Knowledgebase Article 1027

Question:

How does COMPRESS determine hydrostatic test pressure? Is the effect of the test static head on components such as ANSI flanges considered?

Answer:

There are four options for specifying the hydrostatic test pressure.

  • 1.3 times the vessel design pressure (P)
  • 1.3 times the vessel MAWP per UG-99(b)
  • 1.3 times the calculated test pressure (MAP) per UG-99(c)
  • User Defined Test Pressure (gauge, top)

Hydrotest at 1.3 times the vessel design pressure (P). The test pressure at the top of the vessel is reported to be 1.3 times the design pressure. For chambers with varying design pressures the lowest design pressure is used.

Hydrotest at 1.3 times the Vessel MAWP. COMPRESS determines the MAWP (maximum allowable working pressure) for each component. The MAWP is calculated by using the Code allowable stress at operating temperature and subtracting the static head acting on the component assuming the vessel is in its normal operating position. The smallest of the component MAWP's is taken to be the vessel MAWP. The component MAWP values can be found in the pressure summary. The vessel MAWP is then multiplied by the factor 1.3 to determine the test pressure required at the top of the vessel.

Hydrotest at 1.3 times the Vessel MAP. COMPRESS determines the MAP (maximum allowable pressure)for each component. The MAP is calculated by using the Code allowable stress at test temperature without consideration of the operating liquid static head. The smallest of the component MAP's is reported as the vessel MAP. The component MAP values can be found in the pressure summary. The vessel MAP is multiplied by the factor 1.3 to determine the test pressure required at the elevation of the governing component. The static head due to the test liquid is then subtracted from this pressure to determine the required pressure at the top of the vessel during the test.

User Defined Test Pressure (gauge, top). Using this option, the user specifies the test pressure. Static head due to test liquid on the individual components will be added to the value input by the user.

Note: For vessels designed prior to the ASME A99 Code Addenda, a hydrostatic test factor of 1.5 is used in place of 1.3.