Point level detection and high level alarm, switch technologies for preventing silos overfilling
This article seeks to help to clarify some of the choices and technologies you can use for overfill prevention when filling silos.
You can put solids level switches into two main categories, active devices and passive devices.
Passive devices can, of course, detect a point level, but they have no means of detecting their own condition, or faults which may prevent them from switching at the next high level. Such a technology for bulk solids in silos are mechanical or electro mechanical devices; they are low tech and low cost, such as paddle and tilt switches. Generally, they cannot detect if they have lost their actuators: paddle switches need the resistance of a small rotating plate to trip a clutch and micro switch; a tilt switch has a magnetic switch mechanism with a ball or flap inside to actuate when the rising level tilts the dangling body over. If the paddle or tilt body gets damaged or broken off, very seldom is this able to be detected: the rod just keeps spinning, the tilt body is gone. Even a simple power failure is not always detectable, especially with tilt switches. In high demand applications, mechanical wear of these devices is also a consideration.
Capacitance also usually falls into the passive device category. However, capacitance and admittance switches have been around for a very long time, and there are many modern refinements to the technology. In their basic form, they use a capacitance circuit, formed by a rod or probe and the vessel wall, detecting when it is covered or uncovered by the process. They often require manually ‘tuning in’ to an initial setting to calibrate, ideally with product filling. This technique can offer excellent robustness and resistance to build up - using admittance technology to detect it, and internally retuning the device to ignore it. There is no fail-safe, self-monitoring capability inherent to the technology, although testing these systems by introducing a known level of capacitance (i.e. a switch to put a capacitor into the circuit) to simulate a level being reached, can be used. Sometimes special circuits are used to detect broken off probes, but these only work for specific applications. Another limitation to be aware of is the detection of any large changes to the electrical conductivity of the product can cause unreliable switching: a powder shifting from dry to wet, or a waste stream changing in its composition depending on its source e.g. dry plastic to wet paper. These situations can also mean site personnel can start 'retuning' or adjusting devices to get them working again, only to find that they don't when the product electrical properties shifts again. It is always advisable to consult with a manufacturer about an application to ensure you get the right probe and system for a reliable solution.
Active Devices are those that can do the job of level detection, but also have diagnostics to warn the user about their condition and ability to function, as well as, testing their internal functionality. Over the last 30 years, the introduction - and subsequent development - of vibrating level switches for solids has transformed the reliability of point level switches for high (and low) level detection in silos. By measuring the vibration, they detect a covered and uncovered state to make a switch output, and they don't need ‘tuning in’ with product filling. This vibration-based sensing also extends to self-monitoring: the frequency and amplitude must remain within certain parameters for a device to be in a healthy situation. As an active switch it must be vibrating to function; if the vibration stops completely, it indicates damage or power failure, rendering the probe fail safe. The technique also enables self-testing - or ground level testing - where a button press initiates a series of self-test cycles. These tests simulate, through changing vibration patterns, various states of covered, uncovered and damage for full assessment of the vibrating element and electronics, before returning the device back to its operational state.
Both rod and fork versions of vibrating switches are offered. Forks are designed with high sensitivity for powders and very light bulk densities, with very good tolerance for coating and build up. Rods are designed for grains and granulates, as they resist jamming between forks, but offer slightly less sensitivity and a lower tolerance to build up. The attainment of SIL and other overfill certification adds to the credibility of this technology.
Special Solutions: There are other technologies for point level, but these are mostly in a higher cost band. However sometimes these solutions are essential for process reliability. Examples are contactless systems using microwave and nucleonic based technology. Both detect when an energy beam, passing from a sender to a receiver, is interrupted by product. This change in energy is detected and turned into a switch output. This technology is ideal for extreme applications - whether that be temperature, pressure, confined space, abrasiveness or toxicity; or to avoid probes causing a blockage in themselves. Although not fail-safe as such, they have diagnostic functions and, where they are needed, they can be very effective; even cost effective, as they deliver a reliable solution where before nothing would work. Again, consulting with a manufacturer is a wise consideration, especially if the process conditions listed above prevail.
Of course, another option to explore is using a continuous level indication device, to measure and detect the level in a silo and to anticipate an approaching high or low level. Can you use this instead, or even on its own? We will explore this in the next blog: Part 4.
Before you consult your supplier, always get a good overview of the product/material; ask operators and process engineers about its behaviour; get details of the process conditions’ temperatures, pressures, and any likely changes in process conditions. Ask if it is the same product all year round; think about where and how you can install a device and its impact on the process, both in the factory and wider environment. And remember, if an option seems expensive but it's technically the best - it could be money well spent.
Have you read the previous articles on silo level monitoring? If not you can find them at:
Silo level monitoring Part 2: When a Silo High Level Alarm is not enough…
Any questions, queries or enquiries arising from this blog post, please mailto:info.uk@vega.com email us or send us your questions via our contact form