Weld integrity is the single most decisive factor in the safety, leak-tightness and service life of manual bellows globe valves used in toxic, flammable, high-vacuum and high-purity services. This article explains where welds occur on these valves, which samples are submitted for evaluation, and how visual, penetrant, magnetic particle, radiographic, ultrasonic and helium leak testing are applied in accordance with ISO, EN, ASTM, GB and ASME standards. Whether you are a valve manufacturer, welding engineer or quality inspector, it provides a practical roadmap for qualifying, examining and accepting every weld on a bellows seal globe valve.
Technical Overview
A manual bellows globe valve is a handwheel-operated shut-off and regulating valve in which a metallic bellows assembly forms a dynamic, frictionless barrier between the process fluid and the valve stem, replacing conventional packing as the primary stem seal. The bellows is typically a hydroformed multi-ply component or an edge-welded stack of thin diaphragms made from austenitic stainless steel (304, 316L, 321) or nickel alloys such as Alloy 600 and Alloy 625. Typical products cover sizes from DN 10 to DN 300 and pressure classes from PN 16 to PN 420 or ASME Class 150 to 2500, with pressure-temperature ratings governed by ASME B16.34 or equivalent EN and GB standards.
Because the bellows is the only barrier preventing fugitive emissions along the stem, every weld in the bellows seal system is both pressure-retaining and fatigue-critical. Each opening and closing stroke flexes the bellows convolutions, and the weld toes at the bellows ends experience the highest stress concentration. A single pinhole, crack, or area of incomplete fusion can destroy the barrier function immediately, while poor weld geometry can dramatically reduce cycle life. Weld inspection is therefore not an optional quality step but an essential safety requirement, and it is commonly witnessed or performed by accredited third-party laboratories on behalf of end users.
The principal welds subject to examination on a manual bellows globe valve include:
- Edge welds at the inner and outer diameters of welded-bellows diaphragm stacks
- Bellows-to-weld-ring attachment welds at the upper and lower ends of the bellows
- Weld-ring-to-stem and weld-ring-to-bonnet (or valve cover) joining welds
- Body shell and body-to-bonnet welds in welded-body valve constructions
- Butt-weld and socket-weld end preparations supplied for pipeline attachment
- Seat and disc hardfacing overlay welds, such as cobalt-base or nickel-base alloys
Testing Samples
Bellows Subassemblies
The complete bellows cartridge, including the bellows, upper weld ring and lower weld ring, is the most important inspection sample. It is submitted after final welding, solution or passivation treatment and cleaning, so that both weld quality and post-weld surface condition can be evaluated. Sampling is normally carried out on a lot basis agreed between manufacturer and purchaser, with destructive metallographic checks performed on representative sacrificial pieces.
Edge-Welded Bellows Stacks
For welded bellows, short stack sections containing internal and external diaphragm welds are cut from production or witness material. These samples allow direct measurement of weld penetration, nugget width, and internal bead geometry, which cannot be verified by non-destructive means on the finished part.
Valve Body and Bonnet Weldments
Pressure-retaining shell welds in welded-body constructions, and any bonnet closure welds, are inspected on the component before assembly. These welds are thicker than bellows welds and are candidates for volumetric examination by radiographic or ultrasonic testing.
Weld End and Overlay Coupons
Butt-weld and socket-weld end samples are examined for bevel angle, wall thickness, out-of-roundness and surface condition in accordance with the applicable end-standard, such as ASME B16.25 or EN 12627. Hardfacing coupons are checked for overlay thickness, dilution and bond integrity.
Procedure and Production Qualification Coupons
Welding procedure qualification records (WPQR) and production test plates welded with the same procedure, filler metal and position provide the tensile, bend, impact and macro-structural evidence required before serial production begins.
Finished Valve Assemblies
Complete valves are submitted for hydrostatic shell and seat testing, helium leak testing, fugitive emission testing and cycle-life verification, either in-house under third-party witness or directly at accredited third-party laboratories.
