Online Density Measurement for the AFT Tower (High-Concentration Desulfurization Slurry Processing Tower)

AFT Tower Slurry Density Reading Too High? How to Avoid Entrained Gas When Several Pumps Run Together

Chirped ultrasonic acoustic-impedance measurement for concentration monitoring and discharge interlocking on near-saturated crystallising slurry

Scope: the AFT tower (high-concentration slurry treatment tower) in power plant flue gas desulphurisation and advanced FGD wastewater treatment systems — sump circulation line, discharge line and crystalliser feed

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Sump concentration is the only on-line criterion for the timing of discharge

The AFT tower is a key item of equipment in power plant flue gas desulphurisation and advanced FGD wastewater treatment systems, commonly used for the circulating absorption, concentration or crystallisation pre-treatment of high-concentration slurry and by-product salt. In a typical process the gas enters at the bottom of the tower and passes counter-current to the slurry sprayed down from several levels; the circulation pumps repeatedly return sump slurry to the spray banks, producing intense gas-liquid mass transfer, and the concentration of the sump slurry rises steadily as a result. When the sump reaches its set concentration, the slurry must be discharged promptly to the downstream crystalliser and centrifuge, where the by-product salt crystallises out and is taken away.

This section depends on density measurement structurally — nothing can be seen inside the sump and sampling lags behind, so whether discharge is early or late rests entirely on this single quantity, the sump concentration:

▪Discharge triggered too early — the slurry is not concentrated enough, the crystalliser feed is dilute, by-product salt yield falls and specific energy consumption rises;

▪Discharge triggered too late — the sump concentration passes the process upper limit, viscosity climbs sharply, crystallisation intensifies and the risk of blocking the circulation pump and pipework rises markedly;

▪Crystalliser feed stability — fluctuations in feed density directly affect the crystal size distribution and the separation performance of the centrifuge;

▪The limits of sampling and laboratory analysis — once a near-saturated slurry sample has been drawn, any change of temperature causes further crystallisation, so the laboratory value itself carries lag and offset and cannot support discharge decisions on a minute-by-minute basis.

The slurry in this section shows the classic combination of three highs and one tendency: high concentration, high viscosity, high abrasiveness, and a strong tendency to crystallise at the measuring interface. Real-time, continuous and reliable monitoring of slurry density in the sump circulation line is the core basis for safe operation of the AFT tower and for automating the discharge sequence.

▍1.2 Limitations of Existing Measurement Methods

▪Differential pressure density meters: impulse lines block very easily in a viscous, readily crystallising slurry, and once blocked the reading "freezes" at some plausible-looking value that is hard to detect in time; stable operation is barely achievable.

▪Vibrating fork / vibrating-element density meters: when the slurry temperature changes at the measuring interface, crystals form rapidly and encase the fork tines; the vibration frequency drifts with the amount of deposit and the reading becomes falsely high in one direction, or fails altogether.

▪The common problem of insertion-type probes: hard crystals suspended in the slurry continuously abrade the leading edge of the inserted part and wear it quickly, while material build-up and crystal growth at the probe root create a new source of blockage.

▪A wide span: from the start of circulation to the moment of discharge, sump concentration can swing by tens of percentage points, so a single instrument has to combine a wide range with usable accuracy; splitting the range between two instruments creates a hand-over problem at the changeover point.

▪Gamma (nuclear) density gauges: resistant to crystallisation and erosion, but classed as radiation-based measuring equipment requiring a licence and carrying the cost of annual inspections, personnel qualification and source decommissioning and disposal; new projects now generally prefer a non-nuclear solution from the outset.

Figure 1 On-line density measurement of AFT tower sump slurry and the discharge interlock arrangement

2 The PS7000 Technical Approach

▍2.1 Measuring principle: the chirped (linear frequency modulation) acoustic-impedance method

When an ultrasonic wave travelling in a medium meets a solid-liquid interface, the echo amplitude is determined by the difference in acoustic impedance across that interface, and acoustic impedance Z = medium density ρ × sound velocity c is directly related to the density of the medium. The PS7000 uses a single self-transmitting, self-receiving sensor that continuously emits chirped (linear frequency modulation) ultrasonic pulses and acquires the echoes in real time. The wide bandwidth of the chirp signal helps the pulse penetrate bubble layers and suppresses multiple-reflection interference, and the transmitter then resolves the acoustic-impedance signature of the echo to recover the density of the medium.

The fundamental difference from the gamma-ray method is that the PS7000 contains no radioactive source of any kind. It is a non-nuclear instrument: no radiation safety licence is required, and there is no annual source inspection, no radiation-worker qualification and no source decommissioning and disposal.

