Piping and Vessels Preparation for Commissioning - Part 2

Continue of Part 1 - Introduction.

2. Leak tests

2.1 General

In no case are leak tests to be confused with pressure tests/hydrotest; the latter are carried out on spools during precommissioning in order to verify the integrity of welds and material. Leak tests are carried out on entire systems, after flushing and cleaning operations, and completion of the installation, to prove the integrity of the following:

  • Gaskets and other flange joints
  • Valves packings
  • Miscellaneous connections such as vents, drains, safety valves, instruments
  • Connections between the system being tested and inter-related systems.
  • Integrity of the critical valves (“functional seal” leak test)

Leak tests must also establish that any equipment associated with the system, which may have been previously dismantled for any reason, has been correctly reinstalled. At last leak tests should also allow to detect passing valves.

2.2 Test specifications

2.2.1 Equipment to be leak tested General rule

Every piping, vessel, process and utility equipment, and all their associated connections should be leak tested. A leak detection system by measurement of helium traced in nitrogen should be used especially in the following cases: presence of H2S in gas, low temperature systems, confined areas. Critical valves passing leak tests

A list of critical valves should be defined according to list as follows (but not limited to):

  • Emergency Shut Down Valves
  • Blow down valves
  • Any critical controlled or manual valves which could induce hazardous situation during operation if leaking (pig trap manual valves, etc). These valves should be tested according to a dedicated procedure in order to check if the valves are passing or not. Exceptions

Flares stacks and vents, heaters casings, sprinklers, halon piping and deluge network downstream the control valves, atmospheric storage vessels, ancillaries of packages previously run at the Manufacturer's.

2.2.2 Leak test pressure General rule

Leak tests should be carried out at the equipment Maximum Normal Operating Pressure (if not defined, 90% of the design pressure should be considered). Exceptions

  • Pumps should be isolated and tested at their maximum suction operating pressure.
  • Centrifugal compressors should be isolated and tested at the compressor shutdown balance pressure.
  • Systems working under vacuum should be leak tested at 10% below the set point of the pressure safety valves.

2.2.3 Tightness criterion General rule

The leak test should be considered successful if, at the test pressure, the fall in pressure is equal or less than 0.05 b/hr. Case of systems operating under vaccum

The test criteria should be a pressure increase equal or less than 0.02 b/hr. Case of process gases with more than 100 ppm H2S

The accepted fall in pressure should be reduced to 0.03 b/hr.

The achievement of the tightness criterion should be in any case complemented with a leak detection system by measurement of helium traced in nitrogen, the tightness criterion should be as follows: every tested point should have a leakage rate <50 SCF/year.

2.2.4 Systems test limits General rule

The test limits should be the subsystem limits as indicated by the commissioning marked-up PID's which define the subsystems. Exceptions

The subsystem may be divided into portions, in the case where the size of the subsystem makes the leaks search unmanageable.

The test limits should take into account the different levels of operating pressure inside a subsystem, hence there should be at least one test by level of operating pressure.

It may be practicable to carry out a single test encompassing the facilities of two or several subsystems, when those are of limited size, and operated at the same pressure.

Test subdivisions should be planned in such a way that no flanged joint is disturbed following the leak test.

As limit valves cannot be relied upon to be bubble tight, the installation of blinds should be preferred wherever possible. These system limits isolation blinds should have the appropriate thickness.

2.2.5 Pressure recording

The system should be provided with suitably-ranged, calibrated pressure gauges. These gauges must be correct to within 1%.

Preference should always be given to the use of permanently installed instruments provided they have been calibrated, that their accuracy is compatible with the test criteria, and that a recording means is available (DCS print-outs being acceptable).

If no recording mean is available by construction, the system should be fitted with a portable chart/pen recorder with either circular or ribbon type chart paper. Such instruments are powered by clockwork or low voltage DC batteries. The pressure recorder impulse line should be connected to any convenient point on the system, such as a vent or drain.

2.2.6 Relief systems

In all cases the system must be protected from overpressures which can easily occur due to human error, instrument failure, etc.

On all medium and high pressure systems the equipment must be protected by the normal system safety valves or a temporarily installed safety valve if the system in question does not comprise a safety valve.

For systems which normally operate at very low pressures, or under vacuum, the temporary installation of a barometric seal leg, capable of holding back the test pressure, is recommended.

This type of protection is required on vacuum systems as they are first subjected to a low pressure leak test.

