Pressure Tests of Piping systems-Hydrotest Vs Pneumatic Test

A pressure test is always required for a new pressure system before use or an existing pressure system after repair or alteration  to ensure the following:

Nomally two methods for pressure testing: hydrostatic and pneumatic, are widely used in process industry. 
A hydrostatic test is performed by using water as the test medium, whereas a pneumatic test uses air, nitrogen, or any non-flammable and nontoxic gas. Pressure tests (both hydrostatic and pneumatic) must always be performed under controlled conditions, following an approved test plan, and documented in a test record. A single approved test plan could be used for several similar tests, but for each test a separate test record is required.

When to perform a Pressure Test:

As per the governing code, pressure test has to be performed on piping system after all hot works have been completed on a certain piping system. Here the term hot work means  everything related to welding or the post weld heat treatment (PWHT). PWHT also has a potential of degrading the mechanical properties of piping, in case not addressed properly. And this is the reason why code calls for NDT after PWHT operation. In case after carrying out pressure testing, some modification has to be made requiring hot work, it calls for a retest as per code. Here code specifies that the minor repairs / modifications may be waived off provided adequate measures have been taken to ensure sound construction. Now taking this decision as to which repairs or modifications may be waived off should be taken very carefully. Normally, the maximum extent of repairs not requiring retesting shall be the tack welding of any piping support or pad. Anything else shall be done following a retest.

Pressure testing and conducting 100% radiography or ultrasonic inspection shall not be interchanged. In case carrying out of hydrostatic or pneumatic test stands impractical then 100% radiography or ultrasonic testing may be performed but in addition to this it is advisable to check that the whole piping and its components have been supplied against acceptable ASTM standards and required test certificates are available. Conducting 100% radiography of all the weld joints assure that your weld joints are defect free but can never provide you with the assurance of mechanical integrity of a system. This is also to be noted that radiography / ultrasonic inspection shall also not to be waived off if the pipeline is to be hydrostatically tested. This may pose an additional safety risk during the test. Moreover, some defects in the weld zones may prove to be detrimental way after taken in service due to severe extended service conditions.

Comparison of Hydrostatic and Pneumatic Testing:

Hydrostatic Testing
Pneumatic Testing
Test pressure is normally 30% higher than the design pressure
Test pressure is normally 10% higher than the design pressure
Recommended for high pressure applications
Recommended only for low pressure applications
Test media (Water) used is not compressible by pressure application
Test media (Air) used is compressible by pressure application
Energy stored per unit volume of water under test pressure is very negligible
Energy stored per unit volume of compressed air under test pressure is very high
Needs thorough cleaning after test to
eliminate moisture especially for service which are reactive to moisture / fluids
Easy to clean after testing
Pressure Relief devices are recommended to control sudden increase in pressure during testing
Pressure relief devices are must during test to ensure no over pressurisation
Chances of equipment/ Pipe / test apparatus
failures are very low
Chances of equipment/ Pipe / test apparatus
failures are high
Weight of equipment along with test medium as
water is high hence special attention should be given to floor and supporting arrangements
Weight of equipment with test medium
as air is comparatively less
Needs verification and examination of joints and connections before testing
Needs very careful checking of weld joints thoroughly before testing
Test media can be reused or transferred to other place after testing
Test media can not be reused or transferred to other place after testing
Skilled and semi skilled personnel can carry out the test
Needs involvement of senior experienced staff to monitor the test.
Recommended where large volumes are to be tested at same time (example pipe lines )
If pipe lines are tested should be done with small segmental lengths at a time.
Damages made by failures are less compared to failures in pneumatic testing
Damages made by failures in testing are very huge and extensive
Hydrotest is a regular day to day practice and safe procedure and it can be followed in any work site
Needs special attention and safety precautions
 Pressure changes finite amount by infinitesimal change in volume
 Pressure change proportional to volume change 

Difficulties with hydrostatic testing:

Difficulties with Pneumatic testing:

Pneumatic tests are potentially more dangerous than hydrostatic tests because of the higher level of potential energy stored during compressing the gas. Care must be exercised to minimize the chance of brittle failure during testing by initially assuring the system is suitable for pneumatic testing. Pneumatic tests could be performed only when at least one of the following conditions exists:

Using a pneumatic test instead of hydrostatic requires approval from proper authority or body.

Test Pressure:

ASME Code B 31.3 provides the basis for test pressure. The minimum hydrostatic test pressure for metallic piping shall be as per the following equation:

Pt=minimum test gauge pressure
Pd=internal design gauge pressure
St=allowable stress value at test temperature
Sd=allowable stress value at design temperature.
The maximum allowable value of St/Sd is 6.5

The pneumatic test pressure shall be as per the following equation:   Pt=1.1*Pd

Normal Basis for Test Method Selection:

Normally the following basis is followed in process piping industry for selection of hydro or pneumatic tests.

  1. If the fluid handled by piping system is liquid then the pipe must be hydrotested.
  2. If the fluid handled is vapour or gas then the internal design pressure dictates the testing method. For pipe design pressure 10 bar and above the pipe is hydrotested. For pipe design pressure below 10 bar pipe is pneumatically tested.
  3. If fluid handled is stream then hydrotesting is suggested.
  4. If fluid handles is two-phase flow then hydrotesting is suggested. However for big size flare headers pneumatic testing is performed.
Test Duration:

Normally every EPC company have their own standard for duration of tests. However the recommended practice is that a QC inspector have to walk through the whole piping system and check for leaks. Every single length of piping, welds, bolted connections shall be visually examined for any leakage. Duration of this activity varies with the span of piping system. For larger piping system time taken for this activity is enough to clear the pressure test. In case of piping system having smaller span, 1 hour time may be made as standard practice for hydrostatic testing. For Pneumatic test the test time is far lesser.

Few important points to consider while hydrotesting:

  • The tower overhead lines which are normally hydro tested shall be decided by stress engineer group whether to hydro test after erection or at grade, based on the capacity of standard clip available as per engineering specification for pipe supports.
  • During hydrotest in case of big bore it should be decided first that line is to be hydrotested on rack or grade if hydrotest is done on rack , hydrotest load should be considered in case of vapour lines while providing structure / rack loading information.

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