Flow

JITENDRA SURVE PIPING GUIDE 01/04/03

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FLOW MEASUREMENT • Flow measuring instruments are:

1. Orifice plate. 2. Variable area flow meter or Rotameter. 3. Magnetic flow meter. 4. Turbine flow meter. 5. Positive displacement meter.

• Flow measurement is the most important variable in the plant operation.

• Without flow measurement, plant material balance, quantity control and operation of continuous or batch process would be almost impossible.

• Most flow rates are determined by inferential measurements. There are

: 1. Head type: Inferred from differential pressure measurement. 2. V.A. Meter or Rotameter : Inferred from position measurement

resulting from weight force balancing the velocity force. 3. Magnetic meter : Inferred from velocity measurement. 4. Turbine meter : Inferred from velocity measurement. (Velocity is

converted into rotation). • Discrete flow measurement is used for a small percentage of industrial

flow measurement applications. 1. Positive displacement meters. 2. Positive displacement metering pumps.

HEAD TYPE FLOW METER • Most commonly used method in the chemical plants. • Almost 90-95% flow measurement is used with this type of

measurement. • The principle of operation is that a restriction in the pipeline of flowing

fluid produces a differential pressure across the element proportional to the flow rate.

• The basic equation derived from Bernouli’s theorem for retriction devices is :

Q = KA * SQRT(H/D) Where: K = constant A = Area of the pipe.


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H = D.P. across orifice. D = Density

• Several different types of restriction elements are used for head flow measurement.

• The orifice plate is the most commonly used. • Other primary devices include venturi tubes, flow nozzles, pitot tubes

and Annubars. • In case of orifice plates, different types of tapping are used depending

on the size of the pipeline. • For pipe size o f 25 and 40 NB corner taps are used. • For pipe size of 50 NB and upto 150 NB usually flange taps are used. • D and D/2 taps are used for pipe size of 200 NB and above.


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VARIABLE AREA METER (ROTAMETER)

• Consists of a vertical tapered tube with a float which is free to move up or down within the tube.

• The measured fluid enters the tube from the bottom and passes upwards around the float and out at the top.

• As the flow varies, the float rises or falls varying the area of the annular passage between the float and tube.

• Borosilicate glass metering tubes are commonly used for relatively low pressure and temperature services, for nonhazardous fluids such as water and air.

• Metal metering tubes are used for application where glass is not applicable.

• A magnet inside the float is used to operate the pointer mechanism which gives indication of the flow.

• Rotameter sizing is based on the flow of air for vapor/gas service and on water for liquid service.


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• Actual flows of fluid are converted into lpm water equivalent or NM^3/hr air equivalent for selection of the rotameter.

• Scales are provided in actual fluid flows or in lpm of water ir NM^3/hr of air.


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MAGNETIC FLOW METERS • Utilizes the principle of faraday’s law of induction which states that an

electrical potential is developed by the relative motion at right angles between a conductor and a magnetic field.

• The meter consists of an electrically insulated tube of pipe with a pair of electrodes at opposite ends of the tube diameter mounted flush with the inside walls.

• Electric coils are mounted around the tube so that a magnetic field is generated in a plane mutually perpendicular to the axis of the meter body and to the place of electrodes.

• The fluid can be conceived as a group of parallel conductors. • The voltage generated is proportional to the average fluid velocity

giving a true volume measurement of the fluid. • The measurement is independent of viscosity, temperature and

pressure. • The only condition is that the fluid should have a minimum conductivity

which varies between 20 microohms to 0.1 microohms depending on the manufacturer’s interfacing equipment to handle signal strength.

• Magnetic meters are available in sizes of 15 mm to any higher size with wide choice of materials.


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TURBINE FLOW METERS • Turbine meter consists of a straight flow tube within which a turbine is

free to rotate about its axis which is fixed along the center line of the tube.

• The velocity of the flowing stream imparts a force to the turbine blades which rotate at a speed proportional to flow rate.

• In most units a magnetic pick-up system senses the rotation of the rotor through the tube wall.

• As each turbine blade passes the magnetic pick-up coil, one pulse of A.C. voltage is induced, each of these pulses representing a definite flow quantity.

• Basically, a turbine meter consists of 3 separate components namely, the housing, the rotor assembly, and magnetic pick-up coil.

• One of the major advantage of turbine meter is its adaptability for flow totalizing .i.e. pulses can be counted and manipulated by digital technique to give final total quantity.

• One of the disadvantage of turbine meter is the entire rotating mechanism is subjected to dirt and corrosive nature of the flowing fluid.

• Hence turbine meter is preferred only for clean and non-corrosive liquids.