Testing Items
A complete weld inspection program for manual bellows globe valves covers appearance, dimensions, surface and volumetric discontinuities, leak tightness, strength, metallurgy and corrosion resistance. The principal items are:
- Visual examination for cracks, undercut, porosity, spatter, arc strikes, craters and weld surface finish
- Dimensional verification of weld width, reinforcement, misalignment, bellows free length, pitch, ply count and total stroke
- Surface non-destructive testing by liquid penetrant testing (PT) or magnetic particle testing (MT)
- Volumetric non-destructive testing by radiographic testing (RT) and ultrasonic testing (UT) for pressure shell welds
- Leak integrity verification by helium mass spectrometer leak detection and bubble testing
- Strength and tightness verification by hydrostatic shell test and seat leakage test
- Mechanical testing of qualification coupons: transverse tensile, bend and impact testing
- Metallurgical examination: macro-etch and micro-structure of weld cross sections, hardness survey, delta ferrite content
- Corrosion testing: intergranular corrosion per ASTM A262 and pitting corrosion per ASTM G48 for stainless components
- Functional qualification: cycling endurance, operating torque and fugitive emission class per ISO 15848
Testing Methods
Visual and Dimensional Examination
One hundred percent visual inspection per ISO 17637 is applied to all accessible weld surfaces using adequate illumination and low-power magnification, with weld gauges to measure undercut, reinforcement and mismatch. Internal surfaces of end welds are examined with borescopes. Visual acceptance is generally aligned with quality level B of ISO 5817 for critical welds, with tighter requirements prohibiting any cracks, lack of fusion or incomplete penetration on bellows welds.
Liquid Penetrant Testing
Magnetic Particle Testing
For ferritic and martensitic components such as certain valve bodies, stems and weld ends, magnetic particle testing per ISO 9934-1 or ASTM E709 is applied, with acceptance criteria to ISO 23278. Fluorescent wet continuous methods provide the best sensitivity for fine fatigue cracks in weld toes.
Radiographic Testing
Radiographic testing per ISO 17636-1, using Class A or Class B techniques with image quality indicators per the EN 462 series, is applied to full-penetration butt welds of bodies, bonnets and weld ends. It reliably detects porosity, slag, incomplete penetration and lack of fusion. Because of the thin sections and complex geometry of bellows edge welds, radiography is not normally used for the bellows itself; penetrant testing and destructive cross-sectioning are used instead.
Ultrasonic Testing
Ultrasonic testing per ISO 17640, with acceptance to EN 1712, is applied to shell welds above approximately 8 mm wall thickness, and phased-array ultrasonic testing improves sizing and coverage on nozzle and overlay welds. Ultrasonic and eddy-current methods are also used to verify hardfacing overlay soundness and thickness.
Helium Mass Spectrometer Leak Testing
Leak tightness of the bellows seal system is verified with a helium mass spectrometer leak detector. The evacuated-chamber (hood) method on the bellows subassembly provides the highest sensitivity, down to 1 × 10⁻⁹ Pa·m³/s, while the sniffer or tracer method is used on assembled valves. Typical acceptance limits range from 1 × 10⁻⁶ Pa·m³/s for general chemical service to 1 × 10⁻⁹ Pa·m³/s or tighter for vacuum and toxic service, as specified in the purchase specification.
Hydrostatic and Seat Tightness Testing
Each assembled valve undergoes shell testing at a minimum of 1.5 times the allowable pressure and seat tightness testing at 1.1 times the rating per ISO 5208 or GB/T 13927, with leak rate A (no visible leakage) normally required for bellows seal valves. Low-chloride water or nitrogen is used for stainless steel components to prevent pitting and stress corrosion.
