By David W. Spitzer
Measuring the level of solids such as plastic pellets, coal, coke or flour is not necessarily easy, and the difficulty starts before the measurement system is even designed. Simply put, why are you making this level measurement? Do you want to measure the level in the bin or hopper or the amount of material? These two questions may appear to be same—but their implications can be quite different.
To put things in perspective, consider the measurement of water in a tank. The water will tend to fill any voids below the surface, so its surface will appear to be horizontal. The water level is the same at all locations in the tank, so the liquid level can be measured anywhere in the tank. The liquid level measurement and tank dimensions can then be used to calculate the amount of water in the tank. In this application, the amount of water in the tank can be inferred from the level measurement. Conversely, weight measurements could be used to infer the water level.
The Solids Challenge
Measurement of solids is not so straightforward. For starters, solids typically do not flow downward as freely as water, so their level will vary in the bin. Ratholing tends to occur when solids are removed from the bin because the material is removed from the space directly above the point of removal. Material above this point will typically move downwards and out of the bin, but material farther (horizontally) from the point of removal will not readily flow to fill the void that is formed above the point of removal. Under these conditions, level measurements made directly above the point of removal will reflect neither the level nor the amount of material in the bin.
Bins are often designed with their feed points above their point of removal, so material entering the bin will tend to fill the rathole formed by material exiting the bin. However, when the rathole becomes large, its sides can collapse and fill in part or all of the rathole. In addition to reflecting neither level nor the amount of material in the bin, level measurements made directly above the point of removal can increase 10% to 20% within seconds! Weighing the bin can avoid these complications and measure the amount of material in the bin. However, ratholing and other effects can cause weight measurements to infer the level in the bin incorrectly.
Figure 1. Laser Level Measurement in a bin.
Simply put, solids level measurement systems can exhibit significant errors—even when their level transmitters measure level perfectly and without error—so the level transmitter should be strategically located to reflect the desired level or amount of material.
Bins with non-uniform shapes can complicate level transmitter location. Further, additional level transmitters may be installed in applications when accurate level measurement is required.
What Do You Want To Know?
Joe Lewis, vice-president of marketing and sales at Monitor Technologies, says users need to be clear about what they really want.
“Level measurement of bulk solids is often for inventory management purposes, and there is often a disconnect between level measurement and the expectations of people have in the user organization,” Lewis says. “Many customers want to know the amount of material, so the level measurement provides an input to calculate mass. But how accurate can the mass measurement be in a silo that (at best) is cylindrical with center fill and discharge? The accounting department expects 100% accuracy, but operations show mass balances with losses.
“I contend that the level measurement supplier does not necessarily know the accuracy of the calculated mass in the bin because so many factors outside of the level measurement device cannot be defined. A few examples include the flow characteristics in the silo, angle of repose, sensor location, material properties, and fill and discharge locations. In other words, user expectations may not be met, and there can be significant problems even if the level is measured perfectly and without error. Customer goals and expectations should be discussed before a particular level technology or supplier is selected.”
Valentine “Tiny” Sonnier, product manager for ultrasonic level products at Siemens Energy and Automation adds that, “The properties of the material cannot be underestimated. The angle of repose of a material is the angle at which a stack of material will form when filling and be maintained when emptying. Each material has its own angle of repose dependent upon its properties and, to some degree, the bin geometry. Therefore, the location of the measurement is critical because moving the measurement only a short distance can result in a different level measurement due to the effects of ratholing or bridging.”
Sonnier notes that neither contact nor non-contact level gauges can determine whether there is ratholing or bridging in the hopper. In this sense, non-contact level gauges are just as accurate as contact level gauges.
Ultrasonics and Lasers
John Mikos, instrumentation technician at ArcelorMittal I/N Kote in New Carlisle, Ind., uses ultrasonic level transmitters in powder applications. He agrees that the angle of repose and ratholing can be problems. He mitigates part of the problem by calibrating the level transmitter zero to be at or just above the top of the cone on the bottom of the hopper. By not attempting to measure the level in the cone, the non-linearity associated with the relationship between cone level and the amount of material in the cone is eliminated. The level measurement is more stable because the process doesn’t to operate in the cone where rathole and filling effects would be more prevalent.