Variable area flowmeters

A variable area flowmeter 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).

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.

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.

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.

KROHNE offers SIL 2 suitable versions with NAMUR limit switches for low demand mode for its entire product portfolio of variable area flowmeters. This applies to metal devices, e.g. the H250 M40, and the small flow meters DK32, DK34 and DK37 as well as the glass devices. Both the VA40 glass device for simple industrial applications and the DK46, DK47, DK48 and DK800 small flow meters are available in a SIL 2-suitable version for use in Safety Instrumented Systems (SIS).

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.

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.

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

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.

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.

Even though variable area flow meters basically function as purely mechanical flow meters (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 flow meter 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.