Pressure Testing & Cryogenics — Complete Guide
A complete engineering guide to pressure testing and cryogenic engineering: hydrostatic and pneumatic pressure tests, leak testing methods (helium, bubble, pressure decay), burst and proof testing, cryogenic materials and equipment, vacuum technology, pressure fatigue, and standards.
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Pressure Testing & Cryogenics
Hydrostatic · Pneumatic · Leak Test · Burst · Cryogenic · Vacuum · Proof · Fatigue
💧 Hydrostatic Pressure Test
Hydrostatic Principles
Water / OilSafe
PrincipleFill the pressure vessel with liquid (water/oil) + raise pressure to the test value
Test Pressure1.25×–1.5× working pressure (MAWP) — depends on standard
Hold Time10–30 minutes minimum (depends on wall thickness and volume)
MediumClean water (with corrosion inhibitor) / oil / glycol
Key AdvantageA non-compressible liquid — failure doesn't cause an explosion!
MeasurementA calibrated gauge ±1% FS — at the highest point
Pass CriterionNo pressure drop, no visible leakage, no distortion
TemperatureMinimum 5°C above MDMT (brittle fracture analysis) — prevents brittle failure
▸ Air Release: Opening a vent at the highest point before testing is mandatory
▸ Safety: ← no personnel near pressure lines during pressurization
▸ Draining: after testing — drain + dry + preserve
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▸ Safety: ← no personnel near pressure lines during pressurization
▸ Draining: after testing — drain + dry + preserve
Hydrostatic Test Equipment
Test PumpManual/electric hydro pump — up to 700 bar
Pressure GaugeCalibrated, range 1.5×–4× test pressure, accuracy ±1%
Safety ValveRelief Valve — calibrated to 1.1× test pressure
ValvesBlind Flanges / Test Caps / Expansion Plugs
Pressure RecorderChart Recorder / Data Logger — continuous documentation
Fill PipingHigh-Pressure Hose — rated ×4 test pressure
ManufacturersMaximator, Haskel, Parker, Graco, Rice Hydro
Piping Hydro Test
ASME B31.3
Test Pressure1.5 × Design Pressure (ASME B31.3)
Time10 minutes minimum after stabilizing
PreparationDisconnect sensitive instruments, safety valves, expansion joints
VentingOpen an air vent at the highest point — mandatory!
DocumentationTest Pack: P&ID, Test Boundary, Gauge Cert, Results
ReinstatementRestoring instruments + safety valves after testing
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Vessel Hydro Test
ASME VIII
Test Pressure1.3 × MAWP × (σ_test/σ_design) — ASME VIII Div.1
Time30 minutes minimum
Visual InspectionInspect all welds + nozzles at test pressure
DistortionDiameter measurement before/after — permanent distortion = failure
Head TypesEllipsoidal (2:1) / Torispherical / Hemispherical / Flat
MDMTMinimum Design Metal Temperature — Charpy Impact Test
▶ YouTube
💨 Pneumatic Pressure Test
Pneumatic Principles
High Risk!Air / N₂
PrincipleRaising pressure with air/inert gas — high stored energy!
Test Pressure1.1 × Design Pressure (ASME B31.3) — lower than hydro!
When RequiredWhen water isn't allowed (catalyst, O₂ lines, drying not feasible)
Danger!Compressed gas = explosion! ×1000 the energy of liquid!
ProtectionBarricade / safety distance / remote monitoring
PressurizationGradual! 25% → 50% → 75% → 100% with a hold at each step
Inert GasN₂ preferred — ← ← prevents oxidation + safe
▸ Prohibited: pneumatic testing without safety engineer approval!
▸ Relief Valve: mandatory at 1.1× test pressure
▸ Stored Energy: E = P×V — a ⌀6" pipe 100m long at 10 bar = like TNT!
