Typical Applications of Slurry Density Measurement in the Iron and Steel Metallurgy Industry

Measuring BOF OG Sludge Slurry Density: The Selection Limits of a Highly Abrasive 80~90℃ Duty

The ultrasonic acoustic-impedance method in place of differential pressure density meters — highly abrasive iron-bearing dust sludge and high-temperature slurry duties

Scope: iron and steel works — the discharge line from the sludge collection sump of basic oxygen furnace (BOF) gas wet cleaning (OG process), the thickener feed line, and the recirculating slurry lines of sinter flue gas desulphurisation absorbers

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Two density control points in wet slurry handling at a steel works

The iron and steel industry is a priority sector for air pollution control, and its wet slurry handling is concentrated in two areas. The first is sinter flue gas desulphurisation, mostly by the limestone-gypsum wet process or a semi-dry process; the principle is close to that of power plant FGD, but sinter flue gas carries a high dust load and a high SO₂ concentration with sharp temperature swings, which makes the duty considerably more severe. The second is wet cleaning of BOF gas, predominantly by the OG process: venturi scrubbing and a secondary cooling tower wash out the iron-bearing dust sludge, which passes to a sludge collection sump and is then dewatered through a thickener and vacuum filters and returned to the sinter blend as an iron-bearing raw material.

Slurry density is a process control parameter in both sections — the density of the recirculating absorber slurry governs desulphurisation efficiency and the quality of the gypsum by-product, while OG sludge slurry density governs thickening and filtration performance and hence the recovery of the iron-bearing sludge. Compared with power plants, the difficulties of the steel works duty are concentrated in the following points:

▪Severe abrasion: OG sludge slurry has a high proportion of iron-bearing dust (site data commonly show an iron content above 50 %), with particles heavier and harder than those in gypsum slurry, so abrasion of wetted measuring components is markedly more severe;

▪High temperature: OG sludge slurry runs continuously at 80 ~ 90 ℃, above the 0 ~ 80 ℃ medium temperature range of the standard instrument, so the high-temperature custom version must be selected — this has to be confirmed at the enquiry stage and cannot be ordered as a standard unit;

▪Strong acidity: once the scrubbing water has absorbed the acidic components of the flue gas the pH is commonly 2 ~ 3, which combined with the high temperature accelerates ageing of seals and corrosion of materials;

▪Site environment: heavy dust, strong vibration and a complex electromagnetic environment; the BOF gas area also carries hazardous-area classification requirements.

In addition, several steel groups have written the reduction of nuclear sources into their equipment management plans, so existing gamma-ray density gauges are being decommissioned to a schedule, and the demand for non-nuclear replacements follows from this.

▍1.2 Limitations of Existing Measurement Methods

▪Differential pressure / diaphragm density meters: the diaphragm is in direct contact with the slurry. In a highly abrasive medium such as OG sludge slurry, this plant reported that the original differential pressure density meters were worn through and scrapped after about four months on average, and perforation can also cause leakage; in sinter desulphurisation recirculating slurry the main problems are blocked pressure tappings and zero drift, requiring frequent shutdowns for maintenance.

▪Vibrating fork / rotary density meters: the tines or the rotor protrude into the flow path, the leading edge is continuously scoured by the iron-bearing dust sludge and material builds up at the root; in dusty, gas-laden desulphurisation recirculating slurry the reading jumps and is difficult to use reliably in a control loop.

▪Gamma (nuclear) density gauges: the measurement performs well, but a radiation safety licence, annual inspection, radiation-worker qualification and source decommissioning and disposal are all required. Under group plans to reduce nuclear sources, new installations are restricted and existing ones face scheduled decommissioning.

▪The combination of high temperature and strong acid: 80 ~ 90 ℃ together with pH 2 ~ 3 shortens the actual service life of insertion instruments still further below the values found at ambient temperature — life data from other industries cannot be carried across to OG sludge slurry.

Figure 1 Schematic layout of the two typical slurry density measuring points in iron and steel works (left: sinter flue gas desulphurisation recirculating slurry; right: BOF OG sludge slurry discharge and thickening)

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 above the 0 ~ 80 ℃ range of the standard version, so the high-temperature custom version must be selected; please confirm the exact upper limit with Pisonics at the selection stage.

▍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.

▪The OG sludge slurry measuring point should be clear of the section immediately downstream of the venturi and the cooling tower: the slurry there carries large quantities of scrubbing bubbles and the flow pulsates strongly. We recommend moving to a stable section at the discharge of the sludge sump pump, or to the vertical upward-flow section of the thickener feed line.

▪High-temperature duty must be declared separately at the selection stage: where the medium temperature exceeds 80 ℃ the high-temperature custom version must be selected, and the corrosion resistance of the wetted materials and the lining to pH 2 ~ 3 must be confirmed at the same time. Note also that the slurry temperature must be steady during calibration — a change in temperature affects the measurement signal, and calibration points should not be taken while the temperature is swinging widely.

