A collection of variable area flowmeters from KROHNE

Variable area flowmeters

For simple and cost-effective flow measurement of gases or liquids without auxiliary power

  • Glass and metal tube purgemeters for low-flow applications <DN15 / ½" and process meters up to DN150 / 6"

  • Optional switches, 4...20mA, HART®, FOUNDATION™ Fieldbus, Profibus-PA

  • Globally approved for use in hazardous areas and suited for safety related SIL 2 applications

Products

When are variable area flowmeters used?

Variable area flowmeters, also known as VA meters or rotameters, are widely used instruments for measuring the volumetric flow rate of conductive and non-conductive liquids and gases. These devices are characterised by their simple but effective mechanical operating principle, which basically requires no additional power supply.

Variable area flow measurement technology is the most widely used and most suitable solution, particularly for measuring low flow rates of clean liquids or gases, where the focus is not on the highest absolute accuracy, but on cost-effectiveness and a compact installation situation. KROHNE has been developing and producing variable area flowmeters since 1921.

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How do variable area flowmeters work?

Design and measuring principle

A variable area flowmeter – also known as VA meter or rotameter – consists of an upright, tapered measuring tube with the wider end at the top. This tube contains a specially shaped float that can move freely up and down. The level of the float forms a defined annular gap with the conical tube, the size of which varies with the position of the float.

If there is no flow, the float is located at the lower end of the measuring tube due to gravity. As soon as the medium flows through the pipe from bottom to top, the float is lifted. The position of the float is adjusted so that the gravity force (G) of the float is kept in equilibrium by the buoyancy or lifting force (A) and the flow force (S).

VA schematic representation

There are essentially three forces acting on the float:

  • The lifting force A, which depends on the density of the medium and the volume of the float and is constant (at constant density).

  • The gravity force G, which depends on the mass of the float. Floats are made of stainless steel, aluminium, titanium or hard rubber, for example.

  • The flow force S depends on the float shape, the flow velocity in the annular gap and the density and viscosity of the medium

The higher the float rises, the larger the annular gap and thus the cross-sectional area through which the medium can flow. This leads to a decrease in flow velocity and flow force until a new state of equilibrium is reached. The height of the float is therefore a direct measure of the volume flow rate.

Display of the flow rate

With glass cones, the flow value can be read directly from a scale at the level of the float reading line. In the case of metal cones, where the float is not visible, the display is realised via a magnetic coupling. A magnet in the float transmits its position to a coupling magnet, which converts the linear movement into a rotary movement of a pointer that indicates the flow rate on a scale.

Electronical processing

In some variants, the height of the float is detected by magnetic field sensors and electronically processed. This enables a more precise and versatile use of the flowmeters, especially in automated systems.

What are the particular benefits of variable area flowmeters?

In addition to cost-effective measurement, variable area (VA) flowmeters are characterised above all by their robustness. At extremely high pressures of up to 1000 bar / 14504 psi or even in the low-pressure range at extremely low pressures of a few millibars and extremely low flow rates, rotameters are often the only option for flow measurement where other flow technologies can no longer be used under these conditions. In addition, VA flowmeters are suitable for use with high media temperatures or cryogenic process conditions. For example, KROHNE's H250 M40 can be designed for temperature ranges from -192°C to +400°C / -313.6°F to +752°F. The closed measuring tube design of the all-metal VA meters allows reliable measurement without potential leaks.

What are the limitations of rotameters?

Variable area (VA) flowmeters or rotameters are a cost-effective and long-term stable technology for flow measurement, but do not offer the same high absolute accuracy as ultrasonic flowmeters or Coriolis mass flowmeters. Their measurement depends on viscosity, density, pressure and media temperature, which is why stable process conditions are required. They are particularly suitable for clean media, whereas they are unsuitable for media with a high solids content. In addition, the devices are limited in their nominal width, although KROHNE can manufacture VA flowmeters up to DN150.

What are typical applications of flowmeters with floats?

