1. FUNDAMENTAL GOVERNING EQUATION
1.1. Working Principle:
The classical electromagnetic flow meter is contact type flow device that consists of a pair of coil and pair of electrode. The coil generates magnetic flux. The flux is cut by flowing fluid through pipe and resultant flux is measured by electrodes.
The works on principle of faradays law of electromagnetic induction, which is given by,
E= CBLV………………………(1)
E= Induced emf
C = Non-dimensional constant
B= Magnetic flux density
L= Effective length of flow meter (here it is D, diameter of pipe).
V= velocity of fluid inside pipe
Basic sketch:
2. Specifications
2.1 Customer Requirements:
Service Provided: water.
Inlet pressure: 10 kg/cm2
Inlet Temperature: Normal.
Flow Rate: 20000m^3/hr (maximum)
Line size: 10 inches (250cm).
2.2 Vendors Provision:
a) Design Point: customer provided velocity of flow.
b) Accuracy: 1%
c) Material of construction:
d) End connection: (Flanges)
e) Pressure Tap connection: (NPT, AN, subtends etc)
3. Extended Equation
Design Equations
a) E = -BLV
b) Q = ((∏*d^2)*V)/4
A= (∏*d^2)/4=Area under coverage = constant
Q = Volumetric Flow Rate
E α Q
c) Sensitivity Calculations:
Sv = U/V=B*D
where
U = o/p voltage
V = average velocity
B = Magnetic field strength
D = Diameter of pipe
d) Flux Generations:
E (Induced) =-N dΦ/dt
Φ= Flux induced
N = No. of turns of coil.
e) Φ =BA
f) Transformers equation for voltage:
N1/N2=V1/V2=I2/I1
4. Design Calculations:
a) Flow rate given —– 20,000 m^3/h=5.56m^3/sec
b) Velocity calculated from equation b—–Vavg= 2000m/s
And considering upper limit ————-Vmax= 2500 m/s
c) Deciding sensitivity of emfm =150*10^-6
Sv = U/V
For V = 2000m/s
U = 300 mv
For V = 2500 m/s
U = 375 mv
d) design of coils
V in = 230 V ac supply
V Out = 24 V ac
V in / V out =230/24 = 9.583 = 9.6(approximately)
N1/N2 =9.6
So, N1 =1
N2 =10
I2/I1 =10
Input current rating of transformer of transformer =I1=0.08 A
Therefore I2 = 0.08*10 = 0.8A
Material of coil = Cu
Diameter of coil = diameter of pipe
e) Electrode Design: SS-314 material M3 type of screw.
5 . Configurable Parameters with design:
1. Material of pipe: It for specifying that pipe is conducting or non-conducting. According to that internal material of insulation is decided. Flow rate is a considerable factor for that. Insulation material is of different type PTFE, Neopropane, Polyurethane.
Here one table is given for insulation of PTFE for different pipe diameters
2. Material of flange : Flange are used for attachment of sensor assembly with pipe line .Usually flanges are made from same material as that of pipe material. Flanges are used for grounding purpose in case of non-conducting pipe.
3. Length of pipe: Length of pipe is important for placement of electrode in order to avoid turbulence effect and measurement of accurate average velocity.
A table is give here showing standard values for pipe diameter (mm), length of pipe, weight of sensor and flanges material
4 . Electrode Design: The electrodes are indirect contact with process liquids. Their material needs to be adequately resistant to corrosion and must allow good electrical
Contact with process liquids. Depending upon process requirement and cost electrode material is decided. Electrodes are of Cr-Ni alloys ,tantalum, SS-304,SS-314,Nickel,Platinum electrodes are used for critical applications.
As our applications are not critical we are using ss-314 electrodes.
5. Design of coil: Coil design is important for getting constant B resulting output voltage propornality with average velocity. Some of the factors for coil designing are explained below.
5.1. Relation between supply voltage and supply current is shown below
5.2. Excitation of coil: It is powered by either alternating or direct voltage. When ac excitation is used, line voltage is applied to magnetic coils.
As a result, the flow signal will also look like a sine wave. The amplitude of the velocity.
In dc excitation design ,a low frequency (7-30 Hz) dc pulse is used to excite the magnetic coils. When the coils are pulsed on (Figure 4-2), the transmitter reads both the flow and noise signals. In between pulses, the transmitter sees only the noise signal. Therefore, the noise can be continuously eliminated after each cycle.
Pulsed ac meters have also been introduced recently, eliminating
- the zero stability problems of traditional ac designs. These devices contain circuitry that periodically disrupts the ac power, automatically zeroing out the effects of process noise on the output signal.
