Testing requirements for piping systems are covered in para. 345 of ASME B31.3 code. As per ASME B31.3, prior to initial operation and after completion of applicable examinations as per para. 341, each piping system shall be tested to ensure tightness. The recommended test is a hydrostatic leak test.

If a hydrostatic leak test is deemed impracticable by the owner, then either a pneumatic test or a combined hydrostatic-pneumatic test may be substituted, recognizing the hazards associated with stored energy in compressed gas. This article will discuss the hydrotest procedure for piping. The requirements related to pneumatic test will be covered in a separate article.

Hydrotest Fluid

The default hydrotest fluid in most cases is water. Water used for hydrostatic testing of piping shall be fresh, clean and free from suspended solids and other foreign matter. Water may not be used as a hydrostatic test medium under the following circumstances:

  • the presence of water in the piping system may have adverse effects during plant start-up and operation.
  • the piping material or its lining may deteriorate in the presence of hydrotest fluid.
  • there is a likelihood of test water to be contaminated during testing and its disposal may pose environmental hazards.
  • drop in ambient temperature may cause water to freeze during the test.

If the test fluid temperature produces condensation on the piping exterior surface, the water shall be heated to a temperature above the dew point or the test shall be delayed until such time that condensation will not occur on the external surface of the piping system under test. Water used for testing of stainless steel pipework shall not contain more than 30ppm chlorides. Some owners permit up to 50pmm chlorides and some owners impose a maximum limit of 15ppm chlorides. This caution is exercised to avoid pitting and stress corrosion cracking in 300-series stainless steels.

Water used for testing may be treated with chemical additives such as oxygen scavenger, biocide and corrosion inhibitor. Hence, disposal of test fluid shall be carried out in accordance with the local environmental regulations.

Flushing

Prior to the hydrotest, the system shall be flushed with the dedicated test medium to remove all dirt and foreign matter. Recommended methods of flushing are:

  • for lines up to 6” diameter, liquid volume flushing. For this method the minimum velocity in the largest pipe diameter to be not less than 3 meters/second.
  • for lines over 6” diameter, hydroblast by means of self climbing high pressure water jet nozzles (Dynorod method) or approved equivalent.
  • for air, halon and hydraulic lines, air blast cleaning ensuring a minimum velocity of 10 meters/second at the outlet end is achieved.

All necessary precautions shall be taken to avoid debris being pushed into associated equipment or “dead ends”.

Flushing shall proceed in an intermittent manner allowing time between flushes for debris to be cleared from line. Flushing shal continue for sufficient time to ensure that line is cleaned to the satisfaction of Company.

Pressure Test Manifold

A pressure test manifold is used for performing a hydrotest. The pressure test manifold has the required provisions to test and protect the piping system subject to hydrotest and comprises of the following:

  • Manifold provided with end caps suitable for the system pressure testing. It is recommended that all joints in a test manifold are of welded type and should be subject to 100% non-destructive examination by MPI and Radiography.
  • Pressure relief valve of adequate capacity to relieve the system of any overpressure.
  • A manual bleed valve for emergency bleeding and depressurisation of the piping system, should the relief valve fail.
  • A pressure gauge, pressure recorder and temperature recorder connection. The pressure gauge should be calibrated preferably within 4 weeks of the test being carried out. The hydrotest pressure should preferably fall within 35-75% of the full range of pressure gauge.
  • A connection to connect the hydrotest or pneumatic pump to the manifold.
  • A connection to connect the test manifold to the low point of the piping system for filling and pressurisation.
  • Hoses that connect the pumps to the test manifold and the test manifold to the piping system shall be rated for the test pressure and fitted with safety chains of sufficient strength that will prevent whipping of the hose, should the coupling get disconnected.
  • All valves and fittings on the manifold shall be of the next higher rating.
  • The strength of the test manifold should be validated by testing the manifold between 1.2 to 1.4 times of its rated system test pressure. Note that ASME B31.3 code does not provide any guidance on the recommended test pressure. These are based on construction industry practices.
Hydrotest Manifold
PRESSURE TEST MANIFOLD

Preparation and requirements prior to hydrotesting

  1. A pretest check shall be made to ensure that pipe/support “contact” has been achieved at all support locations. This includes temporary supports provided only for the purpose of hydrotest.
  2. The temporary supports should be designed (specially for large bore heavy piping) to ensure they are able to sustain the hydrostatic loads. This verification may not be critical for small bore piping systems. Removal of temporary support should be done only after the test is complete and the lines are fully drained.
  3. All lines shall be checked to ensure the entire system can be completely drained after testing.
  4. Vents or other high point connections shall be opened to displace air from lines once the water filling starts during the hydrostatic test.
  5. All joints including structural attachments shall be left uninsulated/primed and exposed during hydrostatic testing. Some company procedures may allow exceptions to this requirement subject to certain conditions being met such as extended test durations and use of highly qualified welders.
  6. All equipments which are to be excluded from the test shall be either disonnected from the system or positively isolated using spectacle blinds or blind flanges.
  7. Expansion joints and spring supports shall be provided with temporary restraints or stops to prevent excessive travel or deformation under the hydrostatic test loads.
  8. In-line valves should be in full open position during the hydrostatic leak test. Some companies recommend that ball valves should be in partial open position as this allows the valve body cavity to be subjected to the pressure test. 
  9. Orifice plates, spray nozzles and similar restrictions should normally be installed after completion of tests.
  10. When dismantling of piping is necessary to perform testing, less expensive gaskets suitable for the test pressure may be substituted. After completion of the test, the test gaskets shall be replaced by the gaskets required as per piping specifications.
  11. Prefabricated spacers and spool pieces shall be fabricated and supplied to the correct test pressure rating by the contractor for replacing in-line instrumentation during plant testing and flushing, except for equipment such as control valves, with butt welding ends (which will be installed initially without internals and provided with a blind flange). Special attention shall be given to the provision of spacers and spool pieces required for equipment packages.

