Hydrostatic Testing of Pressure Piping
Hydrostatic testing of pressure piping is a mandatory activity before finalization of any new or modified piping system. It is the final check of mechanical integrity of the whole system and should be followed religiously as after this activity the piping system has to be commissioned. This test is carried out at a pressure 1.5 times higher than the design pressure of a system regardless of the service conditions of a piping system. This article discuss some of the major requirements of hydrostatic testing of pressure piping as per ASME B31.3 for process piping and specifically provides guidelines on some issues not directly addressed in the code.
Following the code, hydrostatic test has to be performed after all hot works have been completed on a certain piping system. When we say hot work that includes everything related to welding or the post weld heat treatment. Post weld heat treatment also has a potential of deteriorating the mechanical properties of piping, if not addressed properly. And this is the reason why code calls for NDT after PWHT operation. In case after carrying out hydrostatic 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 repair / modification may be waived off should be taken very carefully. Normally, the maximum extent of repair not requiring retesting shall be the tack welding of any piping support / pad. Anything else shall be done following a retest.
Hydrostatic 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.
Piping readiness for hydrostatic test shall be checked against standard documentation. Normally, during project activities, test packs are prepared before conducting any hydrostatic test. Same type of checklists for pre-checks should be made available if hydrostatic test is to be performed on a running plant. In addition to Project Engineers and QC Engineers, HSE Engineers also take a look at that. The very basic checks shall include checking of hydrostatic test limits and all the signed QC reports for weld joints, heat treatment and any other hot work. QC team shall also verify the limits of piping in the field.
Hydrostatic test limits shall not include any piping component which may hinder water filling, flushing, pressurization or draining. All instruments shall be taken off; all internals for check valves shall be removed preferably and if not possible, pressurization point shall be on the upstream and system should be drained from the downstream side. Preferably, all valves shall be in open condition and they must not be used as a test limit. If you are left with no option, then check that the valve seat test pressure (normally done at 1.1 of rated pressure) is greater than the system test pressure. Refer to 16.34 to gather such information then to proceed for inclusion of valve as a test limit in closed position. Strainers can be included whether with mesh or without mesh, no issues. Remove venturimeters, orifices or other flow measuring devices from the test limit.
Preparations of hydrostatic test shall be made very sensibly as these preparations have to face the same hydrostatic pressure which original piping would face. The most important thing to be noticed is the selection of blinds which are to be installed at test limits. Contractor assigned for conducting hydrostatic test shall be well versed with the requirements and shall select blinds very carefully. The recommended practice is to use standard blind flanges as per ASME 16.5 or 16.47 at the test limits but to make them available in different sizes & ratings for larger projects would not be an easy job. Custom blinds may then be prepared and made available at site. Attachment details shall be followed as per Fig. UG-34 of ASME Sec. VIII Div. 1 referred by ASME B 31.3. Thickness of blind flanges shall be calculated as per applicable formula mentioned in UG-34. This shall be verified by engineering department and then QC must also be consulted for any NDT if its attachment with piping needs any welding. Incorrect non-code compliant blinds may lead to any safety incident having fatality potential.
Duration of hydrostatic test as mentioned is 10 minutes as a minimum. The recommended practice of a QC inspector is 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 hydrostatic test. In case of piping system having smaller span, 1 hour time may be made as standard practice.
Leaks identification, if any, is easier for those systems where 100% excess to whole piping system is available and to declare the hydrostatic test OK does require much of working.
For buried piping, leak identification is tricky. It is recommended to use automatic pressure-temperature recorders and pressure must be kept for 24 hours. Pressure variations then may be examined to see whether they are due to temperature gradient or some leak is present. For this calculations are to be made from the formulas mentioned below:
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