▍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

The PS7000 is in service in the sinter desulphurisation and BOF OG cleaning sections of several Chinese steel works. Feedback from an application on the OG sludge slurry discharge line at a large coastal steel works: the original differential pressure density meters were worn through by the iron-bearing dust sludge and scrapped after about four months on average; since being replaced by a PS7000, the instrument had run for 18 months up to the time of the report without failure, and density data for the sludge thickening stage has become continuously available.

The selection basis needs to be stated explicitly: the medium temperature at this point is 80 ~ 90 ℃, above the 0 ~ 80 ℃ medium temperature range of the standard version, so what was actually supplied was the high-temperature custom version. For equivalent duties the maximum medium temperature, the size of the temperature swings and the wetted materials must be confirmed at the enquiry stage; the standard specification cannot be used to select the instrument directly — this is the most common oversight when specifying instruments for the OG section.

Calibration and verification during operation were carried out online by the gravimetric sampling method. It is worth noting that the representativeness of OG sludge slurry samples is strongly affected by settling of the sludge: the sampling point should be as close as possible to the measuring point and the sample should be taken at the same moment as the displayed reading is recorded, otherwise the deviation introduced by transport lag and by stratification within the sample is easily mistaken for instrument error.

▍Verifiable Benefits for the Customer

▪The OG sludge slurry measuring point has escaped the quarterly cycle of diaphragms wearing through and being replaced; the plant reports 18 months in service without instrument failure, and the associated shutdowns for replacement and the spares consumption have been eliminated with it.

▪The absorber recirculating slurry measuring point no longer requires monthly shutdowns for blocked pressure tappings and drift, and the density signal can be used continuously for monitoring desulphurisation operation.

▪The non-nuclear solution fits the group plan to reduce nuclear sources: new measuring points need no radiation safety licence, annual inspection or source decommissioning and disposal, and a flameproof version is available for the BOF gas area.

▪With OG sludge slurry density continuously available, the thickening and filtration stages now have stable process data to work from, and process adjustments no longer depend solely on intermittent sampling and laboratory analysis.

5 Frequently Asked Questions

Q1 OG sludge slurry is at 80 ~ 90 ℃. Can the standard version be used? How is a high-temperature duty specified?

A The standard version cannot be used directly. The medium temperature range of the standard version in the manual is 0 ~ 80 ℃ (ambient temperature -30 ~ 60 ℃), and OG sludge slurry runs continuously at 80 ~ 90 ℃, which is already outside that limit, so the high-temperature custom version must be selected. The exact temperature limit, the sensor and wetted materials and the lining have to be confirmed item by item against the actual duty at the selection stage — please provide the maximum medium temperature, the size of the temperature swings, the pH, the solids content and the particle size distribution. A related point: if you see a temperature rating quoted in selection material without the conditions it applies to, ask for the corresponding model number and the source of the figure — our position is consistently that the standard version is rated 0 ~ 80 ℃ on the medium, and that anything above this goes through a high-temperature custom build confirmed order by order, with no blanket commitments. High temperature also affects calibration — the manual lists "steady temperature" as one of the preconditions for calibration, and calibration points should not be taken while the slurry temperature is swinging widely.

Q2 The iron-bearing dust sludge is extremely abrasive. How long will the sensor last?

A Start with the geometry: the PS7000 sensor is mounted on the measuring spool, wetted by the medium but not protruding into the flow path, with 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 apply. That is exactly the structural difference from a differential pressure diaphragm or a vibrating fork in OG sludge slurry duty, and the reason the differential pressure type wore through in about four months while this measuring point has run continuously. But not facing the flow does not mean no wear at all: near-wall flow still erodes the spool bore and the sensor face, so the sensor is made from a specially selected material and the measuring spool can be supplied with a lining to suit the duty. We do not quote a guaranteed service life in years — the wear rate depends on velocity, solids content, particle size and hardness, and a life figure divorced from the duty is meaningless. The 18 months without failure reported by the steel works above can be taken as a reference for equivalent duties, not as a guaranteed value. We recommend including the condition of the sensor face in routine inspection and, if scaling or deposits are found, rinsing with clean water and re-verifying the calibration.

Q3 The sinter desulphurisation absorber is full of bubbles and heavily dust-laden. Where are the limits of the measurement?

A There are two cases to distinguish. Fine dispersed bubbles, the fraction left in the recirculating slurry after oxidation air agitation — the manual explicitly permits bubbles in the liquid and the wideband chirp signal penetrates this kind of bubble well, so the measurement is normally stable; this is also the advantage over differential pressure tappings. Large quantities of free gas bubbles — for example a measuring point taken directly from the oxidation air sparging zone, from a region where slurry falls and entrains air inside the absorber, or from the negative-pressure inlet of a recirculation pump — will raise the echo energy, showing up as a systematically high reading with fluctuation. This is an inherent limitation of the acoustic-impedance method; it cannot be compensated with calibration coefficients and can only be avoided by the choice of measuring point, preferably a stable section downstream of the recirculation pump discharge or a vertical upward-flow line. Sinter desulphurisation slurry also carries a high dust load: if the medium contains large quantities of suspended or settled coarse particles the instrument is not applicable, which the manual states explicitly. Measuring points with a high risk of coarse particles or scaling should be discussed with us while the point is being chosen, rather than by adjusting parameters after commissioning.

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.