2.2.7 Principle of fluid to be used

The principle is to test a system by using an inert fluid close as far as possible of the characteristic of the target fluid.

  • Non-Hydrocarbon system: All “non Hydrocarbon” system should be tested with the target fluid, as follows:
    • water should be used for Fire water, water services
    • air should be used for air services
    • nitrogen should be used for nitrogen services
    • etc.
  • Hydrocarbon system: For the system processing hydrocarbon or chemical, principle should be defined as follows: 
    • oil system at low pressure (#150 series), assumed as single phase oil to be tested with water - chemical product system, particularly those one blanketed by gas or nitrogen to be tested with nitrogen 
    • Sweet gas system (fuel gas, gas treatment), propane, single phase oil from #300 series and multi-phase oil to be tested with nitrogen 
    • All system processing fluid with presence of H2S to be tested with nitrogen traced with helium

For technical reasons, nitrogen with traced helium should be used for sweet gas system & multiphase oil, particularly in the case of high pressure (from #600 series).

2.3 Test procedures

2.3.1 Detailed procedure

Commissioning should issue, for each test, a detailed leak test procedure including:

  • Marked up PID's and sketches showing test limits and isolations
  • Step by step action check lists
  • List of safety precautions.

These procedures should include in particular the specific points addressed hereafter.

2.3.2 Pressurization points

  • A suitable vent or drain line should be selected and a line connected from the pressurization system to the system to be tested.
  • More than one pressurization point may be used for large volume systems.
  • When leak-testing water systems, the normal supply may be used.

2.3.3 Low pressure leak test typical procedure

This typical procedure applies to all systems the maximum operating pressure of which can be reached by the instrument air or service air supply.

The test should be performed as follows:

  • Line up the system to be tested (i.e. all system isolations fitted, all parts of the system connected, safety valves operational, all instrument valves opened, drain valves closed, etc.).
  • Pressurize the system to its maximum operating pressure and place a new recorder chart on the pressure recorder. Ensure the chart is turning. The chart should be marked with the system number, date, and time of test start.
  • Commence checking flanges, valve gland packings, vent plugs, etc., for leaks using soap solution. Pull-up flange bolts, etc., as required. If necessary, change gaskets or packings (after depressuring the system).
  • When a stable pressure indication is achieved, mark the recorder chart. The test should be continued for at least 4 hours, preferably during daytime when ambient temperatures are relatively steady.
  • Following a successful test, depressure the system and attach the pressure recording chart to the commissioning file documentation. Remove limit blinds as required and delete them from the blind list.

2.3.4 High pressure leak test typical procedure

Whenever the maximum operating pressure of the system is above the available air pressure, other means of pressurization are required to perform the leak test.

Nitrogen should be used (either from nitrogen cylinders or liquid nitrogen containers handled by a specialized Contractor) as the leak testing means.

This high pressure nitrogen test should be synchronized with the inerting of the system and of other lower operating pressure systems.

Following a pressure test carried out with Nitrogen, the Oxygen content should be around 1% (and lower for high pressures). Therefore the N2 high pressure leak test should normally be sufficient for inerting purposes.

However if very low O2 contents are necessary (this is often the case with catalysts where O2 ≤ 0.2% is required) it may be necessary to inert partially the system before leak testing. A very simple calculation should give the O2 concentration to be reached before starting the leak test.

2.3.5 Case of systems operating under vacuum

The leak tests should be carried out in two stages:

  • 1st stage: using instrument air, by pressurizing the system up to 10% below the set point of the PSV
  • 2nd stage: under normal operating vacuum conditions by running the installed vacuum system.

Typical procedure

If considered necessary, install a barometric seal leg to protect the system from overpressure and isolate vacuum pressure gauges and transmitters.

Pressurize the system to the desired positive pressure using instrument air.

Soap test flange joints, valves, etc., and rectify leaks as necessary.

Following leak rectification carry out a four-hour hold-test using a suitably calibrated pressure gauge. If hold-test fails, the leaks must be found and rectified. If successful, de-pressure the system, remove the barometric seal leg and disconnect the air pressurization line, de-isolate pressure gauges and transmitters.

If a portable vacuum recorder is available it should be connected at this time. If not it may be possible to use the systems normal instrumentation to record the vacuum test. If neither are available a calibrated vacuum gauge must be installed at a convenient point in the system.

Ensure that the vacuum equipment for the system has been correctly commissioned and is ready for use. Following the normal operating procedures, start up the equipment after lining-up the system correctly (condensers must be also in line).

