Shell strength testing is a mandatory pressure test that verifies the pressure-containing envelope of a bellows globe valve—including the body, bonnet, joints, and bellows assembly—can withstand loads well above its rated working pressure without leakage or permanent deformation. This guide explains the governing standards such as ISO 5208, API 598, EN 12266, and GB/T 13927, together with test pressures, holding durations, procedures, and acceptance criteria commonly applied by manufacturers and accredited third-party laboratories.
Technical Overview
A bellows globe valve is a linear-motion shut-off valve in which a welded metal bellows—typically a multi-ply convolution of austenitic stainless steel or nickel alloy—seals the stem passage between the body cavity and the atmosphere. Because the bellows travels with the stem and forms an all-metal static barrier, these valves achieve near-zero fugitive emission performance and are therefore specified for toxic, flammable, radioactive, high-purity, and high-vacuum services where conventional packing alone cannot guarantee long-term tightness.
Shell strength testing, also called the body test or hydrostatic strength test, subjects the complete pressure-retaining envelope of the valve—the body, bonnet, body-bonnet joint, end connections, and all other walls in contact with the process fluid—to a pressure significantly above the maximum allowable working pressure. The objective is to demonstrate structural integrity: the envelope must not rupture, crack, visibly leak, or undergo permanent deformation. Since the internal bellows is loaded by external pressure during the test, the shell test also provides an indirect check of the bellows assembly's resistance to external-pressure collapse.
Shell strength testing of bellows globe valves is typically performed in accordance with one or more of the following standards:
- ISO 5208 – Industrial valves – Pressure testing for metallic valves
- API 598 – Valve Inspection and Testing
- EN 12266-1 and EN 12266-2 – Industrial valves – Testing of metallic valves
- GB/T 13927 – Pressure testing of industrial valves
- ASME B16.34 – Pressure-temperature ratings for flanged, threaded, and welding-end valves
- API 623 – Corrosion-resistant, bolted bonnet globe valves for petroleum and natural gas industries
- EJMA – Standards of the Expansion Joint Manufacturers Association, for bellows design and qualification
Testing Samples
Complete Valve Assemblies
Production and type-test samples representing the full manufacturing range, for example cast steel (WCB, WC6, WC9), forged steel (A105, F11, F22), and stainless steel (CF8M, F316L) bellows globe valves in sizes from DN 8 to DN 600 (1/4 inch to 24 inch) and pressure ratings from PN 16 to PN 420 or ASME Class 150 to Class 2500. End connections may be flanged, butt-weld, socket-weld, or threaded. Valves are tested in the fully assembled condition, complete with bonnet, bellows, stem, handwheel or actuator interface, and, where fitted, secondary packing.
Pressure-Retaining Components
Bodies, bonnets, and end connectors taken from casting or forging qualification batches may be individually proof-tested, including components retested after weld repair. This verifies the soundness of the pressure boundary material itself before final assembly, complemented by non-destructive examination such as radiographic, ultrasonic, or liquid penetrant testing.
Bellows Subassemblies
Two-ply to five-ply formed and welded bellows in materials such as 316L, 321, or Inconel 625, supplied with end fittings, are subjected to component-level tests including internal pressure burst, external-pressure collapse, and helium leak detection. These qualification tests precede and support the shell strength test of the finished valve.
Testing Items
Depending on the purchase specification and product standard, a complete test program for a bellows globe valve typically includes the following items:
- Shell strength (body) hydrostatic test
- Seat (closure) tightness test
- Bellows external-pressure stability and leakage verification
- Material verification, wall-thickness measurement, and dimensional checks
- Functional checks such as operating torque and smoothness of stroking after depressurization
Shell Strength (Body) Test
The primary test applies a hydrostatic pressure of at least 1.5 times the rated pressure to the entire pressure envelope with the valve partially open, so that both the upstream and downstream cavities as well as the bonnet side are pressurized. Acceptance requires no visible leakage and no permanent deformation.
Seat Tightness Test
Although distinct from the shell test, the seat test is almost always performed in sequence. With the valve fully closed, pressure is applied at approximately 1.1 times the rated pressure and leakage across the disc-seat interface is measured against a defined leak-rate class, such as Rate A of ISO 5208.
Bellows Integrity Verification
The inspection confirms that the bellows shows no leakage at its welded seams, no inter-ply weeping, and no collapse under the external pressure imposed during the shell test. For critical services this is supplemented by helium mass-spectrometer leak testing at component level.
Testing Methods
Hydrostatic Shell Strength Test Procedure
The standard hydrostatic procedure follows these steps:
- Clean the valve interior, verify cleanliness, and perform a visual and dimensional inspection before mounting the valve on the test bench with blind flanges or adaptors.
- Set the valve partially open so that the test medium reaches both sides of the seat, the bonnet cavity, and the exterior of the bellows.
- Fill the valve with filtered, low-chloride water, with corrosion inhibitor where permitted, through the low-point connection and vent all trapped air through the high point.
- Increase the pressure gradually to the specified shell test pressure using a calibrated hydrostatic pump.
- Hold the pressure for the minimum duration determined by valve size, while monitoring calibrated pressure gauges for any decay.