Hydrostatic Test Pressure

The hydrostatic test pressure at any point in the system to be as follows :

Not less than 1.5 times the design pressure, except as provided in paragraph below.

For a design temperature above the test temperature, the minimum test pressure shall be calculated by the following equation, except that the ratio of ST/S shall not exceed 6.5.

PT = \(\frac{1.5PS_{T}}{S}\)

where :

PT =      Minimum Hydrostatic Gauge Pressure

P  =      Internal Design Gauge Pressure

ST =      Allowable Stress At Test Temperature

S  =      Allowable Stress At Design Temperature

(See Table I, Appendix A of ASME B31.3 for values of S and ST)

Where the test pressure as defined above would produce a stress in excess of 95 percent of the yield strength at test temperature, the test pressure will be reduced to the maximum pressure at which the stress will not exceed 95 percent of the yield strength at the test temperature.  The maximum test pressure at which the stress produced will not exceed 95 percent of the yield strength will be calculated by equation for internal pressure in para. 304.1.2 of ASME B31.3.

When determining the test pressure, due consideration should be given to the applicable static head in the system subjected to the test. The required test pressure should be achieved at the lowest point in the system. If it is achieved at the highest point, the test pressure at the lowest point will exceed the stipulated test pressure. Test pressures may be taken at any point on the system being tested, provided a correction for static head is agreed with the owners site representative. Where static head is significant, the datum elevation should be the lowest point being tested.

Preliminary Pneumatic Test

Para 345.2.1 of ASME B31.3 includes provision to perform preliminary pneumatic test using air at no more than 170 kPa (25 psi) gage pressure prior to performing hydrostatic test to locate major leaks.

Requirements during pressure test

During the pressure test, the pressure shall be gradually increased to allow time for material to equalize the strains. After the piping subject attains the required test pressure, it is recommended to stay clear from the test area for a period of 10 minutes before examining the joints and connections for leaks. As per para. 345.2.2 of ASME B31.3, a hydrostatic leak test shall be maintained for at least 10 minutes and all joints and connections shall be examined for leaks. Many company specifications prescribe a minimum hold time of 30 minutes to detect small seepage leaks and to permit a thorough inspection of joints and connections.

For internally coated piping such as FBE coated piping system, it should be ensured that coating is not damaged when subject to the full test pressure. 

When the test has to be maintained for a period of time during which thermal expansion of the test medium might occur due to changes in ambient temperature, the contractor should ensure that the test system is not subjected to a pressure greater than the specified test pressure.

A flanged joint at which a spectacle blind or blank is inserted to isolate other equipment during the test need not be examined for leaks. These joints should be subject to a flange management procedure and leak tested during initial service.

Requirements after pressure test

All piping should be completely drained of all test fluid. Special attention may be required to ensure draining of pockets where the test fluid is likely to be trapped. While draining the system due care should be exercised to open one of the vent valves to ensure that the piping system is not subject to vacuum and subsequent collapse. All piping tested with water, in which the presence of water is detrimental to the process start-up and operation should be thoroughly dried. The piping shall be blown-down with air to ensure that there are no trapped water pockets and the piping is dry.

Sensitive Leak Test

Where the Owner considers both hydrostatic and pneumatic leak testing impracticable, the alternative specified in para. 345.9 of ASME B31.3 which requires the system to be subjected to a Sensitive Leak Test may be used if both of the following conditions apply:

  • a hydrostatic test would damage linings or internal insulation, or contaminate a process which would be hazardous, corrosive, or inoperative in the presence of moisture, or would present the danger of brittle fracture due to low metal temperature during the test
  • a pneumatic test would present an undue hazard of possible release of energy stored in the system, or would present the danger of brittle fracture due to low metal temperature during the test

Piping required to have a sensitive leak test as per line list will be tested by the Gas and Bubble Formation Testing Method specified in Article 10, Section V of the ASME Boiler Pressure Vessel Code.  Sensitivity of the test shall not be less than 10-3 atm-ml/sec (100 Pa-ml/s) under test conditions.

The test pressure shall be at least the lesser of 1.00 bar (100 kPa) gage, or 25% of the design pressure.

The pressure shall be gradually increased until a gauge pressure the lesser of one-half the test pressure or 1.70 bar (170 kPa) is attained, at which time a preliminary check shall be made. Then the pressure shall be gradually increased in steps until the test pressure is reached, the pressure being held long enough at each step to equalize piping strains.

If a Sensitive Leak test is required, joints will also be left unprimed and unpainted.

Waiver of Hydrostatic Test

Waiver of hydrostatic test in most organisations requires approval of the owners integrity assurance department. A waiver of hydrotest may be given due consideration by the integrity department if it can be proven that the contractor has exhausted all possible options available to perform testing of the piping system.