Today, dc excitation is used in about 85% of installations and ac magmeters claim the other 15% when justified by the following conditions:
- When air is entrained in large quantities in the process stream;
- When the process stream is a slurry and the solid particle sizes are not uniform and/or the solid phase is not homogeneously mixed within the liquid; or When the flow is pulsating at a frequency under 15 Hz.
- When any of the above three conditions exist, the output of a pulsed dc meter is likely to be noisy. In some cases, one can minimize the noise problem (hold the fluctuations within 1% of setpoint) by filtering and damping the output signal. If more than 1 to 3 seconds of damping is required to eliminate the noise, it is always better to use an ac meter.
6. Compensations
1. Temperature compensations: To ensure effective sealing at high temp by simply mean, at least one end of stainless steel casing is bent to form a concentric inner ring and a steel compensating ring surrounds the inside of inner ring. The steel from which the compensating ring is made a coefficient of thermal expansion which is between that of material of measurement of tube and stainless steel casing.
2. Conductivity: Conductivity doesn’t have any adverse effect on performance of EMFM
3. Viscosity: Viscosity does not directly affect the operation magnetic flowmeters , but in highly viscous fluids, the size should be kept as large as possible to avoid excessive pressure drop across the meter.
4. Surrounding Magnetic field: To avoid effect of surrounding magnetic field EMFM can have enclosures around e.g. SS enclosure. Considering cost point of view and application we will cover it with wooden cover with wood inside to avoid magnetic noise surrounding.
7. Properties of EMFM
Comparison between various sensors
8. Measurement
Output is nothing but change in flux which is captured by electrodes placed perpendicular to pipe diameter. Output of electromagnetic sensor is in millivolt range. It requires further amplification for bringing it in range. This signal is converted to (4-20 mA) or frequency output (0-10,000 Hz) at or near the flow tube. Transmitters are used for that purpose.
9. Installation Related Issues
1. Certain upstream and downstream pipe diameter required for installation of emfm
2. Magnetic flow meter is heavy instrument so precautions should be taken while carrying it for installation
3. When emfm is processing hot fluids, the instrument itself may become ex-extremely hot. So cooling arrangement should be done.
4. When dealing with toxic fluid avoid contact with fluid and avoid inhaling with any residual gas.
Wiring Precautions
- All cables ends must be provided with round crimp –on terminals and terminals and be securely wired.
- Always connections from power supply and signals should be put in steel conduit tubes.
- Four –core cables are used for wiring. Keep conduits or flexible tubes watertight using sealing tape.
- Ground remote flow tube and the converter separately.
10. Environmental Conditions
1. Ambient Temperature:
Avoid installation the instrument in location with constantly fluctuating temperature. If the location subject to radiant heat from the plant, provide heat insulation or improve ventilation.
2. Atmospheric Condition :
Avoid installation the instrument in a corrosive atmosphere. In situation where this is corrosive atmosphere. In situation where this is unavoidable
Consider ways to improve unavoidable consider ways to improve ventilation and to prevent rainwater from entering and being retained in the conduit pipes.
3. Vibrations or Shocks:
Avoid installation the instrument in a place to shocks or vibrations.
11. Advantages of Electromagnetic Flow Meter
1. Free pipe cross-sectional area
- No additional pressure drop therefore can be used for gravity feed applications
- Allow measurement of media with high solid contents
2. No Mechanically moving parts
3. High Accuracy even in difficult operating condition
4. Bidirectional measurement
5 .Largely independent of Viscosity and Density
6. Independent of flow profile
12. Limitations of Electromagnetic Flow Meter
1. Uniform Field: It can be shown in theory the theory only field that is uniform is one from a magnet of infinite extent.
2 . Axisymmetric Profile: All fully developed pipe flow profiles are axisymmetric. But it is not always convenient to allow the necessary upstream pipe length to ensure a developed profile.
3. Minimum Conductivity: Process liquid should have minimum electrical conductivity from 0.05 to 50µS/cm depend on process liquid, application area.
13. Applications In Various Areas:
14. Resources Used:
- B.G Liptak. “Instrumentation engineers handbook- process measurements and analysis, 3rd edition, chiton book company I Randor, pensylvania ”
- Norman A Anderson – Instrumentation for process measurements and control 3rd edition.
- BHEL- Transformers- 2nd edition
- Flow meter Handbook – R.C.Baker
- www.Sciencedirect.com
- www.omega.com
- www.google.com
