Online Measurement of Gypsum Slurry Density in Absorption Towers

FGD Gypsum Slurry Density Never Reads Right? The Causes of Bubbles and Post-Shutdown Settling, and What to Do About Them

WFGD field practice: the ultrasonic acoustic-impedance method replacing differential pressure, vibrating fork and gamma-ray density meters

Scope: wet flue gas desulphurisation (WFGD) at thermal power plants — the absorber slurry pool, the gypsum discharge pump outlet and the gypsum hydrocyclone feed line

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Gypsum slurry density governs by-product gypsum quality and the slurry balance in the absorber

Wet flue gas desulphurisation is currently the mainstream desulphurisation process at coal-fired power plants. Raw flue gas enters the absorber from the bottom and passes countercurrent to limestone slurry sprayed from above; the SO₂ is absorbed by the CaCO₃ in the slurry and is then force-oxidised by the oxidation air blowers in the slurry pool at the base of the tower to form gypsum (CaSO₄·2H₂O). When the gypsum solids content in the slurry pool has built up to the process setpoint (typically 20 % ~ 30 %), the gypsum discharge pump starts and sends the slurry to the hydrocyclones and the vacuum belt filter, where the by-product gypsum is separated out.

The slurry density on the gypsum discharge line is the only continuously available solids concentration signal in this cycle, and it constrains four things at once:

▪How far gypsum has built up in the slurry pool — this sets when the gypsum discharge pump should start and stop. Discharging before enough gypsum has accumulated makes dewatering difficult, while allowing it to build up too far aggravates scaling in the absorber and wear on the equipment;

▪Solids loading on the discharge line — this sets the concentration of the material sent to the hydrocyclones and the vacuum belt filter, and directly affects the moisture content and purity of the by-product gypsum;

▪Slurry concentration balance in the absorber — the quantity discharged and the quantity of fresh slurry added have to match each other, and a distorted density signal upsets this balance, in severe cases triggering knock-on failures such as blocked hydrocyclones or a belt filter running off-track;

▪A leading indicator of gypsum quality — moisture content and purity have to be sent to the laboratory and the results come back late; the density on the discharge line is the leading indicator that operators can see in real time.

The FGD absorber area is a demanding environment for inline density measurement: the oxidation air blowers deliver air into the slurry pool continuously and several slurry recirculation pumps draw from it at the same time, so large quantities of free gas bubbles are present in the slurry at all times; gypsum slurry is itself abrasive and prone to scaling, and the medium temperature stays at 50 ~ 60 ℃ all year round. Whether a measurement solution succeeds here depends far less on the nominal accuracy of the instrument than on whether the measuring point has been chosen correctly.

▍1.2 Limitations of Existing Measurement Methods

▪Gamma (nuclear) density gauges: the measuring accuracy meets the requirement, but they are radiation-based measuring instruments requiring a Radiation Safety Licence from the ecology and environment authorities, together with annual inspection, qualification training for radiation workers and the cost of source decommissioning and disposal. To simplify management and compliance approval, power plants generally want to phase them out.

▪Vibrating fork / vibrating element density meters: the absorber slurry is agitated by the oxidation air and carries large quantities of fine bubbles; the vibration frequency of the tines is disturbed by the bubbles, the data jump frequently, and the signal is difficult to use reliably in a control loop.

▪Differential pressure / diaphragm density meters: the flanged diaphragm is in direct contact with strongly oxidising, chloride-bearing gypsum slurry; after long service the diaphragm surface scales, corrodes and wears, and zero drift is pronounced. Bubbles collecting in front of the diaphragm introduce further error, and shutdowns for cleaning and recalibration are required.

▪Manual sampling and laboratory analysis: limited in both representativeness and timeliness. A single analysis often takes several hours, which cannot support decisions on starting and stopping the gypsum discharge pump, so it can only serve as a means of comparison against an inline instrument.

Figure 1 Schematic process layout for inline density measurement of absorber gypsum slurry (the measuring point is on the straight run at the gypsum discharge pump outlet)

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 medium temperature of this duty is 50 ~ 60 ℃, inside the 0 ~ 80 ℃ range of the standard version but already close to the limit; the heat dissipation conditions for long-term operation should be checked at the same time during selection. Where the absorber slurry has a high chloride content, the spool lining and the sensor material should be built to suit the actual water chemistry.

▍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.

▪FGD measuring points should preferably be on a stable straight run at the gypsum discharge pump outlet: a point near the absorber wall or around the slurry pool is subject both to aeration from the oxidation air blowers and to suction from the negative-pressure inlet section of the slurry recirculation pumps, both of which carry large quantities of free gas bubbles into the medium. Bubbles raise the echo energy, which shows up in the field as a reading that is high overall with pronounced spikes. This is an inherent limitation of the acoustic method: it cannot be compensated with calibration coefficients and can only be avoided by the choice of point. When laying out the measuring points, keep clear of the area directly above the sparging zone and of the pump inlet section, and take a stable straight run at the gypsum discharge pump outlet instead.

