How to Hydrotest a Pipeline: Procedure

A hydrostatic test (hydrotest) on a pipeline verifies the structural integrity and leak-tightness of the entire pipeline system, including pipe, fittings, flanges, and welds, before it is placed in service. This guide covers the step-by-step hydrotest procedure per ASME B31.3 (Process Piping), ASME B31.4 (Pipeline Transportation Systems for Liquids), and ASME B31.8 (Gas Transmission Pipelines).

Hydrotest Procedure: Step by Step

Step	Action	Details
1	Prepare test package	Document test boundaries, test pressure, hold time, ambient temperature limits, and equipment list. Submit for approval.
2	Isolate test section	Install blinds or spectacle blinds at test boundaries. Remove or isolate instruments, relief valves, and expansion joints not rated for test pressure.
3	Install test equipment	Connect calibrated test pump, pressure gauge(s), chart recorder (or data logger), and temperature gauges. Use two independent pressure gauges.
4	Fill with water	Fill from the lowest point. Use clean, treated water (inhibited to prevent corrosion, especially for stainless steel; use chloride-free water <50 ppm Cl).
5	Vent all air	Open high-point vents until water flows freely. All trapped air must be removed for an accurate test.
6	Pressurize gradually	Increase pressure in stages: 25%, 50%, 75%, then 100% of test pressure. Hold at each stage for 5 minutes to check for leaks.
7	Hold at test pressure	Maintain test pressure for the required hold time (minimum 10 minutes per ASME B31.3; 4-8 hours is common for pipelines per ASME B31.4/B31.8).
8	Visual inspection	Walk the entire test section during the hold period. Inspect all welds, flanges, fittings, and connections for leaks, weeping, or deformation.
9	Record and evaluate	Record pressure vs. time and temperature vs. time on a chart recorder. Minor pressure variations due to temperature changes are acceptable if they can be explained.
10	Depressurize and drain	Reduce pressure gradually. Drain and dry the pipeline. Dispose of test water per environmental regulations.

Test Pressure Requirements

Code	Test Pressure	Hold Time (Minimum)
ASME B31.3 (Process Piping)	1.5 x design pressure (adjusted for temperature)	10 minutes
ASME B31.1 (Power Piping)	1.5 x design pressure	10 minutes
ASME B31.4 (Liquid Pipelines)	1.25 x MOP (maximum operating pressure)	4 hours
ASME B31.8 (Gas Pipelines)	1.25 x MOP (Class 1-2) to 1.5 x MOP (Class 3-4)	8 hours
ASME B31.12 (Hydrogen Piping)	1.5 x design pressure	10 minutes
EN 13480 (European standard)	1.43 x design pressure	30 minutes

Pro Tip

Temperature stability is critical during the hold period. A 1 deg C change in water temperature can cause a pressure change of approximately 1-3 bar in a rigid pipeline. Schedule hydrotests during periods of stable ambient temperature (early morning or overcast days), and allow the water to equalize in temperature before starting the hold clock.

Key Points

• The test pressure per ASME B31.3 is calculated as: P_test = 1.5 x P_design x (S_test / S_design), where S_test and S_design are the allowable stresses at test temperature and design temperature, respectively.
• Never exceed 90% of the specified minimum yield strength of the pipe during the test.
• All test equipment (gauges, recorders) must be calibrated with traceable certificates.
• A successful hydrotest is documented in a hydrotest certificate, which becomes part of the pipeline’s permanent records.

The hydrotest is a key milestone in the piping inspection process and must be completed before commissioning.

Read the full guide to hydrostatic testing