All are aware that one of the important activity in any process or power plants are leakage testing (Pressure testing). Subsequent to repairs and heat treatment and prior to initial operation or commissioning of any plant it has to be ensured that there is no evidence of leakage. To start this important activity i.e, leakage tests like hydro or pneumatic testing, piping network, and connected equipment must be prepared for leakage tests. Codes like ASME B31.3/ ASME B 31.1 and local company instructions normally provide guidelines for such preparation.
Piping systems that are normally open to the atmosphere, such as drains, vents, discharge piping from pressure-relieving devices, sewers, and stack downstream of the seal drum, need not be subjected to the piping test pressure.
For some countries, The provincial local inspector shall be notified at least 48 hours in advance of pressure testing of piping under the jurisdiction of the provincial safety code for witnessing at his/her option.
The following equipment shall be excluded from all piping pressure tests:
- Vessels and tanks
- Heat exchangers
- Rotating machinery, such as pumps, compressors, and turbines
- Equipment and Supplier furnished piping specifically recommended by the manufacturer not to be tested
Underground portions of piping systems may be tested and covered before testing aboveground portions.
Roughly, The following steps should be followed for preparing the piping system for leakage testing.
Comparison of P&ID/PEFS and Isometric Drawing
The first step is to compare the piping isometrics with the P&ID drawings to check if any discrepancy exists. All valve types, flow directions, branch tie-ins, and any material changes, etc are reviewed thoroughly. In-line components are verified and ensured that they are able to withstand the test pressure.
Thorough Inspection before testing
All sorts of ultrasonic and radiographic inspections or checks must be carried out before the actual pressure test is started. 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 be waived off if the pipeline is to be hydrostatically tested.
Joints shall be exposed
All joints, welds (including structural attachment welds to pressure-containing components), and bonds shall be left uninsulated and exposed for examination during leak testing as per Section 345.3.1 of ASME B31.3, except that joints previously tested in accordance with this Code may be insulated or covered.
Provision of Temporary Supports
If required Pipings designed for vapor or gas services shall be provided with additional temporary supports, to support the weight of test liquid as the test liquid is heavier than service fluid.
Spring Supports in Piping System
All Spring supports shall be kept in the locked condition during hydrostatic testing. Holding pins shall not be removed from spring supports until testing is completed and the system is drained. Care shall be taken to avoid overloading any parts of the supporting structures during hydrostatic testing.
- All in-line Instruments shall be either removed or blocked prior to hydro testing to prevent damage during hydro testing (e.g. meters).
- Instrument take-off piping and sampling system piping, up to the first block valve, shall be tested with the piping to which it is connected.
- Instrument lead lines, between the first block valve and the instruments to which they are connected, shall be pressure tested to the test pressure of the associated piping. Testing may be performed separately or at the same time as the piping is tested, but the instruments shall be disconnected.
Piping with Expansion Joints
If there is an expansion joint in the piping system under the leakage test, the following criteria shall be applied.
- An expansion joint that depends on external main anchors to restrain pressure end load shall be tested in place in the piping system.
- A self-restrained expansion joint previously shop-tested by the manufacturer
- A piping system containing expansion joints shall be leak tested without temporary joint or anchor restraint at the lesser of
- 150 % of design pressure for a bellows-type expansion joint, or
- the system test pressure determined in accordance with para. 345
In no case shall a bellows-type expansion joint be subjected to a test pressure greater than the manufacturer’s test pressure.
- When a system leak test at a pressure greater than the minimum test pressure specified in (c), or greater than 150% of the design pressure within the limitations of para. 345.2.1(a) is required, bellows-type expansion joints shall be removed from the piping system or temporary restraints shall be added to limit main anchor loads if necessary.
Limits of Tested Piping
Equipment that is not to be tested must be either disconnected from the piping or isolated by blinds or other means during the test. A valve may be used provided the valve is suitable for the test pressure.
