FLOW measurement of slurries has always been one of the most challenging applications in flow metering. Slurries occur in many different guises in several industries.
Mining slurries are the most common, but fruit juice, paper pulp, alumina, sewage, fly ash in suspension, are among the many products considered as slurries.
There are two main types of flowmeter commonly used for measuring the flow of slurries: standard electromagnetic flowmeters (magmeters), slurry magmeters (developed specifically for this type of application).
The appropriate flowmeter to be selected is determined by three primary considerations:
1. Content and properties of solids in the carrier fluid.
2. Chemical composition of the carrier fluid.
3. Flow velocity and profile.
All three factors will influence the performance and lifetime of a flowmeter, so it is important to consider these aspects carefully to determine which flowmeter is best suited to the particular application - certainly there is a cost implication.
The decision is further complicated since the flow tube (sensor) and transmitter should be considered separately. The content and properties of the solids need to be taken into account when considering the flow tube:
Solids content: Depending on the chemical or mechanical nature of the process, the solids content can vary from as little as a few percent to in excess of 80%. If the percentage is low, this may mean a standard magmeter - for example Endress+Hauser's Promag 10/50/53 range - will be suitable. If it is high, this may suggest the selection of a slurry magmeter, such as the new Promag 55.
Properties of the solids: If in the slurry, the carrier fluid contains very fine solids, a standard magmeter can be used even for very high solids content. In contrast, if the slurry contains coarse solids, it is more likely that a slurry magmeter would be more suitable. This is because rough solids cause mechanical noise as they flow past the electrodes and this interferes with the measured signal.
But it is not only the size and shape of the slurry particles that must be considered - solids too can determine the abrasiveness of the slurry. The abrasiveness will also determine whether the flow meter can have a standard liner or a liner that can cope with high abrasiveness. Because of the principle of operation of a magmeter, the conductive carrier fluid must be electrically isolated from the body of the magmeter. This is achieved by lining the inner tube with a non-conductive liner such as PTFE, PFA, Linatex, rubber, PU or ceramic. The liner material therefore must be a major consideration regarding abrasiveness of the slurry.
Milled mining slurry, for example, is normally not very abrasive. But it is a different story if coarsely crushed ore is involved. In general, the finer the solids the less likelihood there is that the slurry will cause difficulties. Particle sizes less than 0.5mm are normally not critical to the measuring accuracy. But if the particles are sharp edged (for example, coal slurry or ash) they can be quite abrasive.
Chemical composition of fluid
Many manufacturing processes require chemicals to be contained in the slurry - for example, the pulp and paper industries and the mining industry. Some of these chemicals can be quite corrosive. Soft liners typically used in abrasive applications are thus inappropriate as they have a limited ability to withstand corrosive fluids.
If high abrasion is combined with corrosion only a ceramic liner would be suitable. But even these have their limits if the slurry is coarse and/or line diameters are >300mm. Ceramic is also known to be sensitive to mechanical shock and severe temperature changes which can make the liner crack. Furthermore, installation of this type of liner (mainly the bolting of the meter) is critical, as it is easy for ceramic-lined meters to be destroyed as a result of unqualified installation procedures.
If chemicals in slurries are not well mixed, they can create nasty signal noise due to significant and rapid changes in the conductivity of the slurry. It is therefore advisable to install flowmeters before the injection point or far enough downstream to allow proper mixing with the slurry. For chemicals that are not well mixed, a slurry magmeter is recommended.
Flow velocity and profile
The higher the flow velocity of an abrasive fluid, the higher the wear. The velocity should therefore be kept reasonably low (<2m/s) in order to extend the lifetime of the meter. On the other hand, flow velocities >4m/s are often required to avoid settling of the solids that would lead to blocking of the pipe. There is always a compromise between the lifespan of the liner and process safety, and this must be considered in any selection decision.
If a flowmeter is installed shortly after a bend (or another flow profile disturbance) it is likely the particles in the slurry will be forced to the wall of the pipe and therefore cause excessive wear. It is advisable to install the flowmeter well downstream, not so much to increase the accuracy but to allow the particles to concentrate in the centre of the profile again and therefore minimise asymmetrical wear.
Slow moving slurries can cause excessive wear at the bottom of the pipe (and flowmeter). By rotating the flowmeter occasionally it is possible to extend the life time of the liner. If the meter is installed in a vertical flow these effects are minimised. (This is the recommended installation in any flow application and in slurry applications in particular.)
Choosing the right liner
Increased process demands, both abrasive and corrosive, coupled with the increasingly harsh requirements of cleaning and sterilisation in the food and pharmaceutical industries, have given rise to a number of new electromagnetic flowmeter liners and liner technologies.
No single liner technology can be used for the entire range of nominal diameters from DN2 to DN2000, and each customer application is unique. Endress+Hauser have therefore adapted four different liner materials: rubber (NR), polyurethane (PU) and two different fluoro plastics - polytetrafluoro ethylene (PTFE) and perfluoroalkoxyethylene (PFA).
Historically, rubber has been the liner of choice. One of the most popular liner materials in Australia is Linatex, for which Endress+Hauser's NR1300 liner is the equivalent. This liner is mainly used in coarse and chunky slurry.
Natural rubber, together with brush electrodes, is a unique and powerful solution if the slurry is expected to be abrasive. Endress+Hauser is the only manufacturer offering this option. It is a premium option, however, and if a cheaper alternative is sought tungsten-plated standard electrodes may be appropriate.
PU (polyurethane) is very abrasion resistant but if the electrode heads are ground away, the sensor will fail. Generally it is a good idea to choose Hastelloy C electrodes (a standard in the Promag 55) since this material is relatively abrasion proof compared with stainless steel. The PFA liner offered by Endress+Hauser up to NB200 is a very useful option for fine and corrosive slurries, as its temperature resistance is unsurpassed.
Choosing the electronic evaluation unit
Having selected the most suitable flow tube, we need to consider the appropriate transmitter/evaluation unit. Slurry properties causing mechanical and electrical signal noise at the electrodes are important criteria in flowmeter selection. Possible applications are:
* coarse slurry (eg in the mining industry)
* pulp with long fibres (eg in the paper industry)
* sewage sludge with more 7% solids
* chemicals in the slurry (if not well mixed)
* enclosed air
* inhomogeneous liquids in general
The degree of noise filtering, relative magnetic field strength, the design of the electrodes and other factors determine the selection of the most reliable transmitters - and the cost.
How to choose the ideal flowmeter
There is no clear cut answer as to which flowmeter should be selected for a specific application depending on the criteria. A standard meter Promag 50/53 might give acceptable results in some critical applications, although it may be difficult to accurately predict the quality of the signal. A safer but more expensive option in these situations is the Promag 55.
As a general rule, the more aggressive the slurry, the more complex the decision making. And this is where Endress+Hauser have the experience and tools to assist with the best solution.