Determination of Spray Jet Distance for High-Flow Desulfurization Nozzles

High-flow desulfurization nozzles are the core atomization components of wet flue gas desulfurization (FGD) absorber towers, and their spray jet distance directly determines slurry coverage, gas–liquid contact efficiency, and SO₂ removal performance. This article explains the technical principles, test samples, measurement items, test methods, and industrial applications of spray distance determination for high-flow desulfurization nozzles, as performed in accredited third-party laboratories. Understanding these testing procedures helps nozzle manufacturers, power plant operators, and EPC contractors verify product quality and optimize absorber tower design.

Technical Overview

In limestone-gypsum wet flue gas desulfurization systems, absorber spray towers rely on arrays of high-flow nozzles to circulate large volumes of slurry and create dense droplet curtains that absorb sulfur dioxide from the flue gas. These nozzles, typically manufactured from sintered silicon carbide (SSiC) or other wear- and corrosion-resistant ceramics, operate at relatively low pressures (commonly 0.05 MPa to 0.20 MPa) with single-nozzle flow rates ranging from tens to more than one hundred cubic meters per hour.

The spray jet distance, also referred to as jet penetration distance, is defined as the maximum distance from the nozzle outlet to the point at which the spray jet still maintains effective momentum, coverage density, and a defined trajectory. It is a critical geometric parameter in absorber tower design: an insufficient spray distance leads to poor coverage of the absorption zone and gas channeling along the tower wall, while an excessive spray distance may cause slurry impingement on the tower shell, accelerated lining wear, and excessive droplet carryover toward the mist eliminators.

Spray distance determination measures the actual free-jet penetration of a nozzle under controlled pressure and flow conditions, providing objective data for product acceptance, type testing, and CFD model validation. Testing is typically carried out by accredited third-party laboratories in accordance with relevant national standards (GB series) for industrial spray nozzles, together with internationally recognized guidance on spray characterization and droplet analysis published by organizations such as ISO and ASTM.

Testing Samples

Silicon Carbide Spiral Nozzles

Spiral (pigtail) nozzles are the most widely used high-flow desulfurization nozzles, producing multiple concentric hollow-cone spray sheets with flow rates typically between 30 m³/h and 160 m³/h. Both bare nozzle bodies and complete assemblies with connecting flanges can be submitted for testing.

Tangential (Vortex) Hollow Cone Nozzles

Tangential-inlet nozzles generate a swirling hollow-cone pattern through a large internal vortex chamber. Their large free passages resist plugging by gypsum crystals and fly ash, making them common in high-solid-content slurry service. Samples of various nominal diameters and spray angles (such as 90°, 120°, and 150°) are tested.

Large-Flange Nozzle Assemblies

Complete assemblies comprising the nozzle, flange, gasket seat, and reinforcement bushing are tested in their as-installed configuration to evaluate the influence of connection geometry on jet trajectory and effective spray distance.

Submitted samples should be new, unused, and free of cracks or shipping damage. Each sample must be clearly marked with model, nominal flow rate, spray angle, and material, and a sufficient quantity (normally two to three units per model) is recommended to assess manufacturing consistency.

Testing Items

Depending on the client's specification and the purpose of the evaluation, spray distance determination is usually combined with the following performance measurements:

• Free-jet spray distance at rated operating pressure and at multiple verification pressures
• Flow rate versus pressure characteristic curve and flow coefficient (K-factor or discharge coefficient)
• Spray angle and spray envelope geometry
• Coverage diameter and spray pattern uniformity at defined distances from the nozzle outlet
• Spray density distribution (water flux distribution across the spray cross-section)
• Optional droplet size analysis, including Sauter mean diameter (SMD/D32), performed by laser diffraction in line with ISO 13320 and related ASTM practices for liquid drop size data processing
• Visual and dimensional inspection of the nozzle bore, spiral surfaces, and mounting interface

Testing Methods

Test Rig and Instrumentation

Testing is performed on a dedicated open-air or enclosed spray test stand consisting of a closed-loop water supply system, a variable-frequency pump, a buffer tank to dampen pressure pulsation, calibrated electromagnetic or turbine flow meters, precision pressure transmitters located immediately upstream of the nozzle, and a temperature monitoring point. Instruments are calibrated and traceable to national metrology standards, and measurement uncertainty is reported with the results.

Measurement Procedure

The nozzle is rigidly mounted on a horizontal boom at a defined height above a gridded test basin or catchment floor, ensuring free development of the jet without obstruction. The system is first conditioned at each test pressure — typically 0.05 MPa, 0.10 MPa, 0.15 MPa, and 0.20 MPa — until stable readings of pressure, flow rate, and water temperature are obtained.

The jet trajectory is recorded using a calibrated background grid combined with high-speed imaging and/or laser ranging. The spray jet endpoint is defined as the point at which the local water flux falls below an agreed threshold (for example, 5% of the peak flux) or at which the jet trajectory intersects a defined horizontal reference plane. The horizontal distance from the nozzle outlet to this endpoint is measured as the spray distance.

Each measurement is repeated at least three times under identical conditions, and the arithmetic mean, standard deviation, and repeatability are calculated. Photographs and video records of the spray pattern at each pressure are appended to the test report.

Data Processing and Evaluation

Test results are compiled into a comprehensive report including the pressure–flow characteristic curve, spray distance at each test pressure, spray angle, coverage data, sample identification, instrument calibration status, ambient and water temperatures, and the evaluation criteria against the client's technical specification or the relevant national and international standards. Conclusions clearly state whether the tested nozzle models meet the specified spray distance requirements.

Applications

Spray distance determination data for high-flow desulfurization nozzles are used throughout the life cycle of wet FGD systems and related scrubbing installations:

• Type testing and factory acceptance of nozzles for coal-fired power plant absorber towers, including ultra-supercritical units and retrofit projects
• Flue gas desulfurization systems in steel sintering plants, coke-chemical plants, and waste-to-energy incineration facilities
• Gas absorption, cooling, and scrubbing towers in the chemical, petrochemical, and metallurgical industries
• Nozzle research and development, including CFD model calibration for spray tier spacing and overlap design
• Procurement verification, bid evaluation, and technical dispute resolution supported by accredited third-party laboratory reports
• Diagnosis of deteriorating tower performance, such as insufficient coverage, wall wetting, and mist eliminator fouling

FAQ

What pressures are used for spray distance testing of desulfurization nozzles? Testing normally covers the actual absorber operating range of approximately 0.05 MPa to 0.20 MPa, with additional points selected according to the client's specification. Because spray distance increases with pressure, multiple pressure points are recommended to establish the full characteristic curve.

How is the endpoint of the spray jet defined? The endpoint is agreed before testing and is commonly defined as the position where the local water flux drops below a fixed percentage of the peak flux, or where the outer edge of the droplet trajectory intersects a defined reference plane. This ensures objective and reproducible measurements.

What samples and information should be submitted? Clients should provide new nozzle samples together with the model designation, nominal flow rate, rated pressure, spray angle, material grade, and any applicable acceptance criteria. Complete flange assemblies are preferred when the installed configuration is to be evaluated.

What standards govern the testing? Testing is conducted with reference to relevant national standards (GB series) for industrial spray nozzles and internationally recognized guidance on spray characterization published by ISO and ASTM, including laser diffraction particle size methods per ISO 13320 where droplet size analysis is required.

How long does the test take? A routine spray distance determination covering multiple pressures with a standard report is generally completed within five to ten working days after sample receipt, depending on sample quantity and the scope of additional measurements such as droplet size distribution.