Pressure Vessel Testing & Inspection – What Really Happens on Site

Pressure Vessel Testing & Inspection – What Really Happens on Site

Here’s the truth:
You can have the best drawings, the cleanest welds, and still end up with a ticking bomb if your testing is sloppy.

Pressure vessel testing isn’t paperwork. It’s the part where you find out if what’s on paper actually holds pressure.
In the field, this is where the vessel proves it’s worth the plate thickness — and where bad welding, poor heat treatment, or cheap gaskets show their face.

Let’s go through how testing really works on site — not just what the ASME book says.

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1. Hydrostatic Test – The Moment of Truth

Hydrotest setup during pressure vessel inspection

Hydrotest is the standard pressure proof test.
You fill the vessel with water, bleed out the air, and raise the pressure slowly — usually to 1.3–1.5× the design pressure.

Example:
Design 10 bar → Test at 13–15 bar.

Procedure on site:

1. Fill vessel with clean water (no air pockets).
2. Pressurize using a hand pump or hydraulic booster.
3. Hold pressure for 10–30 minutes depending on code.
4. Inspect all weld seams, nozzles, and flanges for leaks or deformation.
5. Release pressure slowly — never “pop” it open.

Pro tip:
When pressure drops suddenly, it’s either a leak or trapped air.
If you hear a “ping”, check the nozzle welds first — they crack before the shell does.

Typical Fail Points:

• Fillet welds on saddle supports
• Reinforcement pad welds around nozzles
• Manhole gasket leaks

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2. Pneumatic Test – The Dangerous One

Pneumatic testing of a vessel where hydrotest is not suitable.

If the fluid can’t be exposed to water (like in oxygen or certain chemical systems), we do a pneumatic test — using air or nitrogen.

But make no mistake — this test is risky.
Compressed gas stores huge energy.
A rupture here isn’t a leak, it’s an explosion.

Procedure:

1. Fill the vessel with gas slowly to 25%, 50%, 75%, and 100% of test pressure.
2. Soap test (leak check) on all weld seams and joints.
3. Hold and monitor for at least 10 minutes.

Test Pressure:
Usually 1.1× design pressure, lower than hydro because the risk is higher.

Golden Rule:
Nobody stands near the vessel during pressurization.
If they do, they’re either new or reckless.

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3. Non-Destructive Testing (NDT) – The Invisible Work

Surface inspection using dye penetrant test during vessel fabrication.

NDT is where real quality control happens.
Hydro catches leaks, NDT catches weakness before it leaks.

Overview of four NDT techniques — Visual, Penetrant, Radiographic, and Ultrasonic Testing — with key differences and uses

Method	What It Detects	Notes
RT (Radiographic)	Internal weld defects	Best for root fusion issues
UT (Ultrasonic)	Thickness, cracks inside material	Used for shell & head areas
PT (Penetrant)	Surface cracks	Simple, cheap, effective
MT (Magnetic)	Subsurface defects on ferrous metals	Used for CS & low alloy
VT (Visual)	Obvious surface issues	Done before everything else

Pro tip:
PT is the most underestimated test — half the time, it’s the only one that catches hairline cracks after grinding.

Real Field Tip:
Always clean weld surfaces with acetone before PT.
If you skip it, the dye lies.

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4. Documentation & Traceability

Once tests are done, you’re not finished until the paperwork matches reality.

Checklist:

• Hydro/Pneumatic Test Report (pressure curve, duration, remarks)
• NDT Reports (signed by Level II or III inspector)
• WPS, PQR, WPQ Certificates
• Material Test Report (MTR)
• Final Inspection Record (FIR)

Inspectors from KGS or a third party (LR, BV, TUV) will check every number.
If your PQR or MTR doesn’t line up with the actual batch — it’s an automatic fail.

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5. Field Lessons (You Only Learn Once)

• Never hydrotest on a Friday. If something leaks, nobody stays late to fix it.
• Always bleed trapped air before you pressurize — air kills faster than water leaks.
• Never reuse gaskets for final test.
• For stainless vessels, dry them immediately after hydrotest — residual moisture = corrosion.
• If the gauge reading doesn’t move but the pump’s running, stop. You’re about to burst something.

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Conclusion

Pressure testing is where design, fabrication, and reality meet.
It’s not a formality — it’s the only proof that your vessel deserves that U-stamp.

A vessel that passes hydro and NDT cleanly will run for decades.
One that barely scrapes through will haunt your maintenance logs for years.

So, take your time, do it right, and remember:
“Water doesn’t lie — it finds every shortcut you took.”

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