Thermal Mass Flow Meter Q&A From Magnetrol

Thermatel® thermal mass flow meter
Courtesy Magnetrol®

Sometimes you discover that others do something better than you. When that happens, watch and listen.

Tom Kemme, from Magnetrol®, expertly fielded some questions about thermal mass flow meters in a recent blog post. Mr. Kemme's responses were so useful and clear that I decided, with all the credit flowing his way, to share them here for those of you that may not closely follow the Magnetrol® Blog.

Question: What is the difference between the flow units Nm3/h, Sm3/h, and actual m3/h?

Answer: Actual m3/h is a flow rate at operating temperature and pressure. Normal or standard m3/h (Nm3/h = Sm3/h) is a flow rate at standard temperature and pressure (STP). I tend to reference the natural gas industry, where it is not possible to compare flow rates at every operating condition, so it is preferable to reference all flow rates back to a set of base conditions, such as 60°F and 1 atm. STP is not universal so it may be unique based on the region or industry.

Most flow meters output a flow rate at operating conditions and need to correct this measurement. This may be accomplished with a multivariable transmitter or external to the device. A few examples that do not need to correct the measurement are thermal mass flow meters, such as the ones produced by MAGNETROL, and Coriolis flow meters.

Question: Do you have any certified failure rate data on your units to perform an SIL verification?

Answer: A Failure Modes, Effects, and Diagnostics Analysis (FMEDA) is completed during development to determine failure rates and Safe Failure Fraction (SFF). The SFF is utilized to determine Safety Integrity Level (SIL), which is often the published value.

Question: What should my meter be reading with no air flow in the pipe?

Answer: At zero flow and a dry pipe, a thermal mass flow meter should measure zero. Different thermal meters may have varying stability at no flow due to differences in operation.

There are two different types of operation: constant temperature (CT) and constant power (CP). CT devices start with a low power and this power increases with the flow rate to maintain the constant temperature difference (ΔT) between the RTDs. CP devices start with a high ΔT between RTDs at low flow and the ΔT decreases as the flow rate increases. CP may lack stability at zero flow due to possible convection currents associated with the high ΔT. CT will hold zero better, particularly devices that add less heat. For example, the maximum surface temperature of a TA2 probe is 4 C above process temperature. This is extremely low heat, eliminating convection currents due to the sensor. Convection currents could also occur through the pipe due to temperature variations.

It is also possible for a thermal meter to measure above zero during a no flow condition when there is pressure buildup in the line (typically a valve closed downstream). There may be low flow cutoff settings that can be changed to ignore nuisance measurements.

You can easily tap into Magnetrol® expertise to solve your flow measurement challenges. Reach out to a product specialist and combine your process knowledge with their flow measurement expertise to develop effective solutions.

Factors For Selecting a Variable Area Flow Meter (Rotameter)

Three of many configurations of
variable area flow meters.
Courtesy Brooks Instrument

Industrial processes have many instances where fluid product components, liquid or gaseous, are moving within pipes. Processing is about control, so it follows that an input to the control, measurement, or data logging centers of the facility will answer the question, "How much is flowing through that pipe?".

There are numerous methods employed for quantifying flow in the industrial process measurement and control field, each with particular attributes that may be considered advantageous under  certain operating conditions. All flow measurement methods are indirect, as their actual measurement is of a property that is impacted in a predictable manner by a change in the flow. Flow measurement is an essential element that, combined with other fluid attributes, is used to calculate the total mass of a fluid that has traversed the measurement point.

One time tested method of measuring flow is the variable area meter, also called a rotameter.

Operation of the variable area meter (also referred to as a VA meter) is based upon creating an equilibrium between an upward force, produced by the fluid motion, and a downward force, gravity. The device includes a tapered glass or metal tube that encases a specially shaped float, often referred to as a shaped weight. VA meters must be installed vertically, with the media flowing from the bottom upward, so that the gravitational force necessary for operation is properly aligned with the flow direction. As fluid flows upward through the specially tapered tube, it creates drag on the float contained within the tube, lifting it upward. As the float rises, the free area between the float and the tube wall increases, causing a reduction in the fluid velocity and drag force. For any given flow volume, the flow velocity within the tube will cause the weight to rise until the drag force created by the flow reaches equilibrium with the countervailing force of gravity on the float. Proper design of the tube and the float allow for direct indication of flow volume.