For rotary machinery, such as pumps, compressors, or steam turbines, have lube and seal oil systems that could be impaired by the presence of water. These systems shall not be subjected to the piping test pressure.
Clean water that will not corrode and/or damage the test system shall be used for hydrostatic tests. If municipal water is not available, supply water may be obtained from a native water supply. Water containing silt or suspended material shall not be used, and a suitable filter with 40 mesh should be provided in the filling line. A mixture of glycol-water shall be used where the ambient temperature may reach less than 0°C (32°F) during testing or prior to dry-out
Temporary Spades and Blanks
If Temporary spades and blanks are installed for testing purposes, those must be designed to withstand the test pressure without distortion. The presence of spades shall be clearly visible during testing. The recommended practice is to use standard blind flanges as per ASME B16.5 or B16.47 and spades acc. to ASME B16.48.
Check Valves shall have the piston or flap removed for testing, where pressure can not be located on the upstream side of the valve. The locking device of the flap pivot pin shall be reinstated together with the flap and a new cover gasket shall be installed after completion of the test.
Completion of Hot Work
Wherever applicable All hot works, related to welding or the post-weld heat treatment have to be completed before the Hydrostatic test.
Installation of Barriers
It is necessary to install safety barriers around the piping system under test prior to starting pressurization related to testing. Public Address announcements and access restriction procedures such as permit to work shall be implemented. Under no circumstances should anyone other than an authorized person be allowed within the safety barriers.
Prior to the leakage test, Control Valves and soft-seal block Valves shall be removed from the piping and replaced with pipe spools. However, All welded Valves need to be tested along with pipings otherwise Radiographic Testing of golden joints (welded) will be required.
Prior to test following should be checked :
- All gravity supports installed.
- Proper pipe routing.
- Vents or other high point connections shall be opened to eliminate air from and plugged during the test.
- lines that are to receive a hydrostatic test.
- Completed and torqued flanges with no missing bolts or gaskets.
- Proper material type verified using color codes or markings, and heat numbers recorded if required by the codes.
- Correct valve type and orientation.
- Vents and drains installed to allow proper filling and draining
- When a pressure test is required to be maintained for a period of time during which the testing medium in the system would be subject to thermal expansion, the provision shall be made for the relief of any pressure greater than the maximum test pressure.
- All required piping stress relief, weld examinations, and welding documentation completed and acceptable. A relief valve to be added such that the test pressure is not exceeded beyond a safe amount.
- When conducting tests at freezing temperatures, the test shall not take more than 4 hours, and special precautions, such as using glycol/water mixture, shall be observed to avoid freezing damage
The individual system documentation i.e. test pack shall be available prior to any testing and shall include information such as test limits, test pressure, test medium, duration, test blinds, blind flanges, vents, and drains.
The use of marked up P&IDs coupled with isolation registers should be utilised to identify the locations of blinds, Valves, vents and drains.
Records shall be made of each piping system test. In general, these records shall include the following information:
- Date of test
- Identification of piping system
- Test medium
- Test pressure
- Minimum ambient temperature
- Test medium temperature
- Certification by examiner
- Test duration
- Pipe standards or specifications for the test
- Pressure and temperature recording charts and logs
- Location and cause of leaks/failures
- Repair procedures used in the event that the pipe leaked of failed
- Test results
- Name of test operator
- Minimum metal temperature (if applicable)
- Test gauge calibration date
Forms, when completed, shall be retained in the Construction Contractor’s QC file as a permanent record.
Testing equipment such as pumps, manifold, pressure, and temperature recorders, pressure gauges should be within calibration/certification (as per company procedures) and connected to the lowest convenient connection within the system to ensure the best results. To know the basics of pressure test please refer to my earlier two articles on Hydrotesting with the subject “Pressure Tests of Piping systems-Hydrotest Vs Pneumatic Test” and “THE HAZARDS OF PRESSURE TESTING“