Hydrostatic Testing: Pressure Vessels & Piping

What Is a Hydrostatic Test?

A hydrostatic test fills pressure equipment with water (or another incompressible liquid) at pressures higher than normal operating conditions to verify structural integrity. If the equipment holds pressure without leaking, deforming, or rupturing, it passes. This non-destructive test is mandatory for pressure vessels, pipelines, boilers, and storage tanks.

The Hydrostatic Test

Hydrostatic test (Source: Nigen International)

Purpose and Advantages

Purpose	Details
Leak detection	Ensures no loss of containment through cracks, pinholes, or weld defects
Strength verification	Confirms equipment can withstand maximum operating pressure safely
Durability assessment	Evaluates structural integrity after repairs, modifications, or extended service
Regulatory compliance	Satisfies mandatory safety certifications in most industries

Special Considerations

• Water-sensitive equipment: Some equipment cannot tolerate water exposure-consider pneumatic testing as an alternative (higher risk)
• Test water disposal: Water with additives or dyes must be disposed of per environmental regulations
• Correct test pressure: Excessive pressure damages equipment; insufficient pressure provides inadequate verification

Equipment Requiring Hydrostatic Testing

Equipment Type	Application
Pressure vessels	Tanks, boilers, air tanks, gas cylinders
Pipelines and piping systems	Oil & gas, chemical plants, water distribution
Storage tanks	Above-ground and underground (water, chemicals, oil, gas)
Fire extinguishers	Periodic requalification for emergency readiness
Gas cylinders	Industrial, medical, and diving applications
Sprinkler systems	Fire suppression systems
Industrial hoses	Firefighting and high-pressure applications
Valves	High-pressure valve bodies and seats
Heat exchangers	Shell-side and tube-side verification
Boiler components	Tubes, drums, and pressure parts

Test Execution Process

Hydrostatic tests are executed by skilled personnel using chlorinated water (or colored fluids) to detect leaks in pressure-containing equipment. Test fluid chemistry must be compatible with equipment materials.

Step	Actions
1. Preparation	Ensure PPE, visually inspect for damage, clean equipment, isolate test section from system
2. Filling	Slowly fill with water (or compatible liquid), eliminate all air pockets-trapped air compresses and can cause dangerous depressurization
3. Pressurization	Gradually increase pressure to test level (above operating pressure), monitor gauges continuously
4. Inspection	Check for leaks or deformation at welds, joints, and connections; use dye penetrant or UT if needed
5. Pressure hold	Maintain test pressure for specified duration per applicable standard
6. Depressurization	Slowly release pressure-rapid release can damage equipment
7. Documentation	Record test parameters, findings, and any corrective actions for compliance records
8. Post-test	Complete repairs if needed, perform final inspection before returning to service

Pro Tip

Always request a pre-test meeting with the client and contractor before major hydrotests. Align on hold points, test duration, acceptance criteria, and who signs off. Misunderstandings here cause expensive delays.

Air Pocket Hazard

Never pressurize a system with trapped air. Compressed air stores energy that can release violently during depressurization or failure. Always vent high points and verify the system is completely liquid-filled before pressurizing.

Field Note

For stainless steel systems, always use demineralized or chloride-free water - even short exposure to chlorinated water above ambient temperature can cause stress corrosion cracking. This has killed equipment on more than one project.

Applicable Standards

Organization	Standard	Coverage
ASME	B31 (Pressure Piping)	Design and testing of pressure piping systems
ASME	BPVC (Boiler & Pressure Vessel Code)	Fabrication, inspection, and testing of vessels/boilers
API	510 (Pressure Vessel Inspection)	In-service inspection, repair, and rerating of vessels
API	570 (Piping Inspection)	In-service piping inspection and hydrostatic testing
API	5L (Line Pipe)	Hydrostatic test requirements for PSL1/PSL2 pipes
ISO	14692 (GRP Piping)	Testing for glass-reinforced plastic piping
EN	13480 (Metallic Industrial Piping)	European requirements for metallic piping systems

Safety Best Practices

Practice	Requirement
Risk assessment	Identify hazards and implement safety measures before testing
Personnel qualification	Only trained personnel familiar with equipment and standards
Safety perimeter	Establish exclusion zone around test area
Emergency procedures	Have documented response plans for equipment failure

Common Mistake

Leaving test blinds and temporary spools in the system after hydrotest. Create a checklist of every temporary item installed - and verify removal before commissioning. I’ve seen plants start up with test blinds still in place.

Alternatives to Hydrostatic Testing

When hydrostatic testing is not feasible (water damage risk, drying difficulty, or quick turnaround needed), alternative methods can be used:

Method	Principle	Pros	Cons
Pneumatic testing	Uses air/nitrogen instead of water	No drying required, quick return to service	Higher risk-compressed gas stores more energy
Radiographic (RT)	X-rays or gamma rays detect internal flaws	Finds internal defects without pressure	Does not verify pressure integrity
Ultrasonic (UT)	Sound waves detect internal defects	Precise flaw detection and sizing	Does not verify pressure integrity
Magnetic particle (MPI)	Magnetic flux reveals surface cracks	Fast, effective on ferromagnetic materials	Surface/near-surface only
Dye penetrant (DPI)	Dye seeps into surface cracks	Simple, cost-effective	Surface flaws only
Acoustic emission (AET)	Monitors stress-induced sound waves	Can test at lower pressures	Requires specialized equipment
Helium leak testing	Tracer gas detected by sensitive sensors	Extremely sensitive leak detection	Does not assess structural strength

Combination Testing

No single method replaces hydrostatic testing completely. For critical applications, combine multiple NDT methods to comprehensively assess both defects and pressure integrity.

Related Articles

Testing and Inspection

• Non-Destructive Testing (NDT) Types - VI, LP, MP, UT, RT, and Eddy Current methods
• Mill Test Certificates (EN 10204) - MTC types 2.1, 2.2, 3.1, and 3.2
• Pipe Inspections - Dimensional, metallurgical, and marking verification

Related Standards

• API 5L Line Pipe Specification - Hydrostatic test requirements for line pipe
• ASME B36.10/B36.19 Pipe Sizes - Dimensional standards for steel pipes