Hydrostatic Pressure Testing of Alloy Desulfurization Nozzles

Hydrostatic pressure testing of alloy desulfurization nozzles is a critical quality control procedure that verifies the structural integrity, leak tightness, and pressure-bearing capacity of corrosion-resistant alloy spray nozzles used in flue gas desulfurization systems. This article provides a comprehensive technical overview of applicable test samples, testing items, testing methods, acceptance criteria, and typical industrial applications, in accordance with recognized standards frameworks such as ASTM, ISO, EN, and GB.

Technical Overview

Alloy desulfurization nozzles are precision-engineered spray components installed in wet flue gas desulfurization (WFGD) absorber towers, where they atomize limestone-gypsum slurry, seawater, or alkaline scrubbing liquor to absorb sulfur dioxide from industrial exhaust gas. Because these nozzles are fabricated from corrosion-resistant alloys, they must withstand not only aggressive chemical media but also sustained internal hydraulic pressure generated by circulating slurry pumps.

Hydrostatic pressure testing, commonly referred to as a water pressure test, is a non-destructive verification procedure in which the nozzle is filled with a controlled liquid medium and pressurized to a predetermined level above its rated working pressure. The test confirms that the nozzle body, spray orifice, connection threads, flanges, and weld zones are free from through-wall defects, porosity, and manufacturing flaws that could cause leakage or premature failure in service.

The testing philosophy follows established pressure equipment practice, referencing standards frameworks such as ASTM material specifications, ISO and EN pressure testing methodology, and GB national standards for pressure-bearing components.

Testing Samples

Spiral Spray Nozzles

Spiral (helix) desulfurization nozzles produce hollow or full cone spray patterns through a helical swirl chamber. Available in sizes typically ranging from DN15 to DN150, these samples are tested at their threaded, flanged, or welded inlet connections after internal passages have been verified as unobstructed.

Tangential Whirl Nozzles

Tangential nozzles generate spray rotation through an internal swirl vane or tangential inlet channel. Samples include single-swirl and double-swirl designs used for high-capacity absorption towers and high-velocity gas streams.

Common Alloy Grades

• Duplex stainless steel: UNS S31803 / S32205 (2205) and super duplex UNS S32750 (2507)
• Super austenitic stainless steel: UNS N08904 (904L) and UNS N08926 (1.4529)
• Nickel-based alloys: UNS N10276 (C-276) and UNS N06022 (C-22)
• Investment cast alloys: CN7M and CY-40 cast nozzle bodies

Testing Items

• Shell hydrostatic strength (proof) test at 1.5 times the maximum allowable working pressure
• Leak-tightness verification of the nozzle body wall and spray orifice
• Connection integrity test for threaded, flanged, and welded joints
• Pressure-holding stability over a defined dwell period
• Permanent deformation and dimensional stability assessment after pressure release
• Post-test visual inspection for weeping, sweating, blistering, or crack indications

Testing Methods

Test Medium and Sample Preparation

Clean water with a chloride content generally limited to below 30 ppm, or as specified by the applicable material standard, is used to prevent chloride-induced stress corrosion cracking of stainless steel and nickel alloy samples. The water temperature is maintained between 5°C and 40°C. Each nozzle is cleaned, dried, and visually examined before mounting on a sealed test fixture, with the spray orifice blanked or captured by a containment head.

Pressurization Procedure

The nozzle cavity is completely filled with water to expel all trapped air, since entrained air stores hazardous compressed-gas energy. Pressure is then increased gradually in controlled stages to the specified test pressure, typically 1.5 times the design working pressure and never less than the minimum value agreed in the product specification. Calibrated pressure gauges or transducers with an accuracy of 1.6 class or better are used to monitor the applied load throughout the test.

Holding and Inspection

The test pressure is held for a minimum of 3 to 10 minutes depending on nozzle size and specification requirements. During the holding period, examiners inspect the external body surface, connection interfaces, and transition radii for visible leakage, moisture weeping, or visible deformation. A permissible pressure-decay allowance is defined in advance, and any decay exceeding this limit constitutes failure of the test.

Acceptance Criteria

• No visible leakage or moisture weeping through the nozzle body wall
• No leakage at threaded, flanged, or welded connection points
• No permanent deformation, bulging, or cracking detected after pressure release
• Pressure decay within the specified allowable tolerance during the holding period

Applications

Hydrostatically verified alloy desulfurization nozzles are deployed across a wide range of emission control and gas cleaning installations, including:

• Coal-fired power plant limestone-gypsum WFGD absorber towers
• Steel plant and sintering machine flue gas desulfurization scrubbers
• Refinery and petrochemical acid gas removal units
• Marine exhaust gas cleaning systems installed on oceangoing vessels
• Waste incineration plants and industrial boiler DeSOx systems
• Cement kiln and glass furnace tail-gas treatment plants

FAQ

Why is water used instead of air for the pressure test?

Water is nearly incompressible and stores very little elastic energy, so any failure during testing releases minimal energy. Compressed air can store dangerous levels of energy and is therefore prohibited for routine proof testing of pressure-bearing components.

What test pressure is typically applied?

The test pressure is normally set at 1.5 times the maximum allowable working pressure of the nozzle, subject to agreement between the manufacturer and purchaser and to any minimum value stated in the applicable product standard.

How long must the pressure be held?

A holding time of 3 to 10 minutes is common, extended for larger nozzles or when explicitly specified. The dwell period ensures that slow-developing leaks and time-dependent defect behavior can be reliably detected.

Why is chloride content in the test water controlled?

Austenitic, duplex, and nickel alloys are susceptible to chloride stress corrosion cracking. Residual chloride-bearing water trapped in crevices after testing can initiate cracking, so low-chloride water together with complete drainage and drying after the test is required.

What happens if a nozzle fails the test?

Failed nozzles are rejected, quarantined, and subjected to root-cause analysis, which may include liquid penetrant inspection and metallographic examination. Rework is permitted only where the applicable specification allows repair by qualified welding, followed by complete re-testing and re-verification.