This blog will try to highlight the pros and cons of different gas flow measurement technologies available in the market which will help process instrumentation engineers to effectively choose the best flow metering technology for defined application which results in optimum results w.r.t cost versus performance ratio.

Below are the Gas flow measurement technologies available :

1. DIFFERENTIAL PRESSURE-BASED ORIFICE/AVERAGING PITOT TUBE/VENTURI/AEROFOIL
2. THERMAL MASS–INSERTION TYPE
3. CORIOLIS
4. VORTEX
5. TURBINE
6. ULTRASONIC – CLAMP-ON TYPE

The process instrumentation engineer’s task is to identify which of the above will be the best suitable technology.

Below are some important factors to be considered & evaluated with comparing technologies for optimum selections:

• Pipe sizes
• Process conditions such as flow rate, pressure, temperature, density, viscosity, dirt & moisture, etc.
• Installation conditions such as horizontal, vertical, or inclined & available straight lengths, time & efforts, etc.
• Accuracy & repeatability needed
• Process turndown ratio needed
• Budgeted price

Based on the above factors we will try to summarize the PROS and CONS of different flow technologies & its a possibility for selection. However, the ultimate choice is of the process engineer who has some specific technologies dominance and/or experience.

1. DIFFERENTIAL PRESSURE-BASED FLOW METER (ORIFICE / AEROFOIL / VENTURI / PITOT  TUBE)

PROS

• Established Standard BS-1042/ISO5167 for Volumetric flow measurement
• Suitable for 15mm to 10mtrs or higher  pipe sizes
• Rugged designed for any process conditions of industries
• Any orientation possible
• Highly Repeatable
• Low budget upto 300mm pipe size
• Site calibration easy

CONS

• Higher pressure drop
• Needs periodic maintenance
• Lower Accuracy 3% FSD & may drift time to time
• Lower turn-down ratio 4:1
•  Lower flow sensitivity
• Susceptible to clogging
• High wear factor

2. THERMAL MASS FLOW METER – INSERTION TYPE

PROS

• Works on constant temperature anemometry (thermal dispersion)
• Pipe sizes suitable 15mm to 10000mm
• Insertion is rugged and works up to 400⁰C & 16bar or more
• Any orientation possible
• Better accuracy ±2%RD
• Highest turndown ratio 100:1 or better
• Insertion type is adjustable & versatile
• Insertion has a lowest pressure drop
• The low initial cost above 100mm pipe sizes

CONS

• Mechanically vulnerable to damage
• Susceptible to turbulence & vibrations
• Flow straightener recommended
• Affected by high moisture (>10%Vol.) & dirt/dust needs periodic cleaning or purging system

3. CORIOLIS FLOWMETER

PROS

• Works on Coriolis effect by oscillating tube provides direct mass flow rate
• Suitable from 2mm to 300mm pipe sizes
• Suitable for high pressure upto 400bar or more
• Any orientation possible
• Highest accuracy (±0.1%- ±0.5%RD) & Repeatability (±0.02%)
• Highest turn down ratio 100:1 or more

CONS

• High-pressure drop
• Not suitable larger pipe dia than 300mm
• Not suitable for higher temperatures above 120⁰C
•  Sensitive to vibrations
• High Initial Cost

4. VORTEX FLOWMETER

PROS

• Derives volumetric flowrate by vortices frequency measurement principle
• Suitable from 15mm to 300mm
• Horizontal or vertical orientation is possible
• Suitable up to 300⁰C temperature & pressure up to 40bar
• Better accuracy up to (±1.5%RD) & Repeatability (±0.02%)
• Low Initial cost up to 80mm size

CONS

• Higher pressure drop
• Lower turn-down ratio 10:1
• Installation needs utmost care
• Susceptible to vibrations & turbulence
• Needs periodic maintenance
• High Initial cost 100mm & above size

5. TURBINE FLOWMETER

PROS

• Derives volumetric flowrate by moving the rotating turbine frequency
• Pipe sizes suitable: 6mm to 300mm
• Suitable for high pressure 250 bar & temperature up to 200⁰C
• Horizontal or vertical orientation is possible
• High accuracy up to ±1% RD & repeatability ±0.25%
• Low initial cost

CONS

• Gas must be dry and clean
• Susceptible to vibrations & turbulence
• Moving parts needs periodic maintenance
• High wear factor

6. ULTRASONIC FLOWMETER – CLAMP-ON TYPE

PROS

• Derives volumetric flowrate by ultrasonic beam transit-time measurement – Non-Intrusive &  Fixed Type
• Pipe sizes suitable : 10mm to 1600mm
• Suitable for high pressure 200 bar & temperature upto max 200⁰C
• Horizontal or vertical orientation possible
• Better accuracy upto ±1%-3% RD & repeatability ±0.25%
• Turn down ratio better than 100:1
• Low Installation cost

CONS

• Gas must be dry and clean
• High Initial Cost
• Installation need good technical knowledge
•  Not suitable for pipes with inner lining

*NOTE: Above article don’t claim accuracy and it is for general guidance on flow technologies as per practical experience there may be improvements or different configuration possible from different manufacturers which may eliminate some disadvantages.

