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DIFFERENTIAL PRESSURE FLOW METERS SUBMITTED TO: Dr- Ing Naveed ramzan SUBMITTED BY: 2007-chem-31
Fluid Flow Fluid flow is the movement of liquids in pipe or channels and gases or vapors in pipes and ducts. OR Flow can be defined as the motion characteristic of fluids due to which it moves smoothly with unbroken continuity . There are two types of flow measurements • Volumetric flow rate • Mass flow rate.
Q = A V = ft = * ft ² sec * ³ ft sec ft ³ sec ft sec where: Q = volumetric flow A = cross sectional area (ft ) V = average fluid velocity ² ( ) Volumetric Flow (all fluids)
m = Q = A V = ft = * * * lbs lbs ft ³ ³ sec ft ft ² * * lbs sec where: lbs m = mass flow ( ) = density ( ) Q = average fluid velocity ( ) A = cross sectional area ( ft ) V = average fluid velocity ( ) sec ft ft sec sec ² Mass Flow
Reasons for Flow measurement • 1.)To ensure the correct proportions of raw materials are combined during the manufacturing process • 2.)To ensure that ingredients are supplied at the proper rate during the mixing and blending of the materials • 3.)To prevent a high flow rate that might cause pressure or temperatures to become dangerous, overspills to occur, or machines to over speed
FLOW MEASUREMENT • Methods for flow measurement INDIRECT DIRECT The measurement is obtained indirectly with the use of more than other property . The measurement is obtained directly without the use of any other property
FLOW METERS Definition: Flow meters are a device used to measure the rate of fluid movement at a given point in the pipe or tube. The flow meter is usually secured to a break in the pipe and the fluid is allowed to move through it.
Differential Pressure Flowmeters • They use the difference in pressure resulting from a constriction as an indication for the measurement of flow • The operating principle of a differential flow meter 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 for example orifice plates, flow nozzles, venturi tubes etc.
Differential pressure flow meters Our main concern is with the differential pressure flow meters having the following types • ORIFICE PLATE • FLOW NOZZLES • ROTAMETER • VENTURIMETER
Working principle • Bernoulli’s equation states that energy is approximately conserved across a constriction in a pipe • Static energy (pressure head) • Kinetic energy (velocity head) • Potential energy (elevation head)
Bernoulli’s equation • P/(ρ•g) + ½v2/g + y = constant • P = absolute pressure • ρ = density • g = acceleration of gravity • v = fluid velocity • y = elevation Equation of Continuity • Q = A•v • Q = flow (volumetric) • A = cross-sectional area • v = fluid velocity (average
Apply the equation of continuity and Bernoulli’s equation for flow in a horizontal pipe • Acceleration of gravity is constant • No elevation change • Apply Bernoulli’s equation upstream and downstream of a restriction • P1 + ½ ρ•v12 = P2 + ½ ρ•v22 • Solve for the pressure difference and use the equation of continuity (P1 - P2) = ½ ρ•v22 - ½ ρ•v12 = ½ ρ [v2 2 - v1 2] = ½ ρ [(A1/A2) 2 – 1]•v12 = ½ ρ [(A1/A2) 2 – 1]•Q2/A 2 = constant • ρ • Q 2
CONCLUSIONS: • ΔP = constant • ρ • Q2 • Pressure drop is proportional to the square of the flow rate. OR • Q = constant • (ΔP/ρ)½ • Flow rate is proportional to the square root of the differential pressure produced
ORIFICE PLATE FLOWMETER: • DEFINITION:The orifice plate is a differential pressure flow meter (Primary element).The velocity of fluid passing through the orifice is proportional to the square root of the pressure loss across it. • WORKING PRINCIPLE:To measure the differential pressure when the fluid is flowing, connections are made from the upstream and downstream pressure tappings to a secondary device known as a DP (Differential Pressure) cell.
ADVANTAGE: • Low cost, especially on large sizes • No need for recalibration • Widely accepted DISADVANTAGE: • Poor turndown (4:1 typical) • Long installations (20D to 30D) • Accuracy dependant on geometry.
The Flow Nozzle DEFINITION:A flow nozzle consists of a restriction with an elliptical contour approach section that terminates in a cylindrical throat section. • WORKING PRINCIPLE: Pressure drop between the locations one pipe diameter upstream and one-half pipe diameter downstream is measured.
ADVANTAGES: • The flow nozzle is dimensionally more stable than the orifice plate. • They are applicable to some slurry systems • The flow nozzle, like the venturi, has a greater flow capacity than the orifice plate and requires a lower initial investment than a venturi tube. DISADVANTAGE: • They are comparatively expensive • They are not very accurate.
ROTAMETER: • DEFINITION: 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 • WORKING PRINICIPLE: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 • The float rises until the annular area between the float and tube increases sufficiently to allow a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity factors.The height of the float is an indication of the flow rate.
ADVANTAGE: • The accuracy may be as good as 1% of full scale rating. • Magnetic floats can be used for alarm and signal transmission functions • No output for data transmission • Inexpensive • It is somewhat self-cleaning DISADVANTAGE: • Sensitive to differing gas types and changes in Temperature and pressure
VENTURI TUBE: • 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. • WORKING PRINCIPLE :In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference • After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered
ADVANTAGE: • The venturi tube is suitable for both clean, and dirty fluids • Pressure loss is low. • Typical accuracy is 1% of full range. DISADVANTAGE: • Viscosity effect is high • Relative cost is high
DIFFERENTIAL PRESSURE FLOWMETERS : • ADVANTAGES • Use On Liquid, Gas, and Steam • Suitable for Extreme Temperatures and Pressures • No Moving Parts • Low Cost • DISADVANTAGES • Limited Rangeability • Effected By Changes In Density, Pressure, and Viscosity • Maintenance Intensive
Is the fluid a gas or a liquid? Is the fluid corrosive? Is the fluid conductive or not? Does the fluid contain a slurry or large solids? What is the viscosity of the fluid? Will the fluid density or viscosity change? Is there a need for a noninvasive approach? What is the cost? Selecting a Flowmeter When selecting a flowmeter, the following issues need to be considered: