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3. Flow Measurement
The physical properties of fluid which are important in flow metering ๏ Pressure: Force per area (N/M) ๏ Density: Weight divided by volume (pound per cubic feet) ๏ Velocity: It is defined as rate of flow per area of pipe (Ft/sec) ๏ Viscosity: Its physical resistance to the flow (centipoise) Flow: flow is defined as fluid in motion. ๏ Fluid such as liquid and gases. ๏ Flowing media may be liquid, gaseous, granular solid Types of flow:
1. Laminar flow: - Laminar flow occurs when the fluid flows in infinitesimal parallel layers with no disruption between them. For laminar flow Reynolds number Re < 2300 OR The flow in which fluid flows smoothly such that fluid layers are parallel to each other OR No streamlines intersect each other; such type of flow is known as laminar flow. OR When all the molecules of flow are parallel to each other, it is called Laminar flow. 2. Turbulent flow: Turbulent flow occurs when the fluid does not flow in parallel layers, the lateral mixing is very high, and there is a disruption between the layers. Re > 4000 OR When all the molecules of flow are scattered without fixed position it is called Turbulent flow. OR The flow in which fluid flows in zig-zag manner and fluctuate irregularly in such a way that its velocity changes irregularly, such type of flow is known as turbulent flow. 3. Reynolds no.: The Reynolds number is the ratio of inertial forces to viscous forces and is a convenient parameter for predicting if a flow condition will be laminar or turbulent.
It is a dimensionless number comprised of the physical characteristics of the flow. Re = ฯVmD/ฮผ Where; V- velocity D- Diameter of pipe ฯ- Fluid density ฮผ- Viscosity If the Reynolds number is less than 2000, the flow is laminar. Any Reynolds number over 4000 indicates turbulent flow. Classification of flow: Variable Head Flow Meter: Working principle: Variable flow meter works on the principle that a restriction in the pipe of a fluid, by using orifice plate or venture tube is proportional to the flow rate. Construction and working:
- Here, the proportionality is not linear, but has a square root relation ship in that the flow rate proportional to the square root of the differential pressure.
- The relationship derived from Bernoulliโs theorem
V=Kโ 2 ๐โ/๐
Q=KAโ 2 ๐โ/๐
W=KAโ 2 ๐โ/๐
V= Velocity of Flowing Fluid Q= Volume of Flow Rate W= Mass of Flow Rate A= Cross Sectional Area of Pipe g = Acceleration due to gravity ๐ = ๐๐๐๐ ๐๐ก๐ฆ ๐๐ ๐กโ๐ ๐๐๐ข๐๐
1. Concentric orifice plate: a. A concentric sharp edge orifice plate is the simplest and least expensive of the head meter. b. It is usually made by stainless steel other material are nickel, monel etc. used when good corrosive property is required. c. Accuracy of these types of orifice plate is very high 2. Eccentric orifice plate: a. It is similar to the concentric plate except for the offset hole. b. It has offset hole bored tangential to the circle. c. The location of bore prevents accumulation of solid material so it is useful for measuring fluids contains solid, oils contains water and wet stream. 3. Segmental orifice plate: a. This is used for the same type of services as an eccentric orifice plate. b. It has a hole which is a segment of a circle. c. It is used with fluid containing more solid particles in it. They are more expensive than the eccentric orifice. 4. Quadrant edge orifice plate: a. This type is used for flows of dense fluid such as crudes, slurries, and viscous flows. Its edges are rounded to form a quarter circle. b. They are useful in the pipe have a low diameter (less than 2 inches). c. The drain hole is provided or located the bottom when gases are measured, to allow condensate to pass in order to prevent its building up at the Orifice plate. d. Similarly, a vent hole is located at the top when liquids are measured so that gases can pass and thus avoid gas pocket.
Advantages and disadvantages of orifice plate Advantages:
- Simple in construction, the standards for construction and calibration are readily available.
- They are cheap; their price does not increase dramatically with size.
- Easy to install and remove.
- They are efficient and reliable for many years.
- Available in many materials.
- Easy to measure flow rate, because it produces the considerable pressure drop across the orifice.
- They can be used in a wide range of sizes.
- Their characteristics are well known and predictable. Disadvantages:
- Relatively high permanent pressure drop.
- Their characteristic may change due to erosion and corrosion.
- It may cause clogs in slurries. Not good with fluid having high viscosity.
- The fluid must be in single phase and homogeneous.
- Square root characteristics. Venturi tube: It is a primary element of differential pressure Flow meters.
- It consists of a straight inlet section, a converging conical inlet section, a cylindrical throat and diverging recovery cone.
- Straight inlet section has same diameter as pipe. In converging conical inlet section, the cross-section of stream decreases & velocity increases.
- In cylindrical throat, flow velocity will be maximum & static pressure will be minimum.
- In diverging recovery cone flow velocity decreases
- The pressure taps are located at straight edge section and at cylindrical throat where pressure is minimum thus the maximum Pressure Gauges across this point.
