What Is Pre-Commissioning?

Pre-commissioning is the set of activities performed on a system after mechanical completion (MC) and before commissioning to verify that the installation is safe, clean, leak-free, and ready for functional testing. Pre-commissioning focuses on the physical integrity of the system—not its functional performance.

For piping systems, pre-commissioning includes flushing, chemical cleaning, hydrostatic testing, leak testing, drying, and reinstatement. These activities remove construction debris, verify pressure integrity, and prepare the system for introduction of process-compatible fluids.

Pre-Commissioning Activities

Activity	Purpose	Applies To
Piping flush	Remove debris, weld slag, dirt, scale	All piping systems
Chemical cleaning	Remove mill scale, rust, grease with acid or alkaline solutions	Carbon steel systems, boiler tubes, lube oil systems
Hydrostatic test	Verify pressure integrity at 1.5x design pressure	All pressure-containing piping
Pneumatic leak test	Detect leaks using air or nitrogen at low pressure	After hydro test reinstatement; instrument air systems
Drying	Remove residual water to prevent corrosion or process contamination	Stainless steel systems, instrument air, gas service
Nitrogen purge	Displace oxygen to prevent oxidation	Stainless steel, alloy systems, oxygen-free service
Megger testing	Verify electrical insulation integrity	Cables, motors
Continuity check	Verify cable-to-cable and cable-to-terminal connections	Instrumentation and electrical circuits

Pre-Commissioning Sequence for Piping

Step	Activity	Duration (Typical)
1	Install temporary flushing circuits (blinds, spades, temporary strainers)	1-3 days
2	Water fill and flushing at high velocity (1.5-2.0 m/s minimum)	4-8 hours per system
3	Inspect strainer screens; reflush if debris found	1-2 days
4	Hydrostatic pressure test at 1.5x design pressure, hold for minimum 2 hours	4-12 hours
5	Depressurize, drain, and inspect joints for leaks	2-4 hours
6	Reinstatement: remove blinds, install permanent gaskets, torque bolts	1-3 days
7	Pneumatic leak test at reinstatement joints	4-8 hours
8	Drying (if required): air blow, nitrogen purge, or vacuum drying	1-5 days
9	System preservation (nitrogen blanket, dehumidified air) until commissioning	Ongoing

Hydrostatic Test Requirements

The hydrostatic test is the most critical pre-commissioning activity for piping. Key parameters:

Parameter	Requirement
Test pressure	1.5x design pressure (ASME B31.3)
Test medium	Clean water (potable or treated). Chloride content below 50 ppm for stainless steel
Hold time	Minimum 2 hours for small systems; 4-24 hours for large systems
Ambient temperature	Above 5 deg C (risk of freezing); metal temperature above MDMT
Pressure gauge calibration	Calibrated within 6 months, range 1.5-4x test pressure
Acceptance criteria	No visible leaks, no pressure drop beyond thermal expansion/contraction

Common Mistake

Using untreated water for hydrostatic testing of stainless steel piping. Chloride ions in tap water cause pitting corrosion if the water is left in contact with stainless steel for more than 24-48 hours. Always use demineralized water or water treated to less than 50 ppm chloride content. Drain and dry stainless steel systems immediately after the test.

Pre-Commissioning Documentation

Document	Content
Flushing report	Flushing medium, velocity achieved, strainer inspection results
Hydrostatic test pack	Test pressure, hold time, pressure chart, signed by QC and TPI
Leak test report	Pneumatic test pressure, medium, duration, results
Chemical cleaning report	Solution type, concentration, contact time, passivation record
Drying report	Method, dew point achieved, nitrogen purity
Reinstatement checklist	Blinds removed, permanent gaskets installed, bolts torqued

Pre-commissioning is the bridge between construction and commissioning. Skipping or rushing these activities leads to contamination, leaks, and corrosion that delay commissioning and damage equipment. For procurement documents related to pre-commissioning consumables (chemicals, nitrogen, temporary materials) and inspection requirements, see the detailed guides.

Read the full guide to pipe classes and specifications