▍2.2 Non-intrusive measurement: the sensor is wetted but does not protrude into the flow path

The PS7000 measuring spool is flanged into a straight run of the process line and the ultrasonic sensor is mounted on the spool. This is the key structural difference from conventional insertion-type instruments, and the point most often misunderstood during selection, so it is set out here exactly as it is worded in the Operating Manual:

▪Non-intrusive (non-intrusive is not the same as non-contact): the sensor must be wetted by the medium in order to work, but it does not protrude into the flow path — there is nothing projecting into the bore. The manual is explicit on this point: for horizontal installation with the sensor on top of the pipe, a full pipe must be guaranteed, otherwise the sensor loses contact with the liquid and the measurement fails; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe so that it stays wetted.

▪There is therefore none of the leading-edge erosion, build-up at the probe root or downstream flow disturbance associated with insertion probes, and no risk of blocking the line.

▪The acoustic window of the sensor is sapphire: a fine surface finish, wear-resistant and corrosion-resistant. The measuring spool can be supplied with a lining (PTFE / rubber / ceramic) to suit the duty.

▪Heavily scaling media should still be included in routine inspection — the manual lists "scaling on the sensor face" as one of the items to check when calibration fails, the remedy being to clean the sensor and recalibrate.

▪Applicability limits: bubbles in the medium are permitted; the medium should not, however, contain large quantities of suspended or settled coarse particles.

Figure 2 Sectional view of the PS7000 installation — the structural difference from conventional insertion types (schematic drawing of the principle, not a record of measured data)

▍2.3 Key Technical Specifications

※ The process temperature on this site is 5 ~ 20 ℃, within the 0 ~ 80 ℃ range of the standard version. AFT tower process gas is hot, and on some projects the sump slurry temperature may exceed 80 ℃ — such duties require the high-temperature custom version, and the exact upper limit should be confirmed with Pisonics at the selection stage. Wetted materials (Hastelloy, PFA coating and similar), lining and explosion protection type are equally custom items.

▍2.4 Selection and Installation Requirements (important)

The following conditions directly determine the stability of the measurement in the field. We recommend confirming them while the measuring point is still being designed; doing so avoids the great majority of post-commissioning disputes over deviation.

▪Vertical installation with upward flow is preferred. For horizontal installation, confirm that the medium is not stratified and that the measuring point is representative.

▪Flow velocity > 1 m/s; the higher the density of the solid phase, the higher the velocity required. Avoid low-velocity or stagnant duties, in which particle settling disturbs accuracy and stability.

▪Straight run requirement: the installation position should be well clear of pumps, valves, bends and other local resistances; for horizontal installation, 10D upstream / 5D downstream of straight run must be provided (5D upstream / 3D downstream as an absolute minimum).

▪Keep the sensor wetted: for integral horizontal installation with the sensor on top of the pipe, the pipe must run full; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe. Where the pipe does not run full, the remote-mount version is preferred.

▪Avoid points where free gas bubbles accumulate. Aeration, twin-pump suction, negative-pressure inlets and similar conditions introduce large quantities of free gas bubbles; the bubbles raise the echo energy and bias the reading high. This is an inherent limitation of the acoustic method and must be handled by the choice of measuring point, not by compensating in calibration.

▪The comparison sampling point should be as close as possible to the measuring point. A sampling point far from the measuring point introduces transport lag, so the laboratory reference value and the displayed reading no longer refer to the same moment, and the difference is easily mistaken for instrument error.

▪An AFT tower measuring point must avoid regions where entrained gas collects (a lesson learned in the field): on the AFT tower of one power plant FGD system, the density measuring point was on a line taken off close to the tower shell. With a single circulation pump running the reading was steady; with two pumps running, the gas entrained by the spray increased markedly and the reading rose systematically and became spiky. The reason is that the acoustic impedance method is an energy-based method: free gas bubbles and a rising solids content both shift the echo energy in the same direction, so the value alone cannot distinguish "entrained gas" from "high solids" and must be interpreted together with pump status, temperature trend and the fluctuation of echo energy. There are three counter-measures — move the measuring point to a vertical run free of bubble disturbance (or lower the sensor to a section with higher static pressure and less gas, with a DN15 ~ DN25 vent return at the top of the spool); add an entrained-gas criterion or a pump-running enable to the control logic so that the interlock is inhibited under gassy conditions; and keep flow continuous at the measuring point so that gas cannot collect in a stagnant dead end. Never use the calibration coefficients to compensate for the high bias caused by entrained gas.