Pitot tube: Operating principle: Pitot tube is based on the fact that when a solid body is kept centrally and stationary in the pipe line, with fluid streaming down, the velocity of the fluid start decreasing due to the presence of the body , till it is reduced to zero directly in front of the body, this point is known as the stagnation point. Construction and working:
- As the kinetic head pressure is lost by the liquid, it gains a static head. Thus by measuring the difference between the pressure at normal flow line and tha at stagnation point, the fluid velocity is determine.
- As pitot tube consist of a liquid tube with an impact opening which is placed directly in the line of flow and static opening at 90degree from impact of opening as shown in figer.
- The difference pressure across these tap is proportional to the velocity of the fluid.
- For an accurate measurement, the pitot tube is moved across the entire diameter of the pipe to measure the velocity at several points and then true average velocity is calculated. Advantages:
- Economical to install.
- No process loss
- Low cost.
- Easily inserted through a small hole into pipe. Dis advantages:
- Poor accuracy.
- Unsuitable for dirt or sticky fluid.
- Sensitive to upstream disturbance
Application:
- Used in gas flow measurement.
- Used in aircraft and missiles.
- Used in determine flow in very large pipe or duct.
- Used to measure fluid velocity. Sr. No: Point Venture Flow Meter Orifice Plate Meter 1 Working principle Works on venturi effect. The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. When fluid passes through orifice, there is large drop in pressure that is indicative of flow rate 2 construction It has The venturi meter has a converging conical inlet, a cylindrical throat and a diverging recovery cone works on venturi effect: An orifice meter is essentially a cylindrical tube that contains a plate with a thin hole in the middle of it. 3 Cost Expensive, carefully fabricated, purchase from proper manufacture Cheap & easy to install. Homemade orifice plate possible 4 Pressure loss Low high Variable Area Flow Meter: Rotameter:- An variable-area flow meter is one where the fluid must pass through a restriction whose area increases with flow rate. The height of the float is directly proportional to the flow rate Construction:
Ultrasonic Flow Meter: Time Difference Type
Doppler type ultrasonic flow meter:
- The Doppler Effect Ultrasonic Flow meter 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.
- The fluid for which pipe flow rate is being measured must have material like particles or air bubbles that will reflect ultrasonic waves
- A signal of known ultrasonic frequency (0.5 to 10MHz) is transmitted through fluid , which has uniform velocity (v)
- Solids, bubbles or any discontinuity in liquid will reflect back to the receiver
- Because of the velocity of the liquid there frequency, there will be a frequency shift at the receiver end which is protentional to the velocity Advantages:
- They have no moving parts.
- Used for both solid and liquid level measurement.
- It is a non-disturbance technique.
- Offer no obstruction to the flow
- o/p is insensitive to variation in viscosity, density and temperature
- Linear relationship between o/p and i/p
- Used for bidirectional flow
- Excellent dynamic response
- Good accuracy +-2%
- o/p is electrical
Working:
- The flowing fluid impinges on the turbine blades , imparting a force to the blade surface which causes the rotation of the rotor.
- The speed of the rotor is directly proportional to the fluid velocity, and hence to volumetric flow rate when it is at steady rotation speed.
- The speed of rotation is mounted by magnetic pick up coil.
- As each rotor blade passes the magnetic pick up coil, it generates a voltage pulse which is measuring of flow rate and the total number of pulses given measure of the total flow.
- The number of pulses generated per gallen of flow is given as K=
Tkf
Q
Where K= pulses per volume unit Tk=time constant in minute Q= volumetric flow rate F= frequency in Hz Advantages:
- Good accuracy
- Provide excellent repeatability and reliability
- Allow fairly low pressure drop.
- Easy to install and maintain.
- Gives good temperature and pressure range Disadvantageous: 1. High cost. 2. Its use is limited for slurry. 3. It face problems caused by non-lubricating fluid. Applicion: 1. Widely used in military application 2. Effective in aerospace and airborne application. 3. Used in petroleum industry. Hot Wire Anemometer:
Principle: When fluid flows over a heated surface, heat transferred from th e surface and there for its surface temperature reduces. The rate of reduction of temperature is related to the flow rate Construction:
- It consist of a short length of fine tungsten wire stretched between two supports as shown in figer
- Stream of gas is kept perpendicular to the fine wire. The wire carries current set to a low value to prevent burn out and high enough to give adequate sensitivity to velocity
- Heat will be transferred between the two mainly by convection
- Heat loss is depends on velocity and specific heat of fluid.
- Thus, flow of velocity can be determined.
- This type of flow sensing element is called hot wire anemometer There are two types A. Constant current type hot wire anemometer: In which current kept constant through the wire. Variation in the flow results in change in wire temperature. Hence resistance of wire changes which then measures flow. B. Constant temperature type hot wire anemometer: In this method, servo system is employed to maintain wire resistance, hence temperature of wire becomes constant by adjusting current wire. The required current, then measures flow rate. Advantages: 1. Excellent accuracy 0.1% 2. Excellent dynamic characteristics 3. Low pressure loss 4. The scale can be analog and digital Dis- Advantages:
- Complicated circuit and need skill full operator.
- Cannot sense reverse or forward flow
- Sensitivity of instrument changes due to dust, dirt on the hot wire. Application: The hot wire anemometer is a device that is mostly used in research application to study varying flow condition. Vortex Flow Meter:
