How Do You Measure Discharge Density on a Cutter Suction Dredger? Shipboard Answers to Coarse Particles and Partly Filled Pipes
Shipboard engineering practice: chirped ultrasonic acoustic-impedance measurement replacing a nucleonic density gauge
Scope: dredgers including cutter suction dredgers, trailing suction hopper dredgers and sand pumping vessels — the main discharge line at the dredge pump outlet, for port expansion, channel maintenance, river and lake desilting and land reclamation
▍Project Snapshot / Process Conditions
Customer industry | Dredging / cutter suction dredger (port and channel construction) |
Region | A major coastal port expansion project in China |
Measured medium | Dredged sand and silt slurry (discharge line at the dredge pump outlet) |
Medium temperature | Site ambient (instrument rating 0 ~ 80 ℃) |
Pipe size / installation | DN700 ~ DN1000 discharge line; measuring spool flanged into a straight run, non-intrusive sensor, remote version where the pipe may run partly full |
Range / accuracy | 0 ~ 80 % concentration by weight, ± 1 % FS (published in the datasheet as ± 1 % concentration by weight), calibrated on the actual medium; density output also available |
Field calibration | Gravimetric sampling method; on-line calibration with no need to stop the vessel |
Replaces | Nucleonic (gamma) density gauge |
Operating record | One year in service, the vessel still running (as reported by the customer) |
1 Process Background and Measurement Challenges
▍1.1 Production = discharge density × discharge flow
Dredging is central to port construction, channel maintenance, land reclamation and river and lake desilting. A cutter suction dredger cuts submerged deposits with its cutter head, the dredge pump draws the soil-water mixture in, and the discharge line carries it over long distances to the reclamation area or spoil ground. The output of the whole working chain — cubic metres of soil per unit time — is set jointly by "discharge slurry density × discharge flow", which makes discharge density both a production indicator and a safety constraint on the dredge pump and pipeline:
▪Density too low — too little soil in the mixture, so most of the diesel and pump power goes into moving water; output per unit time falls and energy per cubic metre rises;
▪Density too high — slurry resistance and the required head rise together, deposits and blockages form easily in a long discharge line, and in severe cases the pump stalls against a plugged line and has to be stopped for back-flushing;
▪Instantaneous changes in density — show whether cutting depth, swing speed and soil type are matched, and give the bridge a direct basis for adjusting ladder depth and swing speed;
▪Parallel discharge lines — the difference in density between lines shows whether the soil-water split is balanced, and is the first-hand signal for tracing a blockage on one side.
The working density band generally used in the dredging industry is 1.15 ~ 1.35 g/cm³. This band varies considerably with soil type (silt, fine sand, medium to coarse sand, clay lumps) and discharge distance, so it should be based on trial dredging data from the vessel and project concerned rather than taken directly from another vessel's experience.
▍1.2 Limitations of Existing Measurement Methods
▪Nucleonic (gamma) density gauges: carrying a radioactive source on board involves waterway transport permits, crew radiation protection, annual inspections and qualification training; cross-border work (projects in South-East Asia, Africa, the Middle East and elsewhere) also requires import and use approvals from the destination country, which take a long time and are difficult to amend when the vessel is redeployed. Radiation safety licence approvals for new domestic projects have also tightened in recent years.
▪Differential pressure / diaphragm density meters: the diaphragm is directly exposed to sand-bearing slurry, dredged sand is highly abrasive, and field reports generally give diaphragm service lives of less than 6 months; once the diaphragm is worn through, slurry enters the pressure chamber and the meter fails completely, and replacement on board means stopping work.
▪Insertion-type instruments such as vibrating forks and rotors: a part protruding into the flow path of a coarse-particle discharge line is both a target for erosion and a point for material build-up, and in a large-bore pipe it also creates local turbulence that disturbs the downstream flow pattern.
▪Constraints common to shipboard installations: dredge pump vibration, salt spray on deck and electromagnetic interference from variable-frequency drives act together, so onshore installation practice cannot simply be copied; the signal transmission distances between bridge, engine room and deck also govern the choice of communication method.
