi, flow nozzle, & Dall flow tube. Segmental Orifice Plate http://sageoilfield.com/page.php?page=292 sites.google.com/site/ncpdhbkhn 14
Differential Pressure (Obstruction-Type) Meters (4) http://www.stepbystep.com/difference-between- venturi-and-orifice-103728/ Venturi sites.google.com/site/ncpdhbkhn 15
Differential Pressure (Obstruction-Type) Meters (5) http://www.stepbystep.com/difference-between-venturi-and-orifice-103728/ • Has a precision-engineered tube of a special shape. • Inaccuracy: ±1% • Permanent pressure loss: 10 – 15% of (P1 – P2) • The most expensive of all the obstruction devices. sites.google.com/site/ncpdhbkhn 16
Differential Pressure (Obstruction-Type) Meters (6) http://www.stepbystep.com/difference-between- venturi-and-orifice-103728/ Flow nozzle sites.google.com/site/ncpdhbkhn 17
Differential Pressure (Obstruction-Type) Meters (7) http://www.stepbystep.com/difference-between-venturi-and-orifice-103728/ • Of simple construction  less expensive than either a Venturi or a Dall flow tube. • Pressure loss: 35 – 50% of (P1 – P2). sites.google.com/site/ncpdhbkhn 18
Differential Pressure (Obstruction-Type) Meters (8) http://smartcontrol.blogspot.com/2012/03/flow- measurement.html Dall flow tube sites.google.com/site/ncpdhbkhn 19
Differential Pressure (Obstruction-Type) Meters (9) http://smartcontrol.blogspot.com/2012/03/flow-measurement.html • Consists of 2 conical reducers inserted into a fluid-carrying pipe. • Has a very similar internal shape to the Venturi, except that it lacks a throat. • Much easier to manufacture  cheaper than Venturi. • Inaccuracy: ±1.5% • Its shorter length makes the engineering task of inserting it into the flow line easier. • Pressure loss: 5% of (P1 – P2). sites.google.com/site/ncpdhbkhn 20