- Reliability of operation
- and leak tightness of pressure systems
- Also The last physical quality test in the fabrication process
Nomally two methods for pressure testing: hydrostatic and pneumatic, are widely used in process industry.
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
- Supply & disposal of water, disposal of fluid additives
- Water leak can cause equipment damage
- Freeze susceptibility
- Structural support limitations
- Operational impacts – process contaminant
- Affect dry-out of internal refractory linings
- When the systems are designed in such a way thet it cannot be filled with water.
- When the systems are such that it is to be used in services where traces of the testing medium cannot be tolerated.
Using a pneumatic test instead of hydrostatic requires approval from proper authority or body.
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 6.5The pneumatic test pressure shall be as per the following equation: Pt=1.1*PdNormal Basis for Test Method Selection:
Normally the following basis is followed in process piping industry for selection of hydro or pneumatic tests.
- If the fluid handled by piping system is liquid then the pipe must be hydrotested.
- 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.
- If fluid handled is stream then hydrotesting is suggested.
- If fluid handles is two-phase flow then hydrotesting is suggested. However for big size flare headers pneumatic testing is performed.
- 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.