Airtightness Testing of Alloy Desulfurization Nozzles: Methods, Standards, and Acceptance Criteria

Airtightness testing of alloy desulfurization nozzles is a critical quality control procedure that verifies the sealing integrity of nozzles used in flue gas desulfurization (FGD) systems, preventing slurry leakage and ensuring long-term performance in corrosive environments. This article provides a comprehensive technical overview of sample types, testing items, test methods in accordance with ISO, ASTM, and GB standards, typical applications, and frequently asked questions to guide engineers and procurement specialists in specifying and evaluating leak tests for alloy spray nozzles.

Technical Overview

Alloy desulfurization nozzles are precision-engineered components installed in spray absorption towers, quench ducts, and circulation lines of flue gas desulfurization systems. They are typically manufactured from corrosion-resistant alloys such as duplex stainless steel (UNS S31803/S32205), super duplex stainless steel (UNS S32750), 904L (N08916), high-molybdenum austenitic alloys (N08904, N08926), and nickel-based alloys (N10276, N06022) to withstand continuous exposure to acidic slurry, chlorides, and abrasive limestone-gypsum suspensions.

Because these nozzles operate under sustained internal pressure—commonly between 0.2 MPa and 1.6 MPa depending on the system design—any leakage through the nozzle body, weld seams, threaded connections, or flanged joints results in slurry loss, mist carryover, corrosion of surrounding structures, and accelerated failure of the spray system. Airtightness testing (also referred to as leak testing or sealing integrity testing) is therefore performed during manufacturing, after repair welding, and periodically during plant maintenance outages to confirm that the nozzle assembly remains leak-tight under its rated pressure conditions.

Airtightness testing evaluates the nozzle as a complete pressure boundary. It complements other quality verification activities such as material composition analysis, radiographic testing of welds, dimensional inspection of the spray orifice, and flow-rate characterization. Depending on the required sensitivity, tests are conducted using compressed air or nitrogen as the pressurizing medium, with detection methods ranging from simple bubble emission observation to high-precision helium mass spectrometry.

Testing Samples

Newly Manufactured Nozzle Bodies

Complete nozzle bodies produced from forged, cast, or machined alloy stock are tested prior to assembly and shipment. This sample category covers spiral spray nozzles, tangential swirl nozzles, dual-flow air-atomizing nozzles, and large-diameter header nozzles. Testing at this stage identifies casting porosity, machining defects, and through-wall material discontinuities before additional components are installed.

Welded Nozzle Assemblies

Assemblies incorporating welded connections—such as nozzle-to-flange welds, boss welds, and reinforcing pad welds—are tested after all welding and post-weld heat treatment (where applicable) are complete. Weld seams are the most common leak paths in fabricated alloy nozzles, and airtightness testing verifies the combined integrity of the base metal and the welded joints in accordance with acceptance criteria defined in ISO 5208 or GB/T 13927.

Threaded and Flanged Connection Joints

Nozzles with threaded inlets or bolted flange connections are tested as assembled units with the designated gaskets and fasteners installed at the specified torque. This verifies the sealing performance of the mechanical joint itself, since thread run-out, gasket seating damage, and uneven bolt loading can all produce leakage even when the nozzle body is sound.

Repaired or In-Service Nozzles

Nozzles recovered from absorber towers during maintenance outages, particularly those that have undergone repair welding of erosion thinned areas or replacement of the swirl insert, are retested before return to service. Repeated thermal cycling and slurry erosion in service can initiate cracks that only become detectable as leaks under renewed pressurization.

Testing Items

The airtightness test program for alloy desulfurization nozzles typically includes the following evaluated parameters:

• Test pressure: Generally 1.5 times the maximum allowable working pressure, or a minimum of 0.6 MPa gauge for low-pressure spray circuits; the specific value is agreed between manufacturer and purchaser based on system design data.
• Test medium: Clean, dry compressed air or nitrogen with a dew point low enough to prevent internal condensation on alloy surfaces.
• Stabilization and holding time: A pressure stabilization period of 2 to 5 minutes followed by a holding period of 5 to 15 minutes, during which pressure variation is recorded.
• Pressure decay rate: The permissible pressure drop over the holding period, commonly limited to 5% of the test pressure or to a specified value in kilopascals per minute for high-sensitivity applications.
• Leak rate (quantitative methods): For helium testing, the acceptance leak rate is typically specified as 1 × 10⁻⁶ Pa·m³/s or tighter for critical service nozzles.
• Bubble emission observation: Absence of continuously growing bubbles at any location on the nozzle surface, weld seams, and joints during immersion or surfactant-solution application.
• Visual and dimensional verification: Confirmation of permanent or non-permanent deformation, test medium residue, and post-test condition of markings and gasket surfaces.

Testing Methods

Pressure Decay Method

The nozzle is sealed, pressurized with air or nitrogen to the specified test pressure, isolated from the pressure source, and monitored with a calibrated pressure transducer or precision gauge over the holding period. Any pressure decrease beyond the allowable decay rate indicates leakage. This method is well suited to production-line testing of moderate volumes, is non-destructive, and leaves no residue inside the nozzle. Temperature fluctuations during the test must be controlled or compensated, since gas pressure in a sealed volume is sensitive to ambient temperature drift.

