Complete Guide To Compare Density Meter Technology For Process Control And Slurry Measurement
Density meters are vital measurement instruments used in almost every industrial process. No matter what industrial sector one considers, if material is flowing through a pipeline, it most likely must be measured in some way. Understanding the media in the pipeline is the foundation of process control. The following information will allow you to compare modern and traditional density measurement technology.
The variability of media type, flow rate, pipe sizes and other factors warrant the existence of several different types of density meter. Below you can find a guideline for the methodology of each. Some of these instruments have been around for decades, whereas some are new and disruptive technologies. Each instrument tends to have it’s “sweet spot” for specific applications depending on the process, environment and budget. This guideline will help to insure that your process is using the most efficient instrumentation available.
In this guide we focus on:
Coriolis Density Meter
The Coriolis design is an old but proven one that can measure mass flow from which density can be deduced mechanically. The instrument is underpinned by the “Coriolis Effect” – an effect whereby a mass moving in a rotating system experiences a force (the Coriolis force) acting perpendicular to the direction of motion and to the axis of rotation.
It performs by splitting the pipeline into two and curving the separated pipes before joining them again. The separated measuring pipes perform oscillations on a section of the curved path. When mass flows through the pipes, the pipes deform proportionally to the mass flow rate. The change in vibration is measured by two sensors and forms the basis of the measurement results. Essentially, the sensors register changes in pipe oscillation and can equate that to mass flow.
Though accurate, the coriolis design changes the characteristics of the flow. Consistency throughout the pipeline allows for a more accurate understanding of what is in the pipe, so changing the flow is something many companies might prefer to avoid. Additionally, the change in flow makes this instrumentation unideal for abrasive slurry. Since coriolis relies on pipe wall movement, the pipes have to be thin, which is not ideal when used with an abrasive slurry. The separation of pipes can lead to clogging and faster wear rates. Also, the internal diameter for the pipes of a coriolis meter become prohibitively expensive beyond 12 inches – making this design best suited for measuring homogenous liquid media.
Recently, new types of coriolis meters that follow a straight, almost unaltered path have been developed – but just like their original design, come at considerable cost. Coriolis meters can be very cost effective, but only at a small diameter. The average size of meters in the field is 0.5 inches. Because of their prohibitive pricing for larger pipe diameters, many coriolis applications are measuring a sample line. Although the methodology is highly accurate, in the real world it fails to give accurate measurements of the full volume of pipe.
Nuclear Density Meter
The nuclear density meter (or nuclear density gauge) is an energy based instrument known for its flexibility within processing applications. It can endure up to 50 percent solid slurry. Nuclear density meters are not appropriate, however, for processes involving edible materials, mobile processes or high precision applications. It’s most limiting factor is that, like anything else with a nuclear core, it is unstable and constantly emitting radiation. Nuclear density meters have a relatively long life time. However, when the nuclear source means emissions are too weak to provide function it requires proper disposal. The clamp on design of this unit and its operating principles allow it to be installed in a wide array of applications including most standard piping materials as well as either horizontal or vertical piping configurations.
It functions by emitting a measured amount of radiation from the source, through the pipe and into the receiver. The difference between the radiation emitted from the source compared to the radiation accounted for at the receiver is then used to calculate density. The radiation emitted from the source follows a straight or cone shaped path through the pipe towards the receiver, but the pathway does not cover the entire pipe. This means some media will continue through the system without being measured.
As the nuclear core degrades over time, the accuracy is affected. To account for this, wipe test and calibrations are very common and often scheduled in advance. However, once the nuclear core degrades to its half life, it requires these interventions even more frequently. In many cases, the user will choose to remove the instrument from service at this stage. Removal or disposal of the nuclear density meter has it’s own set of challenges, as strict protocols must be followed and are almost always very expensive.
The nuclear density meter requires monitoring and maintenance from a trained and certified Radiation Safety Officer. This “RSO” is responsible for not only any maintenance or intervention, but also the corresponding paperwork and documentation. In addition to this, the RSO also oversees liaising with the regulatory bodies and instrument suppliers. The national average salary for a Radiation Safety Officer in the United States is $71,000 per year.
Red Meter Process Characterization Device
The Red Meter, more accurately described as a continuous process characterization device rather than solely a density meter, takes multiple direct measurements of wet and dry processes. Often used as an alternative non nuclear density meters in slurries, the inline measurement instrument’s technology is based around it’s patented flexible cartridge. As material flows through the pipe, the cartridge bends slightly under the weight of the media. This bend or flexion is minimal, think 1/60th of a piece of paper, minimal.