▸ Sensitive Leak Test: soap solution / Snoop — bubble testing
▶ YouTube
▸ Relief Valve: mandatory at 1.1× test pressure
▸ Stored Energy: E = P×V — a ⌀6" pipe 100m long at 10 bar = like TNT!
▸ Sensitive Leak Test: soap solution / Snoop — bubble testing
Hydrostatic vs. Pneumatic Comparison
| Parameter | Hydrostatic | Pneumatic |
|---|---|---|
| Medium | Water / Oil | Air / N₂ |
| Test Pressure (B31.3) | 1.5× Design | 1.1× Design |
| Explosion Risk | Very low | Very high! |
| Stored Energy | Minimal | Enormous |
| Leak Detection | Good (water visible) | Needs soap/Helium |
| Drying | Required after | Not required |
| Weight | Heavy (water) | Light (air) |
| Special Approval | No | Yes — safety engineer! |
🔎 Leak Testing Methods
Bubble Test
SimpleCommon
PrincipleSoap solution / Snoop on joints + internal pressure
Sensitivity~10⁻³ mbar·L/s — medium
UseGas piping, HVAC, flange connections, welds
ProductsSnoop / Leak-Tec / regular soap solution
StandardASME V Art.10 / ASTM E515
Helium Leak Test
High SensitivityAerospace
PrincipleHelium (a very small molecule) + a mass spectrometer
SensitivityUp to 10⁻¹² mbar·L/s — the most sensitive!
Sniffer ModeHe inside, sniffer outside — fast, less sensitive
Vacuum ModeHe outside, vacuum + MS inside — ↑↑sensitivity
BombingHe pressurized → removed → measure escaping He (sealed components)
UseSpace, nuclear, semiconductors, industrial HVAC, medical
StandardASME V Art.10 / MIL-STD-1246 / ASTM E499
ManufacturersPfeiffer, Agilent (Varian), Leybold, Inficon
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Pressure Decay
Mass Production
PrinciplePressurize → wait → measure the drop over a fixed time
Sensitivity~10⁻³–10⁻⁴ mbar·L/s — medium-high
AdvantageAutomatic, fast, Go/No-Go — production line
DrawbackTemperature sensitive! ΔT=1°C → significant ΔP
ImprovementDifferential Pressure — a Reference Volume cancels ΔT
ManufacturersATEQ, InterTech, Cincinnati Test Systems
▶ YouTube
Flow Test
PrincipleMeasuring gas flow through a leak — Mass Flow Meter
Sensitivity~10⁻²–10⁻⁴ mbar·L/s
AdvantageFast, not temperature-sensitive
UseAutomotive parts, faucets, valves, connectors
Dye Penetrant
NDT
PrinciplePenetrant dye → cleaning → developer → glowing cracks
VisibleRed dye — regular light
FluorescentUV (black light) — ↑↑sensitivity
LimitationSurface-open cracks only
StandardASME V Art.6 / ASTM E165 / ISO 3452
ManufacturersMagnaflux, Sherwin, MR Chemie, Helling
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Ultrasonic Testing
NDTThickness + Cracks
PrincipleSound waves 1–25MHz → reflection from a defect/back wall
Straight BeamWall thickness — 0° probe
Angle BeamWeld cracks — 45°/60°/70° shear wave
TOFDTime of Flight Diffraction — ↑defect sizing accuracy
Phased ArrayPAUT — full electronic scanning
CouplantGel / water / glycerin — mandatory!
StandardASME V Art.4+5 / ASTM E114 / ISO 17640
ManufacturersOlympus, GE/Baker Hughes, Sonatest, Zetec
▶ YouTube
💥 Burst Test / Proof Test
Proof Test
Non-Destructive
PrincipleRaising pressure above working pressure, below yield — proving integrity
Pressure1.25×–1.5× MAWP — depends on standard
DistortionPermanent deformation prohibited! (= Fail)
UsePressure vessels, pipes, gas cylinders, fittings
StandardASME VIII / PED 2014/68/EU / EN 13445
Burst Test
Destructive!
