Hydrostatic Leak tests of welded piping systems serve the following purposes: a] qualify joints not included in the UT or radiographic examination and b] determine the leak tightness of the welded and flanged joints.


Before a plant facility is put into operation, each piping system is subject to a hydrotest leak test after all the applicable examinations are carried out in accordance with the code. Following exceptions are permitted by ASME B31.3 code:

  • Initial service leak test in lieu of hydrostatic leak test may be carried out for piping systems in Category D fluid service.
  • A pneumatic test or a combined hydrostatic-pneumatic test may be proposed as an alternate test if the owner considers a hydrostatic leak test to be impracticable. The hazards associated with such tests (due to stored energy in a compressed gas) shall be taken into consideration whle carrying out such tests.
  • If the piping system cannot be subject to hydrostatic leak test (e.g. a hydrostatic test would damage the lining or contaminate a process fluid or require extensive modifications to pipe support design) and a pneumatic test would present an undue hazard (e.g. due to release of stored energy or present a danger of brittle fracture due to low temperature during test), the alternate leak test may be proposed for owner's approval.
  • Lines open to atmosphere such as vents or drains downstream of last shutoff valve need not be leak tested.

Hydrostatic Leak Test Exceptions

Hydrostatic Test Pressure

The hydrostatic test pressure at every point in a metallic piping system shall not be less than 1.5 times the design pressure.

When the piping system design temperature is greater than the test temperature, the minimum test temperature shall be calculated using the equation: 

Test Pressure Pt = \(\large\frac{1.5*P*S_t}{S}\)....................1(a)

P = Design pressure

St = Allowable stress at Test temperature for the piping material

S = Allowable stress at Design temperature for the piping material

Pt = Minimum Test pressure

The 2004 edition of code stated that the value of St/S shall not exceed 6.5. This limitation has been removed from subsequent versions of the code.

  • Testing pressure may be reduced to the maximum pressure that will not exceed yield strength at test temperatures.
  • Piping subject to external pressure shall be tested at an internal pressure of 1.5 times the differential pressure, but not less than 105 kPa (15 psig).

Example: Hydrotest Pressure for elevated Design Temperature

Problem: Determine the hydrotest pressure for a NPS 8 (DN 200) ASTM A106 Grade B pipe with nominal thickness of sch 40 (8.18 mm). The design pressure of the pipe is 400 psig and design temperature is 500°F. The test temperature is 100°F. Consider zero corrosion allowance, E = 1, W = 1 and Y = 0.4. Mill tolerance for ASTM A106 Grade B pipe is 12.5%.


Step 1: Calculate Test Pressure

From Table A-1 of ASME B31.3 - 2018 edition, St = 20 ksi and S = 18.9 ksi @ 500°F

As per above equation 1(a), Test pressure Pt = \(\large\frac{15*400*20}{18.9}\) = 635 psig

Step 2: Confirm Hoop Stress for minimum wall thickness is less than Yield Strength

The next step is to perform a pressure stress or hoop stress calculation to determine if the test pressure of 635 psig calculated above would produce stress value less than the yield strength of ASTM A106 Grade B material at test temperature.

The yeld strength at 100°F for ASTM A106 Grade B pipe is 35 ksi. 

We know Pressure design wall thickness (refer this article) t = \(\large\frac{PD}{2(SEW + PY)}\)....................2(a)

Rearranging above equation 2(a) we get SEW = \(\large\frac{PD}{2t}\) - PY

Substituting E =1, W = 1 and Y =0.4 we get S = \(\large\frac{PD}{2t}\) - 0.4P....................2(b)

Minimum wall thickness is calculated as:

t = 8.18 * (1 - mill tolerance) = 8.18 * (1 - 0.125) = 8.18*0.875 = 7.158 mm.

OD of pipe, D = 219.1 mm

Substituting in equation 2(b) gives

Hoop Stress, S = \(\large\frac{635 * 219.1}{2*7.158}\) - 0.4*635 = 9718.4 - 254 = 9464.4 psi = 9.46 ksi.

