Accurate gas flow measurement begins when selecting the correct gas flow meter for an application. Here are four steps to help choose a natural gas flow meter, along with a flow meter comparison and a flowmeter selection guide.
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Before selecting your flowmeter technology, understand the application. In a recent post, we provided tips to consider when choosing a gas flow meter technology. To paraphrase, better understand the application by determining:
Gathering this information will help you make an intelligent selection to measure your natural gas.
After understanding the application, review our cheat sheet to help guide which technology to consider. Click on the image to view this guide. Additionally, keep in mind that there are four different natural gas flow meter types:
After reviewing the selection guide, you likely have honed in on one or two meter types. Compare these technologies to understand each flowmeter type’s operation principle, advantages, and disadvantages. Here are convenient links to the different meter types to guide your review.
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Our dedicated staff is happy to walk you through the selection process and may have recommendations on some natural gas meters for your application.
Our flowmeter selection guide is designed to help you find the best flow meter for your application. Flow meters are one of the most commonly used instruments in industry.
Since flow is a dynamic quantity, the measuring instrument itself is affected by many factors, such as: pipe, diameter size, shape (circular, rectangular), boundary conditions, physical properties of the medium (temperature, pressure, density, viscosity, dirtiness, corrosiveness) etc.), the flow state of the fluid (turbulence state, velocity distribution, etc.) and the influence of installation conditions and levels.
Therefore, in the face of more than a dozen categories and hundreds of varieties of flow meters at home and abroad, how to make reasonable selection is the prerequisite and foundation for the good application of flow meters.
Our flow meter selection guide can help you quickly master the selection and application of flow meters.
Generally speaking, the analysis should mainly be carried out from the following 6 aspects:
First of all, clarifying your measurement purpose is the key to choosing a suitable flow meter.
Therefore, first you need to clarify the purpose of your traffic measurement and what effect you want to achieve.
Common fluid physical properties include density, viscosity, vapor pressure and other parameters. These parameters can generally be found in the manual, and the suitability of each fluid parameter and the selection of the flowmeter can be evaluated under the conditions of use. But there are also some physical properties that cannot be found. Such as corrosiveness, scaling, clogging, phase change and miscible state, etc.
Detailed enumeration and description are as follows:
1.Fluid types:
The first thing that needs to be clarified is what fluid is being measured. Is the measured medium gas, liquid, or steam? And the measured fluid is single? Or mixed?
2.Density：
Density of FluidsFor liquids, the density is relatively constant in most applications. Unless the temperature changes greatly causing major changes, density correction generally does not need to be performed. In gas applications, the range and linearity of the flow meter depend on the gas density. Generally, it is necessary to know the values under standard conditions and working conditions for selection. There is also a method of converting flow state values to certain recognized reference values. This method is commonly used in petroleum storage and transportation. Low-density gases can be difficult for some measurement methods, especially instruments that use the momentum of the gas to push the detection sensor (such as turbine flowmeters).
3.Viscosity:
Required for liquids. The viscosity of various liquids varies greatly and changes significantly due to temperature changes. Gases are different. The difference in viscosity between various gases is small, and their values are generally low. And it will not change significantly due to changes in temperature and pressure. Therefore, the effect of viscosity must be considered for liquids.
4.Electrical conductivity:
Required for electromagnetic type flow meters. If the measured medium is liquid. Then it is best to confirm the conductivity of the fluid.
5.Contaminants:
Air bubbles, mixed-in foreign objects, slurry, etc.
6.Flow range:
It is best to confirm the minimum-normal-maximum flow rate in the pipeline. Can be mass flow rate or volume flow rate.
7.Fluid temperature and pressure:
Carefully analyze the working pressure and temperature of the fluid in the flow meter, especially if the temperature and pressure changes cause excessive density changes when measuring gas, the selected measurement method may need to be changed. For example, when temperature and pressure affect performance such as flow measurement accuracy, temperature or pressure correction must be made. In addition, the structural strength design and material of the flowmeter housing also depend on the temperature and pressure of the fluid. Therefore, the maximum and minimum values of temperature and pressure must be known exactly. Careful selection should be made when temperature and pressure vary greatly. It should also be noted that when measuring gas, it is necessary to confirm whether the volume flow value is the temperature and pressure under working conditions or the temperature and pressure under standard conditions.
