When we think about using level measurement technology for measuring solids in tanks and vessels, we often seek one device for a given application. For many situations one device fulfills all the needs of the process, but in other applications the process calls for more.
Let’s take a quick look at a liquid level measurement example, see what we can learn from it, and then apply those concepts to solids.
Level in tanks and vessels is measured to keep track of how much product is in the tank at any given time. This approach generally calls for a continuous measurement, where the reading on a scale must provide a certain degree of precision. There are also situations where it is important to know when a specific level has been crossed, to avoid overfilling a tank or drawing out product until the tank is emptied – such as a crude oil tank at a refinery that must be kept between specific limits.
The continuous reading can warn operators when an extreme situation may occur, but if the high and low readings are tied to safety-instrumented functions (SIFs), they must have their own measuring devices. These do not need to be continuous but can instead be switches which change state when immersed in liquid or solid material.
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An application where it is critical to avoid overfilling or emptying may call for five measuring devices: one continuous instrument and four switches (Figure 1). The high- and low-level switches will typically activate alarms and alert operators, whereas the high-high and low-low level switches will trigger a more drastic response, usually automatically shutting down pumps or closing valves to stop product movement before causing an incident.
Do We Need All That?
Situations where solid materials reach this degree of criticality are rarer compared to liquids, but most processes call for at least a continuous reading. Depending on how the continuous reading fits into a larger automation system, it might be possible to use it to set up alarms and auto-stop functions. However, extremely critical functions often merit the addition of level switches.
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 instrument and level switches. Since there are multiple options for both, let’s consider how to make selections.
Understanding Solid Materials
Anyone who works with powders and other bulk solids understands that they have specific physical characteristics, including:
* Bulk density
* Particle size
* Angle of repose
* Moisture absorption
When working with level instruments and switches, these characteristics often take on different considerations. For example, dielectric constant (Dk) may jump to the top of the list when working with radar instruments.
Selecting Level Switches
Typically, installers mount level switches through the wall of a tank or vessel at the critical point, such that the working parts are on the inside. If necessary, they can also be mounted from the top pointing down and extended to the appropriate level. There are numerous types of level switches available, but for all practical purposes, there are only three technologies suitable for solids applications.
Paddle-wheel switches (Figure 2) 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. The current consumption or revolutions per minute (RPM) counting of the motor indicates if it is in contact with process media.
Traditionally, paddle-wheel switches have been popular because they are inexpensive, but they often have a short service life, particularly when used to indicate the presence of solids. Units that spend most of their service life immersed in powder are in a constant stalled-motor state. 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.
Vibrating-fork switches (Figure 3) 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.
Capacitance switches (Figure 4) 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.
There is a lot of overlap among these technologies and there is no one-size-fits-all solution. Usually more than one choice is capable of solving a given challenge.
Selecting Continuous Instruments
While it’s possible that an application may call for just a level switch to warn operators of a high- or low-level condition, most applications need some kind of continuous level reading. Given the characteristics of solids, the vast majority of applications use a top-down approach, and for solids, this means radar. There are two basic kinds: guided-wave radar (GWR) and non-contact radar (NCR). GWR sends a pulse down a metal probe that extends to the tank bottom, while NCR sends its signal through the open head space with nothing contacting the product.
The probe used with GWR (Figure 5) is its greatest strength and drawback. It facilitates a positive and accurate reading where it contacts the product and it is not disrupted by obstacles inside the tank. At the same time, when working with solids it has a unique problem. Depending on how heavy and sticky the product is and how it moves when flowing out of the tank, as the product level falls it can cause a pulling force on the probe. If the product moves as a mass, it has the reverse effect of trying to pull out a tent stake. In extreme situations, it can pull the probe out of the transmitter or pull the top of the tank down. This can happen in tall grain silos where the probe is inserted deeply into the contents.
NCR has no probe to focus the radar pulse, so it reads over a wider area (Figure 6), which usually isn’t a problem. But if the surface is sloped due to a high angle of repose it can cause a weaker return signal. However, this must be drastic to cause a reading problem, so in most situations NCR can deliver a reliable measurement.
As mentioned earlier, there are product characteristics we probably don’t think about very often. One is Dk because of its involvement with radar signal reflection. We tend to think that a radar signal is reflected by a physical surface. This is true to some extent, but part of the reflectability of the surface is due to the change in Dk compared to air. In other words, if the product has a very low Dk, it does not provide a strong reflected signal, even though it appears visually to be a solid surface. So, to radar, it is invisible.
This is not a common situation, but it is frequently encountered with specific types of plastic granules for injection molding. NCR is not suitable for these products, but the challenge can be solved using GWR.
If a GWR transmitter has its probe hanging in free space, a signal pulse will be reflected by the bottom of the probe, sending an echo (Figure 7). Since the probe length is fixed, the echo delay is always the same. If the transmitter is inserted into a tank of a low Dk product normally invisible to radar, the pulse will move through the product but will be slowed down as it passes through. When the transmitter reads the return signal, the elapsed time will be longer than the normal return, and from the time extension, it is possible to determine how much product it has passed through. This probe-end projection capability can solve situations where normal return signals are too weak to support conventional measurement.
The Right Combination
As discussed, solving a solids level measurement problem may require more than one device. While a level measurement may seem simple, a precise measurement can help optimize a critical part of a larger manufacturing facility by reducing material use and keeping closer control of inventory. Plus, for safety considerations such as overfill protection, it may be necessary to add level switches to the operation.
Fortunately, there is a wide enough range of level measuring technologies available to deliver the needed performance and generate the desired benefits.
Andrew Foust is a level business development manager with Emerson (Shakopee, MN). He works with Rosemount level measurement products, focusing on radar, ultrasonic, and level switches. Foust joined the company in 2014 and has 28 years of experience in process instrumentation. For more information, call 800-999-9307 or visit www.emerson.com.