MECH 591 Instrumentation and Measurement Flow Measurements.ppt
- 1. MECH 591 Instrumentation and Measurement Flow Measurements Department of Electrical & Mechanical Wentworth Institute of Technology www.AutomationSys.Org 021-88535820-21
- 2. Contents • Fluid Flow • Differential Pressure Flowmeters • Variable area flowmeters • Mechanical Flowmeters • Electrical Flowmeters • Mass Flowmeters • Open Channel Flow Measurements • Solids Flowmeters Objectives • To present an overview of basic flow metering techniques. • Proper flowmeter selection • Design considerations important for the integration of flow metering system
- 3. Historical Background • The early Greek Philosophers Aristotle and Archimedes studied fluid dynamics. • Archimedes invented a water pump that uses a screw to move water. In his honor it is called Archimedes screw. • Modern studies of fluids began in the late 18th century when Swiss physicist Daniel Bernoulli introduced the idea of conservation of energy in fluid flow. • In the middle of 19th century, Austrian physicist Christian Doppler developed the idea behind Doppler Effect, used in Doppler flowmeters. • In the late 19th century, British Engineer Osborne Reynolds developed the idea of a single number to describe the fluid flow. • Reynolds number is fundamental measure of type of flow.
- 4. Fluid Flow • Fluid flow is the movement of liquids in pipe or channels and gases or vapors in pipes and ducts. • Two types of flow measurements: Flow rate and Total flow. • Flow rate is the quantity of fluid passing a point at a particular moment. • Total flow is the quantity of fluid that passes a point during a specific time interval. • The flow rate of liquids is expressed in volumetric or mass units.
- 5. Fluid Flow (continued) • Common volumetric units used in USA are gallons per minutes (gpm) and gallons per hour (gph) • Other countries used : liters per minute (l/min), cubic meters per hour (cu m/hr), cubic centimeters per minute (cc/min) • The unit of mass in USA is pounds per hr (lb/hr). • The metric unit of mass is kilogram per hour (kg/hr) • The flow rate for gases and vapors is usually expressed in volumetric units. • In USA the gas flow rates are stated as standard cubic feet per hour (scfh) or standard cubic feet per minutes (scfm). • In metric unit the volumetric flow rate is standard cubic centimeters per minutes (sccm). • Steam flow rates are generally expressed as lb/hr or kg/hr.
- 6. Reynolds Number • Reynolds number describes the nature of fluid flow. • The Reynolds number of a fluid is the ratio between the inertial forces moving a fluid and viscous forces resisting the movement. • The Reynolds number has no units of measure and is calculated from velocity or flow rate, density, viscosity, and the inside diameter of a pipe. • Reynolds number commonly range from 100 to 1,000,000. However, they can be higher or lower than these values.
- 7. Laminar Flow vs. Turbulent Flow • Laminar flow is a smooth fluid flow that has a flow profile that is parabolic in shape with no mixing between streamlines. • Laminar flow in a pipe occurs at Reynolds numbers below about 2100. • Turbulent flow is flow in which the flow profile is a flattened parabola, the streamlines are not present, and the fluid is freely intermixing. • Turbulent flow in a pipe typically occurs at Reynolds numbers about 4000. Reynolds Number Re = 1488x (v x d x ρ) / µ v = velocity (in ft/sec) d = diameter of a pipe in ft. ρ = density in lb/cu ft µ = viscosity in cp.
- 8. Laminar flow vs Turbulent Flow (cont)
- 9. Differential Pressure Flowmeters • A pressure difference is created when a fluid passes through a restriction in a pipe. • The operating principle of a differential flowmeter is based on the relationship between the volume flow rate and pressure drop. • A restriction in piping used for flow measurement is called primary element. • The examples of primary elements are: orifice plates, flow nozzles, venturi tubes. pitot tubes.