Considering the above Pros & Cons of different flow measurement principle user can be able to understand its need and compare basic important parameters which will ensure optimum selections. Hope this will help in your future decisions. Pls.do write us a comment on the same.

LEOMI, A make-in-India start-up registered company manufacturing Insertion Thermal mass flowmeter which has many inherent advantages in terms of performance versus cost ratio.

 

The efficient operation of today’s power plant depends largely upon accurate & repeatable measurement of primary and secondary airflow to coal mills, flue gas recirculation flow, overfire air flow, airflow to individual burners, etc.

Flue gas or Stack gas emission flow rate measurement is nowadays important for quantifying emission (CEMS) for environmental reporting for Govt. Authorities for pollution control. Flue gas flow measurement are gases emitted due to the combustion process due to heating of fuel (liquid or solid or gaseous) & Air with a stoichiometric ratio in:

• Boiler Heating
• Process furnaces

Flue gas production due to combustion mainly consists of

• Nitrogen (N2)
• Carbon monoxide (CO)
• Carbon dioxide (CO2)
• Traces of Sulphur dioxide (SO2),
• Nitrogen oxides (NO, NO2)
• Particulate matters (SPM)
• Moisture

Flue Gases are gases emitted due to combustion process due to heating of fuel (liquid or solid or gaseous) & Air in:

• Thermal Power Plant
• Steel & Foundries
• Cement production plant
• Chemical & Fertilizer production process plant, many more..

Why is Flue Gas Flow Measurement so important?

As most flue gases emission contains air pollutants that are harmful to human health. So continuous emission monitoring systems (CEMS) are mandatory for providing reporting to state & central pollution control boards for environmental pollution control. It is important to measure the composition and concentration of polluting gases as well as mass flow rates to arrive at the total emission discharge in the environment.

Flue gas flow rate measurement is imperative for below reasons

• Optimizing ESP performance by maintaining design parameters such as specific collection area, gas velocity, and treatment time within ESP in control.
• Indicates early warning for Preheater condenser failures
• Help regulates harmful pollutants, dust emission controls
• Useful information on optimizing mass balance
• Simple design to operate Helps in energy conservation easily
• Predictive & preventive measures for optimizing their process efficiency and reducing harmful emissions in the environment.

Process conditions of Flue gas in Stack

Process engineers design the lowest possible heat loss into the environment for better thermal efficiency of the power plant. Flue gas process conditions with optimum design have typically as below process parameters:

• Composition: Flue gas with moderate dust/fly-ash particle (Coal-fired).
• Process temperature: 130⁰C to 180⁰C
• Process velocity : recommended approx.12m/s to 20m/s.

Where to monitor Flue Gas Flow Rate?

To get the optimum efficiency it can be a monitor at :

• Chimney or Stack near to the point of sampling for lab analysis.
• Inlet of Flue gas desulfurization plant (Wet FGD / Dry FGD ) in Thermal Power plant
• Process stack in chemical production, fertilizer, steel plants
• Process venting systems

Which are the main technologies available for Flue Gas Flow Monitoring?

• Differential Pressure based Flowmeter (Aerofoil/ Annubar /Pitot-tube)
• Non- Contact Ultrasonic Flowmeter
• Insertion Thermal Mass Flowmeters

Process instrumentation engineer’s task is to identify which of the above will be the best suitable technology.

Factors to consider while selecting a flowmeter

Below are some important factors to be considered & evaluated with comparing technologies for optimum selections:

• Duct or Pipe Dimension
• Insulation thickness, if any.
• Process conditions such as flow rate, pressure, temperature, density, viscosity, dirt & moisture, etc.
• Installation conditions such as horizontal, vertical & available straight lengths, time & effort, etc.
• Accuracy & repeatability needed
• Process turndown ratio needed
• Budgeted price

Based on the above factors process engineer has to understand the working principle, PROS, and CONS  of different flow measurement technology & its possibility for selection as mentioned below:

1. DIFFERENTIAL PRESSURE BASED FLOW METER (AEROFOIL/ ANNULAR/ PITOT-TUBE)

Working Principle 

Aerofoil /Annubar / Pitot-Tube works on the principle of measuring differential pressure measurement by creating restriction in the path of flow and measures differential pressure across primary flow elements and derives volumetric flow rate. With additional continuous pressure and temperature (instantaneous density) compensation mass flow rate can be derived.

