LTM-2 Conductive Slurry Level Monitoring in Sumps and Tanks

December 18, 2014 by Bill Hancock 0 comments

Obtaining consistently accurate and rapid slurry sump and tank level measurement is important for proper process control in mining and process industries. Particularly where consistent feed control is required for downstream processing stable operation. The pumps fed from the sumps and tanks are now typically variable speed controlled by VFD using a sump level measurement as feedback. Accurate and responsive slurry level measurements are often challenging and requires ongoing process operator and engineering attention.

If the slurry pulp level measurement signal provided from a level measuring device is inaccurate, slow responding and/or an inconsistent measurement of the slurry interface level, the pump can cycle leading to variable slurry flow surges that typically cause process imbalances and disruptions, oftentimes resulting in severe negative impacts on process results. Downstream process unit operations are most capable when operated at close to steady state. Proper sump level and pumping control is a very important component in overall plant process control and process performance, the importance of which is amplified when there are multiple sumps circulating slurry streams in a process.

Successfully managing sump-pump operation entails several considerations, the most important as follows:

  1. Froth or foam on the sump surface interferes with level monitoring equipment such as ultrasonic and radar which detects the froth-foam surface, not the underlying slurry interface. Further, the froth and foam do not provide good sonic or radar reflective surfaces needed so the level indication signal can be poor.
  2. Varying pulp depth and % solids significantly negatively impacts pressure differential devices so these are very rarely used.
  3. Ideal sump operation is to establish a setpoint (SP) level as sump % slurry depth consistently operating within a 5-10% of SP. This requires near instantaneous slurry level reading and good VFD-pump capabilities that uses the level measurement signal to control the pump speed/flowrate.

Ultrasonic and radar slurry level measuring devices beam waves in a conical shaped pattern to the sump-tank liquid surfaces and actual level measurement involves the entire circle where the incident wave hits the slurry-water-froth-foam surface. It is not at a point so in highly agitated sump-tank the measured surface is not even or level. Consequently the reflected wave back to the ultrasonic and radar device provides a range of levels resulting in highly variable signal and interference.

LTM-2 Probe Installation-2To massage and reduce the ‘noise’ of ultrasonic and radar signals to better approximate actual sump-tank level, the signals are integrated (rolling averaged) typically ranging from 1-3 seconds based on specific conditions and operator preferences. While this signal dampening reduces the signal ‘noise’, by definition the integrated data signal will always be an averaged reading from the immediate past, not an instantaneous signal of the immediate present level. The result is the sump pump will always be ramping up-slowing down based on past sump-tank level information that often leads to cycling and too often sump-tank overflowing and conversely emptying the sump-tank and, in the case of centrifugal pumps, cavitation.

LTM probes were initially developed for slurry and pulp interface level monitoring in challenging applications such as mineral froth flotation, beverage and wood pulp mill processing where froth or foam at the top of the slurry and pulp level can interfere with accurate level measurement. Characteristics and features of LTM level monitoring and level measurement probes include:

  1. Measures the slurry water interface by conductive principles
  2. Effectively distinguishes the froth-foam and slurry/pulp level interface level
  3. Highly accurate readings
  4. Fast response – provides 10 level signals/second live so for practical purposes is instantaneous
  5. Ideal for adhesive and pasty materials
  6. Good performance even when moderately scaled – measures levels at probe conductivities greater than 1 µS/cm
  7. Probes are supplied up to 3 meters in length
  8. Easy to install, setup and use

More product information is available on the LTM-2 website: Further technical details are provided in the LTM-2 Data Sheet.

LTM-2_Probe-InstallationWhile the probe initially is naturally first considered for flotation cells due to its ability to distinguish between slurry-pulp and froth-foam, LTM probe technology is as effective in sumps and tanks whether these are covered with froth-foam or not versus alternative products. The key characteristics that make LTM probes ideal are accurate slurry-pulp interface identification and measurement and rapid response. Sump level monitoring can be troublesome and inconsistent with ultrasonic, radar and pressure differential technologies. Customer applications have provided the effectiveness of LTM technology in sumps explained as follows:

Very fine cyclone feed sump: A PGM producer makes a 10 micron particle split of flotation tails in their cleaner scavenger circuit. A 6 foot deep sump-pump feeds a cluster of 4 inch diameter cyclones where consistent slurry feed is crucial for maintaining an optimized size split. The ultrasonic level monitoring system that was being used was inadequate because the response was too slow and was also negatively impacted by periodic froth on the slurry surface. The sump level was very difficult to maintain and despite having a 70% sump level setpoint the sump multiple times a day overflowed and/or emptied (with pump cavitation) making consistent cyclone operation a real challenge.

Customer installed a 1.5 meter long LTM probe and improved sump level control results were immediate. At a setpoint of 70%, the sump level was controlled tightly in a 62-74% range for the entire first 30 days the probe was in service. And has continued providing excellent service for over the last 1 ½ years.

Secondary mill discharge sump: A large iron ore mill that was using ultrasonics level measurement was experiencing sump level control issues in their secondary mill sumps due to periodic foam build up in the sump as well as generally inconsistent and variable readings due to operating conditions. These sump-pumps feed cyclones to make a -25 µ (micron) size split so consistent operation is required to ensure consistent cyclone operation. Further, due to flowrate variability downstream, the slurry density meters were running erratically.

The mill installed a 3 meter LTM probe and after running a few days in slave mode, they put the probe into control mode and the improved sump level control was apparent right away, with the sump levels more consistent due the more accurate and instantaneous level signal from the LTM probe. The mill gave the probe the ‘acid test’ by pouring some foaming chemical into the sump which caused the sump to overflow foam and yet the sump level held steady. Under severe foaming conditions such as this the ultrasonic level measurement would have failed.

Consequently, due to the improved sump level control accuracy and consistency resulting from using the LTM level monitoring probe vs. ultrasonic measurement, this iron ore producer is now in the process of replacing the ultrasonic devices with LTM probes in all 22 mill discharge sumps.

It is worthwhile to make note that due the typical higher slurry densities and coarser particles in mill discharge boxes, wear has not been found to be a significant issue after using in multiple applications. The probe itself is made of surgical grade 316 stainless steel which has great wear and abrasion resistance properties. In only one mine application globally has abrasion wear been apparent due to very coarse particles, high solids and high slurry turbulence. If an issue, abrasion wear can be mitigated using deflector plates, pipes, etc. placed to minimize impact on particle suspension.

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Categories: LTM-2 Probe, Pulp Level, Slurry Level, Sumps, Tanks