Note: This equipment could be liquid ring pumps, reciprocating vacuum pumps or steam/air ejectors or a combination of this equipment.

Pull a vacuum on the system which is equal to the normal operating vacuum. Shut down the vacuum producing equipment and isolate completely. Allow the vacuum to stabilize for one hour.

At the end of one hour, start recording the vacuum in the system, either by means of a recorder chart or by pressure gauge and graph. If the latter method is employed, readings must be taken every 15 minutes.

Whilst recording the vacuum, pay close attention to leak checking. Flange leaks are normally easy to find due to a high pitched squealing noise created by the air flowing into the system. Pinpoint leaks with shaving foam. Valve packing should be rigorously controlled for leaks.

During a vacuum test of four hours, and following rectification of leaks the loss of vacuum recorded must not exceed 15 mm Hg or 0.02 bar per hour.

Always endeavour to carry out the test when the air temperature is stable.

When test is satisfactorily completed, attach all relevant test documentation to the commissioning file. In certain cases the vacuum may have to be "broken" with dry inert gas, process gas or dry air to maintain a positive pressure, thus preventing the ingress of wet air until start-up time. Special procedures should be issued in all cases.

2.3.6 Case of water systems at atmospheric pressure

These systems (chemical storages, open drains, oily water treatment plants, etc.) should be simply filled in with drinking water and visually searched for leaks.

2.3.7 Case of pressurized water systems

(Fire water and cooling water pumps, water injection network, deluge network, etc.).

These systems are not normally subjected to complicated leak tests and no pressure recording is necessary in this case, however, the following guidelines should be respected:

  • Carry out a flange check to ensure all are bolted up tightly.
  • Ensure all vents, drains, etc., are closed and capped or plugged.
  • If the system is provided with pressure control instrumentation, ensure that this has been correctly commissioned and is ready to function.
  • Check that the water supply pumps, i.e. cooling water or firewater pumps, are ready for service.
  • Start the water supply pumps and slowly raise the system pressure to normal. If the system is empty at the start of the test, be careful not to overload the pumps. Normally it should be necessary to open only partially the pump discharge valve until the system is filled and under normal operating pressure.
  • Ensure the pressure control system (if provided) functions correctly.
  • Carry out visual checks of piping flanges, valves packings, etc., to check for leaks and rectify as required. If a leaking flange cannot be "pulled up", possibly due to a damaged gasket or flange face, the water supply pumps must be shut down, the system depressured and drained, and the leak rectified.

2.3.8 Flange taping/Soap testing

Systems operated above 10 bar and critical systems (i.e. H2S service) should have all flanged connections taped using commercial grade masking tape in widths suitable for various flange sizes and ratings. The flanges thus taped should first be scrupulously cleaned using, if necessary, a solvent to remove surface dirt and grease.

After application of the masking tape, a small hole should be punctured into the space between the flanges. The hole should be about 2 to 3 mm and preferably on top of the flanges for horizontal piping.

When the system has been pressurized a soap solution is applied to the hole in the flange tape and, if leaking, a bubble is formed.

Special soap solutions are obtainable for flange testing but if not available on the site a solution should be prepared using detergent washing-up liquid and suitable squeeze bottles.

When leak testing, systems which operate under vacuum joints may also be taped, and ordinary shaving foam used to indicate leaks. This foam is also useful for pin-pointing leaks on untaped joints, valve packings, etc.

2.3.9 Critical valves passing leak test typical procedure

In order to optimize the leak test operation, the critical valves passing leak test may be carried out at the end of the global leak test by partial depressurization of the section downstream the critical valve to be leak tested.

The check may be carried by controlling the pressure drop upstream the critical valves or the leak measurement (external measurement or internal with using ultrasonic equipment).

At the end of the passing leak test, a particular attention should be paid to avoid a “wild” depressurization of the section by opening the tested critical valve it self (by-pass or additional depressurization point should be used).

2.4 Reports

2.4.1 General rule

The leak tests should be organized by subsystem, hence all test procedures, drawings and reports related to the leak testing of a given subsystem should be filed in the corresponding commissioning dossier. The reports should also be complemented by a commissioning leak test form (see attachment).

2.4.2 Exception

The leak test may be organized by levels of pressure, which should not exactly match the subsystem limit. A specific Leak Test Report may therefore be compiled and handed separately to the Operator, to which reference should be made in every related subsystem commissioning dossier.

Continue to Part 3: Dry Out.

Tags: Piping Vessel Commission