▪Temperature and stagnation management for near-saturated slurry: when the medium cools at the measuring interface, crystals form and adhere to the sensor face and the reading creeps upwards in one direction. The measuring spool should be insulated and long periods of stagnation avoided; we recommend providing a flushing water connection and including sensor inspection, cleaning and subsequent re-calibration in the scheduled maintenance plan.

▍2.5 Field calibration: the gravimetric sampling method, performed online

Calibration brings the displayed value into agreement with the true density of the medium: several pairs of "instrument reading + corresponding laboratory reference value" are collected and the instrument automatically regresses the correction coefficients. The whole procedure requires no shutdown and can be carried out online provided the process is steady and the sensor is clean.

Process conditions to be confirmed, item by item, before calibration:

The most common situation in the field is that only the process medium is available and clean water cannot be introduced temporarily. In that case two consecutive acquisitions are made on the same medium (about 2 minutes each, about 4 minutes in total): the two measured points plus the virtual zero point added automatically by the system form three regression points, and the instrument uses the goodness of fit r² to cross-check the consistency of the two signals. If bubbles, a step change in flow or an abnormal valve movement during acquisition make the two data sets inconsistent, the system rejects the calibration outright rather than returning an incorrect coefficient. This is the key mechanism that prevents a calibration that looks successful but is in fact wrong.

Recommended calibration interval: calibrate a newly installed instrument once within the first 12 ~ 24 hours of operation; once operation is stable, verify every 6 ~ 12 months; verify every 1 ~ 3 months at measuring points with demanding accuracy requirements; recalibrate immediately after changing the sensor or the medium.

3 Before / After Comparison

4 Field Verification and Operating Record

On a coupled salt chemicals / power plant project in Qinghai, the AFT tower sump holds a near-saturated potassium chloride slurry; the plant reports a crystal content of 30 % ~ 40 % and a temperature fluctuating between 5 ~ 20 ℃. A PS7000 was installed on the sump circulation pump discharge line and a two-week continuous comparison test was run after commissioning: the plant reports a deviation between meter reading and manual laboratory analysis in the range 0.5 % ~ 0.8 %, with no abnormal readings during concentration peaks. The meter has run continuously since, and the density signal is used to judge when to discharge.

Another project on comparable duty provides the counter-example, which is worth considering at the design stage: when both sump circulation pumps run together and gas entrainment increases, the reading rises systematically and becomes spiky (see Section 2.4 and Q1 in Chapter 5). For gas-liquid mass transfer equipment such as an AFT tower, the choice of measuring point matters more than the calibration — if the location is wrong, calibration only freezes the error in place.

▍Verifiable Benefits for the Customer

▪No licence application, annual inspection, radiation-worker qualification training or source decommissioning and disposal for radiation-based measuring equipment.

▪With a continuous on-line value for sump concentration, discharge timing has moved from manual laboratory analysis and experience to judgement by density trend and threshold (the threshold should include hysteresis and a time delay).

▪There are no longer any parts protruding into the flow path or any impulse lines on the circulation line, which removes the cleaning and replacement work caused by crystal encasement and blockage.

▪The plant reports a deviation of 0.5 % ~ 0.8 % over two weeks of continuous comparison and no anomalies during concentration peaks, which makes the signal usable as a reference input for crystalliser feed decisions.

5 Frequently Asked Questions

Q1 AFT tower slurry carries gas by nature — will the reading be biased high?

A Yes, it will — this is a limitation of the principle that has to be stated plainly. We met it on the AFT tower of one power plant FGD system: the measuring point was on a line taken off close to the tower shell, and with one pump running the reading was steady, while with both circulation pumps running it rose systematically and became spiky. The mechanism is that spray entrainment increases as circulation intensifies, the gas enters the take-off with the slurry, static pressure falls in the rising run so the bubbles expand, and they pass across the sensor at exactly that point; when flow stops, gas also collects in the dead end. Because the acoustic impedance method is an energy-based method, entrained gas and a high solids content shift the echo energy in the same direction and the density value alone cannot separate the two, so the reading has to be interpreted together with the process conditions. Usable criteria include which pumps are running, the temperature trend (jumps caused by entrained gas are usually not accompanied by a change in the temperature trend) and the amplitude of the echo energy fluctuation. The remedies are to move the measuring point to a vertical run free of bubble disturbance, or to lower the sensor to a section with higher static pressure and fit a vent return at the top, together with an entrained-gas criterion or a pump-running enable in the interlock logic. Please do not try to compensate for a gas-induced offset with the calibration coefficients — that only puts the previously correct single-pump reading out of calibration.