▪We recommend using the pump running signal to enable the measurement: once the gypsum discharge pump stops, the slurry in the line becomes stagnant and begins to settle, solids gradually build up around the sensor, and the reading creeps slowly upwards — which is easily mistaken for a genuine rise in concentration. There are two criteria in the field: first, the medium temperature falls monotonically at a constant slope (the slurry in the line cooling naturally, rather than a process change), and second, the fluctuation in echo energy disappears. When both appear together, the medium is no longer flowing and the data from that period should not be used for control or for performance assessment. The engineering solution is to wire the pump running contact into the control system and trust the density value only while the pump is running.

▍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

At a combined heat and power plant in Inner Mongolia, the differential pressure density meter previously in use gave large errors because of heavy froth in the upstream pipework and scaling on the sensor, and the unit had repeatedly drawn complaints from downstream customers over variation in gypsum quality. After changing to a PS7000, the instrument was installed on the main gypsum discharge pump outlet line, reusing the mounting position of the original differential pressure meter. Several rounds of comparison were made by the gravimetric sampling method; the displayed values stayed consistent with the manual laboratory results within the deviation band acceptable to the process, the drift caused by bubbles and scaling was resolved, and the instrument has now run continuously for several years.

The real lessons at this measuring point came from choosing its position. At this plant and at similar FGD sites we have repeatedly met installations sited near the absorber wall: aeration from the oxidation air blowers and suction at the negative-pressure inlet of the slurry recirculation pumps carry large quantities of free gas bubbles into the medium, the bubbles raise the echo energy, the reading is high overall with frequent spikes, and no amount of adjusting the calibration coefficients will level it out. Once the point was moved to a stable straight run at the gypsum discharge pump outlet, the reading returned to a steady value. The other phenomenon that recurs is the reading creeping slowly upwards after the pump stops — that is solids building up around the sensor as the stagnant slurry settles, identified by the temperature falling at a constant slope and by the disappearance of the energy fluctuation, and finally resolved by using the pump running signal to enable the measurement. Both of these are now standard siting requirements for our FGD installations.

▍Verifiable Benefits for the Customer

▪Density on the gypsum discharge line can be observed continuously, and the timing of starting and stopping the discharge pump has moved from operator judgement to a density threshold (plant feedback).

▪No licence application, annual inspection, personnel qualification training or source decommissioning and disposal for radiation-based instruments, which reduces the compliance management burden on the FGD island.

▪Shutdowns to clean and recalibrate differential pressure diaphragms have been eliminated, reducing unplanned downtime.

▪Once filtered by the pump running signal, the density trend on the DCS can be used directly for after-the-event investigation of variation in gypsum quality.

5 Frequently Asked Questions

Q1 The oxidation air blowers run constantly and the absorber slurry is full of bubbles. Can ultrasound still measure it?

A There are two cases. Fine dispersed bubbles: the wideband chirp signal penetrates them well and the manual states explicitly that "bubbles in the liquid are permitted", which is a clear improvement over the continuous jumping of a vibrating fork in frothy duties; measurement is normal in this case. Large quantities of free gas bubbles: directly above the oxidation air sparging zone, or in the negative-pressure section at a slurry recirculation pump inlet, for example, the bubbles raise the echo energy and the reading is high overall with spikes — this is an inherent limitation of the acoustic method. We do not claim to eliminate it, and we do not recommend compensating with calibration coefficients, because a coefficient derived that way only holds at the gas content prevailing at the time. The correct response is to avoid it by siting: move the measuring point to a stable straight run at the gypsum discharge pump outlet. Send us the pipework layout drawing while the point is being chosen and we will work it out for you.

Q2 Why does the density reading creep upwards as soon as the gypsum discharge pump stops?

A That is stagnant settling, not a genuine rise in concentration. Once the pump stops, the slurry in the line no longer flows, the solids settle under gravity, the local solids content around the sensor rises progressively, and the reading creeps slowly and monotonically upwards. Two criteria confirm it in the field: first, the medium temperature falls monotonically at a constant slope — the signature of the slurry in the line cooling naturally, since process variation is never that regular; second, the fluctuation in echo energy disappears — the energy signal from a flowing medium always varies, so an end to that variation means the medium has come to rest. When both hold at once, the period can be identified as stagnation after the pump has stopped. The engineering remedy is to wire the running contact of the gypsum discharge pump into the control system and trust the density value only while the pump is running, holding the last valid value or flagging the reading invalid while the pump is stopped; this neither misleads the operators nor contaminates the historical trend.

Q3 Gypsum slurry at 50 ~ 60 ℃ with a high chloride content — will the meter have problems in long-term service?

A On temperature: 50 ~ 60 ℃ lies within the 0 ~ 80 ℃ process temperature range of the standard version, so the meter can be used directly. Two points still need attention. First, the process is already close to the upper limit, so the ambient heat-dissipation conditions around the measuring spool should be checked at the selection stage. Second, a change in temperature itself changes the sound velocity and therefore the measurement signal, so calibration requires a stable temperature and the calibration should be re-checked after any large change in process temperature. On corrosion and scaling: chloride-bearing gypsum slurry imposes material requirements, and the lining of the measuring spool and the sensor material must be determined at the selection stage from the actual water analysis. The sensor uses a specially selected custom material with a smooth, scaling-resistant surface, and because it does not protrude into the flow path there is no leading-edge erosion mechanism. We do not, however, claim maintenance-free operation — the manual lists "scaling on the sensor face" as one of the items to check when calibration fails, so inspection and flushing of the sensor face should be a routine part of shutdown maintenance.

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.