PrincipleRaising pressure to failure — determining Burst Pressure
MediumWater only! (safety)
Safety FactorBurst/MAWP ≥ 4 (ASME) / ≥ 3 (EN)
MeasurementPressure, volume, distortion — real-time
UseDesign validation, R&D, material failure, QA sampling
BarlowP_burst = 2×S×t/D — Barlow's formula for pipe
▸ Barlow: P = 2×UTS×t/OD — thin-wall pipe
▸ Sphere: P = 2×UTS×t/R
▸ SF=4 → MAWP = Burst/4
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▸ Sphere: P = 2×UTS×t/R
▸ SF=4 → MAWP = Burst/4
Charpy Impact Test
ASTM E23Low Temp.
PrincipleA pendulum hammer → a V-Notch specimen → fracture energy (Joules)
TemperatureTested at MDMT or lower
Minimum Value20J–27J for carbon steels (ASME VIII UCS-66)
Specimen10×10×55mm · V-Notch 2mm depth, 45°
Transition CurveThe ductile→brittle transition curve — DBTT
UseCryogenics, LNG, pressure vessels, bridges
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🧊 Cryogenic Engineering
Cryogenic Temperatures — Scale
Below 120K
DefinitionCryogenic = below 120K (−150°C)
LNG−162°C (111K) — liquid methane
LOX−183°C (90K) — liquid oxygen
LN₂−196°C (77K) — liquid nitrogen
LAr−186°C (87K) — liquid argon
LH₂−253°C (20K) — liquid hydrogen
LHe−269°C (4K) — liquid helium
Absolute Zero0K = −273.15°C — absolute zero
Materials for Cryogenics
AllowedProhibited
✅ SS 304/304LAustenitic — ↑toughness in cold · the #1 choice
✅ SS 316/316L↑corrosion resistance + cryogenic
✅ 9% Nickel SteelASTM A553 — LNG tanks · down to −196°C
✅ Al 5083/6061FCC — no ductile-brittle transition!
✅ Inconel 718Nickel superalloy — space/rockets
✅ PTFE/Kel-FSeals — flexible even at 77K
❌ Carbon SteelBCC → brittle! DBTT above −40°C
❌ Ferritic SSBCC → brittle in cryogenic conditions!
❌ NBR/Buna-NHardens and cracks — replace with PTFE/Viton-GLT
▸ Rule: FCC = good, BCC = bad in cryogenics
▸ Austenitic SS + Al + Cu + Ni = FCC = ductile in cold
▸ Carbon Steel + Ferritic SS + Martensitic = BCC = brittle!
▶ YouTube
▸ Austenitic SS + Al + Cu + Ni = FCC = ductile in cold
▸ Carbon Steel + Ferritic SS + Martensitic = BCC = brittle!
Cryogenic Valves & Piping
Extended BonnetA valve with an extended neck — packing kept away from the cold source
Vacuum JacketPiping with a vacuum jacket — thermal insulation
MLIMulti-Layer Insulation — aluminum layers under vacuum
PerliteGranular insulation under vacuum — LN₂/LOX tanks
Bellows SealedA bellows-sealed valve — zero leak
BOGBoil-Off Gas — evaporating gas — must be managed!
