There are more instruments for measuring liquid level than for solids, but you may have more options for solids than you think. Are you aware of all the most recent developments, and how they can be used to improve measurements?
That’s the main point of my article just posted online and scheduled to run in print in the Powder & Bulk Solids April 2020 issue, How to Improve Level Measurement Through Technology Selection. If you’re working on solving a level application for solids, make sure you explore all the possibilities. Solids level measuring applications may call for a combination of continuous and point level measurements.
For an engineer looking at a new application or trying to improve an existing installation, it may be necessary to look for two types of devices: continuous level instruments and level switches. Since there are multiple options for both, let’s consider how to make selections.
The article looks at both continuous level instruments and level switches. Radar instruments, both guided wave (GWR) and noncontact (NCR), are excellent choices for solids, and the article compares both in this context, so give it a full reading.
For this post, I’m going to concentrate more on level switches. The article looks at three very different technologies, and the selection should hinge on the product in the vessel.
Paddle-wheel switches have an internal electric motor which turns a paddle wheel extending into the tank interior. If the wheel is in free air, it turns at the full speed of the motor. When immersed in liquid or solid material, the rotation will be slowed or stopped entirely. Modern designs use an internal clutch which allows the motor to continue turning when immersed, but with enough drag to sense the difference. They are still the low-cost option in most situations and popular for some applications, such as grain and small aggregate materials.
For this technology, Emerson offers its Rosemount 2501 Solids Level Switch - Rotating Paddle. It can withstand heavy loads and extreme temperatures, and is suitable for full, demand, or empty detection for all bulk media in all types of vessel. It is a simple, robust, and reliable technology that is resistant to dust, electrical charge, adhesion, high temperature, and pressure. It is easy to install and can be mounted in a vertical, horizontal, or oblique position.
But it isn’t the best solution for every application, so other options should be considered for certain types of products.
Vibrating-fork switches use a tuning fork assembly, where the tines extend into the material. An internal piezo-electric crystal causes the fork to vibrate at a specific frequency in free air. If immersed, the vibration will be impeded, which the electronics can sense. These switches don’t have the mechanical complexity of paddle wheels, so they have a long service life, and some are even safety rated. Vibrating-fork switches are particularly well suited for use with fine powders and low bulk density materials.
Here, Emerson offers its Rosemount 2521 Solids Level Switch - Vibrating Fork. It is suitable for fine-grained and powdered media in storage and process vessels where high sensitivity is needed. It is a robust technology and is easy to operate. It has flexible options via configurable specifications for different applications. With an adjustable switching delay, false switching can be prevented.
Different product types may call for a third solution.
Capacitance switches use a probe that extends into the material, and it has specific capacitance characteristics in free air which change when immersed. These tend to end up in the most difficult applications with heavy, wet materials and high temperatures. They’re favored for wood chips, grain meal, and hot products such as cement clinker, lime, or lightweight aggregate.
For these tough applications, Emerson suggests its Rosemount 2555 Solids Level Switch - Capacitance Probe. It is robust and suitable for all solid bulk media in all types of vessels. It can be used for full, demand, or empty detection and is designed for low dielectric medias and extreme conditions such as high temperatures, high mechanical stress, and high tensile forces.
There’s one more option which has been introduced since I wrote the article, Emerson’s Rosemount 2535 Solids Level Switch - Vibrating Rod. This is a variation on the vibrating fork but does not have multiple tines. It is particularly suitable for use with fine-grained and powdery materials, materials with strong "caking" properties, and coarse-grained granulate. Its simple design makes it reliable, maintenance-free, and less prone to clogging compared with other point level measurement technologies.
Clearly there is a lot of overlap of capabilities, so more than one solution may be suitable for a given application. How well do you understand these differences?
Tell us about how your thinking is evolving. You can share with others about your implementations and experiences at the Emerson Exchange 365 community forum, a place where you can exchange ideas and experiences with others in the same situation. It’s a site where you can communicate with experts and peers in all sorts of industries around the world. Look for the Level Community, plus other specialty areas for opportunities to provide input, suggestions, and answers.