Some of the attributes of variable area meters include:

• No external power or fuel required for operation
• Must be installed vertically, with flow entering bottom
• Meters are characterized to a specific substance, at a specific temperature
• Operation is stable, with low pressure drop
• Requires constant gravity for operation
• Direct local readout of flow rate with meter or scale imprinted on tube
• Glass tube based unit flow readings require visibility of float through the medium
• Accuracy is comparatively low for an industrial flow measurement device
• Generally low maintenance, simple construction, low comparative cost

Brooks Instrument, a world renowned manufacturer of flow, pressure, and level measurement instruments, has produced a concise and compact white paper that summarizes the factors to consider when specifying a VA meter, as well as how each factor impacts operation of the unit. The description is practical and easily understood. It is recommended reading for all process stakeholders to build their flow measurement knowledge.

I have included the paper below. Browse the paper. Contact the flow instrumentation specialists to discuss your application requirements and challenges. Combining your process know-how with their product application knowledge will produce a good solution.

Selecting Variable Area Flow Meters for Process Measurement from Miller Energy, Inc.

Industrial Flowmeter Handbook From Yokogawa

Magnetic Flowmeter
Courtesy Yokogawa Corp.

Measuring the volume, mass, density, and temperature of flowing fluids is a common and necessary function of industrial process control. Industrial flowmeters employ various measuring technologies to deliver accurate measurements, which are utilized to make safety, operational, and financial decisions.

Each measurement technology; magnetic, Coriolis, vortex, differential pressure, rotameter, ultrasonic, or another, has specific attributes of meter design and measurement principle making it more suitable to differing application classes. Yokogawa Corporation has summarized the suitability of the different flowmeter types for a range of process and operational conditions in a table that provides the user a consolidated comparative view of flow meter technology for almost every application. It is a useful tool that allows the engineer to quickly focus in on one or two technologies that will best suit project requirements.

Preview the handbook below, and obtain a copy of the handbook from a Yokogawa product expert, from whom you can also get expert level application assistance.

Flowmeter Handbook by Yokogawa from Miller Energy, Inc.

Process Measurement and Control - Selecting the Right Flow Meter Technology

Vortex Flow Meter
Courtesy Yokogawa Corp.

Magnetic Flow Meter
Courtesy Yokogawa Corp.

Many industrial process control operations require fluid flow measurement as an essential element in the process design. Accurate flow measurement can be obtained with the proper application and installation of a flow meter as part of the fluid transfer system.

Industrial flow meters employ differing technologies to directly or indirectly measure fluid flow rates. Among the many available technologies for fluid flow measurement, vortex flow meters, magnetic flow meters, Coriolis flow meters, and rotameter flow meters are among some of the most common found in industry.

Coriolis Flow Meter
Courtesy Yokogawa Corp.

Rotameter Flow Meters
Courtesy Yokogawa Corp.

Each of the separate technologies have attributes which can make them more suitable to certain applications. Selecting the most appropriate flow measurement technology for an application is one of the initial and crucial steps to designing a well functioning fluid measurement system.

Selection criteria, such as fluid temperature, pressure, and velocity will be part of the selection process. Additional considerations include whether the fluid may be abrasive, corrosive, clean or dirty. The state of the fluid, liquid or gas, must also be considered.

The table below provides quick general guidance on which technologies to consider, based upon the factors outlined above. This will help you better focus further product selection research efforts.

Flow Meter Technology Selection Table
Courtesy Yokogawa Corporation of America

Selecting the right valve is not always an easy task, especially since it may be something that you do infrequently. Manufacturers often have knowledgeable local representatives that are experienced in this field and more than willing to contribute their expertise to your selection process.