PROS

• Established Standard BS-1042/ISO 5167 for Volumetric flow measurement
• Suitable up to 5mtrs or higher
• Rugged designed for any process conditions of industries
• Any orientation possible
• Highly Repeatable
• Site calibration easy

CONS

• Higher pressure drop
• Needs periodic maintenance
• Lower Accuracy 3% FSD & may drift time to time
• Lower turn-down ratio 4:1
• Lower flow sensitivity
• Susceptible to clogging
• High wear factor
• High Installation cost

2. NON-CONTACT ULTRASONIC FLOWMETER 

Working Principle

It measures the volumetric flow rate consisting of a pair of ultrasonic trans-receiver that transmits and receives ultrasonic pulses across the flue gas path in both directions, resulting in transit time (time difference) is proportional to stack gas velocity. It depends mainly on the sound velocity of the gas.

PROS

• Derives volumetric flow rate by ultrasonic beam transit-time measurement
• Used for Pipe diameter up to 10 meters
• Suitable for temperature up to max 450⁰C
• Horizontal or vertical orientation is possible
• Turn down ratio better than 100:1

CONS

• Gas must be dry and clean
• Accuracy up to ±1.5%-3% RD & repeatability ±1%
• High Initial Cost
• Installation need good technical knowledge
• Not suitable for pipes with inner lining
• Drift due to change in flue-gas sound velocity
• Technical knowledge for installation must

3. INSERTION OF THERMAL MASS FLOWMETERS 

Working Principle

Thermal mass flowmeters work on the physical principle of thermal dispersion from a heated element to the ambient medium (eg. air or gases). This is affected by the velocity, density (temperature and pressure), and by the characteristic of the medium. The amount of needed energy is a function of the temperature difference ∆T and the mass flow.

Flue gas flows through two RTD Pt-100 one reference (Tref) and the other Heater (Th). The temperature difference (over-temperature) ∆t between the reference sensor (medium temperature) and the heater sensor is controlled constantly. As per King’s Law, the higher the mass flow rate, the higher the cooling effect of the heater sensor, thus higher the power required to maintain the differential temperature constant. Therefore the heater power is proportional to the gas mass flow rate.

PROS

• Works on constant calorimetric temperature anemometry (Thermal dispersion)
• Pipe sizes suitable 15mm to 10 meters
• Insertion is rugged and works up to 400⁰C & 16bar or more
• Any orientation possible
• Better accuracy < ±2%RD of mass flow rate
• Highest turndown ratio 100:1 or better
• Adjustable & versatile
• Lowest pressure drop
• Low cost of ownership against other flow technology
• Online thermal conductivity compensation

CONS

• Mechanically vulnerable to damage
• Flow straightener recommended
• Affected by high moisture (>10%Vol.) & dirt/dust needs periodic cleaning or purging system

NOTE: Above article don’t claim accuracy and it is for general pros and cons guidance on flow technologies as per practical experience there may be improvements or different configuration possible from different manufacturers which may eliminate some disadvantages.

CONCLUSION

Considering the above advantages and disadvantages & process conditions with respect to different flowmeter technologies for the flue gas flow measurement process engineers can decide on suitable flowmeters for optimum performance.

Insertion thermal mass flowmeters are a preferable choice for stack diameter up to 8 meters or less with moderate wet or dust load conditions with a suitable purging system can be a good & cheaper alternative against DP Type flow meter.

Ultrasonic gas flow meters can be a good alternative for large stack diameters above 8 meters against DP  & Thermal Mass Flowmeters.

Request to kindly write us your inquiry related to the above application and will happy to assist with flow solutions at our best.

 

A gas flowmeter is used to monitor the volumetric or mass flow rate of various process gases in industrial plants and machinery. The flow rate refers to the speed at which fluid is moving through closed pipes or ducts at a given time. Modern instrumentation & control engineering design needs monitoring of fluid flow rates to optimize the use of gases and improve the efficiency of processes and equipment. Therefore, a gas flow meter device must be calibrated against a reference or a master calibration system.

Due to process & environmental conditions flow meters may have some drift in flowrate readings which needs recalibration periodically for the flow range it is designed for, to achieve the desired accuracy for smooth process operation. Some organizations have an ISO audit system in place which needs recalibration at a specific time frame.

Types of gas flowmeter available in market

Various flowmeter technologies are available currently in the market which needs to be calibrated and there are different methods for calibration based on product design.

• Turbine flow meter
• Vortex flow meter
• Coriolis flow meter
• Thermal Mass flow meter
• Ultrasonic flow meter

and there are different methods for calibration based on product design.

Types of calibration procedures? 