A high precision laser is used to detect the displacement in the deflection of the cartridge. Acting as a scale, the measurement of deflection is equated to a measurement of mass. In a fixed volume, the change in mass is equated to the change in density. A variety of additional sensors measure pressure, temperature, wear and even velocity in the line to establish direct measurements of the essential statistical process control variables. When combining these measured variables, an operator can not only truly measure their process, but also accurately and efficiently make changes downstream and/or automate troubleshooting.
These measurements provide the first true process characterization device, allowing operators to use as much or as little of the processing data to achieve their desired level of process control.
A Red Meter is highly accurate and repeatable, in addition to being easy to install and use.
One key advantage of the Red Meter is that the entire volume of pipe is measured as opposed to a sample. Available in pipe diameters from 2” to 60” a Red Meter can withstand highly abrasive slurries as well as high percent solids. The wear sensor on the cartridge allows for efficient process control by alerting when the cartridge needs replacing in advance.
Slurry processing data from the Red Meter is displayed in a graphical format either on the device’s display screen or integrated within the operator’s control room. It is impervious to flow rate changes, percent solid changes, pressure changes, and environmental factors.
The ruggedness of a Red Meter makes it appropriate for a wide range of applications including dry bulk and wet slurries.
Ultrasonic Density Meter
The Ultrasonic density meter operates in a similar manner to a nuclear density meter. Energy is emitted through the pipe and to a receiver. The density can be inferred from changes in the energy levels accounted for at the receiver. It can also be placed vertically or horizontally depending on the processes needs.
The main difference between ultrasonic density meters and more commonly used nuclear density meters is that the energy being transmitted through the pipe is soundwaves, not gamma rays. Another key difference is that ultrasonic density meters are placed in-line and do not have the same capabilities of handling solid materials. They can withstand abrasive slurries, however. This instrument works best with lower density slurries and lower solids content slurries.
A reasonably priced instrument, ultrasonic density meters perform best on smaller pipelines. Although fairly accurate, they have a few clear limitations. The ultrasonic density meter is highly influenced by fluctuating solids content and entrained gases. As such, it works best with homogenous mixtures.
Microwave Density Meter
The first non-nuclear density meter ever developed was the microwave density meter. It is another energy based instrument that operates by measuring how much energy is lost on its way from the source, through the diameter of the pipe, to the receiver. The density can be calculated from the measured quantity.
Just like a microwave that heats food, a microwave density meter affects whatever materials flow through. This limits the potential applications greatly by eliminating any process that involves temperature sensitive materials, or any other material whose chemistry would be altered by microwaves. A hindrance in measurement for this instrument is the low power nature of microwaves- a significant factor when considering high density media or high solids content
High solid percentage slurries are not highly compatible with the microwave density meter as it works best with liquid containing low density materials. These devices are relatively expensive considering other available options.
Tomography Density Meter
This instrument is perhaps the most unique in terms of measurement methodology. Its an energy based meter that functions by electrical resistance tomography or mapping. A series of electrical sensors are dispersed around the pipe. Electromagnetic fields are then emitted from the sensors creating fields of view.
A great factor of the tomography density meter is its’ brilliant image display of what is flowing through the pipeline. However, a physical display does not always equate to accurate numerical measurements. This type of meter requires a slower moving slurry and therefore acquires measurements slowly. Though it is a pioneer for display density meters, it comes at a high cost and functions most accurately with low density materials.
Acoustic Density Meter
The most affordable of all the density meters is the acoustic meter. It is a clamp on, mechanical metal plate that sits on top of the pipe. Inside there’s a canister that holds a metal hammer which continuously taps the side of the pipe. Based on resonate acoustic of the hammer, it can calculate density. The acoustic meter is used in small diameter pipes that have limitations on material of pipe liners. For instance, a rubber lined pipe will diminish the acoustic sound of the hammer therefore altering the measurements gathered.
Another affecting variable is environmental influences. The acoustic meter is open slightly because part of it is connected to the pipe and the other part is completely separate from the pipe. Problems can occur if things get in between the pipe and hammer. Alas, it is still the most affordable of all the meters, so affecting variables are to be expected.
Induced Force Density Meter
This type of density meter can be compared to coriolis style meters. It is a mechanical, inline meter that functions by moving a flexible pipe up and down at a specific pace via an actuator. Based on the amount that movement changes from the set force, it can calculate for density. This is a process altering mechanism by nature, which as previously mentioned, isn’t always ideal. However, there are ways to calibrate for the process changes.
Less is known about this type of technology, as it is fairly new. It is the most expensive density measuring technology available on the market today.
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