Destructive and Metallurgical Verification
Welding procedures are qualified per ISO 15614-1 or ASME Boiler and Pressure Vessel Code Section IX, and welders per ISO 9606-1, before production. Cross sections of production bellows welds are macro-etched to confirm penetration and to exclude cracks and inclusions. Hardness surveys per ISO 9015-1, ferrite content measurement, intergranular corrosion per ASTM A262 and pitting per ASTM G48 complete the metallurgical verification package. For sour service, compliance with ISO 15156 governs hardness and material selection.
Applications
Manual bellows globe valves with verified weld integrity are specified wherever zero-leakage performance and long maintenance-free service are mandatory. Typical applications include:
- Petrochemical and refining units handling toxic, flammable or carcinogenic media where fugitive emission control is regulated
- Chemical processes involving chlorine, phosgene, hydrogen fluoride and vinyl chloride monomer
- Steam, thermal oil and heat transfer systems requiring packing-free stem sealing
- High-vacuum systems and semiconductor process lines demanding leak rates below 1 × 10⁻⁹ Pa·m³/s
- Hydrogen, oxygen and specialty gas service with stringent cleanliness and tightness requirements
- Pharmaceutical and food-grade plants using clean steam and sterile process fluids
- Power generation and nuclear auxiliary systems with high cycle-life demands
- LNG and cryogenic service with appropriately qualified austenitic weldments
FAQ
Why is weld inspection so critical for bellows globe valves?
The bellows is the only dynamic barrier between the process fluid and the atmosphere. Any weld defect such as a pinhole, crack or lack of fusion immediately compromises the barrier and allows fugitive emissions, while poor weld geometry accelerates fatigue failure of the bellows. Systematic weld inspection is therefore essential for safety, environmental compliance and service life.
Which non-destructive testing method is most suitable for bellows welds?
Liquid penetrant testing per ISO 3452-1 or ASTM E165 is the primary method because bellows materials are thin, non-magnetic austenitic alloys. It detects surface-breaking defects with high sensitivity and is repeated after pressure testing. Magnetic particle testing is reserved for ferritic components, and volumetric methods are applied to thicker shell welds.
What helium leak rate is typically accepted?
Acceptance limits depend on the service. General chemical service commonly allows 1 × 10⁻⁶ Pa·m³/s, toxic media around 1 × 10⁻⁷ to 1 × 10⁻⁸ Pa·m³/s, and vacuum or high-purity applications 1 × 10⁻⁹ Pa·m³/s or tighter, verified by the evacuated-hood method with a calibrated helium mass spectrometer.
How are welds qualified before production begins?
Each weld joint is covered by a qualified welding procedure specification supported by a procedure qualification record per ISO 15614-1 or ASME Section IX, executed by welders certified to ISO 9606-1. Production test coupons and periodic destructive cross-sections confirm that the qualified conditions are maintained during serial manufacture.
Can radiographic testing be applied to edge-welded bellows?
In practice, no. The overlapping thin diaphragms and curved geometry make radiographic interpretation unreliable. Manufacturers instead rely on one hundred percent penetrant testing, helium leak testing and destructive metallographic sectioning of representative samples to prove weld quality.
Which standards govern the inspection of these valves?
Key references include ISO 5208 and GB/T 13927 for pressure testing, ISO 15848 for fugitive emissions, ISO 17637 for visual testing, ISO 3452-1 and ASTM E165 for penetrant testing, ISO 9934-1 and ASTM E709 for magnetic particle testing, ISO 17636-1 for radiography, ISO 17640 for ultrasonic testing, ISO 5817 for weld quality levels, ASTM A262 and ASTM G48 for corrosion, and ASME B16.34 for pressure-temperature ratings, together with GB/T 12235 for globe valve requirements.
Can the testing be outsourced?
Yes. Manufacturers and purchasers routinely engage accredited third-party laboratories to perform or witness NDT, helium leak testing, fugitive emission testing and type approvals, providing independent certification accepted by end users, engineering contractors and regulatory authorities.