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POSITIVE DISPLACEMENT METERS • P.D. meters are devices that separate a flowing stream into individual

volumetric increments and count these increments. • The meters are machined such that each volumetric increment is

accurately known and the summation gives the total volume flowed through the meter.


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RECIPROCATING PISTON METERS • Similar to reciprocating steam engine and is used in chemical

industries as metering pumps. • The stroke of these pumps can be varied and the volume pumped can

be changed. • The disadvantage of this pump is that it produces pulsating flows.


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FLUOR DANIEL STANDARD

FLANGE TAP ORIENTATION CLEARANCE AND METER RUNS


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1. This drawing is to be used for layout and design of orifice type, vortex type, cariolis type (See note 8), ultrasonic type, thermal type and magnetic type meter.

2. Single plane definition - The directional change preceding the meter run is in the same plane.

3. Double plane definition - The directional change preceding the meter run is in a different plane.

4. The upstream and downstream dimensions shown are minimum and shall be increased where practicable.

PIPING DESIGN GUIDE PROCEDURE:

5. Determine whether the meter run is single or double plane. 6. Determine whether the run preceding the metering device is over or

under the required pipe diameters run length for the applicable plane.

7. Select the correct letter (B.C.D. or E.) & check the meter run table for the required straight run length.

8. When the necessary straight run length is not available, consult the control systems engineer for permission to decrease the straight run length.

9. When a reducer is located near the meter run, it should be placed with consideration to economics of location. Example: reducer may be very close to upstream elbow weld, so place reducer on weld of elbow.

10. Installing block valves in meter runs should be avoided if possible; when block valves are required to be installed in meter runs B,C,D & E. A fully open gate valve may be installed.

11. Installing globe type control valves in meter runs should be avoided.

12. This orifice meter run guide is based on a d/D ration of 0.70. If any other d/D ratio is to be used, consult with the control systems engineer.

13. Dimensions and details for meter runs not shown on this drawing shall be developed by detail engineering contractor.

14. Consult the control systems engineer for the upstream and downstream straight run requirements of coriolis type meters.

15. Prior to "approved for design", submit final flow meter run design to control systems engineer for approval.


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VORTEX, ULTRASONIC, THERMAL AND MAENETIC TYPE METER RUN NOTES.

1. Adhere to the straight run length requirements as listed in the above chart.

2. Where the requirements for upstream straight run of vortex meters cannot be met, a minimum of 5D is acceptable. Piping engineering shall advise the control systems engineer of the actual straight length available and the type of "First disturbance". This information


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shall appear on the vortex meter specification sheet and the supplier shall factory set a "K" factor compensating the actual piping configuration.

3. For mounting of vortex meter electronics in horizontal lines for liquid service, mount electronics above vortex. In case of liquid flow with entrained gases, special attention must be paid to the installation, since it might be advantageous to mount electronics below the vortex, provided there is not dirt or sediment present. This will have to be co-ordinated between piping and control systems.

4. For mounting of vortex meter electronics in horizontal lines for superheated steam, mount above vortex. (Insulation only between vortex and transmitter)

5. For mounting of vortex meter electronics in horizontal lines for condensable vapors and saturated steam mount electronics above vortex.

6. For installation of vortex meters in steam services, insulation between vortex meter and transmitter part shall be applied (Vertical lines only). Flow direction for vertical lines for vortex meter shall be : Liquid- upward flow: and gas/steam- downward flow.

7. Flow direction shall be upward. 8. Electronics and remote mounted where meter is elevated or

otherwise inaccessible.


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ORIFICE TYPE METER RUN FLANGE TAP NOTES:

1. Clearance allotment for orifice flange tap piping. 2. Orifice tap block valve shall be 1/2" 3. Alternate tap locations . 4. Clearance for line mounted flow transmitter shall be modeled in the

PDS electronic model. 5. Center for transmitter approximately 1300 above grade, platform or

walkway. 6. When a liquid contains dissolved gases, the impulse lines shall slope

downwards to the instrument so that as is automatically vented back into the process. In such cases the instrument shall be mounted in a vertical line. If this is not possible, the process tapping should be installed at the bottom (Downwards) of the horizontal line.

7. For vertical piping, the tappings shall point in the direction from which they are accessible. Assuming solid is heavier than liquid.


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RESTRICTION ORIFICE • This is generally used for effective distribution of fluid in piping

network. • The restriction orifice plate is nothing but a plate having a defined

aperture size in a pipeline sandwiched between two flanges to control the upstream distribution network.

• This can be broadly interpreted as regulating valve locked in a cracked open position in a piping network.

• This device controls the flow of upstream system by virtue of pressure drop across restriction orifice plate.


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