8.Chemical corrosion:
The problem of chemical corrosion of fluids sometimes becomes a deciding factor in our choice of measurement method and use of flow meters. For example, certain fluids can corrode parts in contact with the flow meter, cause surface scaling or precipitation of crystals, and produce electrolytic chemical effects on the surface of metal parts. The occurrence of these phenomena will reduce the performance and service life of the flow meter. Therefore, in order to solve the chemical corrosion situation, a flowmeter made of anti-corrosion materials is selected.
9.Scaling situation:
Due to scaling or crystallization on the flow meter cavity and flow sensor, the clearance of the moving parts in the flow meter will be reduced, and the sensitivity or measurement performance of the sensitive components in the flow meter will be reduced. For example, in ultrasonic flowmeter applications, the scale layer will hinder the emission of ultrasonic waves. In electromagnetic flowmeter applications, the non-conductive scale layer insulates the electrode surface and makes the flowmeter unable to operate.
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Different types of flow meters have different advantages, disadvantages and performance parameters. For details, please refer to: 11 Flow Meter Types and Their Advantages and Disadvantages. Before we choose a flow meter, it is best to understand the performance parameters of various flow meters. Mainly from the following aspects:
There are two types of flow measurement, namely instantaneous flow and cumulative flow.
For example, the crude oil in the distribution station pipeline belongs to the custody transfer, or the continuous proportioning of the petrochemical pipeline, the process control of the production or production process, etc. need to measure the total amount. Occasionally supplemented by observation of instantaneous flow.
In some workplaces, instantaneous flow measurement is required to control flow. Therefore, the selection should be made according to the needs of on-site measurement.
This accuracy class refers to the maximum error of the flow meter.
When calibrating a flow meter, it generally only marks two points: zero point and span. During calibration, five points are generally checked, namely 0%, 25%, 50%, 65% and 100% of the full scale. The error of these five points is less than 1%, which means it is qualified and allowed to leave the factory. The actual errors of these five points may be 0.1%, 0.3%, 0.5%, 0.8%, and 0.4% respectively, but the error of the entire machine is subject to the largest error.
For example: the measuring range of the flow meter is 10m³/h.
Then an accuracy of level 1.0 means that the difference between the measured value of the flow meter at any measuring point and the real flow rate is within plus or minus 1m³/h. For example, when the measured value of the flow meter is 5m³/h, the actual flow rate is between 4-6m³/h.
Therefore, the higher the accuracy of the flow meter, the more accurately it represents the true flow rate. Level 0.5 is definitely more accurate than level 1.0.
Repeatability refers to the ability to produce the same results repeatedly under the same conditions. That is, the flow meter should produce the same reading when operating under the same variables and conditions. This is also expressed as ± percentage.
Generally, the measurement performance requirements in the calibration regulations not only stipulate the accuracy level of the flow meter, but also stipulate the repeatability. The general stipulation is: the repeatability of the flow meter shall not exceed the maximum allowable error specified in the corresponding accuracy level. 1/3～1/5.
However, in practical applications, the repeatability of the flow meter is often affected by changes in fluid viscosity and density parameters. Sometimes these parameter changes have not reached the level that requires special correction, and it is mistakenly thought to be poor repeatability of the flow meter. . In view of this situation, a flowmeter that is not sensitive to changes in this parameter should be selected.
The linearity of the flow meter refers to the degree of consistency between the flow characteristic curve and the specified straight line within the flow range.
It can be expressed by the following formula:
δ=±(Kmax-Kmin)/(Kmax+Kmin)*100%
In the formula:
δ—linearity;
Kmax—the maximum value of the instrument coefficient at each measuring point within the flow range;
Kmin—the minimum value of the instrument coefficient at each measuring point within the flow range;
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The output of the flow meter mainly includes linear and nonlinear square root.
Generally speaking, the nonlinear error of the flow meter is not listed separately, but is included in the error of the flow meter.