Gas flowmeter calibration systems mainly depend on product technology to be calibrated. Some widely used calibration procedure is as below:

•  Reference Master Meter calibration
• Primary Air by Bell / Piston prover calibration
• Critical Flow Venturi (Sonic nozzle) calibration
• Wind Tunnel Air Velocity calibration
• Onsite validation by calibrated reference gas flowmeter

Gas flow meter calibration is done as per above various calibration systems which involve comparison and adjustment of the flowmeter under test confirming to National & International standards mainly following ISO 17025 standards for calibration methods.

Reference Master Meter Calibration Procedures

Reference Master meter calibration compares with duly calibrated master gas flowmeter traceable to national accredited calibration laboratories with a gas flowmeter under test at desired flow ranges and adjusts its calibration as per desired accuracy tolerances.

Reference Master meter calibration steps :

• Place the reference master meter in series with the flow meter under test.
• Compare the readings of the master flow meter and flow meter using air or standard gas flow rate with a close loop calibrated volumetric tank storage system.
• Calibrate the flowmeter under test to conform with the master flow meter calibration.

Piston / Bell Prover Gravimetric Calibration Procedures

Gravimetric calibration is one of the most accurate and cost-effective volumetric and gas mass flow meter calibration procedures at low pressure. Piston prover-based gravimetric method is ideal for gases for low flow rates whereas Bell Prover calibration is mainly used for a high flow rate of air calibration. The calibration flow laboratory is always maintained at controlled ambient conditions such as temperature, humidity for master flow meters’ best performance.

In the piston or bell prover flow meter calibration procedure, a known volume of fluid is forced through a flow meter under test. The piston /bell prover is a cylindrical device with known internal diameter. The piston/bell prover contains a piston/ bell which produces volumetric flow by positive displacement. The piston prover method is ideal for Low flow rate gas or liquid thermal mass flow controller, ultrasonic flow meter & turbine flow meter calibration to a high degree of accuracy.

Piston / Bell prover Gravimetric calibration steps:

• Fix the flowmeter in a series of piston/bell prover systems for calibration.
• Desired gas flow via test flowmeter enters in the piston/bell prover which will move piston/bell upwards with detecting distance between sensors for known volume between is measured.
• Measuring travel time, pressure, temperature, and flow rate at reference conditions is calculated via software systems.
• Compare the calculated flow rate with the flowmeter under test and adjust it to the measured flow rate.

Critical Flow Venturi ( Sonic Nozzle ) Calibration Procedures

Critical Flow Venturi Nozzle (Sonic Nozzles) is used for different flow rate capacities designed as per ISO 9300:2015 as a reference standard. Sonic nozzles are considered the best reference standard for calibration of precision flow meters used for custody transfer. Gas flow accelerates to the critical velocity at the throat (known as sonic velocity) where it will reach a steady flow state of single-phase gases. This sonic velocity is a specific point velocity at a certain pressure which is accurate as a reference velocity for calibrating a flowmeter under test. It is used for almost all fixed type flowmeters mainly vortex flowmeter, turbine flowmeter, thermal mass flowmeter, Coriolis flowmeter, etc.

Sonic nozzle calibration steps:

• Fix the flowmeter in series of the pipeline where calibrated sonic nozzle
• Start air compressor in the testing system
• When sonic velocity reaches the measuring section, the flowmeter under calibration needs to adjust its reading & set it. Repeat the same steps for different sonic velocity points for a specific flow rate.

Comparing flow rate from a sonic nozzle with flow meter under calibration.

Wind Tunnel Calibration Procedures

The wind tunnel is designed for the characterization of air velocity, air turbulence for various different turbomachinery and instruments calibration and simulation. There are different proprietary & standard designs of wind tunnels installed. It has an open test chamber for calibration or test of the device under test. It is widely used for various types of flow sensor calibration such as hot-wire/vane anemometer, insertion thermal mass flowmeter, pitot tube, measuring probes, etc.

Wind tunnel calibration steps:

• Install a flow sensor probe in the test chamber with desired insertion depth and correct orientation.
• Step by step increasing reference velocity and measure & record flow sensor under test.
• Based on reference velocity, plot a curve and suitable interpolation & extrapolation flow calculations upload curve into a device under test.

Onsite validation & calibration procedures

Onsite validation and calibration of gas flowmeters are also possible in some industrial applications. This method is not accurate as the above methods but can validate gas flowmeters in use having high errors or non-repeatability.

Mainly two ways are possible for onsite validation:

• Pump-up test with pressure vessel
• With calibrated reference gas flowmeter to be installed in series

Pump-up test steps:

• The pressure vessel in series of gas flowmeter
• Start Gas flow and note gas flow meter reading and timer for filling a pressure vessel
• Pressure vessel pressure with respect to volume capacity and gas flow meter totalizer to be compared
• Validation is done by comparing both results and errors.

Calibrated reference gas flowmeter:

• Install calibrated reference gas flowmeter in series of gas flowmeter under validation
• Compare both flowmeter result and compute the error

For any technical queries, we would be pleased to provide you related information.