This article describes the criteria and procedures, including safety precautions for carrying out pneumatic testing of piping system in lieu of hydrostatic test. In cases where hydrostatic testing is considered impractical, the Engineer may propose pneumatic test or hydrostatic-pneumatic test for review and approval to the Owner.
The following systems may be considered for pneumatic testing:
- Relief or flare systems outside the plant area.
- Piping with internal linings which may be subject to damage by the hydrotest fluid.
- Piping systems in which moistue is undesirable or cannot be tolerated such as in instrument air systems or refrigerant systems.
- Large bore piping carrying gas (example flare gas) over long runs for which the supporting structure is not designed to take care of hydrostatic test loads.
Section 345.5 of ASME B31.3 addresses the requirements of pneumatic testing.
Precautions and Preparations for Pneumatic Testing
All testing shall be conducted in accordance with the approved testing procedure and as per the Owners work permit procedure. Pneumatic tests above a defined pressure (defined by Company Standards) will require a risk review and approval by Company Safety Department.
While performing pneumatic testing, the minimum pipe metal temperature should be as listed below to avoid brittle failures:
| Nomial Wall Thickness | Minimum Temperature |
|---|---|
| Carbon Steel ≤ 38 mm | 15°C |
| Post-Heat Treated Ferric Alloy Steels ≤ 25 mm | 15°C |
| Austenitic Stainless Steels ≤ 25 mm | 12°C |
Testing shall not be conducted if the ambient temperature falls below the above stated values.
For test pressures exceeding 50psig, the construction contractor will be required to provide calculations for the stored energy of the system prior to conducting the test to determine the associated hazards in case of piping rupture.
The test assembly shall be provided with the requisite instrumentation to provide a permanent record of the test pressure and metal temperature for the entire duration of the test. The range of the pressure gauge shall be such that the test pressure is within the middle second-third of the reading range.
It shall be ensured that the piping connection between the compressed air source and test assembly is safely designed and supported.
The test area shall be cleared for safety, while bringing the piping system up to test pressure. It may be recommended to conduct the test during night or weekend when minimum construction or operating personnel are present in the vicinity of the testing area. Piping within the test limits shall be either tied down or firmly secured to prevent uncontrolled pipe movement in the event of rupture or sudden loss of pressure.
During pneumatic testing, care shall be exercised not to exceed the specified test pressure. As per ASME B31.3, paragraph 345.5.2, a pressure relief device shall be provided, having a set pressure not higher than the test pressure plus the lesser of 3.45 bar or 10% of the test pressure. Safety relief valves shall be sized to provide adequate relief capacity from the system being protected.
The maximum test pressure for pneumatic testing shall not exceed 110% of design pressure of the piping system.
Prior to soaping the joints, the entire line shall be examined to determine if there is any audible evidence of leakage. If any leaks are found at this time they shall be marked and repaired after depressurizing the line. The test will be temporarily suspended until the required repair work is carried out.
Bolting shall not be tightened while systems being tested are pressurized.
The area around the test assembly, to a minimum distance defined as exclusion zone shall be cordoned off by red and white barried tapes. All unauthorized personnel should be away from the exclusion joint prior to commencement of the test.
Pneumatic Testing Procedure
For pneumatic test, section 345.5.4 of ASME B31.3 stipulates that the test pressure shall be 110% of the design pressure. The test media shall be clean dry air or nitrogen. Any other test media may be proposed by the Engineer as long as it is non-flammable and does not adversely affect the piping system or cause a safety hazard. Oxygen shall never be used a test media.
As per ASME B31.3, the pneumatic test pressure is required to be increased gradually until the pressure reaches the lower of 25psi or half the test pressure. Thus, if the design pressure is 100psi, the required test pressure is 110psi. Since half of this value is 55psi, the initial pressurisation would not exceed 25psi. After attaining this pressure, all threaded, bolted and other mechanical joints shall be examined as per para. 34.4.1(a). Some engineering specifications conservatively recommend a lower value of initial pressurisation to 10-15psi and checking the system for major leaks. Subsequently, the test pressure is gradually increased in steps, holding the pressure at each step for sufficient time to allow the piping strains to equalize. The minimum recommended pressure steps are lower of 25psi or 25% of test pressure and a holding time of 5-10 minutes at each step is recommended as well by many specifications. The test pressure is then reduced to the design pressure and all joints are examined for leaks.
All joints and welds are examined using a commercially available bubble forming solution such as "SNOOP" that will facilitate quick detection of any leakage. One common method of testing a flanged joint is to mask it with a tape and check for leaks through a pin-hole punched in the masked tape. The pressure shall be maintained at the design pressure for sufficient length of time to allow all welded, flanged and threaded joints to be visually inspected. After all the joints have been tested, depressurization will be carried out slowly until the pressure reaches atmospheric. Depressurization shall not be carried out by loosening flanged connections or plugs. Also it shall be ensured that the temperature of the test system at the vent point does not fall below sub zero temperatures.
A double block and bleed (DBB) valve arrangement shall be included in the pressurising line on the system being tested. A test recording pressure gauge shall be provided downstream of the DBB arrangement. After each pressure step has been reached, close the block valves and open the bleeder to atmosphere. If after a five minute period the “Step Pressure” is maintained, proceed to the next “Step Pressure”; if not, examine the entire system for leakage.
Repairs prior to repeating the Pneumatic Test
If any leaks are found in a welded, flanged or threaded joint, the test shall be aborted and the piping system shall be depressurised for remedial action. Welded joints if found leaking shall be repaired using the approved weld repair procedure and subject to 100% radiograpy prior to repeating the pneumatic test. Leakage at flanged joints shall be investigated to ensure that the correct flange tightening procedures and bolt torques have been applied. Excessive torquing shall not be used to achieve leak-tight seal of a flanged joint.