Coriolis flowmetering comes of age

When the first Coriolis mass flowmeters appeared on the market in the early 1980s, they were widely welcomed in industries where their ability to measure mass directly - rather than infer it from a knowledge of volumetric flow rate and fluid density - was seen as an immediate benefit.

Unfortunately, many early designs of Coriolis meters did not live up to that early promise. The conventional designs at the time - bent or looped tubes, or split-flow tubes - proved unreliable on many applications where their direct-mass-reading capability should have been of most benefit. In the food processing industry, for example, the convoluted flow patterns and installation restrictions required of such meters meant they could not cope with the viscous, non-Newtonian, shear-sensitive or solids-bearing fluids typical of the industry.

All this changed, however, in 1994 when Krohne introduced the world's first single straight-tube Coriolis mass flow meter. Compared with those earlier designs, the main advantage of this meter was its low pressure drop and a mechanical design that lent itself to hygienic materials of construction and operation. And now Krohne has launched a new family of Coriolis meters that could open up many more application areas for the technique.

The main advantages of the new Optimass design are its ability to adjust to changing fluid densities through 'Adaptive Sensor Technology', improved signal processing, and an ease of installation far better than conventional Coriolis meters. The new range is also the first of its kind to provide wetted parts in titanium, stainless steel or Hastelloy, offering corrosion resistance for almost any fluid.

The Optimass family consists of two styles of single-tube meters: the straight-tube Series 70 and, for extremely low flow rates, the Z-tube Series 71.

Unlike those earlier meters, the straight-tube Series 70 meters have no installation limitations. The meter can be sited anywhere in the pipe run and can even be supported on its own body - inconceivable with most mass flowmeters where external influences have to be avoided at all cost.

Behind the design of the straight-tube Optimass meters lies Krohne's newly patented AST (Adaptive Sensor Technology). The net effect of an AST design has been to deliver an instrument that has end pipes strong enough to withstand the thermal and loading stresses and G-forces associated with Coriolis meters, yet weak enough to decouple the instrument from outside interferences.

The adaptive nature of AST also automatically compensates for changes in the density of the flowing fluid. Temperature compensation is also achieved far more rapidly than competing instruments thanks to Krohne's patented use of strain gauges mounted on the sensing tube.

In addition to this almost 'fit and forget' capability, the Optimass range has also been designed with all new electronics. A sensor signal processor module - common to all meters, straight- and Z-tube - is mounted permanently on the meter with direct analogue connection to the sensors. The main converter or transmitter can then be mounted either integrally or remotely up to 300m away, using just 4-core cable. Both sets of electronics store the sensor data on Eeprom so that, should the need arise to change either set, data can simply be downloaded.

One of the main advantages of the new electronics, however, is the 'dramatic improvement' in the turndown capabilities of the meters. Far lower flows than before can now be measured, accurately and reliably, without having to undersize meters for the duty - which has been a problem with Coriolis meters in the past on some food and pharmaceutical applications.

Also of interest to the food and pharmaceutical industries should be the new patented design of hygienic coupling available for the Optimass range. As with most Coriolis meters, the standard Optimass uses a seamless titanium alloy (ASTM Grade 9) for the measuring tube. Connecting this to the stainless steel fittings required in hygienic operations has long been a problem for meter manufacturers. Conventional attempts at adding hygienic couplings to meters have proved extremely cumbersome, because they effectively introduced an additional set of seals.

The Optimass patented design, however, has overcome this titanium/stainless steel welding problem by means of a special adaptor that simply screws on to the meter's welded end connections to accommodate DN 11864-2 connectors, without any need for extra 'O' rings.

The Optimass meters are designed to comply with a broad range of industry standards such as ASME Bio processing equipment, EHEDG and 3A.

Although each meter is essentially 'made to order' at Krohne's UK operation in Wellingborough, Northamptonshire in the UK, Krohne is aiming to have them available on a two to four week delivery schedule. Sizes in the Series 70 range cover mass flow rates from 20 up to 560 000 kg/h, while the three low-flow Series 71 models range from 0.3kg/h to 455kg/h.

Adaptive sensor technology

Krohne's previous designs of straight-tube Coriolis meters tackled the problem of 'decoupling' the instruments from outside interferences by use of a spring-loaded counterbalancing mass absorber. This was fitted around the outer casing of the sensing tube to minimise 'noise' generation.

Although different for each size of meter, this compensating mass does not allow for changing product densities.

In the Optimass design, however, the end connection tubes now act as the spring for the absorber mass system. This 'spring' is self-compensating - as the density of fluid flowing through the meter changes, so does the dynamic stiffness of the tube-ends. No vibration escapes from the meter and the zero stability of the meter is greatly improved.

The patented Adaptive Sensor Technology, AST, is at the heart of this design. Krohne's team of researchers, led by technical director Dr Yousif Hussain, formulated two dimensionless number groups that mathematically link together the required length of the connecting tube, its Young's modulus, bending moment of inertia, the mass of the inner cylinder and the water-filled tube's natural frequency.

Using finite element analysis to solve for these groups results in the adaptively tuned meter design, independent of external forces and density.