Bubble Emission (Immersion) Test

Conducted in accordance with the principles of ASTM E515 and related practices, the pressurized nozzle is either fully immersed in a water bath or coated with a low-surface-tension surfactant solution. The operator observes the entire external surface for a defined observation period—commonly 1 to 3 minutes per section—watching for a steady stream of bubbles that indicates a through-wall leak path. Single non-repeating bubbles caused by entrained air are distinguished from true leakage. This method provides direct visual localization of the leak and is widely used as a final acceptance test.

Differential Pressure Method

A high-sensitivity variant of pressure decay testing in which the test specimen and a reference volume are pressurized simultaneously and a differential pressure sensor measures the divergence between them. This approach cancels ambient temperature effects and detects leak rates approximately one order of magnitude smaller than conventional gauge-based decay testing, making it suitable for nozzles with small internal volumes where a tiny leak produces a very slow pressure change.

Helium Mass Spectrometry Leak Detection

For critical-service nozzles in ammonia-based desulfurization or marine exhaust gas cleaning systems, helium tracer gas testing per the general framework of ASTM E499 offers the highest sensitivity. The nozzle is either pressurized with helium or helium mixture (pressure technique) or evacuated and screened with an external helium spray (sniffer or vacuum technique). Leaking helium is detected by a mass spectrometer tuned to the helium mass peak, providing quantitative leak rates down to 10⁻⁹ Pa·m³/s. This method verifies leaks far smaller than those that would be functionally significant, providing a margin of safety for thin-wall alloy components.

Hydrostatic Strength Test

Although technically a strength rather than an airtightness test, hydrostatic pressurization with treated water at 1.5 times rated pressure is sometimes performed on large header nozzles and fabricated assemblies in accordance with the philosophy of ISO 5208 shell testing. After the hydrostatic test, the internal passages are thoroughly dried and passivated to protect the alloy surfaces, after which the gas-based airtightness test confirms the final sealing condition. Testing should be performed by the manufacturer's certified inspection personnel or, when independent verification is required, by accredited third-party laboratories.

Applications

Airtightness testing of alloy desulfurization nozzles is applied across a broad range of industrial air pollution control systems, including:

• Coal-fired power plant FGD systems: Limestone-gypsum wet scrubbers in thermal power stations, where large numbers of alloy spray nozzles on the absorption tower spray banks must remain leak-tight to maintain uniform slurry distribution and prevent corrosion of the tower internals.
• Steel and sintering plants: Desulfurization and dedusting spray systems on sintering machine flues and pelletizing lines, which combine acidic condensate with high dust loads and demand reliable nozzle sealing.
• Chemical and petrochemical process scrubbers: Tail-gas treatment units, acid gas scrubbers, and hydrogen sulfide removal towers operating with chemically aggressive media.
• Marine exhaust gas cleaning systems (scrubbers): Shipboard open-loop, closed-loop, and hybrid scrubbers in which seawater-exposed alloy nozzles face severe chloride corrosion and strict classification-society verification requirements.
• Waste incineration and biomass flue gas treatment: Quench and absorption sections where rapid thermal transients make weld integrity verification essential.
• Semi-dry and circulating fluidized bed desulfurization: Atomizing and slurry injection nozzles requiring verified sealing of both liquid and compressed-air passages.

FAQ

Why is airtightness testing necessary when the nozzle material is already corrosion-resistant?

Corrosion-resistant alloys protect against chemical attack, but they do not eliminate geometric defects such as casting porosity, incomplete weld penetration, or gasket seating faults. Airtightness testing detects these through-wall and joint leakage paths that material certification alone cannot reveal. Even a small leak allows acidic slurry to escape, concentrate chlorides on external surfaces, and attack the alloy in crevice conditions that can defeat its passive film.

What test pressure should be specified for a desulfurization nozzle?

Unless the purchase specification states otherwise, a common practice is to test at 1.5 times the maximum allowable working pressure with gas, or at a minimum of 0.6 MPa gauge for low-pressure spray circuits, in line with the test pressure philosophy of ISO 5208 and GB/T 13927 for similar pressure-boundary components. The exact value must always be validated against the nozzle design data and never exceed the pressure-temperature rating of the body, flange, or gasket materials.

How is a bubble test distinguished from a true leak versus trapped air?

Trapped air or dissolved gases typically produce isolated, non-repeating bubbles that grow and detach once. A genuine leak produces a continuous, repeatable stream of bubbles from the same location. Standard practice requires observing each region for a defined dwell time and confirming reproducibility before classifying a discontinuity as a leak.

When is helium leak testing preferable to pressure decay testing?

Helium mass spectrometry is preferred when the nozzle has very small internal volume, when the specified acceptance leak rate is below roughly 10⁻³ Pa·m³/s, or when the consequence of even minimal leakage is severe—such as in ammonia-based systems or classified marine scrubber installations. Pressure decay remains the economical default for routine production acceptance of larger nozzles with moderate sensitivity requirements.

How frequently should in-service alloy nozzles be retested?

Most plant maintenance programs retest nozzles during scheduled absorber tower outages, typically every 12 to 36 months depending on service severity, and always after any repair welding, swirl insert replacement, or observed performance degradation such as abnormal spray pattern or pressure-flow deviation beyond 10% of the characteristic curve.

What documentation should accompany a completed airtightness test?

Test reports should identify the nozzle by serial number and material grade, record the test method, medium, test pressure, stabilization and holding durations, measured pressure decay or leak rate, ambient temperature, instrument calibration references, the applicable acceptance criterion, and the inspector's qualification. Reports issued by accredited third-party laboratories provide independent verification when required by the purchaser or regulatory authority.