Back in the 1970s, Coriolis flowmeters first came into commercial use and ever since they have been developed and refined to meet a growing number of applications across a wide range of industries. However, where fluid density is not required, there is now an alternative that offers all of the accuracy of Coriolis as well as addressing many of its substantial drawbacks.
The development of new flowmeter designs based around Surface Acoustic Wave (SAW) technology is now delivering precision flow measurement from a smooth, uninterrupted flow-through pipe section - meeting all the requirements of a hygienic application.
Ian Webster Hygienic Processing Segment Manager for Bürkert Fluid Control Systems, looks at the latest developments in flow measurement and how they are challenging the more traditional systems.
The demand for more advanced, non-contact measurement technology has seen the rise in ultrasonic, electro-magnetic and Coriolis technology and each of these have their place. However, even these devices have limitations including non-conducting liquids or those containing bubbles or debris. Ultimately it may be the location, space or orientation that may determine the most suitable design for a particular application.
Challenging the constraints of flow measurement
Coriolis flowmeters are regarded as the top specification flowmeter due to their versatile capabilities for both fluids and gases. However, the initial cost of such devices can be very high and they do have some constraints. The design requires the flow direction to be determined before installation and once installed there will be a pressure loss across the flowmeter, depending on the specification.
In addition, the designer may have to choose between drain-ability and accuracy since the more accurate bent-tube Coriolis flowmeter is difficult to drain and may present a contamination issue. In such cases it may be necessary to specify the straight-tube Coriolis device to ensure hygiene standards are met, but this can be at the expense of accuracy.
In terms of liquid flow measurement, there is a clear opening for a lower cost device which can deliver a compact, non-contact measurement which is accurate irrespective of media characteristics, flow direction and flow conditions. In answer to that design engineers have developed SAW technology to create an extremely compact device that has the same hygienic properties and clean-ability as the rest of the process pipework.
Simplicity through advanced design
The main principle of this flow measurement device is based on the wave propagation forms similar to seismic waves, which start from an initial point of excitation and spread along the surface of a solid material. SAW technology uses at least four interdigital transducers which are located on the outside of the measuring tube and therefore have no direct contact with the fluid. Each transducer acts both as a transmitter and as a receiver.
The signals that are received are processed by the on-board electronics and software which have also been developed in conjunction with a university that has been studying SAW technology and its applications. The analysis of all the signals and comparisons based on different criteria such as amplitude, frequency and runtimes, allows evaluation of the quality of the measurement, the existence of gas bubbles or solids as well as the kind of liquid.
The fact that the internal surface of the flow measurement tube can be manufactured to the same specification as the rest of the production pipeline means that hygienic cleaning processes, including clean-in-process (CIP) and sterilisation-in-place (SIP) can be maintained to the highest standard.
Accuracy without compromise
Until now, it has been difficult to match the accuracy levels demonstrated by Coriolis, without introducing other weaknesses to the application. Magmeters rely on the process fluid being conductive, while other designs require components to be in direct contact with the fluid, making it difficult to meet the necessary hygiene standards.
Accuracy and repeatability often rely on laminar flow which can require certain lengths of straight pipe upstream of the flowmeter. Continued testing and development of the SAW technology has minimised this requirement. In addition, specifically designed, on-board software introduces an automatic calibration process that can maximise accuracy, even in applications that require short pipe runs.
Another issue that dominates discussions around flowmeter selection is weight, especially in process skids, where it needs to be minimised in order to retain the portability of the skid. The basic design of Coriolis flowmeters does put them at a disadvantage, in some cases a DN25 unit can weigh over 15kg, compared to just 2.4kg for products that have been created using SAW technology.
In situations where the flowmeter needs to be removed from the installation for any reason, such as cleaning, the implications for manual handling need to be considered. It may require additional mechanical assistance, which will add to the overall time for maintenance or cleaning.
Finally, any discussion regarding Coriolis would not be complete without some mention of the initial cost. Due to the technology and the scale of these flowmeters, even the most basic specification can represent a significant investment. Furthermore, the energy consumption of the two oscillators is considerably higher than more modern flowmeters that use more energy efficient technology.
Improving technology to reduce costs
SAW technology offers a way to measure the flow of a liquid and its temperature without installing a large, expensive, and energy-draining piece of equipment. As well as being a leap forward in terms of size and energy use, it also enables the measuring tube to be free from parts. This cuts the risk of contaminating the medium to zero while also eliminating pressure loss, leakage, and dead zones. Furthermore, SAW technology works regardless of flow direction and conductivity of the medium, and it will be developed further to measure multiple parameters.
From the outset, the aims of this development project were to resolve the traditional issues associated with the Coriolis flowmeter, such as clean-ability, pressure drop, weight, cost and energy consumption. At the same time, the designers were also keen to launch a product that could be improved over time, using more reference data to refine the measurement algorithms for different types of media, which is very much the case with this emerging technology.