- 10. Differential Pressure Flowmeters (contd.) Orifice Plate: • An orifice plate is a primary flow element consisting of a circular metal plate with sharp edged round like hole in it and a tab that protrudes from the flanges. Orifice plate inside the pipe
- 11. Differential Pressure Flowmeters (contd.) Venturi Tubes: • A venturi tube is a primary flow element consisting of a fabricated pipe section with a converging inlet section, a straight throat, and a diverging outlet section. The venturi tube is suitable for clean, dirty and viscous liquid and some slurry services. The rangeability is 4 to 1. Pressure loss is low. Typical accuracy is 1% of full range. Required upstream pipe length 5 to 20 diameters. Viscosity effect is high Relative cost is medium
- 12. Differential Pressure Flowmeters (contd.) Flow Nozzles A flow nozzle is a primary flow element consisting of a restriction shaped like a curved funnel that allows a little more flow than an orifice plate and reduces the straight run pipe requirement. The nozzle is mounted between a pair of standard flanges. The pressure sensing taps are located in the piping a fixed distance upstream and downstream of the flow nozzle.
- 13. Differential Pressure Flowmeters (contd.) Pitot Tubes • A pitot tube is a flow element consisting of a small bent tube with a nozzle opening facing in to the flow. • The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially in air applications as ventilation and HVAC systems, even used in airplanes for the speed measurement. • The pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy.
- 14. Operating Principles Differential Pressure Flowmeters • The differential pressure across a flowmeter is based on the Bernoullis equation, where the pressure drop and the further measured signal is a function of the square flow speed. • Bernoulli determined that at any point in a closed pipe there were three types of pressure head: static head due to elevation, static due to applied pressure, and velocity head. • The Bernoulli equation is an equation stating that for a non-viscous, incompressible fluid in steady flow, the sum of pressure, potential and kinetic energies per unit volume is constant at any point
- 15. Operating Principles Differential Pressure Flowmeters (contd.) • Fluids exiting an orifice plate have a theoretical velocity that is proportional to the square root of the hydrostatic pressure causing the flow. • From the Bernoulli’s equation following equation can be developed for volumetric liquid flow: Volumetric liquid flow: where: Q = flow rate (in gpm); N = constant conversion factor 5.67 D = pipe diameter (in in.); C= coefficient of discharge, depends on the diameter of the pipe and orifice plate. h = differential pressure (in in. water) SG = specific gravity of the liquid relative to water Similar equations could be developed for gas and steam flow. hxSG xCx NxD Q 2
- 16. Selection of Differential Pressure Transmitter. From the equation given in previous slide, you can calculate the differential pressure (in in.water) across an orifice plate, if you the following parameters: Q = flow rate (in gpm); N = constant conversion factor 5.67 D = Pipe diameter (in in.); C= coefficient of discharge SG = specific gravity of the liquid relative to water From the differential pressure you can select a differential pressure transmitter from the vendor specification. Following example will demonstrate the whole process. Sample Problem: A 4 inch pipe carries water that is measured by a concentric, sharp edged, oriifice plate, diameter = 2.00 inches. The flow of water through the pipe is about 135 gpm. Based on the diameter of the pipe and diameter of the orifice plate, the coefficient of discharge is C = 0.1547. Calculate the differential pressure across the orifice plate. Choose a suitable differential pressure transmitter which will provide an output of 4- 20 mA dc. Calculate the signal from the transmitter in mA dc if the flow through the pipe is 135 gpm.
- 17. Selection of Differential Pressure Transmitter (contd.) Volumetric liquid flow: D = 4.0 inches , C = 0.1547, Q =135 gpm, SG = 1 Substituting all the values given, you will get h = 90 inches of water. So, a differential pressure transmitter of 0 -100 in of water will work for this application. The transmitter will be calibrated 0 to 100 inches of water pressure and will output 4 – 20 mA dc. So, at 90 inches of water, the signal from the transmitter will be 18.4 mA dc. You can include this calibration curve into Lab View VI. Rosemount 1151 Pressure Transmitter http:// www.emersonprocess.com/rosemount/ Products/Pressure/m1151.html hxSG xCx NxD Q 2
- 18. Differential Flow Transmitter Installation on a pipe Installation details:
- 19. Variable Area Flowmeter or Rotameter • The rotameter consists of a vertically oriented glass (or plastic) tube with a larger end at the top, and a metering float which is free to move within the tube. • Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. • Three types of rotameters (see pictures on the next slide) Clear Tube Rotameters Plastic Tube Rotameters Metal Tube Rotameters
- 20. ROTAMETERS
- 21. Positive Displacement Flowmeters • The positive displacement flowmeter is a mechanical flowmeter that admits fluid into a chamber of known volume and then discharges it. • The number of times the chamber is filled during a given interval is counted. • This type of meter is commonly used for measuring total flow in homes and factories.