Q2 A near-saturated slurry will crystallise on the sensor face — how is that handled?

A Structurally the sensor does not protrude into the flow path, so it is not subject to the leading-edge erosion of an inserted probe; crystal adhesion, however, still occurs, particularly when the temperature at the measuring interface falls or the slurry stagnates for a long period. It shows up as a reading creeping upwards in one direction, which is easily mistaken for a rising process concentration. The engineering measures are to insulate the measuring spool, avoid stagnation and dead ends, provide a flushing water connection, and include sensor inspection in routine maintenance. The manual also lists "scaling on the sensor face" as one of the items to check when calibration fails — if calibration fails, clean the sensor first and then re-calibrate. This is not a maintenance-free duty; inspect it on a fixed schedule.

Q3 Can one meter cover the whole concentration span from circulation to discharge? Is the accuracy sufficient?

A On range, 0 ~ 80 % is covered by a single instrument, and ± 1 % FS (published in the datasheet as ± 1 % concentration by weight) is a full-range figure. In practice the calibration points are taken across the working range: one at the low end during circulation and one at the high end at discharge, so that the calibration curve spans the real span instead of extrapolating both ways from a single middle point. If the discharge trigger point requires finer resolution, add a calibration point close to that trigger. One further reminder: the discharge interlock threshold should have hysteresis and a time delay so that a spike caused by entrained gas cannot cause a spurious trip — this is the other side of the first question above.

Q4 Does the sensor actually touch the medium? Will it be worn away?

A It does touch it. The manual's formal wording is "non-intrusive", not "non-contact" — the sensor has to be wetted by the medium to work (with horizontal installation and the sensor on top, the pipe must run full or the measurement fails), but it does not protrude into the flow path and there is nothing projecting into the bore, so the main wear mechanism of insertion probes, the leading edge being continuously scoured by fast-moving solids, does not exist. Please note that some material in circulation describes this arrangement as "non-contact", which is not accurate. Within the Pisonics range, the products that genuinely do not touch the medium are the PS7010 clamp-on type (acoustic attenuation method) and the nuclear gamma-ray density gauge.

Q5 How is it calibrated in the field? Is a shutdown needed? How often?

A By the gravimetric sampling method, carried out from the handheld remote menu, with no shutdown — calibration can be done online provided the pipe is full, flow variation is within ± 5 %, no venting is in progress, the temperature is steady and the sensor is clean. Where only one medium is available on site, two consecutive acquisitions are made on that medium (about 4 minutes in total) and the system cross-checks the consistency of the two signals with r², rejecting the calibration outright if they disagree. Interval: first calibration within 12 ~ 24 hours of installation, verification every 6 ~ 12 months once stable, every 1 ~ 3 months for high-accuracy points, and immediate recalibration after changing the sensor or the medium.

Q6 Should the PS7000 or another model be selected for a given measuring point?

A The PS7000 (ultrasonic acoustic impedance) is aimed at the density of bubble-laden, high-solids slurries. If the pipe cannot be tapped, or an ageing line is being retrofitted, choose the PS7010 clamp-on type (acoustic attenuation); for concentration analysis of clean liquids choose the PS7020 (sound velocity); for scaling-prone acids and alkalis in chemical service choose the PS7400 (vibrating fork); for large-bore mains above DN200 the PS7300 (differential pressure) is an option; for custody-transfer accuracy choose the PS7200 (Coriolis); for multi-component concentration analysis choose the PS7100 / 7110 (spectral / refractometric); for aqueous suspensions the PS7600 (microwave) is available. If the selection is not clear-cut, send us the process data and we will work it out for you.

About Pisonics

Xi'an Pisonics Information Technology Co., Ltd. (PISONICS) specialises in ultrasonic inline measurement technology, with products covering slurry density, solution concentration, spectral concentration, level, flow and other industrial process parameters. The PS7000 Series Ultrasonic Slurry Density Meter is the company's own non-nuclear inline slurry density monitor, and is in service in power plant desulphurisation, non-ferrous and ferrous mineral processing, coal preparation, salt lake chemicals, dredging, municipal sludge and other industries.

Ultrasonic · Density · Concentration · Inline Measurement Specialists

※The process data and operating records in this case study are compiled from an actual project; Figures 1 and 2 are schematic diagrams of the principle and the layout, not curves recorded from measurement. The technical specifications of the instrument and its installation and calibration requirements are as stated in the Pisonics Ultrasonic Density Meter Operating Manual supplied with the goods, and are subject to change without notice.