Collection of application pictures of variable area flowmeters vertical

KROHNE variable area flowmeters can be used for gas flow measurements, e.g. of inert gases, sample flows, low-pressure gases or process gases such as air, natural gas or hydrogen, as well as for volume flow measurement of conductive and non-conductive liquids – from water to hydrocarbons such as lubricating oil, petrol, acids, alkaline solutions or ammonia solutions.

The most common applications include

  • Measurement of small quantities of chemicals, additives, alkaline solutions or acids, e.g. mineral acids and hydrogen peroxide

  • Volume flow measurement of liquid hydrocarbons, e.g. benzene, lubricating oil

  • Inertisation with nitrogen

  • Monitoring of sample flows for process analysers

  • Monitoring of sealing gases and barrier fluids for sealing systems

  • Injection of corrosion and scale inhibitors

  • Continuous dosing of purge fluids for measuring systems

  • Gas flow measurement (e.g. of hydrogen) for industrial furnaces and thermal process plants

  • Collection of water, condensate, demineralised or process water

  • Measurement of air, carbon dioxide or other industrial gases

Can variable area flowmeters also be used in hazardous areas?

With the H250 M40, H250 M8, DK32/34 and DK37 M8 as well as the DK46/47/48/800 and VA40 glass tube meters, KROHNE offers a universal Ex concept for a wide range of application zones in gas and dust explosion hazard areas. Protection types intrinsically safe, non-incendive and explosion-proof accommodate the different regional explosion protection strategies.

The devices are ATEX, IECEx and FM/QPS approved for the USA and Canada and NEPSI approved for China. They also have other local authorisations, e.g. INMETRO for Brazil, KCS for Korea or CCOE/PESO for India. This makes the measuring devices ideal for companies with international production sites or machine and plant manufacturers who need to fulfil their local explosion protection requirements.

Can variable area flowmeters also be used in SIL applications?

Icon/Logo Security Integrity Level 2 (SIL 2)

KROHNE offers SIL 2-suitable versions with NAMUR limit switches for low demand mode across its entire product portfolio of variable area (VA) flowmeters. This applies to metal devices, such as the H250 M40, and the small VA flowmeters DK32, DK34 and DK37 as well as the glass devices. Both the VA40 glass flowmeter for simple industrial applications and the small VA meters DK46, DK47, DK48 and DK800 are available in a SIL 2-suitable version for use in Safety Instrumented Systems (SIS). In addition, the H250 M40 is also available as a SIL 2-certified flowmeter with 4…20 mA output (type B).

Can variable area flowmeters also be used in hygienic applications?

Hygienic connection of an H250F M40

With the VA40 and the H250F M40, KROHNE offers two variable area flowmeters in hygienic design for food and pharmaceutical applications. FDA-, EC1935- and GB4806-compliant materials fulfil the strict hygiene and legal requirements. A design without dead spaces and stagnation zones as well as the pickled and optional electropolished surface of the wetted parts with a roughness depth Ra <0.8 µm make deposits more difficult and enable very good cleaning. The outstanding properties of the H250F M40 are utilised by the pharmaceutical industry, e.g. in ultrapure water systems, and by the food industry for cream measurement.

What measuring ranges do KROHNE variable area flowmeters have for process applications?

The typical measuring range span of variable area process devices is 10:1 due to the technology. As an option, the measuring range span of the H250 M40 can be extended to 100:1 - which is unique for measuring devices in this category worldwide. This eliminates the need for additional measuring devices for very small quantities.

How are rotameters installed?

H250 M40 vertical horizontal

Flowmeters with floats (rotameters) must usually be installed vertically in a pipe with rising flow and the medium must flow through them from bottom to top to ensure stable measurement. In addition, KROHNE offers unique versions of the H250 M40 variable area flowmeter that can also be used for horizontal or even downpipes, thus offering maximum design freedom.

Do variable area flowmeters require straight inlet and outlet runs?

When using variable area flowmeters, straight inlet and outlet sections are not required for reasons of accuracy, but can reduce unstable flow indication due to fluctuating flow profiles and extend service life due to reduced mechanical wear. This applies in particular to process devices ≥DN50 / 2", for which inlet/outlet runs of 5D/3D are recommended.