For flowmeters that generally have a wide flow range, have pulse output signals, and are used for total volume accumulation, linearity is an important technical indicator. If a single instrument coefficient is used within the flow range, the linearity difference will be Will reduce the accuracy of the flow meter.
For example, a turbine flowmeter uses an instrument coefficient within the flow range of 10:1. When the linearity is poor, its accuracy will be lower. With the development of computer technology, its flow range can be segmented and fitted using the least squares method. The flow rate-instrument coefficient curve corrects the flow meter, thereby improving the accuracy of the flow meter and expanding the flow range.
The upper limit flow rate is also called the full scale flow rate or maximum flow rate of the flow meter. When we select the diameter of the flow meter, we should configure it according to the flow range used by the pipeline under test and the upper limit flow rate and lower limit flow rate of the selected flow meter. We cannot simply configure it according to the diameter of the pipe.
Generally speaking, the maximum flow rate of fluid in a designed pipeline is determined based on the economic flow rate. If the selection is too low, the pipe diameter will be thick and the investment will be large; if it is too high, the transmission power will be high and operating costs will increase.
For example, the economic flow rate of low-viscosity liquids such as water is 1.5 to 3 m/s, and that of high-viscosity liquids is 0.2 to 1 m/s. The upper flow rate of most flow meters is close to or higher than the economic flow rate of pipelines. Therefore, when selecting a flowmeter, its diameter is usually the same as that of the pipe, and installation is more convenient. If they are not the same, there will not be much difference. Generally, the specifications of the upper and lower adjacent gears can be connected by reducing pipes.
The range is the ratio of the upper limit flow and the lower limit flow of the flow meter. The larger the value, the wider the flow range. Linear meters have a wider range, generally 1:10. The range of nonlinear flowmeter is as small as 1:3.
Flow meters are generally used for process control or custody transfer accounting. If a wide flow range is required, do not choose a flow meter with a small range.
At present, in order to promote the wide flow range of their flow meters, some manufacturers have raised the flow rate of the upper limit flow very high in the instruction manual. For example, the liquid speed is raised to 7~10m/s (usually 6m/s), and the gas speed is raised to 50~ 75m/s (usually 40~50) m/s). In fact, such a high flow rate is not used.
In fact, the key to a wide range is to have a lower lower limit flow rate to meet the measurement needs. Therefore, a wide-range flowmeter with a low lower limit flow rate is more practical.
Pressure loss generally refers to the irrecoverable pressure loss that changes with the flow rate due to the static or movable detection element installed in the flow channel or the change of flow direction in the flow sensor.
Therefore, the flowmeter should be selected based on the pumping capacity of the pipeline system and the inlet pressure of the flowmeter to determine the allowable pressure loss of the maximum flow rate.
Improper selection will restrict the fluid flow and cause excessive pressure loss, which will affect the circulation efficiency. For some liquids (high vapor pressure hydrocarbon liquids), it should also be noted that excessive pressure drop may cause cavitation and liquid phase vaporization, reduce measurement accuracy and even damage the flow meter.
For example, for flowmeters used for water delivery with pipe diameters greater than 500mm, the increased pumping costs due to excessive energy loss caused by pressure loss should be considered.
According to relevant reports, the pumping costs paid for measurement by flow meters with large pressure losses in recent years often exceed the purchase cost of more expensive flow meters with low pressure losses.
The output and display volume of the flow meter can be divided into: ① instantaneous flow (volume flow or mass flow); ② cumulative flow; ③ average flow rate; ④ point flow rate.
Some flowmeters output analog quantities (current or voltage), others output pulse quantities. Analog output is generally considered suitable for process control, and is more suitable for coupling with control loop units such as regulating valves. Pulse output is more suitable for total volume and high-accuracy flow measurement.
Long-distance signal transmission pulse output has higher transmission accuracy than analog output. The mode and amplitude of the output signal should also be compatible with other devices. Such as control interfaces, data processors, alarm devices, circuit breaker protection circuits and data transmission systems.
