|Electromagnetic Flow Meters|
1 ) What is the Principle of operation of Electromagnetic Flow Meters ?
The working of these meters is based on Faraday’s Second Law of Electromagnetic Induction. It states that, a conductor moving in a magnetic field with the direction of its motion perpendicular to the magnetic field generates an EMF across it and the direction of the EMF is perpendicular to both, the magnetic field and the direction of motion. The generated EMF is proportional to the magnetic flux density, B, the velocity of conductor v and the length of the conductor l. In the flow meter the magnetic field is generated by exciting the coils and the conductor is the liquid itself, under measurement with the length equal to the diameter of pipe.
Thus EMF = B.v.d, where d is the diameter of the inner wall of flow meter
2 ) What is the level of conductivity required for the liquid under measurement to get satisfactory operation of the meter?
In general the electrical conductivity of liquid under measurement should have minimum of 5 micro siemens per cm. conductivity. But carefully designed transmitters and primary flow sensors, like Manas, can measure flow rate of liquids having conductivity as low as 1 micro siemens per cm.
3 ) Can Electromagnetic Flow meter be used for measurement of flow of liquids like Petrol, Diesel, Kerosene, and Various types of oil, Edible oil or other organic solvents?
As explained in question 2 above, the required conductivity of liquid is at least 1 micro siemens per cm. The conductivity of liquids under question are nearing to almost zero. E.g. the conductivity of kerosene is in Pico Siemens, where even carefully designed meters also cannot measure the liquid flow.
4 ) Can the Electromagnetic flow meter measure the flow of fluids like, steam, air, gases or any such compressible fluid?
Electromagnetic flow meters are designed as Liquid Flow Meters. They cannot measure flow rates of compressible fluids.
5 ) Can Electromagnetic Flow Meter be used for measurement of Mass Flow Rate?
If we refer to question no.1 above we find the basic equation as,
E = B .v. d
Where E is the Electro Motive Force generated in the liquid because of its average velocity. It is clear that for a given flow meter the magnetic flux density and diameter are constant. The EMF is thus proportional to only average velocity of the liquid. Velocity when multiplied by cross sectional area gives us volumetric flow rate.
The electromagnetic flow meters are thus volumetric flow meters and cannot be used as Mass Flow Meters.
If the density of the flowing liquid is fixed and if there is a facility to feed the density in transmitter the E.M. Meter can be used as a Mass Flow Meter
6 ) What is the typical Accuracy of measurement for Electromagnetic Flow Meters?
The typical accuracy of these meters is +/- 0.3 % of reading. The specified accuracy is +/- 0.5% of reading between 10 % to 100 % of calibrated flow rate range.
7 ) What is meant by Full Bore Meter ?
A Full Bore meter is a circular meter where, when the flow passes by filling the entire bore area, the meter can measure the average velocity inside the meter. Naturally the accuracy of a full bore meter is always much better than any other type.
8 ) What is the difference between Insertion Type and Full Bore Type meter?
As explained in question 7 above the full Bore Meter measures the average velocity. The Insertion Type measures velocity at only one point. So in fact Insertion Meters are velocity meters and not real flow meters. Naturally the accuracy of Insertion type, if used as flow meter, is very poor.
9 ) What is the effect of change in velocity profile through the meter, on accuracy?
As long as the velocity profile is symmetrical around the axis there is no effect on accuracy with change in velocity profile, e.g. from laminar to turbulent or even transitional profile of velocity.
10 ) What is the minimum velocity for which the Electromagnetic Flow Meter works with consistent readings?
General purpose meter manufacturers do not recommend to operate their meters below 0.2 to 0.3 meters/second velocity. Manas make meters, SROAT 1000plus and converter SROAT 1000 A, can comfortably be used for velocities as low as 0.1 meters/second.
11 ) What is the difference between a Wafer Style Meter and Flanged type Meter?
Both types work on the principle of Faraday’s Law. Both are durable and accurate in measurements.
But the lengths of flanged type meters are specified by ISO standard, ISO13359 and those of wafer type are decided by manufacturer.
Wafer type meters are not having flanges and are installed by sandwiching the meter between two flanges of pipe.
There being no flanges to the meter, and length being less the wafer meters are economical compared to flanged meters.
12 ) Are Electromagnetic flow meters suitable for Food and Beverage Industry?
Electromagnetic flow meters are suitable for all liquids having some electrical conductivity. Food and Beverages Industries can comfortably use Electromagnetic Flow meters. The meters used in food applications are with full Stainless Steel Body and having end connections suitable for sanitary applications. Manas offer these meters in its SG-Sroat Series.
13 ) What is meant by "Empty Tube Detection"? How is that useful?
The primary sensor of the flow meter is supposed to be full with the liquid under measurement even when the flow is not passing or when the liquid is under standstill condition. In practice many times one of the following conditions occurs.
a) When flow stops all the liquid gets drained and the primary flow sensor either becomes empty or partially filled.
b) The flowing conditions are such that the pipeline remains partially filled even when Under these conditions the flow transmitter cannot read the flow rate accurately or sometimes even delivers erratic results. Because of this the user gets confused and cannot judge what is going wrong exactly. To help user under this situation some manufacturers (including "Manas") add this empty tube detection feature. Because of this feature partially filled tube or empty tube is detected and the text "Empty Tube" is displayed on transmitter. The user has now two options, one is to stop totalization or continue erroneous totalization with full awareness.