Cooldown Rate50–100°C/hr maximum — ← prevents thermal shock
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Cryogenic Treatment
Cutting ToolsRetained Austenite
PrincipleCooling to −196°C ← converts retained austenite → martensite
Deep Cryo−196°C for 24–36h → slow warming
Shallow Cryo−80°C (CO₂) — less effective
Advantages↑tool life ×2–5 · ↑wear resistance · ↑dimensional stability
UseHSS/carbide cutting tools, F1 engines, gears, valves
MaterialsD2, M2, H13, 52100, 440C — tool and bearing steels
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🌀 Vacuum Technology
Vacuum Ranges
| Range | Pressure | Use |
|---|---|---|
| Rough Vacuum | 1000–1 mbar | Drying, evacuation, packaging |
| Medium Vacuum | 1–10⁻³ mbar | Leak testing, PVD coating prep |
| High Vacuum | 10⁻³–10⁻⁷ mbar | PVD/CVD, silicon, accelerators |
| Ultra-High (UHV) | 10⁻⁷–10⁻¹² mbar | Space, particle physics, MBE |
Vacuum Pumps
Rotary VaneOil-sealed — Rough Vacuum down to 10⁻² mbar
ScrollOil-free (Dry) — clean, quiet
Roots BlowerA booster — ↑flow, needs a backing pump
Turbo MolecularHigh Vacuum — 10⁻¹⁰ mbar, 50,000+ RPM rotation
DiffusionBoiling oil — cheap, reliable, less clean
Cryo PumpGas capture on a cold surface 10–20K — UHV
Ion PumpIonization + sputter — UHV, maintenance-free
ManufacturersEdwards, Pfeiffer, Leybold, Busch, Agilent
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Vacuum Gauges
PiraniThermal — 1000–10⁻³ mbar
Capacitance (CDG)Accurate, gas-independent — 1000–10⁻⁴ mbar
Cold CathodePenning — High Vacuum 10⁻²–10⁻⁹ mbar
Hot CathodeBayard-Alpert — UHV 10⁻⁴–10⁻¹¹ mbar
RGAResidual Gas Analyzer — gas composition analysis
🔄 Pressure Fatigue & Cycling
Pressure Cycling Fatigue
Safety-Critical
PrincipleRepeated pressure cycles → fatigue cracking → failure
S-N CurveStress vs. cycles — ASME VIII-2 Appendix 5
Design CyclesIf N>1000 cycles — fatigue analysis is required!
Stress RangeΔσ = σ_max − σ_min — what determines fatigue
Nozzle/Weld↑stress concentration — the critical point!
AutofrettageOver-pressure → residual compressive stresses → ↑fatigue life
UseGas cylinders, accumulators, heat exchangers, reactors
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Autofrettage
↑Fatigue Life
PrincipleVery high internal pressure → plastic flow → residual compressive stresses
ImprovementFatigue life ×3–10!
HydraulicWater/oil at very high pressure
SwageA mandrel passed through the bore — small piping
UseGun barrels, Common Rail fuel injectors, HP piping
📋 Standards — Pressure Testing & Cryogenics
Pressure Vessel Standards
ASME VIII Div.1Pressure Vessels — fixed rules (the most common)
ASME VIII Div.2Pressure Vessels — analysis + rules (↑pressures, ↓weight)
EN 13445European Pressure Vessels — harmonized with PED
PED 2014/68/EUPressure Equipment Directive — CE marking mandatory
EN 13480Industrial Piping — European
AD 2000German pressure vessel standard — TÜV
Piping Standards
ASME B31.1Power Piping — power plants, steam
ASME B31.3Process Piping — chemical, oil, gas (the most common)
ASME B31.4Pipeline Transportation — oil
ASME B31.8Gas Transmission — natural gas
API 570Piping Inspection — ongoing maintenance
API 510Pressure Vessel Inspection — maintenance
NDT / Testing Standards
ASME VNondestructive Examination — all NDT methods
ASTM E165Liquid Penetrant — PT
ASTM E709Magnetic Particle — MT
ASTM E114Ultrasonic — UT
ASTM E94Radiographic — RT
ASTM E499Leak Testing — He MS
ISO 5817Weld Quality — B/C/D acceptance
SNT-TC-1ANDT Personnel Qualification — Level I/II/III
Cryogenics Standards
CGA S-1.1Pressure Relief Devices — gas cylinders
EIGA Doc 24Liquid Oxygen / Nitrogen Storage
EN 13458Cryogenic Vessels — vacuum insulated
NFPA 55Compressed & Liquefied Gases — safety
ASTM A5539% Nickel Steel — LNG tanks
API 620 App.QLNG Storage — giant LNG tanks