Building new functionality into low-cost Coriolis mass flow meters

01 November 2006

The good news is, low cost Coriolis meters provide general purpose instruments at a competitive price. But on the downside, these meters, introduced five years ago, still have limited functionality, restricted output options or lack the digital processing techniques that are now generally used in the converters. Krohne has addressed these issues with its new Optimass 1000 instrument.

The traditional perception that Coriolis meters are expensive is generally true. And yes, there are applications that only a high-end meter will handle, due to the accuracy required, sophisticated concentration applications, or the high design temperature and pressure requirements.

Krohne, however, has addressed the limitations of the low-cost instrument with its new Optimass 1000 meter. It provides the accuracy of a general purpose meter (0.2 %) but it also has the flexibility and options usually found in high-end meters.

All this is possible through innovative but simple design, using the latest finite element analysis techniques and the experience Krohne has gained with the Optimass 7000 single straight tube design.

Principle of operation
Coriolis forces are generated in oscillating systems when a liquid or a gas moves away from, or towards, an axis of rotation. A Coriolis measuring system is of symmetrical design and consists of two measuring tubes. A driver (located at point C in figures 1 and 2), sets the measuring tubes into a uniform fundamental oscillation mode, usually about 100 Hz.

When the flow velocity v=0 m/sec, then the Coriolis force is also 0; see Fig. 1. At flowing conditions, flow velocity v>0 m/sec, the fluid particles in the product are accelerated between points AC and decelerated between points CB.

This causes a slight distortion of the measuring tube that is directly proportional to the mass flow rate. See Fig. 2.

Density measurement is determined from the frequency of oscillation which changes with changes in fluid density.

Design features
The design incorporates a close-coupled twin tube configuration with an optimised flow splitter. The design had to be robust, easy to manufacture and capable of handling a good flow rate at an acceptable pressure drop. Many elements of the decoupling techniques designed into the single straight tube Optimass 7000 series were utilised in the 1000 series design. This provides a meter that has no installation restrictions and is not affected by external influences, such as vibration, hydraulic noise etc.

A typical configuration is shown in Fig. 3. One of the benefits of the design includes true PED (European Pressure Equipment Directive) certified secondary containment, which is unique in this class. As the chemical industry is one of the fastest growing market sectors for this category of flow meter, it is felt that this
will be a real advantage to the user in the unlikely event of a measuring tube failure. This is of particular importance where fugitive emissions are harmful to the environment.

The flow splitter design has been carefully designed using finite element and fluid dynamic software packages, to minimise the ‘face’ a fluid would encounter when entering the throat of the flow splitter. The pressure drop is minimal when compared to some of the more dished, or ‘pigs snout’ type flow splitters typically seen in other designs.

Flexible converter options.
The Optimass 1000 utilises the new MFC 300 converter together with the tried and tested split architecture concept. What this allows is maximum flexibility to the end user with the minimum of fuss. All the sensors or transducers are fitted and calibrated with the sensor electronics which is always integral to the sensor. To
this the main converter may be added in a variety of options and configurations. The main benefit to the user here is that no restrictions are placed on the functionality of the economy converter. All the options are available such as concentration measurement, and advanced software, which is freely programmable in the functions to be allocated to the outputs. The advanced diagnostic platform is also offered with this generation of converter.

For the truly price conscious OEM customer, the option of using the meter with the sensor electronics only is also possible. This option allows the end user to access all the functionality of the meter via an industry standard Modbus protocol. Here Krohne has also designed a Windows based software tool, which provides the end user full access to all the functions, including the diagnostics, concentration measurement
etc. This makes setting up and trouble shooting the meter very easy.

The Optimass 1000 offers the end user a generous number of options not normally seen in an economy general purpose meter. This includes a number of hygienic process connectors for the upstream pharmaceutical and food and beverage industries. Polishing to a high standard of surface finish is also available.

The standard measuring tube rating is 100 bar and the applicable rated process flanges are also available. Secondary containment is standard to 63 bar, but m100 bar option is also available. This is not only unique in this economy class but in the Coriolis meter market in general. This ensures that the highest level of safety has been designed in.

All sensor-exposed materials to the atmosphere can also be optionally supplied as SS 316 L as well. This includes the converter housing. This is useful for industries where a lot of cleaning is done by hosing the equipment down, as well as the off shore industry where sea air and salt spray is very aggressive.

There are very few limits on the applications for this meter. Liquids (accuracy 0.2%) to gases (accuracy to
0.75%) are measured without any problems. The meter is available in sizes of 15, 25, 40, and 50mm and can handle a density range of 400 to 2,500 kg per cubic meter. Nominal flow rates range from 4,800 to 125,000 kg/hour. The temperature range is - 40 to 130ºC.

The Optimass 1000 now provides the end user with an economical general purpose meter with the maximum flexibility and versatility of features without any restrictions on functionality or performance. Krohne
will continue to develop and invest in Coriolis mass flowmeters.

—Eddie Bridges, International Product Group Manager, Krohne Ltd. UK. Enquiry code N250

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