What are KROHNE variable area flowmeters made of and what materials are used?

KROHNE offers variable area flowmeters in different versions and for different measuring ranges. These include sturdy, break- and shock-resistant flowmeters with a measuring tube and float made of stainless steel or special materials, as well as models with a glass measuring cone.

All-metal variable area flowmeters and purge meters

KROHNE all-metal variable area flowmeters

All-metal flowmeters are specially designed for the measurement of liquids and for gas flow measurements under difficult operating and environmental conditions. Their sturdy, shock-resistant construction enables simple and cost-effective integration into various measuring systems, such as process analysers, without the need for inlet and outlet runs.

With the H250 M40, KROHNE also offers an all-metal VA meter that is also available with many special materials, depending on the product being measured. To ensure the durability of the device even with chemically demanding media, it can be manufactured from special materials such as Hastelloy®, titanium, Monel®, 6Mo, Inconel® etc. For applications with aggressive acids and alkalis, the H250C M40 is also available with a plastic lining made of polytetrafluoroethylene (PTFE). KROHNE also offers special stainless steel display housings for the H250 M40, which can even withstand attacks from salt spray and contaminated precipitation.

Variable area glass flowmeters and purge meters

KROHNE glass tube variable area flowmeters

Variable area glass flowmeters are particularly suitable for measuring very small quantities of inert gases and sample flows up to a pressure of 10 barg / 145 psig. The purge meters offer direct visibility of the medium and therefore an immediate reading of the flow rate. With the VA40 and VA45, KROHNE also offers glass flowmeters for process applications.

What control and communication options do KROHNE variable area flowmeters have?

KROHNE variable area flowmeters offer a wide range of additional options that significantly expand their application options and functionalities. Particularly noteworthy are the options in the area of pressure control valves and limit switches as well as analogue and digital signal outputs.

Pressure control valves

DK37 M8E bottom valve inlet regulator

KROHNE offers variable area flowmeters with integrated pressure control valves that enable precise control of the volume flow rate. These valves are particularly useful in applications where the pressure must be kept constant to ensure a consistent flow rate. The pressure control valves are available in different versions to meet the specific requirements of different applications.

Limit switches

DK800 Switch

In addition to the pressure control valves, KROHNE variable area flowmeters can also be equipped with limit switches. They ensure that certain flow limit values are monitored and trigger a signal if these values are exceeded or not reached. This is particularly important for applications where precise monitoring and control of the flow is required to ensure the safety and efficiency of the processes.

Signal outputs

Even though variable area flowmeters basically function as purely mechanical flowmeters (rotameters) without power supply, KROHNE also offers many versions with an electronic bar graph indicator, analogue or even digital output.

For example, the H250 M40 variable area flowmeter can be used both as a mechanical device without auxiliary power for local flow indication and as an electronic device that can be integrated into systems via various signal output options – via 4-20 mA and HART or fieldbus and Profibus. The intelligent modularity of the H250 M40 enables existing devices to be functionally upgraded in the field by retrofitting or replacing the communication modules without interrupting the process.

What diagnostic options do variable area flowmeters have?

State-of-the-art variable area flowmeters with electronic module can now also utilise diagnostic functions such as function monitoring, error logs and alarms to identify and report problems. To this end, KROHNE H250 M40 variable area flowmeters are equipped with Continuous Float Monitoring (CFM) technology as standard, which enables reliable diagnostics and process optimisation across the entire measuring range. The diagnostic functions include

  • Float blockages due to dirt or pressure bumps.

  • Compressional vibrations of the float in gas measurements, which can influence the accuracy.

  • Pulsating flow in liquid measurements caused by e.g. positive displacement pumps.

  • Incorrect installation of the float after maintenance.

  • External interfering magnetic DC fields that could cause an error in the measurement.

Diagnostics are classified according to NAMUR NE107, from purely informative to device errors with failure current output, to enable suitable corrective measures to be taken. This helps to extend the service life of the flowmeter and improve process reliability.

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