Hoop stress 9.4 ksi is less than yield strength of 35 ksi.
Hence test pressure of 635 psig is acceptable.

Pneumatic Test

The use of hydrostatic leak test may not be permissible in some applications such as a piping system using internal insulation or refractory lining. In such cases, the piping system may be subject to pneumatic testing subject to owners approval. 

  • Pneumatic test pressure shall not be less than 110% of design pressure.
  • A pressure relief device shall be provided with a set pressure not higher than either 110% of the test pressure or the test pressure plus 345 kPa (50 psi).
  • Pneumatic test procedure includes a gradual increase and an intermediate pressure hold at the lesser of half the test pressure or 170 kPa (25 psig) for a preliminary check, including examination of all joints.
  • Hydrostatic-pneumatic if performed shall comply with the requirements of the pneumatic test, except pressure in the liquid filled part of the system is limited by the hydrotest pressure limit.
  • The test fluid, if not air, shall be nonflammable and nontoxic.

Initial Service Leak Test

Initial service leak test is limited to Category D fluid service. The test fluid is the service fluid and test pressure is the operating pressure. 

Sensitive Leak Test

When the owner determines that both hydrostatic and pneumatic testing and test pressures are not achievable, then a sensitive leak test in accordance with the Gas and Bubble Test method specified in Boiler and Pressure Vessel Code, Section V, Article 10 or another test demonstrated to have equal sensitivity may be used. The sensitive leak test is a test at low pressure to check for leaks and the requirements as per the code are:

  • To be performed in accordance with the gas and bubble test method in BPV Code Section V.
  • Test pressure to be at least the lesser of 105 kPa (15 psig) or 25% of the design pressure.
  • The pressure is to be increased gradually, with an intermediate pressure hold for a preliminary check at the lesser of half the test pressure or 170 kPa (25 psig).
  • The sensitive leak test is prescribed in the Alternative Leak Test.

Alternative Leak Test

A final method to test the adequacy of welds and mechanical joints for leakage as allowed by ASME B31.3 is the Alternative Leak Test.

The Alternative Leak Test is permitted when:

  • Exposure of the piping to water would damage linings or internal insulation or would contaminate a process which would be hazardous, corrosive or inoperative in the presence of moisture or
  • A hydrotest would present danger of brittle fracture due to low metal temperature during the test and
  • When a pneumatic test is considered by the owner to entail an unacceptable risk due to the hazard of possible release of stored energy in the system.

For example, explosive rupture of 60m (200 ft) of NPS 30 line containing air at 3500 kPa (500 psig) creates a blast wave roughly equivalent to 35 kg (80 lbs) of TNT.

Alternative Leak Test Requirements

  • Requires owner's approval.
  • 100% radiography of circumferential, longitudinal and spiral groove welds.
  • 100% liquid penetrant or magnetic particle examination of all other welds including structural attachment welds that cannot be radiographed.
  • Formal flexibility analysis of the piping system.
  • The system shall be subjected to Sensitive Leak Test.

Miscellaneous Leak Test Requirements

  • Pressure must be held a minimum of 10 minutes, and then may be reduced to the design pressure and held for such time as necessary to examine all of the joints for leakage.
  • All joints and connections shall be examined for leaks during the test.
  • All joints to be tested shall be left exposed.
  • All joints must be left unpainted for sensitive leak test.
  • All joints to be tested must not be covered with insulation.
  • Special consideration is given to expansion joints as per section 345.3.3.
  • Provide additional support, if required for hydrotest fluid weight.
  • Spring supports should be in locked condition during leak test.
  • A threaded joint that contains a thread compound or lubricant that leaks during leak testing may be seal welded provided all compound is removed from exposed threads.
  • Hydrostatic test pressure may be reduced to as low as 77% of the required piping test pressure if required to prevent exceeding the test pressure of a vessel that must be included in the hydrotest.
  • Test records are required, per paragraph 345.2.7 of ASME B31.3, but need not be retained after completion of the test if a certification by the inspector that the piping has satisfactorily passed pressure testing as required is retained.