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:
- Reliability of operation
- and leak tightness of pressure systems
- Also The last physical quality test in the fabrication process
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 by Pressure Testing.
Types of Pressure Testing
In the process Industry, two methods are widely used for pressure testing of piping or equipment. They are
- hydro-static testing and
- pneumatic testing
What is Hydro-Test or Hydrostatic Test?
Hydro test or Hydrostatic test is a type of pressure test performed on piping and pressure vessels to check system integrity under pressure condition. A hydrostatic test is performed by using water as the test medium.
What is the Pneumatic Test?
The pneumatic test is a type of pressure test for checking system integrity under the pressurized condition and applied to systems where the hydro-static test is difficult to apply. A pneumatic test uses air, nitrogen, or any non-flammable and non-toxic 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 the 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 the 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 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 a detrimental way after taken in service due to severe extended service conditions.
Comparison of Hydrostatic and Pneumatic Testing
The following table lists down the major differences between hydrostatic and pneumatic testing.
|Hydrostatic Testing||Pneumatic Testing|
|Test pressure is normally 50% 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 the 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 a sudden increase in pressure during testing||Pressure relief devices are must during the test to ensure no over-pressurization|
|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 the 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 places after testing||Test media can not be reused or transferred to other places 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 the same time (example pipelines )||If pipelines 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 worksite||Needs special attention and safety precautions|
|Pressure changes finite amount by an infinitesimal change in volume||Pressure change proportional to volume change|
Difficulties with hydrostatic testing
- Supply & disposal of water, disposal of fluid additives
- A water leak can cause equipment damage
- Freeze susceptibility
- Structural support limitations
- Operational impacts – process contaminant
- Affect dry-out of internal refractory linings
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:
- When the systems are designed in such a way that 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.
How to calculate Hydrotest 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 the test temperature
Sd=allowable stress value at design temperature.
The maximum allowable value of St/Sd is 6.5
How to Calculate Pneumatic Test Pressure?
The pneumatic test pressure shall be as per the following equation:
The basis for Pressure Test Method Selection
Normally the following basis is followed in the process piping industry for the selection of hydro or pneumatic tests.
- If the fluid handled by the piping system is liquid then the pipe must be hydro tested.
- If the fluid handled is vapor or gas then the internal design pressure dictates the testing method. For pipe design pressure 10 bar and above the pipe is hydro tested. For pipe design pressure below 10 bar pipe is pneumatically tested.
- If fluid handled is stream then hydro testing is suggested.
- If the fluid handled is two-phase flow then hydro testing is suggested. However, for big size flare headers, pneumatic testing is performed.
Pressure Test Duration
Normally every EPC company has its own standard for the duration of tests. However, the recommended practice is that a QC inspector has 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. The duration of this activity varies with the span of the piping system. For larger piping system time taken for this activity is enough to clear the pressure test. In the case of a piping system having a smaller span, 1 hour time may be made as standard practice for hydrostatic testing. For the Pneumatic test, the test time is far lesser.
Few important points to consider while hydro-testing:
- 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 the hydro test in case of big-bore, it should be decided first that line is to be hydro tested on rack or grade if the hydro test is done on the rack, the hydro test load should be considered in case of vapor lines while providing structure/rack loading information.
As the name signifies, the test has to be performed using clean potable water free from suspended solids. However different codes specify different requirements for water quality. Hence, hydro tests need to be performed following those instructions.
Hydrotest system preparation
The piping network and the connected equipment must be prepared thoroughly before proceeding for pressure tests like hydro or pneumatic testing. 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.
Hydrotest for 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.
Hydrostatic Testing Equipment
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.
Hydrostatic Testing Problems
Even though proper industry-approved methods are followed for pressure testing, still due to some negligence hazards do happen. Click here to know about 5 such hazards.