14 ) Can electromagnetic flow meter work for steam condensate or any other bubbling liquid at high temperature?
The E.M. Flow meter works on the principle of generating electromotive force in all elements of the conductor. Here the conducting elements are liquid elements. Whenever bubbling occurs the chain of conducting elements from one electrode to other breaks and the voltage generated is having the discontinuity at various points in the chain.This prevents the transmitter to read the flow rate accurately. The solution to make the transmitter work under such situations is to avoid bubbling of liquid while passing through the primary flow sensor.
Thus the flow meter can work for steam condensate at high temperature if the bubbling can be avoided. There are several techniques to avoid bubbling due to high temperature.
15 ) What is the difference between magnetic flow meters and magnetically coupled flow meters?
The measurement principle of magnetic or electromagnetic flow meters is based on Faraday’s Law of Electromagnetic Induction. These meters contain no moving parts.While as in magnetically coupled meters, measurement is not based on Faraday’s law, but is based on rotation of wheel with velocity of liquid. The rpm value of this rotating wheel is transferred to the outside measurement scheme by magnetic coupling. Turbine type and peddle wheel type meters usually use this scheme. These meters are having moving parts and s limitations on accuracy as well as durability and capacity to measure minimum velocity signal. Magnetically coupled meters are some times mistaken as magnetic or electromagnetic flow meters.
16 ) Can Electromagnetic flow meter be used for measurement of Boiler Feed Water?
The E.M. Flow Meter can be used for measurement of Boiler Feed water. In this application it is recommended to install the meter on suction side of the feed pump.
17 ) What are the various names of Electro Magnetic Flow Meters?
These meters are known with various names like, Magnetic Flow Meters, Magmeters, Fullbore electromag, Magnetic Inductive Flow meter etc.
|Gas Flow Meters|
1 ) What is the principle of operation of Steam and other Gas Flow Meters?
There are various types of Flow Meters for Steam, Gas, flow measurements. The most popular type is based on the principle of creating differential pressure across the differential pressure element like ORIFICE, VENTURY TUBE etc.
2 ) Why Mass Flow Rate is measured? Why not Volume Flow Rate?
The volume of compressible fluid changes with change in pressure and temperature. The mass remains constant irrespective of changes in pressure and temperature of fluid. It is therefore convenient to measure mass flow rate.
Volume flow rate also can be measured provided the reference conditions of temperature and pressure are well defined. The volume flow rate is then computed for these conditions thus avoiding the errors due to changes in pressure and temperature of fluid.
Steam flow meters display the reading in terms of mass, i. e. Tons/hour or Kg/hour etc. while Air or Gas flows measure flow in terms of Nm³ or Sm³ where N and S stands for reference conditions.
3 ) What are the reference conditions for measuring volume flow rate?
There are two different reference conditions.
One is N.T.P. and other is called S.T.P.
Normal condition of Temperature and Pressure is NTP.
Standard condition of Temperature and Pressure is STP.
The definitions for these conditions are defined but are interchanging in various countries. In India they are defined as follows,
NTP: 0° C Temperature and 1 Atmosphere Pressure.
STP: 15° C Temperature and 1 Atmosphere Pressure.
4 ) Why accuracy gets hampered with variation of temperature and pressure?
As explained in question 2 above the volume of fluid changes with change in temperature and pressure. If we are measuring volumetric flow naturally we will find different readings of flow rate at different pressure and temperature for the same quantity of flow.
If we are measuring Mass flow rate, the density changes with change in temperature and pressure so the flow rate readings will not be accurate (refer to the expression of mass flow rate in question 1 above.
5 ) How to avoid effects of temperature and pressure variations while measuring flow rate of compressible fluid? What is density compensation?
To avoid the effects of variation of temperature and pressure on flow measurement the flowing pressure and flowing temperature is measured. From this data the density is computed and as per the expression given in question 1 above for mass flow rate the value for the same is calculated. Pressure Temperature compensation and Density compensation is one and the same.
For calculating volumetric flow rate first the mass flow rate is calculated and then it is divided by density at NTP or STP conditions as per requirement.
This type of correction is also known as density compensation.
6 ) What is the typical accuracy of this type of systems using orifice as primary element?
There are two standards that govern the flow measuring system using orifice or other restriction elements.
a) BS 1042 b) ISO 5167.
As per these standards the maximum error of measurement is within ±3%. Carefully crafting of the primary element and other components can reduce the error down to 1½ % to 2%.
7 ) What is meant by Beta Ratio? What is its normal range?
Beta (ß) Ratio is the ratio of orifice diameter to pipe internal diameter. Its normal range is from 0.4 to 0.8.
8 ) What are the piping requirements in terms of minimum upstream and downstream straight lengths near the orifice element?
The requirement of minimum straight lengths depends on two factors,
a) The Beta ratio. The more the beta ratio the more is the requirement of straight lengths, especially more on upstream side.
b) The type of disturbance on upstream side, e.g. One elbow in single plane, two elbows in one plane, two elbows in different planes, reducer or expander, regulator or valve operation. It is difficult to tell any thumb rule for straight lengths but knowing the specific case proper straight lengths required on upstream and downstream side can be advised. Requirement of upstream straight length varies between 7D and 24D. The variation of downstream is comparatively much less and varies between 3 to 5 times the internal diameters of pipe (D).
Contact on email@example.com with your specific requirement.