When used in pulsating flow situations, attention should be paid to the response of the flowmeter to step changes in flow. Some applications require the flowmeter output to follow the fluid flow, while others require a slower response output to obtain a comprehensive average.
Instantaneous response is often expressed in terms of time constant or response frequency. The former ranges from a few milliseconds to a few seconds, and the latter is below hundreds of Hz. The use of a display instrument may significantly extend the response time.
It is generally believed that when the flow rate of a flow meter increases or decreases, the asymmetric dynamic response will accelerate the flow measurement error.
Installation issues have different requirements for flow meters with different principles.
When using the flow meter, attention should be paid to the adaptability and requirements of the installation conditions, mainly considering the following aspects. For example, the installation direction of the flow meter, the flow direction of the fluid, the configuration of the upstream and downstream pipelines, valve positions, protective accessories, pulsating flow Impact, vibration, electrical interference and flow meter maintenance, etc.
In the process of selecting a flow meter, surrounding conditions and related changes should not be ignored, such as ambient temperature, humidity, safety and electrical interference, etc.:
1) Ambient temperature
Changes in ambient temperature can affect the electronics and flow sensor portions of the flow meter. For example, temperature changes will affect changes in sensor size, and heat transfer through the flowmeter housing will change fluid density and viscosity. When the ambient temperature affects the electronic components of the display instrument, the component parameters will change.
2) Ambient humidity
The atmospheric humidity in the environment is also one of the problems that affects the use of flow meters. For example, high humidity will accelerate atmospheric corrosion and electrolytic corrosion and reduce electrical insulation, while low humidity will induce static electricity.
3) Security
The flowmeter should be selected in accordance with relevant specifications and standards to adapt to use in explosive hazardous environments, and on-site requirements should be carried out in accordance with explosion-proof standards.
4) Electrical interference
Power cables, motors and electrical switches will all produce electromagnetic interference. If relevant measures are not taken, it will become the cause of errors in flow measurement.
Below is a summary of flow meter options for some common application types. For your preliminary reference:
LiquidsClean LiquidDirty LiquidAbrasive /SlurryCorrosiveHigh PressureDensity, ConcentrationCryogenicHigh TempMass FlowLow Flow Rates <0.1m³/hr (0.44gpm)Low ConductivityCoriolis Flow Meter■■OO■■■■■■■Magnetic Flow Meter (4-wire)■■■■OOoOMagnetic Flow Meter (2-wire)■O■OOOO Capacitance Magnetic Flow Meter■■■■OOO■Vortex Flow Meter■OO■O■■■■Variable Area Flow Meter■O■O■■O■■Differential Pressure (DP)Flow■OOO■OO■■■Gas and SteamClean GasDirty GasCorrosiveLow PressureSaturated SteamSuperheated SteamCryogenicHigh TempMass FlowLow Flow RatesCoriolis Flow Meter■■OO■■■■■Vortex Flow Meter■OO■■■■■■Variable Area Flow Meter■OO■■■■■■Differential Pressure (DP)Flow Meter■OO■■■OOO■■ Designed for this service ; O Applicable for this service under certain conditions -consult manufacturerRefrigerant Flow Measurement for HVAC System
A customer from the United States was looking to purchase a flow meter to measure the flow rate of liquid R134A in their HVAC system. The system requirements and fluid properties were as follows:
After evaluating various flow meter technologies, the customer selected our gear flow meter due to its suitability for their specific application. The gear flow meter provided accurate measurement for the refrigerant and met the temperature and pressure requirements. The selected gear flow meter had the following specifications:
By choosing the gear flow meter, the customer was able to accurately measure and control the flow rate of liquid R134A in their HVAC system, ensuring optimal performance and energy efficiency.
Choosing the right flow meter for your application is critical to achieving accurate, efficient flow measurement. By considering the factors discussed in this Flowmeter Selection Guide and understanding the different flow meter technologies available, you can make an informed decision and find the product that best suits your specific needs.
As an experienced manufacturer and supplier, Sino-Inst offers a variety of flow meters and supports customization to meet your unique requirements.
Contact us today to discuss your flow meter needs and learn how our expertise can help you improve process control and efficiency.
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