Figure 1 Arrangement for on-line density monitoring of discharge slurry on a cutter suction dredger
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
Item | Specification | Item | Specification |
Measuring principle | Chirped ultrasonic acoustic-impedance method | Pipe diameter | DN50 ~ DN1000 |
Measuring range | 0 ~ 80 % concentration by weight / calibratable on the actual medium | Installation | Integral / remote-mount, flanged into a straight run |
Accuracy | ± 1 % FS (the published datasheet figure is ± 1 % concentration by weight) | Dimensions | 288 (W) × 190 (D) × 95 (H) mm |
Power supply | Standard AC220V (175 ~ 285V, 50/60Hz); DC24V optional | Weight | Approx. 2.5 kg |
Power consumption | ≤ 1 W | Display | OLED screen (bilingual Chinese / English menu) |
Ambient temperature | -30 ℃ ~ 60 ℃ | Analogue output | 4-20 mA, 2 channels |
Medium temperature | 0 ℃ ~ 80 ℃ | Digital interface | RS485 / Modbus RTU, approx. 500 m |
Humidity range | ≤ 98 %RH | Ingress protection | IP65 |
Data storage | 2000 real-time records + 400 daily records (customisable) | Remote transmission | Optional 4G module |
Field calibration | Gravimetric sampling method, performed online with no shutdown | Custom options | Lining, range, explosion protection, high temperature and similar options built to the duty |
※ The manual states a pipe diameter range of DN50 ~ DN1000, and the DN700 ~ DN1000 discharge line in this case lies within it; larger main discharge lines require a custom solution to be confirmed at the selection stage. Lining, range, explosion protection and high-temperature versions are all custom options, confirmed point by point for the duty.
▍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.
▪Shipboard installation must avoid the areas of strongest vibration: the manual explicitly requires that the installation location be free of severe vibration. Vibration is worst immediately at the dredge pump outlet next to the pump casing, so the measuring spool should be on a straight run with its own supports, well away from the pump body and from bends; the transmitter can be supplied as a remote version wall-mounted in the engine room, away from direct impact and salt spray on deck.
▪A large-bore measuring spool needs its own supports: a DN700 ~ DN1000 spool is heavy and the slurry momentum is considerable, so both end flanges should be independently supported and properly aligned, and the spool should not be carried by an adjacent hose or expansion joint — otherwise the loaded spool deforms and the position of the sensor relative to the flow path changes.
▍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:
Full pipe | The sensor must be completely immersed in the liquid |
Steady flow | Variation within ± 5 % |
No significant bubbles | No venting operation should be in progress in the line |
Steady temperature | A change in slurry temperature affects the measurement signal |
Clean sensor | No significant deposit or scaling; rinse with clean water if necessary |
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
Aspect | Previous nucleonic (gamma) density gauge solution | PS7000 solution |
Compliance and transport | A radioactive source carried on board requires waterway transport permits, annual radiation protection inspections and personnel qualification; overseas projects additionally require approval from the destination country | A non-nuclear instrument, with no radiation-related approvals, annual inspections or source decommissioning and disposal |
Abrasion and maintenance | Diaphragm and insertion-type parts wear out within months in sand-bearing slurry, and replacement means stopping the vessel | The non-intrusive sensor does not protrude into the flow path, so there is no leading-edge erosion; calibration is completed on line by the gravimetric sampling method |
Blockage risk | Insertion-type parts readily attract material build-up in a coarse-particle discharge line and create local turbulence | Nothing protrudes into the flow path, so the flow field is unchanged and no additional source of blockage is introduced |
Partly filled pipe | — | A remote installation with the sensor on the underside of the pipe copes with some partly filled sections; with an integral version and the sensor on top, the manual requires a full pipe |
Measurement performance | — | ± 1 % FS (published in the datasheet as ± 1 % concentration by weight) across the full 0 ~ 80 % range, calibrated on the soil encountered on the project |
Fleet management | Mainly local indication, with data difficult to consolidate | 4-20 mA × 2 + MODBUS-RTU, optional 4G, integrated into the vessel monitoring system and the fleet platform |
4 Field Verification and Operating Record
On a coastal port expansion project in China, a cutter suction dredger belonging to a major dredging contractor replaced its nucleonic density gauge with a PS7000, the measuring spool being flanged into the main discharge line at the dredge pump outlet. After one year in service the customer reports that the number of plugged-pump incidents fell by more than 60 % against the same period before the retrofit, and that effective production time gained about 1.5 hours per vessel per day. These are the customer's own operating statistics and have not been verified by a third party; vessel type, discharge distance and soil conditions differ widely, so figures for your own project should be based on the working records of the vessel concerned.