- Inspect all external surfaces, the body-bonnet joint, welds, gasketed or pressure-seal joints, and the bellows region for drops, weeping, sweating, or visible deformation.
- Depressurize slowly, drain completely, and dry the interior, paying particular attention to the bellows cavity to prevent residual-water corrosion.
- Issue a test report recording the medium, test pressure, holding time, ambient temperature, gauge calibration traceability, observations, and pass or fail conclusion.
Test Pressure and Holding Duration
Under ISO 5208, GB/T 13927, and API 598, the hydrostatic shell test pressure is at least 1.5 times the rated pressure or 1.5 times the 38 degree Celsius pressure-temperature rating of the applicable ASME B16.34 class. For example, a PN 40 valve is shell tested at a minimum of 60 bar, while its seat test is performed at approximately 44 bar. The minimum holding durations are:
- DN 8 to DN 50: minimum 15 seconds
- DN 65 to DN 150: minimum 60 seconds
- DN 200 to DN 300: minimum 120 seconds
- DN 350 and larger: minimum 300 seconds
The test medium temperature is normally kept between 5 and 40 degrees Celsius, and pressure gauges are calibrated instruments with a range between approximately 1.5 and 4 times the test pressure, as required by API 598.
Pneumatic (Gas) Shell Testing
Gas testing is used only when hydrostatic testing is impractical, for example on valves with moisture-sensitive internals or linings. Because compressed gas stores significant energy, the test pressure is usually reduced, commonly to about 1.1 times the rated pressure, and strict safety measures are enforced, including protective barriers, remote monitoring, and leak detection by soap solution or underwater immersion. Pneumatic shell tests are exceptional and require prior agreement with the purchaser or engineering authority.
Acceptance Criteria
A valve passes the shell strength test when all of the following conditions are satisfied:
- No visible leakage in the form of drops, weeping, or sweating through any pressure-retaining wall or joint
- No structural damage, cracking, or visible permanent deformation of the body, bonnet, or end connections
- No sustained pressure drop that cannot be attributed to temperature stabilization
- No collapse, rupture, or inter-ply leakage of the bellows assembly
- Stable gauge readings maintained for the entire holding period
Bellows-Specific Considerations
During the shell test the bellows is subjected to full external test pressure, so convolution stability must be confirmed by design calculation or prior type testing, particularly for single-ply constructions. Low-chloride or demineralized water should be used for austenitic stainless steel and nickel-alloy bellows to avoid chloride stress corrosion cracking, and the bellows cavity must be drained and dried immediately after testing. It should also be noted that shell strength testing does not replace bellows fatigue (cycle life) qualification, which is a separate type test covering thousands of open-close cycles.
Applications
Shell strength testing of bellows globe valves is demanded wherever absolute containment and verifiable structural integrity are essential. Typical applications include:
- Petrochemical and refining plants – isolation of toxic and flammable hydrocarbons, with fugitive emission compliance to ISO 15848 or TA Luft
- Chemical and specialty chemical plants – corrosive, lethal, and high-value media handling
- Thermal power stations – boiler feedwater, steam vent and drain lines, and lubricating oil systems
- Nuclear power plants – radioactive coolant and auxiliary systems requiring absolute stem sealing
- Oil, gas, and LNG facilities – prevention of methane slip and greenhouse gas emissions
- Hydrogen, vacuum, and heat-transfer (thermal oil) systems where zero leakage is critical
- Pharmaceutical, semiconductor, and high-purity gas distribution systems
FAQ
What is the purpose of shell strength testing for bellows globe valves?
It proves that the pressure-containing envelope of the valve—the body, bonnet, joints, and associated walls—can safely withstand a pressure substantially above its rated working pressure without rupture, visible leakage, or permanent deformation, thereby validating design, material quality, and workmanship.
What test pressure is applied and how long is it held?
The hydrostatic shell test pressure is at least 1.5 times the rated pressure or the applicable ASME B16.34 class rating. The pressure is held for a minimum of 15 seconds on small valves up to DN 50, increasing to 60, 120, or 300 seconds for larger sizes in accordance with ISO 5208, API 598, and GB/T 13927.
Is the test performed with water or gas?
Water is the default medium because it is incompressible and stores little energy. Gas testing is reserved for special cases where water ingress is unacceptable, and it is performed at reduced pressure with stringent safety precautions.
Can the shell test damage the bellows?
When performed correctly, no. Bellows are designed to withstand external pressure at the shell test level, and multi-ply constructions provide an additional safety margin. The external-pressure collapse capability is verified during bellows qualification, and inspectors specifically check for convolution collapse during the shell test.
Which standards govern shell strength testing?
The most widely applied standards are ISO 5208, API 598, EN 12266, and GB/T 13927, supported by the pressure-temperature ratings of ASME B16.34 and GB/T 12224 and bellows design rules of EJMA.
Do bellows globe valves need retesting after installation or maintenance?
Yes. Valves that have undergone weld repairs, bellows replacement, or major maintenance should be retested to the original shell and seat test requirements, and plant inspection programs often specify periodic revalidation during scheduled turnarounds.