Measuring abrasive slurry density the smarter way

Density measurement of flowing (abrasive) slurries is based on physical processes such as absorption of radioactive radiation, reflection of (ultra)sound waves, direct (gravitational) mass/volume measurement or percussion with respect to the slurry

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Abrasive slurries are watery mixtures of insoluble matter consisting of hard, abrasive particles, commonly present in applications such as dredging, offshore, well-servicing industry, metal & mineral mining, maritime, cement, power and tunnel construction.

For example, in dredging, underwater deposits are excavated and the slurry transported to another place. On the other hand, in building construction cement can be transported as a slurry through a pipeline.

Such application areas need productivity monitoring. This can be defined as the amount of material pumped per unit time (in kg/s or ton/s). It is this essential to know the slurry density (in kg/m3 or ton/m3) together with the flow rate (in m3/s) or flow velocity (in m/s) of the slurry through the slurry pipeline. Furthermore, density measurement is necessary for quality control purposes. This will also aid in the monitoring of the process consistency over time.

In order to be able to adjust the process, from an economical point of view it is desirable to have the slurry density available in real time or within a few seconds after the measurement.

Abrasive slurries deviate in at least two ways from ‘average’ slurries. Their most striking characteristic is the presence of abrasive particles. This thus necessitates the use of wear resistant materials for the part of the measurement device that gets in contact with the slurry. Furthermore, the density of these slurries is usually high, up to 2.5 ton/m3. These ‘dense materials’ will put constraints to measurement types that rely on easily penetration of slurries.

Methods of conducting abrasive slurry density measurements

Density measurement of flowing (abrasive) slurries is based on physical processes such as absorption of radioactive radiation, reflection of (ultra)sound waves, direct (gravitational) mass/volume measurement or percussion with respect to the slurry;

Nuclear measurement
This method works through the gamma radiation absorption principle. This principle is the most common. This is because as measurement is conducted at the outside of the pipe, installing of the setup does not interfere with production loss to a large extent.

However, some of the limitations are such that large pipe diameters and high slurry densities may lead to radiation sensing that is in the order of magnitude of background radiation level. Moreover, there are legislation constrains on working with radioactive technology.

Ultrasonic measurement
The method works using the principle of reflection and transmittance of (ultra)sound waves. In ultrasonic density measurements, a transducer made of a piezo-sensitive material sends ultrasonic pulses into the to-be-measured slurry, and a computer analyses the returned echoes.

Percussion technology

By its very nature the Percussion method uses a non-invasive, clamp-on approach. This maintenance-free method uses a small, compact sensing module that is mounted at the outside of the slurry pipe. This module has a built-in striker and receiver. The striker impacts the outside wall of the pipe with pre-determined force and duration. This creates vibrations in the measurement zone of the slurry-filled pipe. The sensor component of this module receives those vibrations and transmits them to the processing module which selects the most informative of the vibration spectrum harmonics generated to produce highly accurate and precise slurry density measurements.

Non-nuclear slurry density measurement
Experts such as Arjan Vriend from Alia Instruments, advocates the use of non-nuclear slurry density measurement as opposed to the other techniques. He points out the main issue in the usage of nuclear material.

According to Mr. Vriend, the radioactive source is the cause of a lot of practical drawbacks with regards to licenses, training, shipment and operators who have their reservations about using such technology. He also points out that the method has an impact on the business management of companies who have to answer to society’s pressure to reduce nuclear waste, and to operate in a sustainable manner.

Industry application
The non-nuclear slurry density measurement is applicable in basically all applications which are covered today by radiometric devices non-nuclear alternatives.

However, there are new technologies on the market, most of which have some downsides. Some of them have troubles with gasses in the process, while others fail to work well with vibrations. Mr. Vriend clarifies that Alia Instruments always considers the amount of wear of the material.

This, he says, is because they supply an inline meter. Some very abrasive processes have proven to be rather incompatible with their technology, as the material would wear the rubber liner in months or even weeks.

Advantages
Some of the compelling reasons that support this cause include the fact that non-nuclear slurry density measurement ensures a more sustainable business management and company reputation.

He further adds that it is a demonstration of social responsibility, as well as responding to society’s pressure to reduce nuclear waste. Mr. Vriend points that employees using this method will not be afraid of performing checks and repairs as opposed to other techniques, citing the risks involved in the same.

The non-complexity of this method removes the need for special RSO trainings or specifically trained personnel. Moreover, there are little to no concerns about more stringent regulations in the future.

Factors to consider when switching to non-nuclear methods
Should a company wish to switch to non-nuclear methods, they ought to consider what demands they have based on their process, and what technology fits best. In the case of Alia Instruments, for instance, Mr. Vriend points out that they have a meter which has the benefit that it really measures all materials, and has no problems with larger parts, sediments or gasses.

On the other hand, most of the potential customers claim they have not switched because of lack of a viable alternative. This is in terms of technology and finances in comparison to the same in nuclear methods. The technological bit poses a challenge especially when you consider misconceptions such like how nuclear density meters are very accurate, and therefore can be used as a reference to other meters.

It is important, however to note that these meters also have their measuring error, especially when people fail to have it re-calibrated often.  The half-life of the source also makes it a requirement that the task is undertaken by the supplier.

Moreover, end users often fail to realize that the signal of a nuclear meter is significantly delayed. This acts to their disadvantage such that by the time they notice a change in the signal, the material has already passed.

Finally, when it comes to finances, it is advisable to make a good comparison between the total cost of ownership of the nuclear meter compared with the alternative. This should not be limited to just the purchase costs.

Instead, end users should consider the costs of RSO certification, the high disposal costs, but most of all, the lost hours of all the hassle people have of importing and transporting the meter, permits and the maintenance by the supplier as well.