- 22. Vortex Flow Meter • An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream. • These alternating low pressure zones cause the obstruction to move towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured.
- 23. Electrical Flowmeters Magnetic Flowmeters: • A magnetic flowmeter or magnetometer, is an electromagnetic flowmeter consisting of a stainless tube lined with nonconductive material with two electrical coils mounted on the tube like a saddle. • A magnetic flow meter is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non- aqueous solutions). Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.
- 24. Magnetic Flowmeter (contd.) Principle of Operation • The operation of a magnetic flowmeter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.
- 25. The Doppler Effect Ultrasonic Flowmeter • The Doppler Effect Ultrasonic Flowmeter use reflected ultrasonic sound to measure the fluid velocity. By measuring the frequency shift between the ultrasonic frequency source, the receiver, and the fluid carrier, the relative motion are measured. • The resulting frequency shift is named the Doppler Effect.
- 26. The Doppler Effect Ultrasonic Flowmeter (contd) • The fluid velocity can be expressed as • v = c (fr - ft) / 2 ft cosΦ (1) • where • fr = received frequency • ft = transmission frequency • v = fluid flow velocity • Φ = the relative angle between the transmitted ultrasonic beam and the fluid flow • c = the velocity of sound in the fluid . • This method require there is some reflecting particles in the fluid. The method is not suitable for clear liquids. Advantages with the Doppler Effect Ultrasonic Flowmeter • Doppler meters may be used where other meters don't work. This might be liquid slurries, aerated liquids or liquids with some small or large amount on suspended solids. The advantages can be summarized to: • Obstruct less flow • Can be installed outside the pipes • The pressure drop is equal to the equivalent length of a straight pipe • Low flow cut off • Corrosion resistant • Relative low power consumption
- 27. The Doppler Effect Ultrasonic Flowmeter (contd) • Limitations with Doppler Effect Ultrasonic Flowmeters • The Doppler flowmeters performance are highly dependent on physical properties of the fluid, such as the sonic conductivity, particle density, and flow profile. • Non uniformity of particle distribution in the pipe cross section may result in a incorrectly computed mean velocity. The flowmeter accuracy is sensitive to velocity profile variations and to the distribution of acoustic reflectors in the measurement section. • Unlike other acoustic flowmeters, Doppler meters are affected by changes in the liquid's sonic velocity. As a result, the meter is also sensitive to changes in density and temperature. These problems make Doppler flowmeters unsuitable for highly accurate measurement applications.
- 28. The Time of Flight Ultrasonic Flowmeter With the Time of Flight Ultrasonic Flowmeter the time for the sound to travel between a transmitter and a receiver is measured. This method is not dependable on the particles in the fluid. Two transmitters / receivers (transceivers) are located on each side of the pipe. The transmitters sends pulsating ultrasonic waves in a predefined frequency from one side to the other. The difference in frequency is proportional to the average fluid velocity.
- 29. Mass Flowmeter • Mass meters measure the mass flow rate directly. Thermal Flowmeter • The thermal mass flowmeter operates independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path where the flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate and the he temperature difference is calculated to mass flow. • The accuracy of the thermal mass flow device depends on the calibrations reliability of the actual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid.
- 30. Mass Flowmeter (contd.) Coriolis Flowmeter • Direct mass measurement sets Coriolis flowmeters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density. With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal meters. • Coriolis Mass Flowmeter uses the Coriolis effect to measure the amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors. • The Coriolis flow meters are in general very accurate, better than +/-0,1% with an turndown rate more than 100:1. The Coriolis meter can also be used to measure the fluids density
- 31. Mass Flowmeter (contd.) Open Channel Flowmeters • A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel. • Common used is the Sharp-Crested Weir, the V-Notch Weir, the Cipolletti weir, the Rectangular-Notch Weir, the Parshall Flume or Venturi Flume.