After the change, carrying a radioactive source on board and the associated waterway transport permits were no longer required, and the group has since used the same configuration on inland waterway desilting and overseas land reclamation projects. Calibration checks during operation were all completed on line by the gravimetric sampling method, and no vessel downtime was scheduled for instrument calibration.
A Note on Comparison Methodology The gravimetric sampling method itself carries method error: the physical distance between the sampling point and the measuring point introduces transport lag, and the representativeness of the sampling operation together with the weighing and drying accuracy in the laboratory also contribute to the deviation. We therefore avoid expressions such as "in complete agreement", and recommend instead that acceptance be based on the mean deviation and the trend consistency across several consecutive comparison sets, with the sampling rules stated explicitly in an annex to the contract. Pisonics can provide a standard field comparison and acceptance procedure. |
▍Verifiable Benefits for the Customer
▪No radioactive source carried on board, no waterway transport permit, no crew radiation protection training and no source decommissioning and disposal, which simplifies customs clearance and entry formalities on overseas projects.
▪The diaphragm replacement work that stopped the vessel every few months with the previous diaphragm-type meter has been eliminated, reducing unplanned downtime.
▪With discharge density displayed on the bridge, adjustments to cutting depth and swing speed have a quantitative basis instead of relying purely on the sound of the dredge pump and on experience.
▪The density signal is fed via MODBUS-RTU (optional 4G) into the vessel monitoring system and the fleet platform, so head office can consolidate discharge density and production data from every vessel.
5 Frequently Asked Questions
Q1 The discharge line carries coarse sand, gravel, even shells and clay lumps — can these be measured?
A It has to be considered case by case. A uniform slurry of fine sand and silt is within the application range — the manual states explicitly that the product is suitable for solid-liquid two-phase slurries containing gas bubbles and for media containing solid particles. The manual also draws a boundary, however: the medium should not contain large quantities of large suspended or settled particles. Where the dredged strata contain quantities of gravel, cobbles, shell beds or clay lumps, the echo is scattered by those particles and deposits may build up inside the spool, which puts the duty outside the applicable boundary. The engineering practice is to provide the geotechnical survey data and particle size distribution at the selection stage; to place the measuring point on the discharge line downstream of the dredge pump where the mixture is thoroughly mixed, rather than on the suction line; and to set expectations in advance for the availability of the measurement in gravel-bearing strata — interference of this kind cannot be absorbed by calibration coefficients.
Q2 When the cutter lifts off the bottom or the suction mouth draws air, the discharge line is not full — is the reading still usable?
A Not directly; this is a limitation inherent in an acoustic method: the sensor has to be immersed in the medium to work, and once the pipe is not full and the sensor is out of contact with the medium the measurement fails. Engineering handles this at two levels. The first is selection: for a large-bore discharge line, choose the remote version and place the sensor on the underside of the pipe so that it stays immersed as far as possible; with an integral version and the sensor on top of the pipe, the manual requires a full pipe. The second is data usage: readings taken while the pump is drawing air do not represent the true soil concentration, and the correct approach is to interlock with the dredge pump running, vacuum or flow signal so that those periods are flagged or masked and excluded from the production statistics. Mixing air-drawing data into the statistics and then questioning the instrument is the most common misjudgement on projects of this kind.
Q3 Vibration and salt spray are severe on board — will the meter stand up to it? Will a step rise in density be seen immediately?
A On vibration, the manual explicitly requires that the installation location be free of severe vibration — the strongest vibration is immediately at the dredge pump outlet next to the pump casing, so the measuring spool should be on a straight run with its own supports, well away from the pump body and bends; the transmitter can be supplied as a remote version wall-mounted in the engine room, away from direct impact and salt spray on deck. The protection rating is IP65, and a protective enclosure is recommended outdoors and in hot, humid conditions. On response, to be clear: the manual gives no response time figure and we make no claims such as "response in seconds". The meter provides two adjustable parameters, measurement damping and measurement interval, which trade response speed against reading stability, and the practical usability of an alarm should be tuned on site against a real step change in density.
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
Company | Xi'an Pisonics Information Technology Co., Ltd.PISONICS |
Address | Room 15B016, Block A, Olympic Building, North Chang'an Road, Beilin District, Xi'an, Shaanxi, China |
Tel | +86 159-0293-2017 |
info@pisonics.com | |
Web | Chinese site www.pisonics.cn | English site www.pisonics.com |
※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.
